Post on 22-Sep-2020
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.