Alkanes

Alkanes are saturated hydrocarbons. By saturated hydrocarbons, it

means alkanes have single hydrogen and carbon atoms in their

chemical formula. Formula of alkane is CnH2n+2. Methane, propane,

ethane, and butane are four alkanes. Let us learn in detail about

Alkanes and their physical properties below. 

Hydrocarbons

Let us first begun by understanding what are hydrocarbons.

Hydrocarbons are compounds that are made up of atoms of carbon and

hydrogen exclusively. The unique nature of the carbon ensures that it

shares a strong covalent bond with hydrogen. Since carbon atoms will

make long chains with relative ease, hydrocarbons can be very big

molecules linking even hundreds of atoms.

Actually, all living things or things that were once alive are actually

made up of hydrocarbons. Some other most common examples are

fossil fuels, natural gas, coal, petrol, and even the butter and cooking

oil we use.

Broadly there are two types of hydrocarbons, namely saturated and

unsaturated. Saturated hydrocarbons have a single bonding between

their atoms. Which means only one pair of electrons is shared between

any two atoms of the compound. Unsaturated hydrocarbons can have

double or triple bonds.

Browse more Topics under Hydrocarbons

● Conformation of Alkanes

● Nomenclature and Preparation of Alkenes

● Nomenclature and Preparation of Alkynes

● Nomenclature and Preparation of Aromatic Hydrocarbons

● Properties of Alkenes

● Properties of Alkynes

● Properties of Aromatic Hydrocarbons

Alkanes Definition and Formula

Alkanes are another name of saturated hydrocarbons. This means they

only have carbon and hydrogen atoms in their chemical formula. And

these atoms are bonded by single bonds only. That means all atoms

share only one pair of electrons with each other.

The general formula for alkanes is CnH2n+2

Here n is the number of atoms of carbon in their chemical structure.

So accordingly the number of hydrogen atoms is 2n+2. This chemical

formula will stand true for all saturated hydrocarbons.

The simplest alkane is methane which is CH4. Here one atom of

carbon is bonded to four atoms of hydrogen with single bonds. So the

four valence electrons of carbon will bond with the one valence

electron of each hydrogen atom. And so a completely saturated

hydrocarbon comes into being.

List of Alkanes

● Methane (CH4)

● Ethane (C2H6)

● Propane (C3H8)

● Butane (C4H10)

● Pentane (C5H12)

● Hexane (C6H14)

● Heptane (C7H16)

● Octane (C8H18)

● Nonane (C9H20)

● Decane (C10H22)

Physical Properties of Alkanes

● Alkanes are non-polar compounds. The difference in the

electronegativities of Carbon and Hydrogen is almost

non-existent, hence they have an almost complete absence of

polarity.

● Alkanes generally have relatively lower boiling points and

melting points. This is because their atoms have weak Van Der

Waals force and so the atomic bonds break easily.

● However, as the molecules get bigger the force gets stronger.

So more complex alkane has higher boiling and melting points.

● They can exist as solids liquids and gases in their natural states.

Unbranched alkanes usually are gases in their natural state. The

examples are methane, ethane etc. The alkanes bigger than

hexadecane are all solids.

● Also, they are completely insoluble in water, again due to the

weak van der Waal forces.

● However, they are soluble in organic solids. Here the van der

Waal forces of alkane break and are replaced by newer van der

Waal forces.

Solved Examples for You

Question: Which of the following alkane(s) do no show isomerism?

A. Methane

B. Ethane

C. Propane

D. All of the above

Solution: The correct option is “D”. Methane, ethane, and propane do

not exhibit any isomerism. The higher alkanes, butane onwards,

exhibit chain isomerism.

Conformation of Alkanes

Conformation of Alkanes deals with the isomers of alkanes that form

due to slight changes in their structure, especially in their

carbon-carbon bonds. The conformations start with ethane and occur

in all alkanes higher than ethane. Let us take a look. 

What is Conformation of Alkanes?

In the organic chemistry, it is found that you can rotate a single bond

of a molecule to change the arrangement of atoms, without changing

the chemical formula of the compound. So isomers of a compound can

be created by different forms of rotation about one single bond. So the

different arrangements of atoms due to such rotations of bonds are

known as conformations.

Browse more Topics under Hydrocarbons

● Alkanes

● Nomenclature and Preparation of Alkenes

● Nomenclature and Preparation of Alkynes

● Nomenclature and Preparation of Aromatic Hydrocarbons

● Properties of Alkenes

● Properties of Alkynes

● Properties of Aromatic Hydrocarbons

Conformation of Alkanes

As we know alkanes have a simple C-C single bond in higher alkanes.

There is no such bond in methane, but ethane onwards all alkanes

have a C-C single bond. So this allows rotation of this single bonds to

form a different spatial arrangement of carbon atoms, forming

different conformations of such alkanes.

Now ideally this rotation is limitless and thus an indefinite number of

conformations should be able to form. But there is a torsional strain to

consider which is a resistance to bond rotation. This strain occurs due

to the repulsive interactions of the other bond in alkanes, which is the

C-H bond. So this will limit the number of conformations that can

form.

Conformations of Ethane

(Source: inorganicchemistry)

As we saw before there can actually be an unlimited number of

conformations when it comes to ethane, due to its C-C single bond.

But we will be focusing on two important conformers of ethane. These

two conformations are the two most extreme and opposite possible

conformations of ethanes.

When you see an ethane structure you will notice the single C-C bond.

And every atom is also bonded to three atoms of hydrogen each. Now

in the first conformation, the hydrogen bonded to the first carbon atom

will line up with the hydrogen of the second carbon atom. The angle

between the two planes i.e. is the dihedral angle is 0°. So if you see

the first hydrogen atom can be said to be eclipsing the second one.

Therefore this is known as Eclipsed Conformation.

Then there is the exact opposite of the eclipsed conformers. Here the

hydrogen atoms bonded to the carbon atoms are as far away from each

other as possible. The dihedral angle between the two concerned

hydrogen atoms here is 60°. This conformation of alkanes also has a

name, staggered conformation.

In ethane conformations, the eclipsed conformation is unstable. The

two hydrogen atoms that line up together have unfavourable

interactions, they repel each other. So staggered conformation of

ethane requires less energy to be maintained and is more commonly

found.

Representation of Conformation of Alkanes

Newman Projections: This is one of the ways to represent

conformations of alkanes. It visually represents the bonding of atoms

from front to back. A Dot represents the first carbon atom (the one

that is nearer). The further carbon atom is a circle. Lines show the

hydrogen atoms bonded to each carbon atoms. This representation

clearly indicates the dihedral angles.

Sawhorse Projections: This represents the conformations of alkanes in

form of a straight line. The first carbon is at the lower end of the line

on the left side. And the second carbon is at the upper end of the

diagram. Here, the lines attached to carbon atoms show hydrogen

atoms.

Solved Examples for You

Question: On mixing certain alkane with chlorine and irradiating with

ultraviolet light, it forms only one monochloroalkane. This alkane

could be:

A. Propane

B. Pentane

C. Isopentane

D. Neopentane

Solution: Option D. The number of monochloro products formed

depends on the number of different sets of hydrogen.

● In propane; CHa3−CHb2−CHa3 , there are two different types

of hydrogens as labelled.

● For pentane; CHa3−CHb2−CH2b−CH2b−CHa3 also there are two

types of hydrogens

● In isopentane; CHa3−CHc(CH3a)−CH2b−CHa3 , there are three

different types of hydrogens.

● In neopentane; CHa3−(CH3a)C(CH3a)−CHa3 , there is only one

type of hydrogen.

Thus amongst the given options, neopentane gives only one

monochloroalkane on chlorination.

Nomenclature and Preparation of Alkenes

Alkenes are necessary chemical compounds that exist in our homes

and daily lives. However, we have never really given much thought to

it. Let’s discuss some very common example of how alkenes have

found their way to our home. We use different forms of plastic bottles

and containers of plastics for storage purpose. Several times we have

come across alkenes like ethane and propene. However, are you aware

of the fact that these simple alkenes are important components of

polystyrene and polyvinyl chlorides (PVC)? These chemical

compounds are necessary for manufacturing many types of plastics.

Alkene such as 1, 3- butadiene is used for making rubber products. In

this topic, we will study the different methods of preparation of

alkenes, a simple but necessary group of hydrocarbons. 

Alkenes

By now, we know that alkenes are important components of rubber

and plastic products but what are alkenes. They are organic

compounds containing double bonds in their chemical structure. They

are interchangeably known as olefins. Alkenes are class of unsaturated

hydrocarbons containing carbon and hydrogen atoms having one or

more carbon-carbon double bonds in its chemical structure.

The unsaturation is because of the presence of one or more double

bonds in its structure. The general chemical structure of alkene is

RC=CR’. Acyclic alkenes are the subdivision of alkene containing

only 1 double bond. However, it lacks any other functional groups,

also refers to as mono-enes.

The general formula of the homologous series of hydrocarbon is

CnH2n. Hence, alkene contains 2 hydrogen atoms less than its

corresponding alkane. Ethylene having the formula C2H4 is the

simplest alkene. The IUPAC name of ethylene is ethane and it has the

largest industrial production among all the alkenes.

Browse more Topics under Hydrocarbons

● Alkanes

● Conformation of Alkanes

● Nomenclature and Preparation of Alkynes

● Nomenclature and Preparation of Aromatic Hydrocarbons

● Properties of Alkenes

● Properties of Alkynes

● Properties of Aromatic Hydrocarbons

Nomenclature of Alkenes

We have to select the longest chain of carbon atoms having the double

bond in order to name an alkene according to the IUPAC system. We

start numbering from the carbon chain near to the double bond. “Ene”

replaces “ane” suffix present in the alkanes. Even though the first

compound of ethane is methene but it has a short lifecycle.

Therefore, the first stable compound is ethylene or ethene (C2H4).

Nomenclature or naming alkenes is very similar to alkanes according

to the IUPAC system. Let us once go through the summary of the

rules for nomenclature of alkenes with an example.

Rule 1

Identification of the longest continuous chain containing the

carbon-carbon double bond is necessary. The parent name of the

alkene compound will be similar to its corresponding alkane. Just the

suffix “ene” will replace the suffix “ane”, in case of the alkene. Suffix

“ene” represents the double of the alkene. For instance, if an organic

compound continuous chain having a double-bond containing five

carbon atoms, the compound will be pentene.

Rule 2

The numbering of the carbon atom starts on the side nearest to the

double bond of the continuous chain. Refer to the example below:

Hence, the numbering starts from right to left because only in this

arrangement the double bonds remain in between the second and third

carbon atom of the chain. On the contrary, if numbering begins from

left to right, the placement of double bond will be in between the third

and fourth carbon of the chain.

Hence, the rule of numbering from the nearest side of the double bond

will not be possible. Therefore, the numbering starts from right to left

in the above example. Moreover, it is important to indicate the

position of the double bond. Thus, it is necessary to place the lower of

the pair of numbers to the double containing carbon atoms before the

name of the alkene. Therefore, in the above example, the name of the

compound can be written as 2-pentene.

Rule 3

It is important to specify the location as well as the name of any

substituent molecule present in the compound. For example, the below

example can be written as 5-chloro-2-hexene.

Rule 4

Another important rule to note in the nomenclature of an alkene is to

correctly indicate the three-dimensional relationship of the organic

compound, in this case of the given alkene. This means that whether

the compound containing double bonds is present in cis or trans

conformation. Therefore, to complete the nomenclature the

compound, assigning the conformation is important. For example, the

below example is cis-5-chloro-2-hexene.

Few alkenes and their IUPAC names are:

Preparation of Alkenes

Preparation of Alkenes from Alkynes

Alkynes undergo partial reduction with a particular quantity of

dihydrogen to produce alkenes. The reaction takes place in the

presence of palladised charcoal that has been deactivated with

poisonous compounds such as quinolone or sulfur compounds, also

known as Lindlar’s catalyst.

The reaction leads to the formation of compounds having

cis-geometric conformation. However, if the same alkynes reduction

takes placed with sodium present in liquid ammonia, the resultant

product will be trans alkenes.

Preparation of Alkenes from Alkyl Halides

R-X or Alkyl Halides undergo heating with alcoholic potash to form

alkenes. Dissolving potassium hydroxide in an alcohol such as ethanol

leads to the formation of alcoholic potash. In this reaction, the heating

of alkyl halides with alcoholic potash will remove one molecule from

halogen acid leading to the formation of alkenes. Thus, it is a

β-elimination reaction example because removal of hydrogen atom

takes place from the β carbon atom.

The rate of reaction is dependent on the alkyl group and nature of

halogen atom. The order, in this case, is iodine > bromine > chlorine.

The rate of reaction according to the alkyl group is tertiary >

secondary > primary.

Preparation of Alkenes from Vicinal Dihalides

Vicinal halides are compounds containing two halogen atoms present

on two adjacent carbon atoms. Vicinal dihalides undergo treatment

zinc metal to form an alkene. The treatment of vicinal dihalides with

zinc metal will help in removal of a molecule from ZnX2 thereby

resulting in the formation of the alkene. The reaction is an example

dehalogenation.

Preparation of Alkenes from Acidic Dehydration

Hydroxy derivatives of alkanes in simple terms refer to as alcohols.

R–OH represents an alcohol, where the formula of R is CnH2n+1.

Alcohol reacts with the concentrated sulphuric acid in heating

conditions to form alkenes. The reaction will remove one water

molecule to form an alkene.

The reaction is an example of acidic dehydration of alcohols because

the reaction results in the elimination of a water molecule in the

presence of an acid, in this case, sulphuric acid. Furthermore, the

reaction is also an example of β-elimination reaction because the

functional group of alcohol (-OH) eliminates extracts one hydrogen

atom from β-carbon atom.

Solved Questions for You

Question 1: Write IUPAC names of the following compound:

Solution: 2, 8-Dimethyl-3, 6- decadiene

Question 2: Write IUPAC names of the following compound:

Solution: 4-Ethyl-2, 6-dimethyl-dec-4-ene

Nomenclature and Preparation of Alkynes

Natural gas has many uses but do you know it acts as a precursor of

one of the most important and commercially dominant alkyne.

Acetylene, commercially dominant alkyne, production is possible by

partial oxidation of natural gas. What is the first naturally occurring

acetylenic substance? The compound dehydromatricaria ester is the

first naturally occurring acetylenic substance. It acts as the precursor

for many compounds like acrylates as well as it acts as fuel. It was

obtained from Artemisia species in the year 1826, according to

Ferdinand Bohlmann. Isolation of acetylene is possible from natural

sources such as plant species, marine sponges, corals, bacteria, and

fungi. One such example is polyynes. In this topic, we will discuss

nomenclature of alkynes and preparation of alkynes. 

Alkynes

Alkynes are unsaturated hydrocarbons containing a triple bond

between two carbon atoms. The least number of hydrogen atoms is

present in alkynes among alkanes, alkenes, and alkynes. The general

formula of this class of organic compound is CnH2n–2. The first stable

of an alkyne is ethyne or acetylene.

Acetylene is essential alkyne and the compound finds its use for many

different purposes such as arc welding. Acetylene in this example is

present in the flame as oxyacetylene flame (mixing acetylene with

oxygen). It is also the precursor for many other compounds.

Therefore, it is essential and interesting to learn about the

nomenclature and preparation of this class of organic compounds.

In the case of alkynes understand the isomerism along with

nomenclature is very important because it plays an important role in

the naming of the compound. Therefore, we will study the isomerism

and preparation of alkynes along with the nomenclature.

Browse more Topics under Hydrocarbons

● Alkanes

● Conformation of Alkanes

● Nomenclature and Preparation of Alkenes

● Nomenclature and Preparation of Aromatic Hydrocarbons

● Properties of Alkenes

● Properties of Alkynes

● Properties of Aromatic Hydrocarbons

Nomenclature of Alkynes

● Common system: According to the common system, different

members of alkynes are named in the form of derivatives of

acetylene.

● IUPAC System: According to the IUPAC system, the

nomenclature of alkynes is very similar to the nomenclature of

alkanes. Hence, the naming is similar just the suffix “yne”

replaces the suffix “ane” of the corresponding alkanes.

Moreover, the numbering of position starts with the first carbon

having the triple bond.

Refer to the below diagram to observe the structure of alkynes along

with their respective common name and IUPAC name.

Isomerism

Isomerism is the phenomenon of two or more compounds

demonstrating same molecular formula but different physical and

chemical properties. We have studied about geometric isomerism and

stereochemistry in the chapter chemical reactions of haloalkanes.

However, to understand the proper nomenclature of alkynes it is

important to understand the structural isomerism.

Isomerism can be divided into two different categories

● Structural Isomerism

● Stereoisomerism

Structural Isomerism

Compounds demonstrating same molecular formula but differs in the

structures. Thus, the structural isomerism has six different categories.

They are chain isomerism, position isomerism, functional isomerism,

metamerism, tautomerism, and ring chain isomerism. Let us discuss

them one by one.

● Chain Isomerism: Compounds exhibiting same molecular

formula but separate skeletons of carbon refer to as chain

isomers. This phenomenon refers to as chain isomerism. Let’s

take an example of the hydrocarbon pentane (C5H12). There

can be three different possibilities of representation. Refer to

the diagram below

● Position Isomerism: The phenomenon in which the compounds

exhibit same molecular formula but have different types of

bonds or functional group is position isomerism. The

compounds exhibiting this phenomenon are position isomers.

Let’s take an example of propanol (C3H8O).

● Functional Isomerism: Compounds exhibiting same molecular

formula but separate functional groups are functional isomers

and the phenomenon is functional isomerism. Let’s take an

example C2H6O. It has the possibility of different functional

groups. Refer to the example below.

● Metamerism: Metamers are isomers having the same molecular

formula but they differ in the alkyl chain present on the sides of

the functional group of the compounds. Therefore, this

phenomenon is known as metamerism. For example, let’s take

the compound C4H10O

● Tautomerism: This is a special type of isomerism phenomenon

which occurs inside the same molecule because of the

migration taking place in the 1,3-migration of a hydrogen atom.

One of the classic examples of tautomerism is keto-enol

tautomerism. In this example of tautomerism, one of the forms

will have enol group and the other form will have the keto

group. Let’s take the example of acetaldehyde and ethanol

● Ring-Chain Isomerism: Ring chain isomerism is the

phenomenon where compounds will have the same molecular

formula but will have different structures. In this type of

isomerism, one of the compounds will have an open chain. On

the other hand, the other compound will have ring-chain, hence

this name ring-chain isomerism.

● Stereoisomerism: In this type of isomerism, the compounds

will have the same molecular formula but will vary in the

spatial arrangement of atoms/group of atoms. It consists of two

different subclasses. They are conformational isomerism and

configurational isomerism. Further classification of

configurational isomerism is geometrical and optical

isomerism.

Isomerism of Alkynes

Ethyne and propyne, members of the alkyne group will have just one

structure. However, the higher members of this group will exhibit

more than a single structure. For example, the compound butyne

exhibit two different structures.

Similarly, pentyne will have three different isomers. Therefore, the

compound pentyne exhibits chain isomerism and position isomerism.

The three isomers of pentyne are pent-1-yne, pent-2-yne, and

3-Methylbut-1-yne. Among these three, pent-1-yne and pent-2-yne

will exhibit position isomerism; pent-2-yne and 3-Methylbut-1-yne

will exhibit chain isomerism; pent-1-yne and 3-Methylbut-1-yne will

exhibit chain isomerism.

● Alkynes Exhibit Functional Isomerism: Alkynes exhibit

functional isomerism consisting of dienes such as but-1-yne

and but-1,2-diene exhibit functional isomerism. Similarly,

but-1-yne and but-1,3-diene are also examples of functional

isomerism.

● Alkynes are Ring-Chain Isomers: Alkynes exhibits ring-chain

isomerism with cycloalkanes. For example, propyne and

cyclopropene.

Preparation of Alkynes

Alkynes from Calcium Carbide

This is the industrial method for the preparation of ethyne. In this

method, calcium carbide undergoes treatment with water to form

ethyne. Preparation of calcium carbide is possible by heating

quicklime and coke. We can obtain quicklime by heating limestone.

Alkynes from Vicinal Dihalides

Treatment of vicinal dihalides with potassium hydroxide (alcoholic)

results in the dehydrohalogenation reaction. In this reaction, removal

of a hydrogen halide molecule takes place thereby forming alkenyl

halide. Furthermore, treatment of Alkenyl halide with sodamide

produces alkyne.

Solved Question for You

Question: Write the structures and IUPAC names of the different

types of isomers corresponding to the fifth alkyne member. Identify

the type of isomerism demonstrated by the compounds.

Solution: The formula for the fifth member of alkyne series is C6H10.

There is the possibility of 7 different isomeric forms.

Hence, different pairs of isomers will exhibit position as well as chain

isomers.

Nomenclature and Preparation of Aromatic Hydrocarbons

Aromatic Hydrocarbon is an essential class of hydrocarbons and

organic substances. However, what are aromatic hydrocarbons and

what is the basis of naming the compounds in such a manner?

Moreover, what characteristics make this class of organic compounds

so unique? In this topic, we will tackle two of the above

questions-nomenclature and preparation of aromatic hydrocarbons. 

Aromatic Hydrocarbons

There was a time when chemists use to smell the compound and

sometimes even taste it to identify chemicals. Many people believe

that Carl Scheele, Swedish Chemist, died because he tasted a

poisonous chemical while experimenting in the lab. Another example

is of the famous scientist “Robert Bunsen” who had developed the

habit of smelling arsenic as he continuously was exposed to it.

Furthermore, the chemical made his tongue black.

During this period of time, chemists and scientist started naming

certain carbon-compounds having a distinct odour as an aromatic

hydrocarbon. One of the major reasons for such a nomenclature

method was because the compounds such as benzene and toluene were

able to emit a sweet smell. This made the scientists name the

compounds as aromatic hydrocarbons (the Greek word for pleasant

smelling compounds).

Therefore, aromatic hydrocarbon consists primarily of carbon and

hydrogen. They are stable and unsaturated cyclic organic compounds.

Aromatic hydrocarbons or arenes have a special smell to the

compound, hence the name aromatic compounds. Mostly the

compounds will contain as well as retain one or more benzene ring,

even after undergoing different reactions.

However, there is another class of aromatic hydrocarbon which do not

contain benzene ring but they have a highly unsaturated ring.

Aromatic compounds with benzene ring refer to as benzenoids and the

compounds which do not contain benzene ring refer to as

non-benzenoids.

Browse more Topics under Hydrocarbons

● Alkanes

● Conformation of Alkanes

● Nomenclature and Preparation of Alkenes

● Nomenclature and Preparation of Alkynes

● Properties of Alkenes

● Properties of Alkynes

● Properties of Aromatic Hydrocarbons

Definition of Aromatic Hydrocarbons

Aromatic Hydrocarbons are compounds having sigma bonds as well

as delocalized pi electrons in between the carbon atoms present in the

ring form. Refer to see the different examples of aromatic compounds

containing a benzene ring. A hydrocarbon can be an aromatic

compound if it follows the Huckel Rule. According to this rule, a

compound can be aromatic if it contains the following distinct

properties:

● Planarity

● Delocalization of the pi-electrons in the carbon ring entirely

● A compound having (4n + 2) π electrons in its structure, where

n is an integer.

Substituted Benzene

Replacement of one hydrogen atom from benzene and addition of

another atom results in the substitution of benzene. The compounds

are known as substituted benzenes. Depending on the number of the

substituents, the compounds can either be monosubstituted benzene,

disubstituted benzene, and trisubstituted benzene.

Isomerism of Aromatic Hydrocarbons

If we consider any “disubstituted benzene”, there is a possibility of the

formation of three different position isomers on the basis of

substituent’s position in relation to the other. Thus, we use

ortho-position to indicate the position of two substituents (1,2-).

Similarly, meta-position will represent the relative position (1,3-) and

para-position will represent the relative position (1,4-). Let’s take the

example of xylene. Refer to the diagram below to observe the

different isomers of xylene (dimethylbenzene) depending on the

position.

Nomenclature of Aromatic Hydrocarbons

IUPAC System

● Rule 1: As per IUPAC nomenclature system, it is important to

place the substituent’s name before the name of the compound

as a prefix in any substituted aromatic hydrocarbon. For

example, nitrobenzene where the benzene ring is present along

with a nitro group.

● Rule 2: You have to attach Greek numerical prefixes such as di,

tri, and tetra to indicate similar substituents group in case of

compounds with more than one substituent group present in the

benzene ring. For instance, a benzene ring with two bromo

groups present on the adjacent carbon atoms of the benzene

ring refers to as 1,2-di-bromobenzene.

● Rule 3: If different substituent groups are present in the

aromatic compounds, it is important to assign number one

position to the substituent of the base. Furthermore, the

numbering direction for the rest of the compound is chosen in

such a manner that the next substituent will have the lowest

numbering position. Moreover, we have to use alphabetical

order for the naming of the substituent. For example, if a

benzene consists of chloro group as well as a nitro group, then

we start with the chloro group and then the nitro groups on the

basis of the alphabetical order.

● Rule 4: In case of aromatic compounds with more than one

substituents, it is necessary to use terms such as ortho, meta,

and para as prefixes to represent the relative positions like 1,2-;

1,3-; 1,4-. For instance, we can rewrite 1,2 di-bromo-benzene

as o-di-bromo-benzene.

● Rule 5: If an organic compound consists of an alkane with a

functional group and aromatic compound, then the aromatic

compound will act as a substituent instead of the parent group.

For instance, when there is a benzene ring joined with an

alkane and a functional group, then the aromatic group is

known as phenyl (Ph-).

Diagrams representing Nomenclature of Aromatic Hydrocarbons

Preparation of Aromatic Hydrocarbons

One of the important commercial preparation methods of benzene is

by isolation of coal tar. However, the laboratory techniques for

preparation of aromatic hydrocarbons are different.

Cyclic Polymerization of Alkynes

Alkynes undergo polymerization reaction similar to alkenes. It can

undergo two types of polymerization reaction- linear and cyclic.

However, only cyclic polymerization can yield ethyne.

Cyclic polymerization of ethyne results in the formation of aromatic

hydrocarbons. It is one of the important chemical reactions in alkynes.

Ethyne undergoes reaction by passing it from the red-hot iron tube at a

very high temperature of 873K to form benzene. This reaction is

cyclic polymerization of ethyne. Refer to the example below

Aromatic Hydrocarbons by Decarboxylation of Aromatic Acids

The sodium salt of benzoic acid and soda lime react under heating

conditions to produce benzene.

Aromatic Hydrocarbon by Reduction of Phenol

Phenol vapours undergo reduction reaction by passing extremely

heated zinc dust. This reaction results in the formation of benzene.

Solved Questions for You

Question 1: Mention the structure and IUPAC name for the tautomer

of phenol. The structure of phenol is

Solution: The name of the tautomer of phenol is

Cyclohexa-2,4-dien-1-one. The structure of the tautomer of phenol is

Question 2: Name the compound which

undergoes polymerization reaction to produce the compound

1,3,5-trimethylbenzene or mesitylene.

Solution: Propyne undergoes polymerization reaction to produce

mesitylene.

Properties of Alkenes

Alkenes are an unsaturated form of hydrocarbons that are formed by

double bonding between the carbon atoms. There is at least one such

double bond in their structure. The simplest alkene with one double

bond is ethene (C2H4). Alkenes are an important part of our everyday

lives, and also have many industrial uses. Let us take a look at some

properties of alkenes.

Physical Properties of Alkenes

Physical properties of alkenes are quite similar to those of alkanes. Let

us take a look at few physical properties

● Alkenes exist naturally in all three states. The first three

alkenes are gases, and the next fourteen are liquids. Alkenes

higher than these are all solids.

● All alkenes are insoluble in water, due to the weak van der

Waal forces.

● But alkenes are soluble in organic solvents like benzene or

acetone because here the van der Waal forces will be replaced

by new ones, making alkenes fully soluble.

● The boiling points of alkenes depend on their molecular

structure. The bigger their molecular chain the higher the

boiling points. So the higher alkenes have very high boiling

points

● The polarity of alkenes will depend on their functional groups

Chemical Properties of Alkenes

Alkenes are unsaturated compounds, which makes them highly

reactive. Most of these chemical reactions occur at the Carbon-Carbon

double bonds. This makes alkenes far more reactive than alkanes.

Alkenes undergo three types of main reactions, which are as follows

Addition Reactions

● Addition of Hydrogen: In the presence of nickel or platinum

alkenes will react to add to its molecular chain one diatomic

molecule of hydrogen (dihydrogen). And in this process, they

become alkanes due to the rearrangement of atoms.

● Addition of Halogens: Halogens will react with alkenes to form

vicinal dihalides. From the halogens, iodine will not react with

alkenes. But Bromine reacts with alkenes and will attach at the

unsaturated site. In fact, the reaction is used to as proof of

unsaturation.

C2H4(g) + Br2 (aq) → C2H4Br2 (aq)

● Addition of Halides: These reactions follow a certain rule, the

Markovnikov rule. This rule states that the negative portion of

the reactant (the molecule which gets added to the chain) will

attach itself to the carbon with the least number of hydrogen

atoms attached. So when a hydrogen halide will react with an

alkene, the hydrogen will attach at the double bond to the atom

with more hydrogen atoms attached. The halide ion, on the

other hand, will attach to that carbon atom that has the lesser

hydrogen atoms attached.

CH3-CH=CH2+ HBr → CH3-CH(Br)-CH3

● Addition of Water: According to the Markovnikov rule, water

will react with an alkene to form alcohols. This happens in the

presence of sulphuric acid.

CH2=CH2 + H2O → CH3CH2OH

Oxidation Reactions

• Combustion Reaction: The combustion of alkenes is very

exothermic, it will give out huge amounts of thermal energy. A

practical example of this reaction is seen in welding of metals. It is

known as oxy-ethylene welding.

CH2=CH2 + 3O2 → 2CO2 + 2H2O

• Oxidation by Pottasium Permanganate: When alkenes are reacted

with cold dilute KMnO4 also known as Baeyer’s reagent, it forms

vicinal glycols. It will also decolourize the pink colour of KMnO4. So

it is used for testing unsaturation in compounds.

Solved Question for You

Question: What are the major organic products of the following reaction?

1. I and III

2. I and IV

3. II and III

4. II and IV

Solution: Option B. Treatment of alkenes with bromine Br2 gives

vicinal dibromide. The bromines add to opposite faces of the double

bond anti addition. Sometimes the solvent is mentioned in this

reaction – a common solvent is carbon tetrachloride CCl4. In the

above compound, bromine should be added in an anti manner. so

major organic products will be 1,4 compounds as they are added in an

anti manner. the other two are in a minor amount.

Properties of Alkynes

The third type of hydrocarbon is Alkynes which contain at least one

triple bond between a pair of carbon atoms. Since it is also an

unsaturated hydrocarbon some of its properties will be similar to

alkenes. Let us take a look at some physical and chemical properties

of alkynes. 

Physical Properties of Alkynes

● The properties of alkynes pretty much follow the same pattern

of those of alkanes and alkenes.

● Alkynes are unsaturated carbon that shares a triple bond at the

carbon site

● All alkynes are odourless and colourless with the exception of

ethylene which has a slight distinctive odour.

● The first three alkynes are gases, and the next eight are liquids.

All alkynes higher than these eleven are solids

● Alkynes are slightly polar in nature

● The boiling point and melting point of alkynes increases as

their molecular structure grows bigger. The boiling point

increases with increase in their molecular mass

● Also, the boiling points of alkynes are slightly higher than

those of their corresponding alkenes, due to the one extra bond

at the carbon site.

Chemical Properties of Alkynes

Acidic nature

Coming to the chemical properties of alkynes, we begin with their

slightly acidic nature. Now Alkynes are slightly electronegative in

nature. The triply bonded carbon atoms in alkynes are sp hybridized,

Whereas like in alkanes the single bond atoms are sp3 hybridized,

causing the difference in the electronegativity. This makes it easier for

them to attract the shared electron pair of the C-H bond.

So when we react a strong base like NaNH2 with ethyne, we will get

sodium acetylide and liberated hydrogen (H2) gas. But such reactions

will not happen in alkanes and alkenes. The conclusion being that the

hydrogen atoms attached to the carbon-carbon triple bond in alkynes

are slightly acidic in nature. It is to be noted the other hydrogen atoms

baring these ones are not acidic.

HC ≡ CH + Na → HC ≡ C– Na+ + 1/2H2

Addition Reactions

Under suitable conditions (temperature and pressure) alkynes will

undergo hydration reactions quite easily. Alkynes will react with

halogens, hydrogen and other such elements to give a saturated

compound as a product. Since they have a triple bond, two atoms of

H2 or halides or halogens can be added to its structure.

1] Addition of Dihydrogen

The reaction occurs in presence of a catalyst such as Nickel or

Platinum or Palladium. Here the addition of hydrogen to the alkyne

gives us an alkene.

C3H4(g) + 2H2(g) C3H8(g)

2] Addition of Halogens

When alkynes and halogens like Bromine react, halogen will add itself

to the structure of the alkynes and result in halogen substituted

alkenes. The resulting product will be tetrabromopropane.

(Source: chemistryassignment)

3] Addition of Water

Just like other hydrocarbons (alkanes and alkenes) alkynes also do not

react with a water molecule. This is called immiscibility. But if

alkynes is bubbled through dilute sulphuric acid (about 40%) in

presence of the catalyst mercuric Sulphate, then a reaction occurs. The

products will be carbonyl compounds, and such a reaction can be

called a hydration reaction.

(Source: chemistryassignment)

4] Polymerization

Alkynes can undergo linear and cyclic polymerization under suitable

conditions. They polymerize to give compounds that have a higher

molecular weight than the original alkyne. Like for example, ethyne

will polymerize to give polyacetylene or polyethene (of higher

molecular weight), This is an example of linear polymerization.

For cyclic polymerization, high temperatures and the presence of a

catalyst is required. Like passing ethyne through a red-hot iron tube at

a minimum of 877K which gives benzene.

Solved Example for You

Question: Which of the following reacts with metal by displacing H

atom :

A. C2H2

B. C2H4

C. C2H6

D. None of the above

Solution: The correct answer is A. Only end alkynes have acidic

hydrogen molecules which react and can be displaced by a metal.

Properties of Aromatic Hydrocarbons

If you do not understand hydrocarbons well, you might find it difficult

to memorize! However, once you get a grasp of the topic, you will

find it absolutely interesting! In this chapter, we will try to make this

topic extremely interesting for you! Let us learn about the properties

of aromatic hydrocarbons.

Aromatic Hydrocarbons

Based on the number and type of substitution of the ring the Aromatic

compounds are named. Cyclic hydrocarbons with delocalized pi

electrons between carbon atoms of the ring are defined as Aromatic

Hydrocarbons. The phenomenon has aromatic nature that is called

aromaticity. Benzene is the simplest aromatic compound. They are

famous and well known for their strong, pungent aromas.

Structure of Benzene

Properties of Aromatic Hydrocarbons

● Properties of Aromatic Hydrocarbons include that their major

sources are Petroleum and coal. They are well known for their

exceptional physical and chemical properties. Poly-aromatic

hydrocarbons are defined as aromatic compounds with more

than one benzene. When they include in atmospheric pollution

then it is known as carcinogenic in nature.

● Aromatic compounds also include amino acids and precursors

to nucleotides. Which are soluble in water they are known as

non-polar hydrocarbons. These hydrocarbons cannot form ions

or H-bonds with water molecules. They are usually unreactive

because of extra stability and for many organic and inorganic

reactions it is widely used as an inert solvent.

● The ratio for carbon-hydrogen is high. They born with sooty

yellow flame because of the presence of high carbon content.

● They go through electrophilic substitution reactions and

nucleophile aromatic substitution.

● Hydrocarbons which have multiple bonds are unsaturated in

nature like alkenes and alkynes. They tend to give addition

reactions due to this unsaturation.

● Due to resonance and give characteristic electrophilic

substitution reactions aromatic hydrocarbons are stable. The

carbon ring acts as a nucleophile in these reactions and to form

a substituted product an electrophile attack on benzene.

● With the coming electrophile, one of the H-atom of a ring is

substituted because of this the product also holds its stability

and aromatic in nature. On the opposite side in the addition

reactions, aromatic compound may lose their aromaticity so

they do not prefer to give such reactions.

Video on Preparation and Properties of Hydrocarbons

Uses of Aromatic Hydrocarbons

● In several industries, aromatic hydrocarbons have wide

applications. For example, for model glues, toluene is used as

solvent while naphthalene is used as mothballs.

● For manufacturing of dyes, explosives, and drugs,

Phenanthrene is an intermediate product which has a different

synthetic process. Trinitrotoluene (TNT) or 2, 4, 6

trinitrotoluene is an important aromatic compound which is

mainly used as explosive along with the preparation of

explosive.

● 1, 2 benzenediols or pyrocatechol is advertised as catechol

which is one of the most important components of a

photographic developer.

Question For You

Q. What are the physical properties of Benzene?

Ans: In organic solvent benzene is soluble but it is immiscible in

water. It is a colourless liquid. Benzene has a typical aromatic odour

because it is an aromatic compound. It burns with sooty flame because

it is highly inflammable.

Based on the positioning of double bond benzene shows the resonance

that it can exist in different form. For this property benzene is stable.

Benzene having density 0.87g cm-3 and it is lighter than water. It has a

moderate boiling point that is 80.5oC and high melting point that is

5.5oC.