Alkanes

Synopsis

Alkanes are saturated hydrocarbons.

Methane is the first member of this family and it is found in coal mines

and marshy places.

∙ General formula of alkanes is CnH2n+2

∙ Alkanes are less reactive and are called as paraffins. (Latin,

Parum =little, affinis = affinity

∙ IUPAC name is alkane (Alk+ane→Alkane)

∙ In these compounds, each carbon show sp3 hybridization and due to

this it form four single covalent bonds.

Bond angle is alkanes is about 109o.281

Types of carbon

Types of carbons =4

a) Primary carbon or 1o carbon - carbon attached to one other

carbon only

b) Secondary carbon or 2o carbon - carbon attached to two other

carbons

c) Tertiary carbon or 3o carbon - carbon attached to three other

carbons

d) Quarternary carbon or 4o carbon - carbon attached to four other

carbons

2, 2, 4-tri methyl pentane has 5 primary, 1 secondary, 1 tertiary and 1

neo carbons.

Alkyl group

a) Group produced due to the removal of one hydrogen from alkane.

b) It has the general CnH2n+1.

Ex :- −CH3 Methyl

−C2H5 Ethyl

−C3H7 Propyl

−C4H9 Butyl

−C5H11 Amyl

IUPAC Nomenclature of alkanes

Lowest Sum rule

1) If the side chain has more carbons in the chains, the carbon atoms

are given separate numbers with number 'I' to carbon attached to the

main chain.

2) The substituents are numbered by treating the side chain as

independent alkyl group.

3) In deciding alphabetical order sec, di, tri, etc., prefixes are not

considered, but iso, neo and cyclo are considered for alphabetical

order

3, 3-diethyl-5-(1-methyl ethyl) -4 methyloctane

OR

3,3-diethyl-5-isopropyl-4-methyloctane

* IUPCA accepts common names isopropyl, isobutyl, sec-butyl etc for

unsubstituted groups

4-ethyl-3,3 dimethyl hexane

This name is given as per the lowest sum rule and the and the longest

continuous chain rule.

Rule : When two different numbering methods give equivalent sets of

locants (equivalent sums) then the carbon which contains the

substituent of first in alphabetical order is given lowest number

Physical properties of alkanes

Alkanes are non-polar hence, only weak Van der waals forces of

attraction exist between their molecule.

They possess very low B. Pts and M.Pts At 298K

a) First four members (C1−C4) are gases

b) C5−C17 are liquids and

c) Above C17 are solids

They are soluble in non-polar solvents

In case of alkanes with similar structures, boiling point increase with

increase in molecular weight.

In case of chain isomers, more branched isomer will have less boiling

point

This is because with side chains, the surface area 'or' size of the

molecules decreases and the molecule tries to get spherical shape so,

attraction among molecules decreases.

Isomerism in alkanes

Chain isomerism in alkanes

∙ The compounds with same molecular

but exhibit different properties due to different structures or orientations

are known as "isomers"

∙ Alkanes having same molecular which differ mainly in carbon chain

length such isomers are known as "chain isomers"

Chain isomers

No. of chain isomers possible with,

(a) C2H10=2, (b) C5H12=3

(c) C6H14=5 (d) C7H16=9

(e) C10H22=75

Conformational isomers of ethane

∙ For an alkane, conformational isomers

are obtained by rotation about C-C bond (single bond)

∙ Conformations are represented by

(a) Newman projections

(b) Line-Wedge

(c) Sawhorse projections

∙ The molecule is viewed as if we were looking down the axis of C-C

bond

∙ The lines radiating from the centre of the circle denote the bonds

between the carbons closes to us and its attached atoms or groups.

∙ Those lines radiating from the circumference (outside) denote the

bonds between the carbon farthest from us and its attached atoms or

groups.

∙ The ratio of two conformations of a particular alkane in equilibrium at

certain temperature T can be calculated by using the ΔG=RT/nKeq

Here ΔG= Gibbs energy difference between two conformations

R is universal gas constant

Keq is equilibrium constant

∙ Rotation about C-C bonds in ethane is though very rapid not

completely free.

Staggered and eclipsed conformations of ethane

∙ In staggered conformation the C-H bonds are

arranged as each one bisects the angle defined by two C-H bonds on

adjacent carbon.

∙ In eclipsed conformation each C-H bond is aligned with a C-H bond

on adjacent carbon.

∙ The staggered and eclipsed conformations are interconvertible by

rotation of one carbon with respect to the other around the σ bond

that connects them

∙ Different conformations of the same molecule are also

calledconformers or rotamers.

Stability of conformers of ethane

From the graph,

we can understand that staggered conformation is more stable than

eclipsed.

This is because in staggered conformation bonds are at maximum

separation. Hence bonded electrons have minimum repulsion.

But in eclipsed conformation, bonds are close and repulsions are

maximum. Hence, it is less stable. In eclipsed conformation, the

destabilisation is due to torsional strain.

∙ As the energy difference between E and S conformation of ethane is

very small (2.9 K.cal/mole) it can overcome this energy barrier easily

even at ordinary temperature by getting sufficient thermal or kinetic

energy through inter-molecular collisions. So, rotation about C-C in

ethane is very rapid

∙ Any other intermediate conformation between staggered and

eclipsed is called "a skew conformation."

∙ In all the conformations, the bond angles and bond lengths remain

the same.

Reactions of Alkane

Preparation methods of alkane

a) By hydrolysis of carbides :-

Al and Be carbides on hydrolysis produce methane.

Al4C3+12H2O→3CH4+4Al(OH)3

Be2C+4H2O→CH4+2Be(OH)2

b) Decarboxylation of sodiumacetate :-

CH3−COONa+NaOH−→−ΔCaOCH4+Na2CO3

c) Reduction of methyl iodide :-

CH3−I+2(H)−→−−−EtOHZn−CuCH4+HI

∙ Methane can not be prepared from sabatier - senderen's, Kolbe's

electrolysis and Wurtz reactions.

Preparation of ethane and other higher order alkanes

∙ Decarboxylation :

CH3−CH2COONa+NaOH−→−ΔCaOC2H6+Na2CO3

CH3−CH2−CH2−COONa+NaOH−→−CaOCH3−CH2−CH3+Na2CO3.

∙ Wurtz reaction :

CH3−I+2Na+I−CH3−→−−−dry etherCH3−CH3+2NaI

CH3−I+2Na+I−CH2−CH3−→−−−−dryetherCH3−CH3+CH3−CH2−CH2−

CH3+CH3−CH2−CH3.

Pure propane can not be prepared by Wurtz reaction.

Kolbe's Electrolysis

∙ Kolbe's Electrolysis :

2CH3COOK+2H2O→C2H6+2CO2+2KOH+H2

∙ At Anode :

2CH3COO−−→−−−2e−2CH3COO→cH3−CH3→C2H6+2CO2↑

∙ At Cathode :

H2O+e−→HO−+H;2H→H2↑

Sabatier - Senderen's reaction (large scale preparation) :

H2C=CH2H2−→−−300oCNiC2H6(ethane) (Ethylene)

CH3−CH=CH2+H2−→−−300oCNiCH3−CH2−CH3.

REDUCTION OF ETHYL IODIDE

C2H5I+2H−→−−−−C2H5OHZn−CuC2H6+HI

Halogenation

Under normal conditions, alkanes are least reactive. Hence, they are

also called paraffins (parum =little, affinis=affinity) Alkanes are inactive towards acids, bases, oxidising and reducing

agents

Under special conditions, alkanes undergo substitution reactions.

a) Halogenation :-

C2H6+Cl2−→hvC2H5Cl+HCl net reaction

C2H6+6Cl2→C2Cl6+6HCl

Mechanism of Halogenation

∙ The rate of reaction of ethane with halogens is of the order of F > Cl >

Br > l

∙ Rate of reaction with respect to alkyl group with a given halogen

is 3o>2o>1o

∙ Fluorination is violent and iodination is very slow and reversible.

∙ Iodination requires the presence of oxidising agents like HNO3,HIO3

Isomerisation

Formation of branched chain

isomers from higher alkanes on heating in the presence of

anhydrous AlCl3 and HCl gas is isomerisation.

Aromatisation or Reforming

CH3−(CH2)4−CH3−→−−−−−−−−−−−−Cr2O3orV2O5orMo2O3500oC,10−20atmC6

H6

n-hexane Benzene

Combustion

As alkanes on combustion release huge amount of heat energy, they

are used as fuels.

General equation for combustion of alkanes is

CnH2n+2+(3n+12)O2→nCO2+(n+1)H2O+energy

If oxygen supply is not sufficient, partial oxidation takes place giving

carbon black which is used in Indian ink etc.

Cycloalkanes

Synopsis

a) Hydrocarbons with no 'or' more carbon rings in their molecules with

only C-C single bonds and sp3 hybridised carbon atoms are

cyloalkanes.

b) Cycloalkanes with only one ring have the general $$C_nH_{2n}$$

IUPAC Nomenclature of Cycloalkanes

They are named by adding

primary prefix cyclo before parent name alkane

Ex :

IUPAC Nomenclature of Bicyclic compounds

∙ If the molecule has two fused 'or' bridged rings, they are named as

bicyclic alkanes.

∙ Bicyclic compounds are named by using the alkane name (on the

basis of total no of carbon atoms) and the prefix bicyclo.

∙ In bicyclo compounds, the carbon atoms common to both rings

(either one or two) are called bridge heads.

∙ Each bonds or chain of atoms connecting the bridge head atoms is

called a bridge

∙ In naming these compounds, in between the words bicyclo and

alkane, the no. of carbon atoms in each bridge is written (in

descending order) in square brackets

∙ If a substituent is present, the bicyclic ring system in numbered

∙ In substituted bicyclic alkanes, the numbering begins with one of the

bridge head atoms proceeds first along the longest bridge, to the

second bridge head atom continues along the next longest bridge to

the bridge head atom and finally completed along the shortest path.

In cyclic compounds, more branched chain carbon should be given

the lowest number possible.

Angle strain

∙ It is the increased potential energy of a cyclic molecule caused by

deformation of bond angle from its lowest energy value.

∙ For example : In cyclic alkanes 'C' atoms are 'sp3' hybridised and

hence the angle should be 109o28' but in cyclopropane, with the

shape of regular triangle the internal angle must be 60o i.e. they are

compressed from 109o"28′ by almost 49.5o

∙ This compression of the bond angle causes some strain known as

angle strain, which increases the potential energy of the molecule.

∙ In cyclopropane and cyclobutane, in addition to angle strain there is

one more strain known as torsional strain.

∙ Torsional strain is caused by repulsions between the aligned electron

pairs of the "eclipsed bonds" in eclipsed conformations of a molecule".

∙ Due to (a) angle strain and (b) torsional strain cyclo propane and

cyclobutane are more reactive than 'cyclopentane' and

'cyclohexane'.

Reactions of cycloalkanes

Preparation of cycloalkanes

By Dieckmann condensation

reaction

This is an intramolecular condensation

5, 6, 7 membered rings can be prepared from esters of dicarboxylic

acids

Freund's method

In this method 1, 3 to 1, 6-dihalo

alkanes react with Na metal 'or' Zn metal to form corresponding

cycloalkanes

Diel-Adler's reaction

Generally, this reaction is between 1)

conjugated diene 2) dienophile

∙ Conjugated diene is a 4−π electron compound

∙ Dienophile in an -ene compound (with double bond (or) 2π electrons)

Here, two new σ bonds are formed at the expense of two π bonds of

diene and dienophile. So, the product (addict) contains a new six

membered ring with a double bond.

Zieglar reaction

It is a general method to

prepare cyclic alkane

a) A dinitrile is treated with diethyl lithium amide.

b) An imminonitrile is formed.

c) This immino nitrile is converted to cycloketone by acid-catalysed

hydrolysis and decarboxylation.

d) Cycloketone is then reduced to cycloalkene.

Alkenes

Synopsis

Alkenes are unsaturated hydrocarbons. These contain a C =C. They

contain two hydrogens less than corresponding alkanes.

Double bonded carbon undergoes sp2 hybridisation.

These are otherwise known as OLEFINS

General of alkenes is CnH2n

Bond angle is 120∘

The C=C bond length in alkenes is 1.34A0 which is less than the C-C

single bond distance of 1.54A0 in alkanes.

In alkenes, the double bond contains one sigma bond and a π-bond.

As Pi(π) electrons are easily available for reagents, they undergo

electrophilic addition reactions .

Due to the presence of a double bond, alkenes are more reactive than

alkanes.

The unsaturation (or) pi bond between carbon and carbon is identified

by following reagents

a) Br2 water (reddish brown colour)

b) Alkaline KMnO4( Pink colour of Baeyer's reagent)

IUPAC Nomenclature of alkenes

The IUPAC name is derived from the

IUPAC name of alkane by replacing ending ane by ene alongwith the

position of double bonds.

b) The suffix used for alkene is - ene

eg :- Alkane - ane+ene= Alkene

CH3−CH=CH−CH3 But - 2 - ene.

c) In case of two double (or) three double bonds, the ending ene of

alkanes is suitably replaced by diene (or) triene.

CH3−CH2−CH=C=CH2, CH2=CH−CH=CH2

Penta - 1, 2-diene Buta -1, 3-diene.

d) Residual part left after the removal of one H-atom from alkene is

known as alkenyl group.

According to nomenclature, these groups are named by replacing

terminal e of alkene by yl

Group Common IUPAC

CH2=CH− vinyl ethenyl

CH2=CH−CH2− allyl orio-e-enyl

e) The longest continuous chain should include both the carbon atoms

of the double bond.

f) The chain is numbered from the end that gives the lower number to

the first carbon atom of the double bond.

eg:- CH3−CH=CH2,CH3−CH2−CH=CH2

Propene But-1-ene

CH3−CH=CH−CH3

But-2-ene,

The compounds containing two double bonds are named as dienes.

CH2=CH−CH=CH2, Buta - 1,3-diene

Geometrical isomerism in alkenes

a) Alkenes exihibit geometrical

isomerism

b) Alkenes with baC= Cab (or) daC= Cab exihibit geometrical

isomerism

c) Trans isomers are more symmetrical than cis isomers.

d) A cis - isomer - has lower melting point than trans isomer.

e) Physical properties like density, dipole moment, refractive index and

heat of combustion are more for cis isomer.

f) Trans isomer is more stable than cis-isomer

Physical properties of alkenes

a) Ethylene is a colourless gas, and dissolves in non polar solvents like

benzene, ether.

b) First three alkenes are gases, next fourteen alkenes are liquids and

higher alkenes are solids

c) B.P. increases with the molecular weight

d) For every CH2 group B.P. increases by 20-30K

e) Boiling point α surface area

α1No. of branched chains

Reactions of alkenes

Dehydration of alcohols

-Dehydrohalogenation of alkyl halides

(R−X)X=Cl, Br, I

CH3−CH2−Br−→−−−−ΔAlcKOHCH2=CH2+HBr Ethyl bromide

In this reaction, H atom is eliminated from

β- carbon atom

∴ it is known as β- elimination reaction

Nature of halogen atom and the alkyl group determine the rate of

reaction.

a) for halogens rate is I > Br > Cl

b) for alkyl group rate is 30 > 20 > 10

Dehalogenation

CH2−CH2+Zn−→−−−ΔAlcoholCH2=CH2+ZnBr2

| | Ethylene

Br Br

1, 2 dibromo ethane

Controlled hydrogenation of alkynes

Chemical properties

a) Ethene undergoes addition reactions.

b) In these addition reactions, the π bond, which is weaker than the

sigma bond, breaks.

c) In addition reaction, pi bond cleavage takes place and as a result

two new sigma bonds will be formed in the product.

Hydrogenation

CH2=CH2−→−−−−−−−Ni/120∘CH2/Pt,Pd(or)CH3−CH3

Chorination

Addition of Hydrogen halide

CH2=CH2→HXCH3−CH2 - X,ethyl halide

(HX = HCl, HBr, HI)

Reactivity order- HI>HBr>HCl>HF

Markovnikov's rule:-

The negative part of the addendum (attacking molecule) attack the

carbon atom which contains smaller number of hydrogen atoms.

Br

|

CH3−CH=CH2+HBr→H+CH3−CH−CH3

2 - Bromo Propane

(major)

+CH3−CH2−CH2Br 1-Bromo Propane

(minor)

The mechanism proceeds through an achiral carbocation

intermediate

b) The stability of carbocation is in the order-

tertiary > secondary > primary carbocation

* Anti Markownikoff's rule :-

In the presence of peroxides during the addition of HBr to an

unsymmetrical alkene the Br atom will join to the carbon carrying more

hydrogen atoms while H-atom will go to the other carbon atom.

a) It is also known as Kharasch effect (or) Peroxide effect.

b) Anti Markownikoff's addition proceeds through since homolytic

cleavage

c) 2∘ free radical is more stable than 1∘, so 1-bromopropane is major

product

CH3−CH=CH2+HBr−→−−−−PeroxideCH3−CH2−CH2Br 1-Bromo propane

(major)

Br

|

+CH3−CH−CH3

2-Bromo propane

(minor)

(d) Only HBr follows peroxide effect, HF, HCl, HI do not exihibit peroxide

effect even in presence of peroxide.

Addition of sulphuric acid

CH3−CH=CH2+H2SO4→CH3−CH−CH3

|

OSO3H

Isoproylhydrogen sulphate

The addition of cold conc. H2SO4 to alkene produces alkyl hydrogen

sulphate by electrophilic addition following Markownikoff's rule

Oxidation reaction

Oxidation by Baeyer's reagent:-

Alkenes on passing through dilute alkaline 1% cold KMnO4, form

dihydroxy compounds (eg. glycols)

CH2CH2−→−−−−−−−−−−−cold dil. alk. KMnO4CH2 OH−CH2OH

Ethylene glycol

Oxidation of alkenes by acidic KMnO4 or K2Cr2O7 gives carboxylic

acids

CH2=CH2−→−−−−−−(O)KMnO4/H+HCOOH+HCOOH

↓(O)

CO2+H2O

KMnO4 decolorises. This is the test for unsaturation.

Acidic KMnO4(or) K2Cr2O7 oxidises alkenes to ketones (or) acids

depending on the nature of alkene

CH3

|

CH3−C=CH2 −→−−−−−−KMnO4/H+(CH3)2C=O+CO2+H2O

2 - Methylpropene propanone

CH3−CH=CH−CH2 −→−−−−−−KMnO4/H+2CH3COOH

2-Butene Ethanoicacid

Ozonolysis

a) It is a test for unsaturation in molecules

b) The formation of Ozonide and its decomposition to giv e carbonyl

compounds is known as ozonolysis.

(c) Ozonolysis is used to detect the position and nature of unsaturation

in a molecule.

e) Symmetrical alkene gives rise to two molecules of same carbonyl

compound

CH2=CH2+O3→C2H4(O3)−→−−−−Zn+H2O2HCHO+H2O2

Ozonide Formaldehyde

CH3

|

CH3−C=CH2−→−−−−Zn+H2OO3CH3COCH3+HCHO

2-Methyl-l-Butene Acetone

CH3

|

CH3−CH=C−CH3 −→−−−−Zn+H2OO3CH3CHO+CH3COCH3

2-Methyl-2-Butene

Polymerisation

Alkenes form the basis of many important polymers.

a) Formation of polythene;

nCH2=CH2−→−−−−−−−O2,200∘C1500−2000atm[−CH2−CH2−]n Polythene

Combustion of ethane

The combustion of alkenes is also exothermic, used for welding

purposes in oxy-ethylene welding.

CH2=CH2+3O2 →2CO2+2H2O ; ΔH=−Ve

With S2Cl2

2CH2=CH2+S2Cl→(ClCH2CH2)2S It is poisonous gas. It is used as war gas

Alkynes

Isomerism in alkenes

Alkynes exhibit chain, position and

functional isomerism (Functional isomer of alkyne is alkadiene).

Ex : CH≡C−CH2−CH2−CH3......I pent-1-yne

CH3−C≡C−CH2−CH3......II pent-2-yne

3-methyl but-1-yne

Structurers I and II are position isomers and structures I & III or II & III are

chain isomers.

Ex : CH3−C≡CH and CH2=C=CH2 functional isomers

Synopsis

Structure Common name IUPAC Name

HC≡CH Acetylene Ethyne

CH3C≡CH Methyl

Acetylene Propyne

CH3CH2C≡CH Ethyl acetylene But-1-yne

CH3C≡CCH3 Dimethyle

acetylene But-2-yne

General of alkynes is CnH2n−2

These are unsaturated hydrocarbons containing (−C≡C−)

Structure

Each carbon atom of ethyne has two sp hybridised orbitals.

C-C sigma bond (σ) is formed by head - on overlapping of two sp

hybridised orbitals of two carbon atoms.

sp hybridised orbital of each carbon atom undergoes head- on

overlapping with 1s-orbital of each of the two hydrogens forming two

C-H sigma bonds.

H-C-C bond angle is 180o

Two p-orbitals of a carbon undergo side - on overlapping with two p-

orbitals of another carbon to form two pi (π) bonds between two

carbons .

Ethyne molecule contains.

one C-C σ bond

two C-H σ bonds

two C-C π bonds

Order of bond energies

C≡C>C=C>C−C

823 KJ 681 KJ 348KJ

order of bond lengths

C−C>C=C>C≡C

154 pm 134 pm 120 pm

Physical properties of alkynes

First three alkynes are gases, the next eight are liquids and the higher

alkynes are solids.

All alkynes are colourless.

Ethyne has characteristic odour and other alkynes are odour less.

Alkynes are weakly polar in nature

Alkynes are lighter than water and immiscible with water but soluble in

organic solvents like ether, CCl4 and Benzene etc.

Their M.P, B.P and density increase with increase in molecular

weights.M.P & B.P of alkynes decrease with increasing of number of

branches.

Order of M.P and B.P:

alkanes > alkenes > alkynes

Acidic nature of alkynes

Sodium metal and sodamide (NaNH2) are strong bases.

* Ethyne or 1-alkyne on reaction with strong bases (Na

or NaNH2)leberates H2 gas hence ethyne or 1-alkyne shows acidic

nature.

* Alkanes and alkenes can not liberate H2 gas with Na

or NaNH2because these are not acidic

* Hydrogen atoms in ethyne are attached to the sp hybridised carbon

atoms, due to highest percentage of s- character (50%) of sp-orbital of

carbon has highest electro-negativity hence, sp-carbon of ethyne

attracts the shared electron pair of C-H bond to release hydrogen

ion (H+). Hence, 1-alkynes and ethyne are acidic.

Reactions of alkynes

Hydrolysis of carbides

CaC2+2H2O−→−−−−−hydrolysisH−C≡C−H+Ca(OH)2

Calcium Carbide Acetelyne

CaC2 can be obtained as follows

CaCO3−→ΔCaO+CO2↑

CaO+3C→CaC2+CO↑

Magnesium carbide on hydrolysis gives propyne

Mg2C3+4H2O→CH3−c≡CH+2Mg(OH)2

Dehydrohaogenation

CH3−CHBr2−→−−−−Alc.KOHH−C≡C−H+2HBr 1,1-dibromo Ethane

(Gem-dihalide)

Br−H2C−CH2−Br−→−−−−−−−−KBr,−H2OAlcoholicKOH

1, 2-dibromo ethane

(vic-dihalide)

H2C=CH−Br−→−−−NaNH2CH≡CH+NaBr+NH3

ethenyl bromide

Dehalogenation

Chemical properties

Alkynes show acidic nature,addition reactions and polymerisation

reactions.

Addition reactions

Alkynes contain a

triple bond so they adduct two molecules of H2.X2,HX,H2O etc

The formation of the addition product takes place according to the

following steps.

The addition product formed depends upon stability of vinylic cation.

Addition in unsymmetrical alkynes takes place according to

Markovnikov rule.

Hydrogenation

Hydrogenation can be

controlled at the alkene stage by using a Lindlars catalyst which is

mixture of palladium and barium sulphate poisoned by quinoline or by

using Na & NH3

Hydrogenation

If the triple bond is

not present at the end of the chain of the molecule, its reduction

produces either a cis alkene or a trans alkene depending upon the

choice of reducing agent.

Note: Sodium dissolve in liquid ammonia give solvated electrons, these

reduces the alkyne.

Bromine water test

Reddish orange colour of

bromine solution is decolourised. This is used as test for unsaturation.

Addition of water

Alkynes on

reaction with one molecule of water at 333K in presence

of HgSO4 and dil- H2SO4

Ozonolysis

In

oxidative ozonolysis, carboxylic acids are formed

H−C≡C−H−→−−−−−−−−−−−ii)H2O+CH3COOHi)O3HCOOH+HCOOH

Combustion

Acetylene burns with smoky flame in excess of air

Under controlled supply of air burns with a bright light called

oxyacetylene torch (or) flame which is used for welding purposes

and gives a temperature of about 35000C

2C2H2+5O2→4CO2+2H2O+flame(1300KJ)

Oxidation with alkaline KMnO4

Acetylene on oxidation with alk-

KMnO4 solution gives oxalic acid

In the above reactions, the pink colour of alkaline KMnO4 ( Baeyer's

reagent ) gets discharged. The above reactions in which triple bond is

completely broken are called degradation or cleavage reactions.

These can be used to locate the position of triple bond.

Hydrogenation

Alkynes react readily with

hydrogen in the presence of finely divided Ni, Pt or Pd as catalysts. The

reaction is called hydrogenation.

Oxidation with acidified KMnO4

Acetylene

converted into formic acid with a rupture of triple bond.

HC≡CH+3(O)+2H2O→2HCOOH

R−C≡CH+3(O+H2O→RCOOH+HCOOH

CH3−C≡C−CH3+3(O)+H2O→2CH3COOH

When the reaction is carried out at high temperature, the triple bond is

completely broken leading to the formation of carboxylic acids and

carbon dioxide depending upon the position of triple bond. The

cleavage occurs at the site of triple bond.

Oxidation with chromic acid

Acetylene is converted into acetic acid and orange colour

of K2Cr2O7changes to light green

CH≡CH+H2O+O→CH3−COOH

Polymerisation

Dimersation : When passed through a solution of cuprous chloride in

ammonium chloride, acetylene dimerises to give vinyl acetylene

CH≡CH+HC≡CH−→−−−NH.ClCuClCH≡C−C=CH2

acetylene vinyl acetylene

−→−−−NH.ClCuClCH2=CH−C≡C−CH=CH2

divinylacetylene

Polymerisation

TRIMERISATION: When passed through

aired hot metal tube, acetylene is converted into benzene

Similarly, propyne trimerises in the presence of sulphuric acid to form

sym-trimethyl benzene or mesitylene.

Polymerisation

TETRAMERISATION:Acetylene

undergoes tetramerisation under high pressure and inpresence of

nickel cynide catalyst to form cyclooctatetraene

Uses

Uses of acetylene

Acetylene is used in the preparation of Benzene, (PVC) vinyl plastics,

acetaldehyde, acetic acid, ethyl alchol, solvents like

westron (Acetylene tetra chloride) westrosol (Trichloroethylene)

Acetylene is used in the form of oxy acetylene flame for arc welding

and cutting tools.

Oxyacetylene flame is obtained with 3500oC by mixing acetylene with

oxygen gas.

Uses of benzene

It is used as solvent for fats and resins.

It is used in dry cleaning.

It is used in the synthesis of phenol, styrene, aniline, insecticides like BHC.

It is used as a motor fuel.

Aromatic Hydrocarbons

Synopsis

Aromatic hydrocarbons can be distinguished from aliphatic

hydrocarbons by flame test.

Aromatic hydro carbons give sooty flame.

Benzene is considered as parent compound of aromatic

hydrocarbons.

IUPAC Nomenclature

In this system, the substituent

name is place as prefix to the word benzene.

IUPAC Nomenclature

The dimethyl benzenes are

xylenes.

IUPAC Nomenclature

More than two substituents

are always given by numbers.

IUPAC Nomenclature

When two substituents are

present, their relative positions are indicated by the prefixes ortho,

meta or para(o-;m-;p-etc). These prefixes are only for di-substituted

compounds but, for more substituted compounds numbers are given

C6H5- group is known as phenyl which is some times abbreviated as Ph

or ϕ

Resonance & Aromaticity of Benzene and its compounds

Resonance of benzene

The carbon to hydrogen

ratio indicates unsaturation in the molecule.

But, benzene does not behave like other unsaturated compounds

(alkenes, alkynes.)

It does not decolourise

a) Br2 water (or) alkaline KMnO4

b) It can't undergo Polymerisation and oxidation under normal

conditions.

c) It undergoes electrophilic substitution reactions rather than addition

reactions and is a stable molecule.

To explain the stability of benzene let us see the hydrogenation

reactions.

(just double that of cyclo-hexene)

Resonance of benzene

If benzene was a hexatriene, then the

energy to be liberated in hydrogenation is

3×28.6=85.8K.cal.mole−1 but actually only 49.8 Kcal is liberated. To

the extent of 85.8−49=36k.cal/mole, benzene is stabilized compared

to cyclohexatriene.

The phenomenon in which two or more structures can be written for a

molecule but none of them represents it accurately is

called resonance.

Various possible alternative structures are known as resonating

structures or Canonical structures.

In these structures only delocalization of electrons takes place.

During this delocalization, some energy is released which is called

resonance energy.

Greater the resonance energy, greater will be the stability.

More the number of resonating structures or canonical structures, more

will be the stability.

For benzene, resonance energy is 36 K.cal/mole or 150.48 KJ/mole.

According to Kekule, benzene is resonance hybrid of two structures,

which differ in the position of double bonds.

Pi bonds in benzene are not static so, benzene is normally represented

by small ring in a regular hexagon.(small ring represents pi electron

cloud)

Orbital model of benzene

* In Benzene, each carbon is surrounded by 3sp2 hybridized orbitals

and one unhybridized 'p' orbital with unpaired electron.

* The six unhybridised 'p' orbitals forms 3 delocalised π bonds by side -

on over lapping.

* In Benzene, the total number of hybridized orbitals and pure orbitals

are 18 & 12 respectively.

* The total number of σ and π bonds are 12 & 3

* The Bond angle is 120o, and the bond length is 1.39A0. This is due to

delocalisation of 'pi' bond (or) resonance.

* The C-C double bond (1.39A0) is intermediate between C-C single

bond (1.54A0) and C=C double bond (1.34A0) length.

Aromaticity - Huckel Rule

The carbon compounds

with the following characteristics can exhibit aromaticity.

i) Planarity

ii) Complete delocalisation of the π electrons in the ring

iii) Presence of (4n+2)π electrons in the ring where n is an integer (n =

0, 1, 2, .......). This is often referred to as Huckel Rule.

Eg:

Reactions of aromatic hydrocarbons

Preparation of benzene - Decarboxylation

Preparation of benzene- Reduction method

Reduction of phenol :

Preparation of benzene from acetylene

From acetylene :

3C2H2−→−−−−−−−−−−−500oC(Trimerisation)red hot Cu/Fe tubeC6H6

It can be catalysed by Al4C3 or carbon.

Desulphonation of benzene acid

Chemical properties of benzene

Benzene undergoes electrophilic substitution reactions predominantly.

Nitration of benzene

Nitration :

C6H6+HNO3−→−−−−−−57oCCon.H2SO4C6H5NO2+H2O

Mechanism of nitration of benzene

It is electrophilic substitution

reaction and electrophile is nitronium ion (−NO+2). It is produced by

transfer of proton from sulphuric acid to nitric acid in the following

manner

Step I :

In the generation of nitronium ion, sulphuric acid serves as an acid and

nitric acid as a base.

The role of H2SO4 in the nitration mixture is to eliminate water

Nitration mixture is Con.HNO3+Con.H2SO4 (1 : 1 by volume)

To get mono nitro benzene temperature should be maintained in

between 50 to 60oC

Halogenation of benzene

Electrophile is halonium ion (X+)

Order of reactivity of halogens is F2>Cl2Br2>l2

Fluorination is not carried out directly as F2 is highly reactive.

Fluorination is carried out indirectly by decomposition of benzene

diazonium fluoroborate.

Chlorination and bromination may be carried out by X2 / halogen

carrier (Lewis acid, Fe-dust, P, l2 etc.), HOCL or

HOBr, Br2/(CH3COO)3Tl,Cl2O/H2SO4, N-chloro or N-

bromosuccinimide.

Iodine reacts very slowly and iodination is carried out by I2/ oxidising

agent (HNO3,HIO3,SO3,H2Oetc.),I2/Cu may also be used

Sulphonation of benzene

The attacking electophile

is SO3 (from fuming sulphuric acid i.e., H2SO4+SO3) Sulphonating agents :

Fuming H2SO4(orH2SO4+SO3),Conc.H2SO4,SO2/ inert solvent.

Reaction is reversible.

Friedel Craft's Alkylation

Benzene reacts with

alkylhalide in presence of anhydrous AlCl3 and gives alkyl benzene.

The reaction is known as Friedel Craft's reaction

Attacking electrophile is R+

Alkylating agents : (i) RX/Lewis acid, (ii) Alkene/protonic acid,

(iii) ROH/H+

Two types of catalyst are used :

a) Lewis acids : Anhydrous AlCl3,AlBr3,BF3etc.

b) Protonic acids : H2SO4, HF etc.

* Order of reactivity of RX :

RF > RCl > RBr > Rl

* Order of activity of Lewis acids :

AlBr3>AlCl3>CaCl2>FeCl3>SnCl4>BCl3>BF3>ZnCl2

* Reaction is not possible if deactivating groups

like −NO2,−COCH3etc. are attached to aromatic ring.

Friedel Craft's Acylation

Electrophle is acylium cation (RC+O) Acylating agents : (i) RCOX / Lewis acid

(ii) RCOOH or (RCO)2O or CH2=C=O/Lewisacid

Product is not re-arranged

With (CH3)3C−COCl/AlCl3 alkylation, not acylation is observed as less

stable (CH3)3C+O decomposes to more stable (CH3)3C+

Halogenation of benzene

In presence of Pt, reaction occurs at

room temperature

Chlorination of benzene

BHC is a powerful insecticide

and is used under the name lindane orGammaxene

Addition reactions-Ozonolysis of benzene

Zn dust destroys H2O2 which may

oxidise glyoxal

Combustion

Benzene is a stable compound it is not attacked by ordinary oxidising

agents (aidified KMnO4, chromic acid)

Strong oxidising agents convert benzene slowly in to CO2 and water,

on heating being inflammable liquid it burns in air with smoking flame

(combustion)

2C6H6+15O2→12CO2+6H2O

Oxidation with V2O5

Benzene is oxidised in to maleic

anhydride in presence of V2O5 at 450−500oC

Directing influence of functional groups

When a group is attached

to benzene ring it produces two effects

a) Activity effects b) Directing effects

On the basis of activity effects, groups may be divided into two types-

ACTIVATING GROUPS :Which increases the activity of aromatic ring in

electrophilic substitution reactions.

There are of three types

a) Strongly activating groups

Eg :- NH2,−NHR,−NR2,−OH,−O−

b) Moderately activating groups

Eg :- NHCOCH3,−NHCOR,−OCH3,−OR

c) :- CH3,−C2H5,−R,−C6H5

DEACTIVATING GROUPS:Which deactivates the benzene ring towards

electrophilic substitution reactions,

These are of three types

a) Strongly deactivating groups

Eg :- NO2,−NR3,−CF3,−CCl3

b) Moderately deactivating groups

Eg :- CN,−SO3H,−CO2H,−CO2R,−CHO,−COR

c) Weakly deactivating groups

Eg :- F, -Cl, -Br, -I

Ortho, Para-Directing Groups. :-

R,−OH,−OR,−SH,−NH2

−SR,−NHR,−NR2,−CH2R,−C6H5,−X,−CH2OH,

−CH2Cl,−CH2NH2,−CH2CN,−CH2COOH,

−CH=CH−COOH,−CH=CH2

Meta Directing Groups. :-

NO2,−SO3H,−SOCl,−COR,−COCOOH,−CX3,

−N+H3,−N+HR2,−N+R3etc.

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