CHAPTER 11 :

INTRODUCTION TO

ORGANIC CHEMISTRY

CHAPTER 11 : INTRODUCTION TO ORGANIC CHEMISTRY

11.1 Introduction

11.2 Empirical molecular and structural formulas

11.3 Functional groups and homologous series

11.4 Classification of carbon atoms in organic molecules

11.5 Isomerism

11.6 Reactions in organic compound

Organic and Inorganic Compound

Organic compound Inorganic compound

were defined as

compounds that

could be obtained

from living

organisms

were those that

came from

nonliving sources

Some examples of carbon compounds in our daily lives :-

CH4

methane (a component of natural gas)

OCOCH3

COOH

CH3 CHCOOH

NH2

Methyl salicylic acid (aspirin-a drug)

alanine (amino acid-a protein component)

NCH3

CO2CH3

OCO

cocaine (a pain killer)

11.2 EMPIRICAL, MOLECULAR AND

STRUCTURAL FORMULAE

Empirical formula is the simplest formula that

shows the relative numbers of the different kinds

of atoms in a molecule.

Molecular formula is a formula that states the

actual number of each kind of atom found in the

molecule. Example : C2H4 , C2H4O2

Quantitative example :-

A sample of hydrocarbon contains 85.7 % carbon and

14.3 % hydrogen by mass. Its molar mass is 56.

Determine the empirical formula and molecular formula

of the compound.

Solution :-

Element C H

Mass (g) 85.7 14.3

Moles (n) 85.7 12.0

14.3 1.0

Smallest ratioSmallest ratio 7.1427.142= 1

14.437.142= 2

= 7.142 = 14.3

Empirical formula = CH2

n (empirical molar mass)= molar mass

n ( 12 x 1 + 1 x 2 ) = 56 n = 4Molecular formula = C4H8

Practice Exercise:

1) A complete combustion of 10.0 g of compound X,

CxHyOz forms 12.0 g H2O and 22.0 g CO2. Its

molar mass is 60.

a) Determine the percentage composition of

C, H and O.

b) Determine the molecular formula of

compound X.

2) A complete combustion of 0.6 g of hydrocarbon,

CxH12 forms 1.98 g CO2. Determine the percentage

of C and H in the hydrocarbon and write the

molecular formula.

Structural formula shows the order in which

atoms are bonded together

Representation of structural formula :-

a) Condensed Structure

b) Expanded Structure

c) Skeletal Structure

d) 3-Dimensional formula

e) Ficher Projection

a) Condensed Structure

In condensed formulae all the hydrogen atoms

that are attached to a particular carbon are

usually written immediately after that carbon

Example :

C4H9Cl CH3CHCH2CH3 or CH3CH(Cl)CH2CH3

Condensed structure

Cl

b) Expanded Structure

Expanded structures indicate the way in which

the atoms are attached to each other and are not

representations of the actual shapes of the

molecules.

Example :

C4H9ClC C C C

H H

H H HCl

H H H H

Expanded structure

c) Skeletal Structure

This structure shows only the carbon skeleton

The hydrogen atoms that are assumed to be present, are not written.

Other atoms such as O, Cl, N and etc. are shown

Example :

CH3CH(Cl)CH2CH3

Cl

=1.

H2C CH2

H2C CH2

2.

=

CH2=CHCH2OH 3. =OH

Practice Exercise :

Rewrite each of the following structures using skeletal formula :-

O

CH3CH2CH2C CH3

(CH3)2CHCH2CH2CH(CH3)CH2CH3

CH2= CHCH2CH2CH = CHCH3

O

CH3 CH2 CH ( CH3 ) CH2 C OH

1.

2.

3.

4.

d) 3 - Dimensional formula (wedge - dashed wedge - wedge)

Describes how the atoms of a molecule are arranged in space

Example :

C

Br

H

H

H(Bromoethane)

C

Br

HH

HC

H

BrH

HC

H

HBr

H

Indication :-

bonds that lie in the plane of the page bonds that lie behind the plane bonds that project out of the plane of the paper

OR OR

e) Fischer Projection

Vertical lines represent bonds that project behind the plane of paper

Horizontal lines represent bonds that project out of the plane of paper

The intersection of vertical and horizontal lines represent a carbon atom, that is stereocentre

Example :

2 – butanol , CH3CH(OH)CH2CH3

CH3

HO

CH2CH3

H

CH3

H

CH2CH3

OHOR

11.3 FUNCTIONAL GROUPS AND

HOMOLOGOUS SERIES

A functional group is an atom or group of

atoms in an organic molecule which characterized

the molecule and enables the molecule to react

in specific ways (determines its chemical properties)

Some important functional groups in organic compounds :-

Homologous Homologous SeriesSeries

Functional Functional GroupGroup

General General FormulaFormula

IUPAC IUPAC nomenclaturenomenclature

Prefix- -suffixPrefix- -suffix

ExampleExample

alkanealkane nonenone CCnnHH2n+22n+2 -ane-ane CHCH44

methanemethane

alkenealkene C = C C = C (double (double

bond)bond)

CCnnHH2n2n -ene-ene CHCH22=CH=CH22

etheneethene

alkynesalkynes C C C C (triple (triple

bond)bond)

CCnnHH2n-22n-2 -yne-yne CH CH CH CH

ethyneethyne

Homologous Homologous SeriesSeries

Functional Functional GroupGroup

General General FormulaFormula

IUPAC IUPAC nomenclaturenomenclature

Prefix- -suffixPrefix- -suffix

ExampleExample

arenearene CCnnHH2n-62n-6 -benzene-benzene

alcoholalcohol ––OH OH (hydroxyl)(hydroxyl)

CCnnHH2n+12n+1OHOH alkanolalkanol CHCH33CHCH22OH OH

ethanol ethanol

etherether ––OR OR

(alkoxy)(alkoxy) CCnnHH2n+22n+2OO alkoxyalkanealkoxyalkane CHCH33OCHOCH33

methoxymethanemethoxymethane

haloalkanehaloalkane ––X X (halogen)(halogen)

CCnnHH2n+12n+1XX haloalkanehaloalkane CHCH33CHCH22ClCl

chloroethanechloroethane

aromatic ring

CH3

methylbenzene

Homologous Homologous SeriesSeries

Functional Functional GroupGroup

General General FormulaFormula

IUPAC IUPAC nomenclaturenomenclature

Prefix- -suffixPrefix- -suffix

ExampleExample

aldehydealdehyde CCnnHH2n2nOO alkanalalkanal CHCH33C=OC=O

ketoneketone CCnnHH2n2nOO alkanonealkanone CHCH33C=O C=O

carboxylic carboxylic acidacid

CnHCnH2n2nOO22 alkanoic acidalkanoic acid CHCH33C=OC=O

C

O

H

carbonylH

ethanal

C

O

carbonylCH3

propanone

C OH

O

carboxylOH

ethanoic acid

Homologous Homologous SeriesSeries

Functional Functional GroupGroup

General General FormulaFormula

IUPAC IUPAC nomenclaturenomenclature

Prefix- -suffixPrefix- -suffix

ExampleExample

acyl acyl

chloridechloride CCnnHH2n+12n+1

COClCOCl

alkanoyl alkanoyl

chloridechloride CHCH33C=OC=O

esterester CCnnHH2n2nOO22 alkyl alkyl

alkanoatealkanoate CHCH33COOCHCOOCH33

amideamide CCnnHH2n+12n+1

CONHCONH22 -amide-amide CHCH33CONHCONH22

amineamine -NH-NH22 CCnnHH2n+12n+1

NHNH22 -amine-amine CHCH33NHNH22

C

O

Cl

acylCl

ethanoyl chloride

C

O

O C

ester

C

O

NH2

ethyl ethanoate

amideethanamide

amino methanamine

11.4 CLASSIFICATION OF CARBON AND HYDROGEN ATOMS IN ORGANIC

MOLECULES

Carbon atom classified primary (1o)

secondary (2o)

tertiary (3o)

quarternary (4o)

depending on the number of carbon atoms bonded to it

A primary carbon – directly bonded to one other

carbon atom

(has 1 adjacent carbon atom)

C

H

H

CH3

H

Example :

1o carbon

1o H

A secondary carbon – directly bonded to two other

carbon atoms

(has 2 adjacent carbon atoms)

C

H

CH3

H CH3

Example :

2o carbon

2o H

A tertiary carbon – directly bonded to three other

carbon atoms

(has 3 adjacent carbon atoms)

C

CH3

CH3

H CH3

Example :

3o carbon

3o H

A quarternary carbon – directly bonded to four other

carbon atoms

(has 4 adjacent carbon atoms)

CCH3

CH3

CH3

CH3

Example :

4o carbon

Similarly, a hydrogen atom is also classified as

primary, secondary or tertiary depending on the

type of carbon to which it is bonded.

1° hydrogen atom bonded to a 1° C atom

2° hydrogen atom bonded to a 2° C atom

3° hydrogen atom bonded to a 3° C atom

Classification of haloalkanes (alkyl halides)

Alkyl halides are classified based on the carbon atom

to which the halogen is directly attached.

1° alkyl halide – the halogen atom is bonded to a primary carbon atom

2° alkyl halide – the halogen atom is bonded to a secondary carbon atom

3° alkyl halide – the halogen atom is bonded to a tertiary carbon atom

H C

H

H

C

H

H

Cl

H C

H

H

C

H

C

H

H

HCl

H C

H

H

C C

H

H

HCl

CH3

1° alkyl chloride

1° C

2° alkyl chloride

2° C

3° alkyl chloride

3° C

Classification of alcohols

Alcohols are classified based on the carbon atom

to which the hydroxyl group is directly attached.

1° alcohol – the hydroxyl group is attached to a 1° carbon atom

2° alcohol – the hydroxyl group is attached to a 2° carbon atom

3° alcohol – the hydroxyl group is attached to a 3° carbon atom

H C

H

H

C

H

H

OH

H C

H

H

C

H

C

H

H

HOH

H C

H

H

C C

H

H

HOH

CH3

1° alcohol

1° C

2° C

2° alcohol

3° alcohol

3° C

Classification of amines

Amines are classified based on the number of alkyl

groups or carbon atoms that are directly attached

to the nitrogen atom

1° amine – N is bonded to one alkyl group

2° amine – N is bonded to two alkyl groups

3° amine – N is bonded to three alkyl groups

H3C N

H

H

H3C N H

CH3

H3C N

CH3

CH3

N bonded to one alkyl group

A primary (1°) amine

N bonded to two alkyl group

A secondary (2°) amine

N bonded to three alkyl group

A tertiary (3°) amine

ISOMERISM

Structural/Constitutional Isomerism Stereoisomerism

Isomerism

Chain isomerism

Positional isomerism

Functional group

isomerism

Geometric isomerism

Optical isomerism

Isomerism is the existence of different compounds

with the same molecular formula but different

structural formulae

Isomers – different compounds that have same

molecular formula

Two types of isomerism

structural isomerism

stereoisomerism

different order of attachment of atoms

different spatial arrangement of atoms in molecules

Structural isomerism

Chain/skeletal isomerism

Structural isomers are different compounds with

the same molecular formula but differ in the order

of attachment of atoms

Positional isomerism

Functional group

isomerism

The isomers differ in the carbon skeleton (different carbon chain)

a) Chain/skeletal isomerism

They possess the same functional group and belong to the same homologous series

Example :

C5H12 :

CH3CH2CH2CH2CH3

CH3CHCH2CH3

CH3

CH3-C-CH3

CH3

CH3

b) Positional isomerism

These isomers have a substituent group in different positions in the same carbon skeleton

Example :

C3H7Cl i) CH3CH2CH2Cl

1-chloropropane

CH3CHCH3

Cl

2-chloropropane

C4H8 ii) CH2=CHCH2CH3 CH3CH=CHCH3

1-butene 2-butene

1,2-dimethylbenzene

iii) C8H10 CH3

CH3

CH3

CH3

1,3-dimethylbenzene

CH3

CH3

1,4-dimethylbenzene

c) Functional group isomerism

These isomers have different functional groups and belong to different homologous series with the same general formula

Different classes of compounds that exhibit functional group isomerism :-

General formulaGeneral formula Classes of compoundsClasses of compounds

CnH2n+2O alcohol and ether

CnH2nO aldehyde and ketone

CnH2n alkene and cycloalkane

CnH2nO2 carboxylic acid and ester

Example :

i) C2H6O CH3CH2OH

ethanol

CH3OCH3

dimethyl ether

ii) C3H6O CH3CH2C-H

Opropanal

CH3C-CH3

Opropanone

iii) C3H6O2 CH3CH2C-OH

O

propanoic acid

CH3C-O-CH3

O

methyl ethanoate

Stereoisomerism

Geometric Isomerism Optical Isomerism

a) Geometric isomerism

occurs only in two classes of compounds :

Alkenes & cyclic compound

(because of rigidity in molecules)

Geometric isomers (also called cis-trans isomers)

are stereoisomers that differ by groups being

on the same side (cis-isomer) or opposite

sides (trans-isomer) of a site of rigidity in a molecule

The requirements for geometric isomerism :

i) restricted rotation about a C=C,double bond, in alkenes or a C-C single bond in cyclic compounds

ii) each carbon atom of a site of restricted rotation has two different groups attached to it

Examples :

H3C CH3

H H

C C=

i)

cis-2-butene

H3C

C= C

CH3

H

H

trans-2-butene

ii) H3C CH2CH3

C= C

H CH3

trans-3-methyl-2-pentene

H3C CH3

C = CH CH2CH3

cis-3-methyl-2-pentene

iii)HH

CH3 CH3

cis-1,2-dimethylcyclohexane

H

HCH3

CH3

trans-1,2-dimethylcyclohexane

Cl

Cl

H

H

iv)

Cl

Cl

HH

cis-1,3-dichlorocyclopentane trans-1,3-dichlorocyclopentane

If one of the doubly bonded carbons has 2 identical

groups, geometric isomerism is not possible.

Examples :

CH3CH2i)

C = C

H

HH3C

2-methyl-2-butene

Hii)

C= C

CH3

CH3Cl

1-chloro-2-methylpropene

cis-trans isomers have similar chemical properties

but different physical properties

They differ in melting and boiling points and

solubility due to different polarity of the molecules

cis-isomers polar molecules

trans-isomers non-polar

• Melting point: trans- isomer > cis-isomer• Boiling point: cis-isomer > trans- isomer

• Stability: trans-isomer > cis-isomer

b) Optical isomerism

If a beam of light is passed through a piece of

polarizer prism, the emergent light vibrates in a

single plane, hence it is called a plane-polarized

light

Optically active compounds have the ability to

rotate plane-polarized light

The angle of rotation can be measured with an

instrument called polarimeter

Schematic representation of a polarimeter containing an optically active sample :

clockwise rotation – plus sign (+) / dextrorotarory

anticlockwise rotation – minus sign (-) / levorotorary

The requirements for optical isomerism :-

i) molecule contains a chiral carbon or chiral centre

(carbon atom with 4 different groups attached to it)

ii) molecule is not superimposable with its mirror image

A representation of a chiral molecule with

3-dimensional formula :-

P

CQ

RS

* PQRS *designates chiral centre

Enantiomers are a pair of mirror-image molecules

that are not superimposable (must have one or more

chiral carbons)

Examples :

i) 2-butanol, CH3CHCH2CH3

OH

C*

CH2CH3

H3C

OHH

C

CH2CH3

CH3HOH

enantiomers

:-

ii) 2-hydroxypropanoic acid, CH3CHCOOH

OH

:-

COOH COOH

OH HO H H

CH3 CH3

enantiomers

A racemic mixture or racemate is an equimolar

mixture of enantiomers which is optically inactive

because the two components rotate plane-polarized

light equally (same degree of rotation but in opposite

direction– so they can cancel each other’s rotation)

A pair of enantiomers have identical chemical and

physical properties but differ in the direction of

rotation of plane-polarized light

A compound with n chiral centers can have a

maximum of 2n stereoisomers

If a molecule contains two or more chiral centers,

diastereomers may exist

Diastereomers are stereoisomers that are not

mirror images of each other

All physical properties of diastereomers are usually

different from one another

Example :

The 4 stereoisomers of 2-amino-3-hydroxybutanoic acid CH3CH-CHCOOH are shown below using

OH NH2

Fischer projection formula:-

COOH COOH

CH3 CH3

H

H

H

H

NH2 H2N

OH HO

enantiomers

COOH

NH2

H H

HO

CH3

COOH

H H OH

H2N

enantiomersCH3

A B

C D

Four pairs of diastereomers are identified :

A and C ; A and D ; B and C ;

B and D

Meso compound is a stereoisomer that has more

than one chiral centres and that is superimposable

on its mirror image because of the presence of an

internal plane of symmetry, hence it is optically

inactive (does not cause a rotation of plane-polarized

light)

Example : Tartaric acid , HOOCCH(OH)CH(OH)COOH

COOH

OH

OH

H

H

COOH

COOH

COOH

HO

HO

H

H

COOH

COOH

H

H

OH

OH

plane of symmetry

plane of symmetry

rotate 180o

P Q

identical

At first glance, P and Q are assumed to be enantiomers

But if compound Q is rotated 180o in the plane of the paper, it is actually identical to compound P, therefore P and Q are superimposable mirror images

P and Q are the same compound

It is a meso compound

COOH

OH

OH

H

H

COOH

COOH

COOH

R S

COOH

COOH

H HO

H OH *not a plane of symmetry

*not a plane of symmetry

rotate 180o

different

H OH

H HO

R and S are related as mirror images and are not superimposable even if rotated 180o

Thus R and S constitute an enantiomeric pair

There are 2 pairs of diastereomers :

P and R & P and S

Further examples of meso compounds:

CH3

CH3

Cl

Cl

H

H

CHO

CHO

HO

HO

H

HH OH plane of symmetry

11.6 REACTIONS OF ORGANIC COMPOUNDS

11.6.1 Types of Covalent Bond Cleavage/Fission

All chemical reactions involved bond breaking and bond making

Two types of covalent bond cleavage :-

Homolytic cleavage Heterolytic cleavage

a) Homolytic Cleavage

Occurs in a non-polar bond involving two atoms of

similar electronegativity

A single bond breaks symmetrically into two equal

parts, leaving each atom with one unpaired electron

Free radicals are formed in homolytic cleavage

X X X + X ≡ 2X•• • • •

free radicals

b) Heterolytic Cleavage

Occurs in a polar bond involving unequal sharing of electron pair between two atoms of different electronegativities

A single bond breaks unsymmetrically and both the bonding electrons are transferred to the more electronegative atom

Cation and anion are formed in heterolytic cleavage

A B••

A ••- + B+ A is more

electronegative

A+ + B••- B is more

electronegative

cationanion

anioncation

Carbocations and free radicals are intermediates in organic reactions.

They are unstable and highly reactive

11.6.2 Reaction Intermediates

a) Carbocation

Also called carbonium ion

A very reactive species with a positive charge on a carbon atom

Carbocation is formed in heterolytic cleavage

Example :

(CH3)3C — Cl

(CH3)3C+

carbocation

+ Cl-

anion

Chlorine is more electronegative than carbon and the C—Cl bond is polar

The C—Cl bond breaks heterolitically and both the bonding electrons are transferred to chlorine atom to form anion and carbocation

b) Free Radical

A very reactive species with an unpaired electron

Formed in homolytic cleavage

Cl – Cl

Example :

uvCl •

free radicals

+ Cl •

C C C • + C•

H3C H CH3 • + H •

11.6.3 Relative Stabilities of Carbocations and Free Radicals

Carbocation and free radical primary

secondary

tertiary

depending on the number of carbon atoms directly bonded to the :-

• positively charged carbon atom (for carbocation)

• carbon atom with unpaired electron (for free radical)

The stability of carbocation increases with the number of alkyl groups present

The alkyl groups are electron-releasing relative to hydrogen, thus help to stabilize the positive charge on the carbocation

Carbocation Stability:

H C H < R C H < R C H < R C R

H H R R

+ + + +methyl cation

primary(1°)

secondary(2°)

tertiary(3°)

Increasing stability

As the number of alkyl groups attached to the positively charged carbon atom increases, the stability of carbocation increases

Likewise, the stability of free radical increases as more alkyl groups are attached to the carbon atom with unpaired electron

Free Radical Stability :

H C H < R C H < R C H < R C R

H H R R

methyl radical

primary(1°)

secondary(2°)

tertiary(3°)

Increasing stability

11.6.4 Reagents and Sites of Organic Reactions

a) Electrophile (E+)

Means ‘electron loving’

An electron-deficient species and electron-pair acceptor that attacks a part of a molecule where the electron density is high

An electrophile can be either neutral or positively charged

Examples of electrophiles :-

1. cations such as H+, H3O+, NO2+, Br+ etc.2. carbocations.3. Lewis acids such as AlCl3, BF3 etc.4. oxidizing agents such as Cl2, Br2 and etc

Examples of electrophilic sites in organic molecules :-

• molecules with low electron density around a polar bond such as :-

+ - + - + -C = O C – X C – OH

carbonyl haloalkanes hydroxyl compound

b) Nucleophile (Nu-)

Means ‘nucleus loving’

An electron-rich species and electron-pair donor that attacks a part of a molecule where the electron density is low

A nucleophile can be either neutral or negatively charged

Examples of nucleophiles :-

1. anions such as OH-, RO-, Cl-, Cn- etc.2. carbanions. (species with –ve charge on C atoms)3. Lewis bases which can donate lone pair electrons such as NH3, H2O etc.

Examples of nucleophilic sites in organic molecules :-

molecules with high electron density around the carbon-carbon multiple bond such as :-

-C=C- (alkenes) , -CC-(alkynes),

(benzene ring) and etc.

11.6.5 Types of Organic Reactions

The four main types of organic reactions are:

• Addition

• Substitution

• Elimination

• Rearrangement

1.Addition reaction

a) Electrophilic addition b) Nucleophilic addition

A reaction in which atoms or groups added to a multiple bond

a) Electrophilic Addition

Initiated by an electrophile, which attacks a nucleophilic site of a molecule

Typical reaction of unsaturated compounds such as alkenes and alkynes

Example :

CH3CH=CH2 + Br2 CH3CHBrCH2Brroomtemperature

electrophile

b) Nucleophilic Addition

Initiated by a nucleophile, which attacks an electrophilic site of a molecule

Typical reaction of carbonyl compounds

Example :

CH3 C CH 3 + HCN

O

CH3 C CH3

OH

CN

+

H+ CN-

2. Substitution Reaction

A reaction in which an atom or group in a molecule is replaced by

another atom or group

a) Free-radical Substitution

b) Electrophilic Substitution

c) Nucleophilic Substitution

a) Free-radical Substitution

Substitution which involves free radicals as intermediate species

Example :

CH3CH3 + Cl2 CH3CH2Cl + HCl uv light

b) Electrophilic Substitution

Typical reaction of aromatic compounds

The aromatic nucleus has high electron density, thus it is nucleophilic and is tend to electrophilic attack

Example :

+ Br2

Fe

catalystBr + HBr

electrophile

Br Br

c) Nucleophilic Substitution

Typical reaction of saturated organic compounds bearing polar bond as functional group, such as haloalkane and alchohol

Example :

CH3CH2Br + OH-(aq) CH3CH2OH + Br-(aq)

nucleophile

3. Elimination Reaction

A reaction in which atoms or groups are removed from adjacent carbon atoms of a molecule to form a multiple bond (double or triple bond)

Elimination reaction results in the formation of unsaturated molecules

CH3CH2OH CH2= CH2 + H2O Conc. H2SO4

Example :

4. Rearrangement Reaction

A reaction in which atoms or groups in a molecule change position

Occurs when a single reactant reorganizes the bonds and atoms

Example :

H C C

H OH

R

tautomerisation

H C C R

H

H O

enol keto (more stable)