Chapter 19 Aliphatic Hydrocarbons Part 1 by TEACHING CARE Online coaching and tuition classes

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“Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are obtained mainly from petroleum, natural gas or coal. Petroleum is a major source of aliphatic hydrocarbon. The important fuels like petrol, kerosene, coal gas, oil gas, compressed natural gas [CNG], LPG etc, all are hydrocarbon and their mixtures”.

Hydrocarbons

Aliphatic or open chain Alicyclic or closed chain or cyclic Aromatic or arenes

Cyclo alkanes Cyclo alkenes

Saturated Unsaturated

Alkanes or paraffines

Alkenes or olefins Alkynes or acetylenes

Mineral oil or crude oil, petroleum [Petra ® rock; oleum ® oil] is the dark colour oily liquid [do with offensive odour found at various depths in many regions below the surface of the earth. It is generally found under the rocks of earth’s crust and often floats over salted water.

(1) Composition

Alkanes : found 30 to 70% contain upto 40 carbon atom. Alkanes are mostly straight chain but some are branched chain
Cycloalkanes : Found 16 to 64% cycloalkanes present in petroleum are; cyclohexane, methyl cyclopentane cycloalkanes rich oil is called asphattic oil.
Aromatic hydrocarbon : found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene
Sulphur, nitrogen and oxygen compound : Sulphur compound present in the extent of 6% and include mercaptans [R-SH] and sulphides [R-S-R]. the unpleasant smell of petroleum is due to sulphur compounds. Nitrogen compounds are alkyl pyridines, quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and Compounds like chlorophyll, haemin are also present in it.
Natural gas : It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), Vapours of low boiling pentanes and hexanes (0.5%) and Nitrogen (1.3%). L.P.G. Contain butanes and pentanes and used as cooking It is highly inflammable. This contain, methane, nitrogen and ethane.
N.G. : The natural gas compressed at very high pressure is called compressed natural gas (CNG). Natural gas has octane rating of 130 it consists, mainly of methane and may contain, small amount of ethane and propane.

Theories of origin of petroleum : Theories must explain the following characteristics associated with petroleum,

Its association with brine (sodium chloride solution). The presence of nitrogen and sulphur compounds in it.

The presence of chlorophyll and haemin in it. Its optically active nature. Three important theories are as follows.

Mendeleeff’s carbide theory or inorganic theory
Molten metals in hot earth’s crust combine with coal deposits and formed
Carbides reacted with steam or water under high temperature and pressure to form mixture of saturated and unsaturated
The unsaturated hydrocarbon in presence of metal catalyst, high pressure and high temperature, undergoes reactions such as hydrogenation, isomerisation and polymerisation to form number of

Reactions :

Ca + 2C ® CaC₂

(Calcium carbide);

Mg + 2C ® MgC₂

(Magnesium carbide)

4 Al + 3C ® Al₄ C₃

(Aluminium carbide);

CaC2 + 2H 2 O ® Ca(OH)2 + C2 H 2 (Acetylene)

Al4 C3 + 12H 2 O ® 4 Al(OH)3 + 3CH4 (Methane) ;

C2 H 2 ¾¾^H¾2 ® C2 H4 ¾¾^H¾2 ® C2 H6

(Ethane)

3[CH º CH] ¾¾^Poly¾^me¾^risat¾^io¾ⁿ ® C6 H6 ;

(Benzene)

CH3 – CH = CH – CH3 ¾¾^Isom¾^eris¾^atio¾ⁿ ® CH3 – CH 2 – CH = CH 2

(1-Butene)

Theory fails to account for, The presence of nitrogen and sulphur compounds. The presence of chlorophyll and haemin derivatives. The presence of optically active compounds.

Engler’s theory or organic theory : Theory is supported by the following facts,
The presence of brine with petroleum,
The presence of optically active compounds,
The presence of nitrogen and sulphur compounds
The presence of fossils in the petroleum

The theory was further supported by the fact that when destructive distillation of fish oil and other animals fats under high temperature and pressure was carried out, a petroleum like liquid was obtained.

Theory fails to account for, The presence of chlorophyll in the petroleum. The presence of coal deposits found near the oil fields. The presence of resins in the oil.

Modern theory : This theory explain nearly all the facts about
The presence of chlorophyll and haemin in
The presence of coal deposits near oil fields suggesting its vegetable
The presence of nitrogen and sulphur compounds along with optically active compounds in
The presence of resins also suggests that oil must have been formed from vegetable
The presence of helium gas in natural gas suggests that radioactive substances must have helped in the decomposition of organic
Mining of petroleum : Petroleum deposits occurs at varying depth at different places ranging from 500 to 15000 This is brought to the surface by artificial drilling.
Petroleum refining : Separation of useful fractions by fractional distillation is called petroleum

*Fraction*	*Boiling range (^oC)*	*Approximate* *composition*	*Uses*
Uncondensed gases	Upto room temperature 30 – 150^o 30 – 70^o 70 – 120^o 120 – 150^o 150 – 250^o 250 – 400^o Above 400^o	C₁ – C₄	Fuel gases: refrigerants; production of
			carbon black, hydrogen; synthesis of
			organic chemicals.
Crude naphtha on		C₅ – C₁₀
refractionation yields,
(i) Petroleum ether		C₅ – C₆	Solvent
(ii) Petrol or gasoline		C₆ – C₈	Motor fuel; drycleaning; petrol gas.
(iii) Benzene derivatives		C₈ – C₁₀	Solvent; drycleaning
Kerosene oil		C11 – C16	Fuel; illuminant; oil gas
Heavy oil		C15 – C18	As fuel for diesel engines; converted to
			gasoline by cracking.
Refractionation gives,
(i) Gas oil
(ii) Fuel oil
(iii) Diesel oil
Residual oil on		C17 – C40
fractionation by vacuum
distillation gives,
(i) Lubricating oil		C17 – C20	Lubrication
(ii) Paraffin wax		C20 – C30	Candles; boot polish; wax paper; etc
(iii) Vaseline		C20 – C30	Toilets; ointments; lubrication.
(iv) Pitch		C30 – C40	Paints, road surfacing
Petroleum coke			As fuel.
(on redistilling tar)

(5) Purification

Treatment with concentrated sulphuric acid : The gasoline or kerosene oil fraction is shaken with sulphuric acid to remove aromatic compounds like thiophene and other sulphur compound with impart offensive odour to gasoline and kerosene and also make them

Doctor sweetening process :

2RSH + Na₂ PbO₂ + S ®

Mercaptan

RSSR + PbS + 2NaOH

Disulphides

Treatment with adsorbents : Various fractions are passed over adsorbents like alumina, silica or clay etc, when the undesirable compounds get

(6) Artificial method for manufacture of Petrol or gasoline

Cracking, (ii) Synthesis

Cracking : It is a process in which high boiling fractions consisting of higher hydrocarbons are heated strongly to decompose them into lower hydrocarbons with low boiling Cracking is carried out in two different ways.

Liquid phase cracking : In this process, the heavy oil or residual oil is cracked at a high temperature (475

– 530^oC) under high pressure (7 to 70 atmospheric pressure). The high pressure keeps the reaction product in liquid state. The conversion is approximately 70% and the resulting petrol has the octane number in the range 65 to 70.

The cracking can be done in presence of some catalysts like silica, zinc oxide, titanium oxide, ferric oxide and alumina. The yields of petrol are generally higher when catalyst is used.

Vapour phase cracking : In this process, kerosene oil or gas oil is cracked in vapour phase. the temperature is kept 600 – 800^oC and the pressure is about 3.5 to 10.5 atmospheres. The cracking is facilitated by use of a suitable The yields are about 70%.
Synthesis : Two methods are applicable for
Bergius process : This method was invented by Bergius in Germany during first world

Coal + H2 ¾¾^F^e2¾^O¾3 ® Mix. Of hydrocarbons or crude oil

450 – 500^o C

250 atm

Fischer- tropsch process : The overall yield of this method is slightly higher than Bergius

H O + C ¾¾¹²⁰¾⁰o¾^C ® CO + H

Water gas

xCO + yH 2 ¾¾® Mix. Of hydrocarbon + H2O .

The best catalyst for this process is a mixture of cobalt (100 parts), thoria, (5 parts), magnesia (8 parts) and kieselguhr (200 parts).

Knocking : The metallic sound due to irregular burning of the fuel is termed as

“The greater the compression greater will be efficiency of engine.” The fuel which has minimum knocking property is always preferred.

The tendency to knock falls off in the following order : Straight chain alkanes > branched chain alkanes > olefins > cyclo alkanes > aromatic hydrocarbons.

Octane number : It is used for measuring the knocking character of fuel used in petrol engine. The octane number of a given sample may be defined as the percentage by volume of iso-octane present in a mixture of iso-octane and heptane which has the same knocking performance as the fuel

CH3 – CH2 – CH2 – CH2 – CH2 – CH2 – CH3

n-heptane; octane no. = 0

CH3

CH₃ — C — CH₂ — CH — CH₃

CH3

; Octane no. = 100

2, 2, 4-Trimethyl pentane or Iso-octane.

For example : a given sample has the knocking performance equivalent to a mixture containing 60% iso- octane and 40% heptane. The octane number of the gasoline is, therefore, 60.

Presence of following types of compounds increases the octane number of gasoline.

in case of straight chain hydrocarbons octane number decreases with increase in the length of the
Branching of chain increases the value of octane number
Introduction of double bond or triple bond increases the value of octane
Cyclic alkanes have relatively higher value of octane
The octane number of aromatic hydrocarbons are exceptionally high
By adding gasoline additives (eg TEL)
Antiknock compounds : To reduce the knocking property or to improve the octane number of a fuel certain chemicals are added to it. These are called antiknock compounds. One such compound, which is extensively used, is tetraethyl lead (TEL). TEL is used in the form of following mixture,

TEL = 63%, Ethylene bromide = 26%, Ethylene chloride = 9% and a dye = 2%.

However, there is a disadvantage that the lead is deposited in the engine. To remove the free lead, the ethylene halides are added which combine with lead to form volatile lead halides.

Pb + Br – CH2 – CH2 – Br ® PbBr2 + CH2 = CH2

Ethylene bromide

Volatile

Ethylene

However, use of TEL in petrol is facing a serious problem of Lead pollution, to avoid this a new compound cyclopenta dienyl manganese carbonyl (called as AK-33-X) is used in developed countries as antiknocking compound.

(4) Other methods of improving octane number of gasoline

Isomerisation [Reforming] : By passing gasoline over

CH3

CH3CH2CH2CH2CH3 ¾¾^A^l^C¾^l3 ® CH3 CHCH2CH3

AlCl₃ at 200^o C .

Pentane (Octane number = 62)

200^o C

CH3

Isopentane (Octane number = 90)

CH3

Alkylation: CH3 CH + CH 2 = CCH3 ¾¾^H2S¾^O¾4 ® CH3 CCH 2 CHCH3

CH3

Isobutane

Isobutylene

CH3

Iso-octane

(Octane number = 100)

CH3

Aromatisation : CH3 (CH2 )5 CH3 ¾¾^Pt ^/¾^A^l2¾^O¾3 ® + 4 H2

Heptane

500^o C

The octane no. of petrol can thus be improved.

Toluene

By increasing the proportion of branched chain or cyclic
By addition of aromatic hydrocarbons (BTX)
By addition of methanol or
By additon of tetraethyl lead (C2H5 )4 Pb

Cetane number : It is used for grading the diesel

CH3 – (CH2 )14 – CH3

Cetane ® cetane no. = 100

CH₂

Cetane no. = 0

a-Methyl naphthalene

The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a mixture of cetane and a -methyl naphthalene which has the same ignition property as the fuel oil

Flash point : The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is referred to as flash point of the

The flash point in India is fixed at 44^o C , in France it is fixed at 35^oC, and in England at 22.8^oC. The flash

point of an oil is usually determined by means of “Abel’s apparatus”.

Chemists have prepared some hydrocarbons with octane number even less than zero (e.g., n-nonane has octane number – 45) as well as hydrocarbon with octane number greater than 100 (e.g., 2, 2, 3 trimethyl-butane. has octane number of 124).

Petrochemicals : All such chemicals which are derived from petroleum or natural gas called Some chemicals which are obtained from petroleum are:

“Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.”

Alkanes are less reactive so called paraffins; because under normal conditions alkanes do not react with acids, bases, oxidising agents and reducing agent.

General formula : Cn H2n+2

Examples are :

CH4

Methane

, C2 H6 ,

Ethane

C3 H8

Propane

Structure : (i) Every carbon atom is

sp ³ hybridized.

The bond length between carbon-

carbon and carbon-hydrogen are

1.112Å respectively.

1.54 Å

and

Bond angle in alkanes are tetrahedral angles having a value of 5^o (109^o.28¢).

Alkanes have planer

3 – D , rather than

C – C

bond dissociation energy is

83 k cal / mol .

C – H

bond dissociation energy is 99k cal / mol .

Isomerism : Only chain and structural Isomerism No. of carbon atom in molecule µ no. of chain Isomers
General Methods of preparation
- By catalytic hydrogenation of alkenes and alkynes (Sabateir and sanderen’s reaction)

Cn H 2n + H 2 ¾¾Ni ® Cn H 2n+ 2 ;

Cn H 2n-2 + 2H 2 ¾¾Ni ® Cn H 2n+ 2

Alkene

heat

Alkane

Alkyne

heat

Alkane

Note : ®Methane is not prepared by this method

Birch reduction :
From alkyl halide

R – CH = CH 2 ¾¾^1. ^N¾^a ^/ ^N¾^H¾3 ® R – CH 2 – CH3

CH₃OH

By reduction :

RX + H 2 ¾¾^Zⁿ ^/¾^H¾^C^l ® RH + HX

With hydrogen in presence of pt/pd :

RX + H 2 ¾¾^Pd ^o¾^r^P¾^t^. ® RH + HX

With HI in presence of Red phosphorus :

RBr + 2HI ¾¾® RH + HBr + I ₂ Purpose of Red P is to remove I

By Zn-Cu couple :

2CH₃CH ₂OH +

Zn-Cu couple

¾¾^C¾^u ®(CH3 CH 2 O)2 Zn+ 2H

Zinc ethoxide

Wurtz reaction : R

RX + 2H ¾¾® RH + HX

R¢ ¾¾^D^r^y¾^e^t^h¾^e^r ® R – R+ 2NaX

Alkyl halide

Alkane

Note : ® R – Br

or RI preferred in this reaction. The net result in this reaction is the formation of even no.

of carbon atoms in molecules.

Frankland’s reaction : 2RX + Zn ¾¾® R – R + ZnX ₂
Corey-house synthesis

CH3 – CH2 – Cl ¾¾^1.¾^Li ®(CH3 – CH 2 )2 LiCu ¾¾^C^H¾3 –C¾^H2 ¾^–¾^C^l ® CH3 – CH 2 – CH 2 – CH3

CuI

Note : ® Reaction is suitable for odd number of Alkanes.

From Grignard reagent

By action of acidic ‘H’ :

RMgX

HOH ¾¾® RH + Mg(OH)X

Alkyl magnesium halide

Water

Alkane

By reaction with alkyl halide : R – X + R¢MgX ¾¾® R – R¢+ MgX2
- From carboxylic acids
Laboratory method [Decarboxylation reaction or Duma reaction]

R COONa + NaOH ¾¾^h^e¾^a^t ® R – H+ Na2 CO3

CaO Alkane

Note : ® NaOH and CaO is in the ratio of 3 : 1.

Kolbe’s synthesis : R – C– O^–

Na+ Electrolysis

Ionization

R – C– O^–

Na ⁺

At anode [Oxidation] : 2R – C – O^– + 2e ^– ¾¾® 2R – C– O

¾¾®

2 R+ 2CO₂

|| ||

O O

2 R ¾¾® R – R (alkane)

At cathode [Reduction] :

2Na⁺ + 2e^– ¾¾® 2Na ¾¾²^H¾2¾^O ® 2NaOH + H2

( )

Note : ® Both ionic and free radical mechanism are involved in this reaction.

Reduction of carboxylic acid : CH3COOH+ 6HI ¾¾^Re ^d¾^u^cti¾^oⁿ ® CH3CH3 + 2H2O + 3I2

Acetic acid

p Ethane

By reduction of alcohols, aldehyde, ketones or acid derivatives

CH3OH

+ 2HI ¾¾^Red¾^P ® CH4 + H2O + I2 ;

CH3CHO+ 4 HI ¾¾^Red¾^P ® C2 H6 + H2O + 2I2

Methanol (Methyl alcohol)

150^o C

Methane

Acetaldehyde (Ethanal)

150^o C

Ethane

CH3COCH3 + 4 HI ¾¾^Red¾^P ® CH3CH2CH3 + H2O + 2I 2 ;

Acetone (Propanone)

150^o C

Propane

CH3 – C– Cl+ 6HI ¾¾^Red¾^P ® CH3 – CH 3 + H 2 O + HCl + 3I 2

Acetyl chloride (Ethanoyl chloride)

200^o C

Ethane

CH3 – C– NH 2 + 6HI ¾¾^Red¾^P ® CH3 – CH3 + H 2 O + NH 3 + 3I 2

Acetamide (Ethanamide)

200^o C

Ethane

Note : ® Aldehyde and ketones when reduced with amalgamated zinc and conc. HCl also yield alkanes.

Clemmensen reduction : CH3 CHO+ 2H 2 ¾¾^Zⁿ^–¾^H¾^g ® CH3 – CH3 + H 2 O

Acetaldehyde (Ethanal)

HCl

Ethane

CH3COCH3 + 2H2 ¾¾^Zⁿ^–¾^H¾^g ® CH3CH2CH3 + H2O

Acetone (Propanone)

HCl

Propane

Note : ® Aldehydes and ketones

(> C = O)

can be reduced to hydrocarbon in presence of excess of

hydrazine and sodium alkoxide on heating.

R R

Wolff–kishner reduction : C = O ¾¾^H2 N¾^N^H¾2 ®

C = NNH2 ¾¾^C2 H¾5 O¾^N¾^a ®

CH2

R¢ – H 2 O R¢

Hydroboration of alkenes

On treatment with acetic acid

180^o C

R – CH = CH2 ¾¾^B2H¾6 ®(R – CH2 – CH2 )3 B ¾¾^C^H¾3CO¾^O¾^H ® R – CH2 – CH3

Alkene

Trialkyl borane

Alkane

Coupling of alkyl boranes by means of silver nitrate

6[R – CH = CH₂

(4) Physical Properties

] ¾¾²^B2¾^H¾6 ®[2R – CH2

-]3

B ¾¾^A^g^N¾^O3¾²⁵o¾^C ® 3[RCH

NaOH

₂CH2

₂CH2 R]

Physical state : Alkanes are colourless, odourless and

Alkanes State

C1 – C4 C5 – C17

Gaseous state

Liquid state [Except neo pentane] [gas]

C18

& above Solid like waxes

Density : Alkanes are lighter than
Solubility : Insoluble in water, soluble in organic solvents, solubility µ 1

Molecular mass

Boiling pts and Melting pts : Melting and boiling pts. µ Molecular mass µ 1

No. of branches

C C C

C C C C

Odd no. of Carbons

C C C

Even no. of carbons

Note : ® Melting points of even > Odd no. of carbon atoms, this is because, the alkanes with even number of carbon atoms have more symmetrical structure and result in closer packing in the crystal structure as compared to alkanes with odd number of carbon atoms.

(5) Chemical properties

Substitution reactions of Alkanes

(a) Halogenation : R – H + X – X ¾¾® R – X + HX

The reactivity of halogen is :

F2 > Cl2 > Br2 > I2

Note : ® Fluorine can react in dark

Cl2 , Br2

require light energy.

I ₂ doesnot show any reaction at room

temperature, but on heating it shows iodination.

Iodination of methane is done in presence of oxidising agent such as neutralises HI .

® Chlorination of methane :

CH4 + Cl – Cl ¾¾^u^.^v^.¾^lⁱ^g¾^h^t ® CH 2 – Cl2 ¾¾^u^.^v^.¾^lⁱ^g¾^h^t ® CHCl3 ¾¾^–^H¾^C^l ® CCl 4

HNO₃ / HIO₃ / HgO

which

– HCl –HCl

Reaction based on free radical mechanism

Nitration :

R – H+ HONO2 ¾¾^Hⁱ¾^g^h ® R – NO2 + H2O

Alkane

temp.

Nitroalkane

Nitrating mixture : (i) (Con. HNO3 + Con. H 2 SO4 ) at 250^o C

(HNO3 vapour at

Sulphonation : Free radical mechanism

400^o – 500^o C).

R – H + HOSO3 H ¾¾^S¾^O3¾® R – SO3 H + H2O Prolonged heating

Note : ® Lower alkanes particularly methane, ethane, do not give this reaction.

Oxidation

Complete Oxidation or combustion : C H

+ æ 3n + 1 öO

¾¾® nCO

+ (n + 1)H O + Q

Note : ® This is exothermic reaction.

Incomplete combustion or oxidation

2CH4 + 3O2 ¾¾^B^u¾^rⁿ ® 2CO + 4 H 2 O

n 2n+ 2 ç

2 ÷ 2 2 2

CH₄ + O₂ ¾¾® C +2H₂O

Catalytic Oxidation :

CH4 + [O] ¾¾^C^u^–¾^t^u¾^b^e ® CH3OH 100 atm / 200^o C

This is the industrial method for the manufacture of methyl alcohol.

Note : ® Higher alkanes are oxidised to fatty acids in presence of manganese stearate.

CH3

(CH

₂)n CH

¾¾^O¾2 ® CH

3 100 -160 ^o C 3

(CH

₂)n COOH

Chemical oxidation : (CH3 )3 CH ¾¾^K^M¾ⁿ^O¾4 ®(CH3 )3 .C.OH

Isobutane Tertiary butyl alcohol

Thermal decomposition or cracking or pyrolysis or fragmentation

CH4

¾¾¹⁰⁰¾⁰o¾^C ® C + 2H

; C2 H

¾¾⁵⁰⁰¾o¾^C ® CH

6 Cr2O3 + Al2O3 2

= CH2

Methane

Ethane

Ethylene

C3 H8 ¾¾® C2 H4 + CH4 or C3 H6 + H2

Note : ® This reaction is of great importance to petroleum industry.

CH3

Isomerisation : CH CH CH CH

¾¾AlC¾l3 +¾H¾Cl ® CH

CHCH ;

¾¾AlC¾l3 +¾H¾Cl ®

Aromatisation :

3 2 2

n– Butane

3 200^o C, 35atm

3 3

Isobutane

2-Methyl pentane heat

2,3 Dimethyl butane

H₂C

CH₃

¾¾Cr O¾/¾Al O¾®

2 3 2 3

+4H

H₂C

CH₂

600^o C / 15 atm

Benzene

n-Hexane

Step up reaction

n-Heptane

¾¾Cr2O¾3 /¾Al2O¾3 ®

600^o C

Methyl cyclo Hexane

¾¾^–^H¾2 ®

Toluene

Reaction with CH2 N2 : R – CH 2 – H + CH 2 N 2 ¾¾^h^v ® R – CH 2 – CH 2 – H

Reaction with CHCl₃ / NaOH

: R – CH 2

H ¾¾^C^H¾^C^l3 ¾^/ ^O^H¾- ® R – CH

: CCl2 2

CHCl2

Reaction with CH₂ = C :

R – CH 2 – H ¾¾^C^H¾2 =C¾^/¾^D ® R – CH 2 – CH3

:CH₂ / –CO

HCN formation :

2CH4 ¾¾^N2 ¾^/ ^el^e¾^c^t^rⁱ^c¾^a¾^r^c ® 2HCN + 3H 2

or CH4 + NH3 ¾¾^A^l2O¾3 ® HCN + 3H2

700^o C

Chloro sulphonation/Reaction with SO₂+Cl₂

CH3 – CH2 – CH3 + SO2 + Cl2 ¾¾^u^.^v ^l¾ⁱ^g¾^h^t ® CH3 – CH2 – CH2SO2Cl + HCl

This reaction is known as reed’s reaction.

Note : ® This is used in the commercial formation of detergent.

Action of steam :

CH4 + H2O ¾¾^Nⁱ ^/¾^A^l2O¾3 ® CO + 3H2 800^o C

Methane : Known as marsh gas.
- Industrial method of preparation : Mathane gas is obtained on a large scale from natural gas by It can also be obtained by the application of following methods,
From carbon monoxide : A mixture of carbonmonoxide and hydrogen is passed over a catalyst containing

nickel and carbon at 250^o C when methane is formed.

CO + 3H2 ¾¾^Nⁱ⁺¾^C ® CH4 + H2O 250^o C

Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of

(C6 H10O5 )n + nH2O ¾¾® 3nCH4 + 3nCO2

Cellulose

Synthesis : q By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at 1200^oC, methane is

2 4

C + 2H ¾¾¹²⁰¾⁰o¾^C ® CH

By passing a mixture of hydrogen sulphide and carbon disulphide vapour through red hot copper, methane is formed.

CS2 + 2H2S + 8Cu ¾¾^Hⁱ^g¾^h ^t^e¾^m^p^e¾^ra^t^u¾^r^e ® CH4 + 4Cu2S

Physical properties

It is a colourless, odourless, tasteless and non-poisonous
It is lighter than Its density at NTP is 0.71 g/L.
It is slightly soluble in water but is fairly soluble in ether, alcohol and
Its melting point is – 5^o C and boiling point is – 161.5^o C .
- Uses
In the manufacture of compounds like methyl alcohol, formaldehyde, methyl chloride, chloroform, carbon tetrachloride,
In the manufacture of hydrogen, used for making
In the preparation of carbon black which is used for making printing ink, black paints and as a filler in rubber
As a fuel and

(2) Ethane

Methods of preparation

Laboratory method of preparation :

C2 H5 I + 2H ¾¾^Zⁿ^–¾^C^u¾^co^u¾^p¾^le ® C2 H6 + HI

Ethyl iodide

C2H5OH

Ethane

Industrial method of preparation : CH2 = CH2 + H2 ¾¾^Nⁱ ® CH3 – CH3

(iii) Physical properties

Ethylene (ethene)

300^o C

Ethane

It is a colourless, odourless, tasteless and non-poisonous
It is very slightly soluble in water but fairly soluble in alcohol, acetone, ether,
Its density at NTP is 34 g/L
It boils at – 89^oC. Its melting point is –172^oC.

(ii) Uses

(a) As a fuel. (b) For making hexachloroethane which is an artificial camphor.

(3) Interconversion of Alkanes

Ascent of alkane series,

Methane to ethane :

CH4 ¾¾^C¾^l2 ® CH3Cl ¾¾^Wu¾^rtz ^re¾^acti¾¾^on ® CH3 – CH3

Methane UV

Heat with Na in ether

Ethane

Butane from ethane :

C2 H6 ¾¾C¾l2 ® C2 H5 Cl ¾¾Wu¾rtz re¾acti¾¾on ® C2 H5 – C2 H5

Ethane (excess)

UV Ethyl chloride Heat with Na in ether

Butane

Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion.

Ethane to methane

C2 H6 ¾¾^C¾^l2 ® C2 H5Cl ¾¾^A^q^.¾^K^O¾^H ® C2 H5OH ¾¾^[¾^O^] ® CH3CHO ¾¾^[¾^O^] ® CH3COOH ¾¾^N^a^O¾^H ® CH3COONa ¾¾^N^a^O¾^H ^/¾^C^a¾^O ® CH4

Ethane (excess)

UV Ethyl chloride

Ethyl alcohol

Acetaldehyde

Acetic acid

Sodium acetate

heat

Methane

Higher ¾¾^C¾^l2 ® Alkyl ¾¾^A¾^q^. ® Alcohol ¾¾^[¾^O^] ® Aldehyde ¾¾^[¾^O^] ® Acid ¾¾^Na^O¾^H ® Sodium salt of ¾¾^Na^O¾^H ^/¾^C^a¾^O ® Lower alkane

alkane

UV halide

KOH

the acid

heat

These are the acyclic hydrocarbon in which carbon-carbon contain double bond. These are also known as

olefins, because lower alkene react with halogens to form oily substances. General formula is

Ex : Ethene C2 H4 , Propene C3 H6 , Butene C4 H8

(1) Structure

Hybridisation of unsaturated ‘C’ atom is sp²
Geometry of unsaturated ‘c’ atom is trigonal planer

Cn H2n .

C – H
C = C
C – H
C = C

bond length is 1.34 Å

bond energy is 143.1 K cal/mol

bond length is 1.10 Å

bond energy is 108 Kcal/mol

(2) Isomerism

Chain Isomerism : CH3 – CH2 – CH = CH2 and (CH3 )2 – C = CH 2
Position Isomerism : CH2 = CH – CH2 – CH3 and CH3 – CH = CH – CH3
Functional Isomerism : [Ring chain] CH3 – CH2 – CH = CH2 and CH2 – CH2

1- butene | |

Geometrical Isomerism : CH₃ – C – H

CH₃ – C – H

cis-2-butene

CH = CH2

and CH₃ – C – H

H – C – CH₃

Trans – 2-butene

CH2 – CH2

Cyclo butene

Optical Isomerism :

H – C – CH₃

CH2CH3

Note : ® Cumulated polythene having even no. of double bonds. Which has = C

system at the both

end can exhibit optical isomerism but cannot exhibit geometrical isomerism.

Cumulated polythene having odd of double bonds which have = C

system at both end can

exhibit geometrical isomerism but cannot exhibit optical isomerism.

(3) Preparation methods

From Alkynes :

R – C º C – H + H2 ¾¾^Lind¾^lar’¾^s ^Ca¾^taly¾^st ® R – C = C – H

Note : ® Poison’s catalyst such as alkene.

Pd. BaSO₄

BaSO₄ , CaCO₃

are used to stop the reaction after the formation of

H H H

| | |

From mono halides :

R – C – C – H + Alc. KOH ¾¾-H¾X ® R – C = C – H

| | |

H X H

Alkene

Note : ® If we use alc. NaOH in place of KOH then trans product is formed in majority because of its stability. According to saytzeff rule.

From dihalides

From Gem dihalides

R – CH

CH – R

¾¾D ® R – CH = CH – R

2 Znx2

Note : ® If we take two different types of gemdihalides then we get three different types of alkenes .

Above reaction is used in the formation of symmetrical alkenes

H H H H

| | | |

From vicinal dihalides :

R – C – C – H + Zn dust ¾¾^D ® R – C = C – H + ZnX 2

| | 300^o C

X X

Note : ® Alkene is not formed from 1, 3 dihalides. Cycloalkanes are formed by dehalogenation of it.

C H2 – CH2 – C H2 ¾¾^Zⁿ ^d¾^u¾^st ®

| |

X X H2C

CH2

Br Br I I

| | | |

By action of NaI on vic dihalide :

C – C

¾¾^N¾^a^I ®

acetone

C– C

unstable

¾¾®

C = C

From alcohols [Laboratory method] : CH3CH2OH ¾¾^H2 S¾^O4¾^o^r ^H¾3 P¾^O¾4 ® CH2 = CH2 + H2O

CH₂COOK

Ethyl alcohol

CH2

443 K

Ethene

Kolbe’s reaction :

CH₂COOK

Potassium succinate

+ 2H2O ¾¾Elec¾trol¾y¾sis ®||

CH2

Ethene

+ 2CO₂ + H₂ + 2KOH

From esters [Pyrolysis of ester] : CH₃ – CO – O

H ¾¾Gla¾ss w¾ool 4¾50¾o ® CH3 – COOH

CH2

liq. N 2

CH2 = CH2

Pyrolysis of quaternary ammonium compounds :

+ –

¾¾^h^e¾^a^t ®(C H )

N + C H

H O

(C2 H5 )4 N OH

2 5 3

2 4 2

Tetraethyl ammonium hydroxide

Triethylamine (Tert. amine)

Ethene

Action of copper alkyl on vinyl chloride :

H2C = CHCl ¾¾^C^u^R¾2 ® H2C = CHR

Vinyl chloride

By Grignard reagents : Mg

X – CH = CH2 ¾¾® MgX2 + R – CH = CH2

The wittig reaction : (Ph)3 P = CH 2 + CH– R ¾¾®(Ph)3 P = O + R – CH

|| ||

O CH 2

(Ph)3 P = CH – R + CH– R ¾¾®(Ph)3 P = O + R – CH = CH – R

From b bromo ether [Boord synthesis]

Br O – C2 H5

| | Br

R – CH– CH

R¢

¾¾^Zⁿ ® R – CH = CH – R¢ + Zn

C4 HgOH

O – C2 H5

(4) Physical Properties

Alkenes are colourless and
These are insoluble in water and soluble in organic
Physical state

C1 – C4 ¾¾® C5 – C16 ¾¾®

gas liquid

C16¾¾®

solid wax

P. and M.P. decreases with increasing branches in alkene.
The melting points of cis isomers are lower than trans isomers because cis isomer is less symmetrical than Thus trans packs more tightly in the crystal lattice and hence has a higher melting point.
The boiling points of cis isomers are higher than trans isomers because cis-alkenes has greater polarity (Dipole moment) than trans
These are lighter than

Dipole moment : Alkenes are weakly The, p-electron’s of the double bond. Canbe easily polarized. Therefore, their dipole moments are higher than those of alkanes.

The symmetrical trans alkenes are non polar and hence have zero dipole moments in these alkene the dipole moment of individual bonds are equal in opposite direction. Therefore these get cancelled resulting zero dipole

moment for the molecule.

CH₃

C = C

CH₃

C = C

CH₃

Trans-2-Butene

m = 0

Cis –2-Butene

m = 0.25 D

Thus symmetrical and unsymmetricals cis alkene are polar and hence have finite dipole moments

CH₃

(5) Chemical properties

C = C

Propene

m = 0.35 D

CH₃ – CH₂

C = C

Bute 1-ene

m = 0.37 D

Francis experiment : According to Francis electrophile first attacks on olefinic

CH₂ = CH₂ + Br – Br ¾¾^C^C¾^l4 ® CH₂ – CH₂

| |

Br Br

¾¾^N^a¾^C^l ® CH₂ – CH₂ + CH₂ – CH₂

| |

Br Br

| |

Br Cl

H H H H

| | | |

Reaction with hydrogen :

R – C = C– R + H2 ¾¾^Nⁱ ® R – C – C– R

| |

H H

Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis

2 2 2 3 2 3

RCH = CH ¾¾^H^–¾^B^H¾2 ®(R – CH – CH ) B ¾¾^H+ ¾^/ ^H2¾^O ® R – CH – CH

Hydroboration : Alkene give addition reaction with diborane which called hydroboration. In this reaction formed trialkylborane, Which is very important and used for synthesis of different organic compound

3R – CH = CH2 + BH3 ¾¾®(R – CH2 – CH2 )3 B

CH₃COOH/Zn

NaOH H2O2

Trialkyl borane HI/H₂O₂

R – CH₂ –CH₃

R – CH₂ –CH₂OH R – CH2 –CH3

The overall result of the above reaction appears to be antimarkownikoff’s addition of water to a double bond.

By treatment with AgNO₃ + NaOH : This reaction gives coupling

CH3

| 6CH₃ – CH₂ – CH₂ – C =

CH3

CH2 ¾¾^B2H¾6 ® 2[CH3 – (CH2 )2 – C –

CH2 ]3 B ¾¾Ag /¾NO¾3 Na¾O¾H ®

CH3

CH3 – CH2 – CH2 – C –

CH3

CH₂ – CH₂ – C –

CH2 – CH2 – CH3

Birch reduction : This reaction is believed to proceed via anionic free radical

R – CH = CH₂

¾¾^N¾^a ® R –

+e ^–

–

C H–

–

C H 2

¾¾^Et^–¾^O^–¾^H ® R – CH – CH3

¾¾^N¾^a ® R – –

C H

+e ^–

– CH3

¾¾^Et^.^–¾^O^–¾^H ® R – CH2

– CH3

Halogenation

CH3CH = CH2

¾¾⁵⁰⁰¾o¾^C ® ClCH – CH = CH

HCl

Propene

Allyl chloride

or 3- Chloro-1- propene

Note : ® If NBS [N-bromo succinimide] is a reagent used for the specific purpose of brominating alkenes at

the allylic position.

CH₂ CH₃ CH=CH₂ + |

N – Br

CH₂ CH₂ – CH = CH₂+ |

N – H

Propene

CH₂ – CO

NBS

Allyl bromide

CH₂ – CO

Succinimide

In presence of polar medium alkene form vicinal dihalide with

H H H H

| | | |

R – C = C– H + X – X ¾¾^C^C¾^l4 ® R – C – C– H

| |

X X

Vicinal dihalide

Reactivity of halogen is

F2 >

Cl2 >

Br2

Reaction with HX [Hydrohalogenation]

C = C

alkene

+ HX ¾¾®

C – C

Alkyl halide

According to markownikoff’s rule and kharasch effect.

H H

| |

CH3 – CH = CH 2 + HBr ¾¾® CH3 – C – C– H Markownikoff rule

| |

Br H

H H H H

| | | |

(Based on F.R.M.) CH3 – CH = CH 2 + HBr ¾¾^Pero¾^xi¾^de ® CH3 – C – C– H + CH3 – C – C– H

| |

Br H

| |

H Br

(minor)

– +

(major)

Reaction with hypohalous acids : CH 2 = CH 2 + H OCl ¾¾® CH 2 OH.CH 2 Cl

Ethylene Ethylene chlorohydrin

Note : ® In case of unsymmetrical alkenes markownikoff rule is followed.

Reaction with sulphuric acid :

CH 2 = CH 2 + H ⁺ HSO^– ¾¾® CH3 CH 2 HSO4

Ethylene Ethyl hydrogen sulphate

CH3 CH 2 HSO4 ¾¾® CH 2 = CH 2 + H 2 SO4

Note : ® This reaction is used in the seperation of alkene from a gaseous mixture of alkanes and alkenes.

Reaction with nitrosyl chloride

C = C

NOCl ¾¾®

C – C

( NOCl is called Tillden reagent)

Note : ® If hydrogen is attached to the carbon atom of product, the product changes to more stable oxime.

| H

C – C ⇌

C – C = NOH ;

C = C

NOCl ¾¾®

C – C

(Blue colour)

| |

Cl Cl

Oxime

C | | C

NO Cl

Oxidation : With alkaline KMnO4

[Bayer’s reagent] : This reaction is used as a test of unsaturation.

H H H H

| | | |

R – C = C – H + [O] + H – OH ¾¾^A^l^k¾^K^M¾ⁿ^O¾4 ® R – C – C– H

–OH

| |

HO OH

glycol

H H O

| | ||

With acidic

KMnO4

: R – C = C – H + [O] ¾¾^a^cid¾^ic ® R – C– O – H + CO2 + H 2 O

KMnO4

Hydroxylation

Using per oxy acid :

CH3

H – C

CH3

2-Butene

¾¾H2O¾2 , H¾CO¾O¾H ®

or HCO₃H

CH3

H – C – OH

OH – C – H

CH3

Trans (racemic)

R H

C R

Hydroxylation by OsO₄ :

||+ OsO₄

Trans

+ ¾¾I ®

H OH

HO H

(±)

Note : ® If per benzoic acid or peroxy acetic acid is used then oxirane are formed.

R – CH = CH – R ¾¾^C6 H¾5 C¾^O3¾^H ® R – CH – CH – R ¾¾^– ^H¾2¾^O ® R – CH – CH– R

or CH 3 CO3 H

| |

OH OH

[Oxirane]

Combustion : Cn

H 2n

3n O

2 2

¾¾® nCO₂

nH 2 O

They burn with luminous flame and form explosive mixture with air or oxygen.

Ozonolysis

C = C

¾¾^O¾3 ®

C C

O O

Ozonide

¾¾^H2O¾^/ ^H¾+ / ¾^Zⁿ ® ZnO +

O O C + C

Application of ozonolysis : This process is quite useful to locate the position of double bond in an alkene The double bond is obtained by Joining the carbon atoms. of the two carbonyl compounds.

Ex.

H H

| |

CH3 – C = O + O = C – CH3 ¾¾® CH3 – CH = CH – CH3

Bute-2-ene

Ethanal

CH3 H

| |

CH₃ – C = O + O = C

– CH₃ ¾¾® CH₃ – C = C

CH3

2- methyl, but – 2-ene

Oxy – mercuration demercuration : With mercuric acetate (in THF), followed by reduction with

NaBH ₄ / NaOH

is also an example of hydration of alkene according to markownikoff’s rule.

(CH3 )3 C – CH = CH 2 + (CH3 COO)2 Hg ¾¾®(CH3 )3 C – CH – CH 2 – Hg ¾¾^N^a^B¾^H4¾^/ ^N^a¾^O¾^H ®(CH 3 )3 C – CH – CH 3

OCOCH3

THF

3, 3-

Dimethyl – 2–butanol

Ex.

CH = CH₂ BH3

Antimarkowni – koff rule

(CH₂ – CH₂)₃ B

(CH₂)₂OH

Hg (CH₃COO)₂

CH CH2

H₂O/H⁺ CH

CH₃

Mercuration markowni – koff rule

OCOCH₃

HgOCOCH₃ OH

H⁺/H₂O

H Å

CH CH₃ ⇌

^Å CH₂

CH₃

H₂O/H ⁺

CH₂ –CH₃ OH

Less stable Carbocation

More stable carbocation

alcohol

Epoxidation

O ||

By O₂ / Ag

: CH2

= CH2

2 2

¾¾^A¾^g ® CH2

C – O – O – H

Epoxidation by performic acid or perbenzoic acid :

CH2 = CH2 ¾¾¾¾¾¾® CH2 – CH 2

Hydroboration

CH3 – CH = CH2 ¾¾^H^–¾^C^–^O¾^–^O¾^–¾^H ® CH3 – CH – CH2

3R – CH = CH₂

¾¾®(R – CH₂ – CH

2 )3

B ¾¾^H2O¾2 / O¾^H¾- ® R – CH

CH₂ – OH + B(OH)₃

Tri alkyl borane

(Anti markownikoff’s rule)

H H

| |

Hydroformylation : R – CH = CH 2 + CO + H 2 ¾¾^Co^H¾⁽^C¾^O¾⁾4 ® R – C – C – H

Note : ® If CO + H₂O is taken then respective acid is formed.

R – CH = CH 2 + CO + H 2 O ¾¾^Co^H¾⁽^C¾^O¾⁾4 ® R – CH 2 – CH 2

COOH

| |

H C = O

Addition of formaldehyde

Å Å

H C = O + ¾¾®[H C =

¬¾®

^Å – OH] ¾¾^R^–^C¾^H ⁼¾^C^H¾2 ® R – Å

HOH

2 H 2

OH H2 C

R – CH

CH₂

C H 2 2

¬¾^H^C¾^H^O¾^/ ^H ¾ R – CH – CH2 – CH2

O O

CH₂

| |

OH OH

Polymerisation

Cyclic acetal

1, 3-diol

H H é H H H Hù

| | ê | | | ú

C = C ¾¾^T^ra^c¾^e ^O¾2 +C¾^a^t^a¾^l^y¾^s^t ®ê- C – C – C – C ú

1500^o / high pressure

| |

H H

ê | |

ëê H H H

Húû n

Note : ® If in polymerisation zeigler- natta catalyst [(R)₃ Al + TiCl₄ ] is used then polymerisation is known as zeigler-natta polymerisation.

Isomerisation : CH3 – CH2 – CH2 – CH = CH2

The mechanism proceeds via carbocation.

AlCl₃

CH₃ – CH₂ – CH = CH – CH₃

Addition of HNO₃ : CH2 = CH2 + HO – NO2 ¾¾® CH2OH.CH2 NO2

Ethene 2- Nitroethanol

Addition of Acetyl chloride : CH2 = CH2 + CH3COCl ¾¾® CH2ClCH2COCH3

(6) Uses

Ethene

4 -Chlorobutanone- 2

For the manufacture of polythene – a plastic material; (ii) For artificial ripening of fruits; (iii) As a general anaesthetic; (iv) As a starting material for a large number of compounds such as glycol, ethyl halides, ethyl alcohol, ethylene oxide, etc; (v) For making poisonous mustard gas (War gas); (vi) For making ethylene-oxygen

These are the acyclic hydrocarbons which contain carbon-carbon triple bond are called alkynes. General formula is Cn H 2n-2 . Ex. Ethyne CH º CH ; Propyne CH3 – C º CH

(1) Structure

Hybridisation in alkynes is sp .
Bond angle in alkynes is 180^o .
Geometry of carbon is
C – C triple bond length is 120 Å
C – H bond length is 108 Å
C – C triple bond energy is 190 Kcal / mol .
C – H bond energy is 38 Kcal / mol .

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(1) Composition

Reactions :

(5) Purification

(6) Artificial method for manufacture of Petrol or gasoline

(4) Other methods of improving octane number of gasoline

(4) Physical Properties

(5) Chemical properties

® Chlorination of methane :

(2) Ethane

(3) Interconversion of Alkanes

(1) Structure

(2) Isomerism

(3) Preparation methods

(4) Physical Properties

(5) Chemical properties

(6) Uses

(1) Structure

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