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Organic Chemistry

• Organic chemistry is the study of carbon compounds, excluding CO, CO2, CS2 and various carbonates, bicarbonates, and cyanides, which are traditionally considered to be inorganic compounds.

• Today, well over 13 million synthetic and natural organic compounds are known. This number is significantly greater than the 100,000 or so known inorganic compounds.

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Carbon can form more compounds than any other element because carbon atoms are able not only to form single, double, and triple carbon-carbon bonds, but also to link up with each other in chains and ring structures.

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Classes of organic compounds

All organic compounds are derived from a group of compounds known as hydrocarbons because they are made up of only hydrogen and carbon.

Aromatic hydrocarbons contain one or more rings with delocalized electrons (commonly benzene).

Aliphatic hydrocarbons do not contain rings with delocalized electrons.

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Aliphatic hydrocarbons are divided into alkanes, alkenes and alkynes.

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Alkanes

• Alkanes have the general formula CnH2n+2, where n = 1,2,3,…

• The essential characteristic of alkane hydrocarbon molecules is that only single covalent bonds between the carbons are present.

• The alkanes are known as saturated hydrocarbons because they contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms present (there are no multiple bonds between carbons).

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Alkane nomenclature

• The nomenclature of alkanes and all other organic compounds is based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC).

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The parent name of the hydrocarbon is that given to the longest continuous chain of carbon atoms in the molecule.

Name of Hydrocarbon

Molecular Formula Number of Carbon Atoms

Methane CH4 1

Ethane CH3 – CH3 2

Propane CH3 – CH2 – CH3 3

Butane CH3 – (CH2)2 – CH3 4

Pentane CH3 – (CH2)3 – CH3 5

Hexane CH3 – (CH2)4 – CH3 6

Heptane CH3 – (CH2)5 – CH3 7

Octane CH3 – (CH2)6 – CH3 8

Nonane CH3 – (CH2)7 – CH3 9

Decane CH3 – (CH2)8 – CH3 10

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There are several different ways to represent an alkane…

Expanded formula

Condensed formula

Ball and stick model

Carbon skeleton model

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Both ball and stick models and Newman projections are useful when an understanding of the spatial relationship is needed

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•Any chain branching off the longest chain is named as an alkyl group, an alkane which has one less hydrogen atom.

•For example, when a hydrogen atom is removed from methane, we are left with CH3-, which is called a methyl group.

What would a CH3 – CH2- group be called?

ethyl

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Common Alkyl Groups

Name Formula

Methyl -CH3

Ethyl -CH2-CH3

n-propyl -CH2CH2-CH3

n-butyl -CH2-CH2-CH2-CH3

isopropyl

t-butyl

(the letter t stands for tertiary)

C

CH3

H

CH3

C

CH3

CH3

CH3

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When one or more hydrogen atoms are replaced by other groups, the name of the compound must indicate the locations of carbon atoms where replacements are made.

The procedure is to number each carbon atom on the longest chain in the direction that gives the smaller numbers for the locations of all branches.

Explain why 4-methyl pentane is not the correct name for this structure.

1 2 3 4 5

Because numbering the carbon chain from the other end would have given a smaller number to the methyl group, therefore the correct name for this structure is 2-methyl pentane. (Note that the branch name and the parent name are written as a single word, and a hyphen follows the number.)

12345

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When there is more than one alkyl branch of the same kind present, we use a prefix such as di-, tri-, or tetra- with the name of the alkyl group.

CHCH3 CH CH2CH2 CH3

CH3CH3

This compound’s name would be

2,3-dimethyl hexane

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What would the name of this compound be?

3,3-dimethyl pentane

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• When there are two or more different alkyl branches, the name of each branch, with its position number, precedes the base name.

• When two or more different branches occur, their names appear in alphabetical order.

For example, CHCH3 CH CH2CH2 CH2

CH2CH3CH3would be…

3-ethyl-2-methylheptane

CH3

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Of course, alkanes can have many different types of substituents.

Names of Common Substituent GroupsFunctiona

l GroupName

-NH2 Amino

-F Fluoro

-Cl Chloro

-Br Bromo

-I Iodo

-NO2 Nitro

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What is the name for the following compound?

CHCH3 CH CH3

NO2Br

2-bromo-3-nitrobutane

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Structural Isomers – Compounds containing the same atoms, but arranged in completely different ways.

Draw two structural isomers of C4H10.

CHCH3

CH3

CH3

CH2CH3 CH2 CH3

Both of these compounds share the same chemical formula, C4H10, but being structural isomers, they differ in their physical and chemical properties.

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Reactions of AlkanesAlkanes are generally not considered to be very

reactive substances. However, under suitable conditions they do react.

Combustion• Methane and other hydrocarbons undergo highly

exothermic combustion reactions. Write a chemical equation showing the complete

combustion of gaseous methane, a process which results in the release of 890.4 kJ of energy.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890.4 kJ

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HalogenationHalogenation – is the replacement (substitution)

of one or more hydrogen atoms by halogen atoms

When a gaseous mixture of methane and chlorine is heated above 100 oC or irradiated with light of a suitable wavelength, methyl chloride and hydrogen chloride are produced. Write a chemical equation showing this reaction.

methyl chloride

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)100 oC

Alkanes in which one or more hydrogen atoms have been replaced by a halogen atom are called alkyl halides. Among the large number of alkyl halides, the best known are chloroform (CHCl3), carbon tetrachloride(CCl4), methylene chloride (CH2Cl2), and the chlorofluorohydrocarbons.

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Optical isomers are compounds that are nonsuperimposable mirror images. (“Superimposable” means that if one structure is laid over the other, the positions of all the atoms will match.)

The structural relationship between two optical isomers is analogous to the relationship between your left and right hands. If you place your left hand in front of a mirror, the image you see will look like your right hand. We say that your left hand and right hand are mirror images of each other. However, they are nonsuperimposable.

Optical Isomerism of Substituted Alkanes

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Optical isomers are described as chiral (from the Greek word for “hand”) because like your left and right hands, chiral molecules are nonsuperimposable.

Nonsuperimposable mirror images of a chiral compound are called enantiomers. Like geometric isomers, enantiomers come in pairs.

Molecules that are superimposable are said to be achiral.

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Optical isomers have identical physical and chemical properties, such as melting point, boiling point and chemical reactivity.

Optical isomers DO differ from each other in their interactions with plane-polarized light.

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Optical isomers rotate a plane of polarized light either to the right or to the left. If the plane of polarization is rotated to the right, the isomer is said to be dextrorotatory (d) or (+) ; it is levorotatory (l) or (-) if the rotation is to the left. Glucose rotates polarized light to the right so it’s also known as dextrose.

The d and l isomers of a chiral substance always rotate light by the same amount, but in opposite directions. Thus, in equimolar mixtures of the two enantiomers, called a racemic mixture, the net rotation is zero

FYI - Chemists can actually identify the sugar in a solution by which direction and how much it rotates the light. Wine makers routinely use a specially calibrated polarimeter to measure the sugar remaining as the wine ages.

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Is CH2ClBr a chiral or achiral molecule? Explain.

It is an achiral molecule because it can be superimposed on its mirror image

Is CHFClBr a chiral or achiral molecule? Explain.

It is a chiral molecule because it is nonsuperimposable with its mirror image.

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Chirality plays an important role in biological systems. Protein molecules have many asymmetric carbon atoms and their functions are often influenced by chirality.

Because enantiomers behave differently in the body, they are of great interest to the pharmaceutical manufacturers. Many prescribed drugs are chiral. In most of these drugs, only one enantiomer of the drug works as medicine, whereas the other form is useless or less effective or may even cause serious side effects.

The best-known case in which the use of a racemic mixture of a drug had tragic consequences occurred in Europe in the late 1950s. The drug thalidomide had to be withdrawn from the market because although the (+) form has sedative properties, (-) thalidomide is a powerful mutagen.

Ibuprofen is also sold as a racemic mixture; the (-) form is a potent pain reliever, the (+) form is ineffective but also harmless.

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CycloalkanesAlkanes whose carbon atoms are joined

in rings are known as cycloalkanes. Cycloalkanes have the general formula

CnH2n, where n = 3, 4, …The simplest cycloalkane is

cyclopropane.

What is the chemical formula for cyclopropane?

C3H6

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Draw the correct structure for cyclobutane.

CH

H

H H

C

H

CC

H

H

H

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Elimination reactions occur when a saturated reactant becomes an unsaturated product.

C C C

Br

C C

C+ HBr

Note the elimination

of HBr

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Alkenes

The alkenes (also called olefins) contain at least one carbon-carbon double bond, C=C.

Alkenes have the general formula CnH2n, where n = 2, 3, …

The simplest alkene is C2H4, commonly referred to as ethylene.

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Alkene Nomenclature

In naming alkenes we indicate the positions of the carbon-carbon double bonds.

The names of compounds containing C=C bonds end with –ene.

The name of the parent compound is determined by the number of carbon atoms in the longest chain that contains the double bond.

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CH2=CH-CH2-CH3

would be 1-buteneNote that the number in the name of the alkene refers to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. (If the double bond is between carbon 1 and carbon 2, you call it 1-butene, not 2-butene.)What would be the correct name for

this compound? CH3-CH=CH-CH2-CH3 2-pentene

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Alkene nomenclature must also specify whether a given molecule is cis or trans if it is a geometric isomer.

Geometric isomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements.

The cis isomer has two particular atoms (or groups of atoms) on the same side of the double bond, and the trans isomer means that two atoms, (or groups of atoms) are on opposite sides of the double bond.

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Cis and trans isomers, like other geometric isomers, generally have quite different colors, melting points and chemical reactivities.

4-methyl-cis-2-hexene 4-methyl-trans-2-hexene

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Properties and Reactions of Alkenes

Alkenes are classified as unsaturated hydrocarbons, compounds with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. (Compounds with more than one multiple bond are considered polyunsaturated.)

Unsaturated hydrocarbons commonly undergo addition reactions, in which one molecule adds to another to form a single product.

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HydrogenationHydrogenation of an alkene is an

example of an addition reaction.

C2H4 + H2 CH3-CH3

Other addition reactions to the C=C bond include

C2H4 + HX CH3-CH2X

C2H4 + X2 CH2X-CH2X

Where X represents a halogen (Cl, Br or I).

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When ethylene gas is bubbled through the aqueous bromine solution, the reddish-brown color gradually disappears due to the formation of 1,2 dibromoethane, which is colorless. This makes bromine an indicator for the presence of an alkene.

Br2(aq)

C = C + Br2 Br – C – C – Br

colorlessReddish-brown colorless

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The addition of a hydrogen halide to an unsymmetrical alkene such as propylene is more complicated because two products are possible:

In reality, however, only 2-bromopropane is formed. This phenomena was observed in all addition reactions with alkenes.

Markovnikov’s rule states that when adding polar reagents to alkenes, the hydrogen will add to the carbon that already has the most hydrogen atoms bonded to itself.

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Alkynes

Alkynes contain at least one carbon-carbon triple bond, C=C.

Alkynes have the general formula CnH2n-2 where n = 2, 3, …

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Alkyne Nomenclature

Names of compounds containing C C bonds end with –yne. Again the name of the parent compound is determined by the number of carbon atoms in the longest chain (containing the triple bond).

As in the case of alkenes, the names of alkynes indicate the position of the carbon-carbon triple bond, as, for example, in

HC C-CH2-CH3 or CH3-C C-CH31-butyne 2-butyne

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Properties and Reactions of Alkynes

The simples alkyne is ethyne, better known as acetylene (C2H2).

Acetylene, an unsaturated hydrocarbon, can be hydrogenated to yield ethylene:

C2H2 + H2 C2H4

It undergoes the following addition reactions with hydrogen halides and halogens:

C2H2 + HX CH2 = CHX

C2H2 + X2 CHX = CHX

C2H2 + 2X2 CHX2 – CHX2

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Alkynes follow Markovnikov’s rule during addition reactions.

What product is formed when hydrogen bromide is added to 1-propyne?

2 - bromopropene You don’t need to state where the double bond is because in propene, the double bond must be between carbon 1 and carbon 2.

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Aromatic Hydrocarbons

Benzene is the parent compound of this large family of organic substances.

Benzene is best represented by both of resonance structures shown below.

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Alternatively, the properties of benzene can be explained in terms of delocalized molecular orbitals. Electrons in delocalized molecular orbitals are not confined to the two adjacent atoms, instead the electrons are free to move around the benzene ring. For this reason, the structure of benzene is commonly represented as a hexagon, (the points of which represent the carbon atoms) containing a circle (which represents the freely moving electrons).

The hydrogen atoms are not represented in this model – but we have to remember they are still there!

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Nomenclature of Aromatic Compounds

The naming of monosubstituted benzenes, that is, benzenes in which one H atom has been replaced by another atom or group of atoms, is quite straightforward, as shown below:

EthylbenzeneChlororbenzeneAminobenzene(aniline)

NitrobenzeneToluenePhenol

OH CH3 NO2 NH2 Cl CH2CH3

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If more than one substituent is present, we must indicate the location of the second group relative to the first. The systematic way to accomplish this is to number the carbon atoms as follows:

Numbering can start at any carbon, and can go clockwise or counter clockwise – depending on the location of the substituted groups.

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The prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote the relative positions of the two substituted groups, as shown below for the dibromobenzenes.

1,2 dibromobenzene

(o-dibromobenzene)

1,3 dibromobenzene

(m-dibromobenzene)

1,4 dibromobenzene

(p-dibromobenzene)

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A group containing benzene minus a hydrogen atom (C6H5) is called the phenyl group. Phenyl can be a branch on a hydrocarbon chain.Thus the following compound is called 2 - phenylpropane

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Properties and Reactions of Aromatic Compounds

Benzene is relatively inert, primarily because of electron delocalization.

Benzene can be hydrogenated, but only with difficulty.

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The most common reaction of halogens with benzene is the substitution reaction, in which an atom or group of atoms replaces an atom or group of atoms in another molecule.

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In another example of a substitution reaction, alkyl groups can be introduced into the ring system by allowing benzene to react with an alkyl halide.

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Chemistry of Functional Groups

Organic functional groups are responsible for most of the reactions of the parent compounds.

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Alcohols

All alcohols contain the hydroxyl functional group, -OH.

Ethyl alcohol, or ethanol, can be formed by the fermentation of sugar or starch in the absence of oxygen.

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Alcohols have countless applications as solvents for organic chemicals and as starting compounds for the manufacture of dyes, synthetic drugs, cosmetics and explosives.

Ethanol, the least toxic of the straight chain alcohols is also a constituent of alcoholic beverages.

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Ethers

Ethers contain the R-O-R’ linkage, where R and R’ are a hydrocarbon (aliphatic or aromatic) groups.

C2H5OC2H5

Dimethyl ether

CH3OCH3

Diethyl ether

CH3OC2H5

Methyl ethyl ether

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Diethyl ether, commonly known as “ether”’ was used as an anesthetic for many years. It produces unconsciousness by depressing the activity of the central nervous system. The major disadvantages of diethyl ether are its irritating effects on the respiratory system and the occurrence of postanesthetic nausea and vomiting. “Neothyl”’ or methyl propyl ether is currently favored as an anesthetic because it is relatively free of side effects.

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Condensation Reaction

Ethers are formed by the reaction between two alcohols.

A condensation reaction is characterized by the joining of two molecules and the elimination of a small molecule, usually water.

Note the formation of

water

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Aldehydes and Ketones

The functional group in these compounds is the carbonyl group, C=O

In an aldehyde at least one hydrogen atom is bonded to the carbon in the carbonyl group.

In a ketone, the carbon atom in the carbonyl group is bonded to two hydrocarbon groups.

R - CH

O

R – C – R’

O

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Aldehydes and KetonesUnder mild oxidation conditions, it is

possible to convert alcohols to aldehydes and ketones.

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The simplest aldehyde, formaldehyde, (IUPAC name is methanal) is a disagreeable-smelling liquid used as a starting material in the polymer industry and in the laboratory as a preservative for animal specimens. Interestingly, the higher molar mass aldehydes, such as cinnamic aldehyde have a pleasant odor and are used in the manufacture of perfumes.

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The simplest ketone is acetone, a strong-smelling liquid that is used mainly as a solvent for organic compounds and nail polish remover.

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Carboxylic acidsCarboxylic acids contain a carboxyl group, -

COOH.

Carboxylic acids are widely distributed in nature; they are found in both the plant and animal kingdoms. All protein molecules are made of amino acids, a special kind of carboxylic acid containing an amino group (-NH2) and a carboxyl group (-COOH).

Unlike the inorganic acids HCl, HNO3, and H2SO4, carboxylic acids are usually weak.

R - COH

O

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Under appropriate conditions both alcohols and aldehydes can be oxidized to carboxylic acids:

CH3CHO + ½O2 CH3COOH

CH3CH2OH + O2 CH3COOH + H2O

These reactions occur so readily, in fact, that wine (which of course contains ethanol) must be protected from atmospheric oxygen while in storage. Otherwise, it would soon turn to vinegar due to the formation of acetic acid.

acetic acid IUPAC name = ethanoic acid

ethanal

ethanol

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EstersEsters have the general formula R’COOR,

where R’ can be H or a hydrocarbon group and R is a hydrocarbon group.

Esters are used in the manufacture of perfumes and as flavoring agents in the confectionery and soft-drink industries. Many fruits owe their characteristic smell and flavor to the presence of small quantities of esters.

R’ - CO - R

O

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In the presence of an acid catalyst, such as HCl, esters undergo hydrolysis (involves the lysis or splitting of water) to yield a carboxylic acid and an alcohol. For example, in acid solution, ethyl acetate hydrolyzes as follows:

CH3COOC2H5 + H2O CH3COOH + C2H5OH

ethyl acetate acetic acid ethanol

However, this reaction does not go to completion because the reverse reaction, the formation of an ester from an alcohol and an acid, also occurs to an appreciable extent.

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CH3COOC2H5 + NaOH Na+CH3COO- + C2H5OHethyl acetate sodium acetate ethanol

On the other hand, when NaOH solution is used in hydrolysis the sodium acetate does not react with ethanol, so this reaction does go to completion from left to right.

For this reason, ester hydrolysis is usually carried out in basic solutions. Note that NaOH does not act as a catalyst; rather, it is consumed by the reaction.

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The term saponification (meaning soapmaking) was originally used to describe the alkaline hydrolysis of fatty acid esters to yield soap molecules (sodium stearate):

C17H35COOC2H5 + NaOH Na+C17H35COO- + C2H5OH

Saponification has now become a general term for alkaline hydrolysis of any type of ester.

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Amines

Amines are organic bases with the general formula R3N, where R may be an H or a hydrocarbon group. As with ammonia, the reaction of amines with water is

RNH2 + H2O RNH3+ + OH-

Where R represents a hydrocarbon group.

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Like all bases, the amines form salts when allowed to react with acids:

CH3CH2NH2 + HCl CH3CH2NH3+Cl-

Ethylamine ethylammonium chloride

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Aromatic amines are used mainly in the manufacture of dyes. Aniline, the simplest aromatic amine, is a toxic compound. A number of other aromatic amines, such as

2-naphthylamine and benzidine are potent carcinogens:

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The End