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

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

File name : Chapter-19-Aliphatic-Hydrocarbons-Part-1.pdf

 

“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  ¾¾H¾2  ® C2 H4  ¾¾H¾2  ® C2 H6

(Ethane)

 

3[CH  º CH] ¾¾Poly¾me¾risat¾io¾n ® C6 H6  ;

(Benzene)

CH3  – CH  = CHCH3  ¾¾Isom¾eris¾atio¾n ® 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

 

 

 

 

FractionBoiling range (oC)Approximate compositionUses
Uncondensed gasesUpto room temperature

 

30 – 150o

 

30 – 70o

70 – 120o

120 – 150o

150 – 250o

250 – 400o

 

 

 

 

 

 

Above 400o

C1 – C4Fuel gases:  refrigerants;  production  of
  carbon   black,    hydrogen;   synthesis     of
  organic chemicals.
Crude          naphtha          onC5 – C10 
refractionation yields,  
(i) Petroleum etherC5 – C6Solvent
(ii) Petrol or gasolineC6 – C8Motor fuel; drycleaning; petrol gas.
(iii) Benzene derivativesC8 – C10Solvent; drycleaning
Kerosene oilC11 – C16Fuel; illuminant; oil gas
Heavy oilC15 – C18As fuel for diesel engines; converted to
  gasoline by cracking.
Refractionation gives,  
(i) Gas oil  
(ii) Fuel oil  
(iii) Diesel oil  
Residual              oil              onC17 – C40 
fractionation by   vacuum  
distillation gives,  
(i) Lubricating oilC17 – C20Lubrication
(ii) Paraffin waxC20 – C30Candles; boot polish; wax paper; etc
(iii) VaselineC20 – C30Toilets; ointments; lubrication.
(iv) PitchC30 – C40Paints, 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

– 530oC) 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 – 800oC 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  ¾¾FeO¾3  ®  Mix. Of hydrocarbons or crude oil

450 – 500o C

250 atm

 

  • Fischer- tropsch process : The overall yield of this method is slightly higher than Bergius
2
2

H  O + C ¾¾120¾0o¾C ® 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

|

 

CH3C CH2CH 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 ¾¾AlC¾l3  ® CH3 CHCH2CH3

AlCl3 at 200o C .

 

Pentane (Octane number = 62)

200o C

 

CH3

|

Isopentane (Octane number = 90)

 

CH3

|

CH3

|

CH3

|

 

  • Alkylation:  CH3 CH + CH 2 = CCH3  ¾¾H2S¾O¾4  ® CH3 CCH 2 CHCH3

 

|

CH3

Isobutane

Isobutylene

|

CH3

Iso-octane

(Octane number = 100)

 

CH3

 

  • Aromatisation : CH3 (CH2 )5 CH3 ¾¾Pt /¾AlO¾3  ®                    + 4 H2

 

Heptane

500o 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  44o C , in France it is fixed at 35oC, and in England at 22.8oC. 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 5o (109o.28¢).

 

  • Alkanes have planer

3 – D , rather than

 

CC

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

RCH  = CH 2  ¾¾1. N¾a / N¾H¾3  ® RCH 2  – CH3

  1. CH3OH

 

  • By reduction :

RX + H 2  ¾¾Zn /¾H¾Cl ® RH + HX

 

  • With hydrogen in presence of pt/pd :

RX + H 2  ¾¾Pd o¾rP¾t. ® 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

Zn-Cu couple

¾¾C¾u ®(CH3 CH 2 O)2 Zn+ 2H

Zinc ethoxide

 

 

  • Wurtz reaction : R

RX + 2H ¾¾® RH + HX

R¢ ¾¾Dry¾eth¾er ® RR+ 2NaX

 

Alkyl halide

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 ¾¾1.¾Li ®(CH3  – CH 2 )2 LiCu ¾¾CH¾3 –C¾H2 ¾¾Cl ® CH3  – CH 2  – CH 2  – CH3

  1. 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 ¾¾he¾at ® RH+ Na2 CO3

CaO              Alkane

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

O

 

  • Kolbe’s synthesis : R CO

||

O

Na         Electrolysis    

Ionization

||

R CO

  • Na +

 

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

¾¾®

2 R+ 2CO2

 

||                            ||

O                                                 O

2 R ¾¾® RR (alkane)

 

At cathode [Reduction] :

2Na+ + 2e ¾¾® 2Na ¾¾2H¾2¾O ® 2NaOH + H2

( ­ )

 

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

  • Reduction of carboxylic acid : CH3COOH+ 6HI ¾¾Re d¾ucti¾on ® CH3CH3 + 2H2O + 3I2

 

Acetic acid

p                          Ethane

 

  • By reduction of alcohols, aldehyde, ketones or acid derivatives

 

CH3OH

+ 2HI ¾¾Red¾P ® CH4  + H2O + I2 ;

CH3CHO+ 4 HI ¾¾Red¾P ® C2 H6 + H2O + 2I2

 

Methanol (Methyl alcohol)

150o C

Methane

Acetaldehyde (Ethanal)

150o C

Ethane

 

CH3COCH3 + 4 HI ¾¾Red¾P ® CH3CH2CH3 + H2O + 2I 2 ;

 

Acetone (Propanone)

O

||

150o C

Propane

 

CH3  – CCl+ 6HI ¾¾Red¾P ® CH3  – CH 3 + H 2 O + HCl + 3I 2

 

Acetyl chloride (Ethanoyl chloride)

O

||

200o C

Ethane

 

CH3  – CNH 2 + 6HI ¾¾Red¾P ® CH3  – CH3 + H 2 O + NH 3  + 3I 2

 

Acetamide (Ethanamide)

200o C

Ethane

 

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

Clemmensen reduction : CH3 CHO+ 2H 2  ¾¾Zn¾H¾g ® CH3  – CH3 + H 2 O

 

Acetaldehyde (Ethanal)

HCl

Ethane

 

CH3COCH3 + 2H2 ¾¾Zn¾H¾g ® 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

Wolffkishner reduction :            C = O ¾¾H2 N¾NH¾2  ®

R

C = NNH2 ¾¾C2 H¾5 O¾N¾a ®

CH2

 

R¢                    – H 2 O               R¢

  • Hydroboration of alkenes
  • On treatment with acetic acid

180o C

¢

R

 

RCH  = CH2  ¾¾B2H¾6  ®(RCH2  – CH2 )3 B ¾¾CH¾3CO¾O¾H ® RCH2  – CH3

 

Alkene

Trialkyl borane

Alkane

 

  • Coupling of alkyl boranes by means of silver nitrate

 

6[R CH = CH2

(4)  Physical Properties

] ¾¾2BH¾6  ®[2RCH2

  • CH2

-]3

B ¾¾AgN¾O25o¾C ® 3[RCH

NaOH

2CH2

  • CH

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

 

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  + ClCl ¾¾u.v.¾lig¾ht ® CH 2  – Cl2  ¾¾u.v.¾lig¾ht ® CHCl3  ¾¾H¾Cl ® CCl 4

HNO3 / HIO3 / HgO

which

 

HCl                                                            HCl

  • Reaction based on free radical mechanism

 

  • Nitration :

RH+ HONO2  ¾¾Hi¾gh ® RNO2 + H2O

 

Alkane

temp.

Nitroalkane

 

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

 

  • (HNO3 vapour at
  • Sulphonation : Free radical mechanism

400o – 500o C).

 

RH + HOSO3 H ¾¾S¾O3¾® RSO3 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  ¾¾Bu¾rn ® 2CO + 4 H 2 O

n      2n+ 2      ç

ø

è

2   ÷   2                        2                     2

 

CH4 + O2 ¾¾® C  +2H2O

 

  • Catalytic Oxidation :

CH4  + [O] ¾¾Cu¾tu¾be ® CH3OH 100 atm / 200o 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

¾¾O¾2  ® CH

3 100 -160 o C               3

(CH

2)n COOH

 

  • Chemical oxidation : (CH3 )3 CH ¾¾KM¾nO¾4 ®(CH3 )3 .C.OH

Isobutane                           Tertiary butyl alcohol

  • Thermal decomposition or cracking or pyrolysis or fragmentation

 

 

 

 

CH4

¾¾100¾0o¾C ® C + 2H

; C2 H

¾¾500¾o¾C ® CH

6 Cr2O3 + Al2O3                2

= CH2

  • H2

 

Methane

Ethane

Ethylene

 

2

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     200o C, 35atm

3                3

Isobutane

2-Methyl pentane                  heat

2,3 Dimethyl butane

 

 

H2C

CH3

CH3

¾¾Cr  O¾/¾Al  O¾®

 

2   3        2 3

+4H

 

H2C

CH2

CH2

600o C / 15 atm

2

 

 

Benzene

 

n-Hexane

 

 

 

 

 

 

  • Step up reaction

 

 

n-Heptane

¾¾Cr2O¾3 /¾Al2O¾3  ®

600o C

 

 

Methyl cyclo Hexane

¾¾H¾2  ®

 

 

Toluene

 

 

  • Reaction with CH2 N2  :  RCH 2  – H + CH 2 N 2  ¾¾hv ® RCH 2  – CH 2  – H

 

 

  • Reaction with CHCl3 / NaOH

: R CH 2

  • H ¾¾CH¾Cl3 ¾/ OH¾- ® RCH

: CCl2                                            2

  • CHCl2

 

 

  • Reaction with CH2 = C :

||

O

O

||

RCH 2  – H ¾¾CH¾2 =C¾/¾D ® RCH 2  – CH3

:CH2 / –CO

 

 

 

  • HCN formation :

2CH4  ¾¾N2 ¾/ ele¾ctric¾a¾rc ® 2HCN + 3H 2

or CH4  + NH3  ¾¾Al2O¾3  ® HCN + 3H2

700o C

 

 

  • Chloro sulphonation/Reaction with SO2+Cl2

 

CH3  – CH2  – CH3  + SO2  + Cl2  ¾¾u.v l¾ig¾ht ® 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 ¾¾Ni /¾Al2O¾3  ® CO + 3H2 800o 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 250o C when methane is formed.

CO + 3H2  ¾¾Ni+¾C ® CH4  + H2O 250o 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 1200oC, methane is
2                               4

C + 2H   ¾¾120¾0o¾C ® CH

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

CS2  + 2H2S + 8Cu ¾¾Hig¾h te¾mpe¾ratu¾re ® 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 – 5o C and boiling point is – 161.5o 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 ¾¾Zn¾Cu¾cou¾p¾le ® C2 H6 + HI

 

Ethyl iodide

C2H5OH

Ethane

 

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

 

 

(iii) Physical properties

Ethylene (ethene)

300o 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 – 89oC. Its melting point is –172oC.

(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   ¾¾C¾l2  ® CH3Cl ¾¾Wu¾rtz re¾acti¾¾on ® 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

CH6  ¾¾C¾l2  ® CH5Cl  ¾¾Aq.¾KO¾H ® CH5OH ¾¾[¾O] ® CH3CHO ¾¾[¾O] ® CH3COOH ¾¾NaO¾H ® CH3COONa ¾¾NaO¾H /¾Ca¾O ® CH4

 

Ethane (excess)

UV          Ethyl chloride

Ethyl alcohol

Acetaldehyde

Acetic acid

Sodium acetate

heat

Methane

 

Higher ¾¾C¾l2  ® Alkyl ¾¾A¾q. ® Alcohol ¾¾[¾O] ® Aldehyde ¾¾[¾O] ® Acid ¾¾NaO¾H ® Sodium salt of ¾¾NaO¾H /¾Ca¾O ® 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 H2.

 

  • 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 : CH3C H

||

CH3C H

cis-2-butene

CH = CH2

|

and CH3C 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

a

system at the both

b

 

end can exhibit optical isomerism but cannot exhibit geometrical isomerism.

a

  • Cumulated polythene having odd of double bonds which have = C

b

 

system at both end can

 

exhibit geometrical isomerism but cannot exhibit optical isomerism.

(3)  Preparation methods

 

 

  • From Alkynes :

H

|

RC º CH + H2  ¾¾Lind¾lar’¾s Ca¾taly¾st ® RC  = CH

 

 

 

Note : ® Poison’s catalyst such as alkene.

Pd. BaSO4

 

 

BaSO4 , CaCO3

|

H

are used to stop the reaction after the formation of

 

H   H                                                                      H

|   |                                           |

 

  • From mono halides :

RCCH + Alc. KOH ¾¾-H¾X  ® RC  = CH

 

|   |                                      |

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 :

RCCH + Zn dust ¾¾D ® RC  = CH + ZnX 2

 

|   |                   300o C

X     X

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

 

C H2  – CH2  – C H2  ¾¾Zn d¾u¾st ®

|                 |

X                           X                                 H2C

CH2

CH2

 

Br    Br                               I      I

|    |                  |   |

 

  • By action of NaI on vic dihalide :

C C

¾¾N¾aI ®

acetone

CC

unstable

¾¾®

C = C

 

 

  • From alcohols [Laboratory method] : CH3CH2OH ¾¾H2 S¾Oor H¾3 P¾O¾4  ® 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] : CH3CO O

|

H       ¾¾Gla¾ss w¾ool 4¾50¾o   ® CH3  – COOH

 

+
|

CH2

  • CH2

liq. N 2

CH2 = CH2

 

  • Pyrolysis of quaternary ammonium compounds :

+        –

¾¾he¾at ®(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 :

 

R

H2C = CHCl ¾¾CuR¾2  ® H2C = CHR

Vinyl chloride

 

  • By Grignard reagents : Mg
  • X CH = CH2 ¾¾® MgX2 + R CH = CH2

X

 

  • The wittig reaction : (Ph)3 P  = CH 2  + CHR ¾¾®(Ph)3 P  = O + RCH

||                                    ||

O                                                             CH 2

O

||

(Ph)3 P  = CHR + CHR ¾¾®(Ph)3 P  = O + RCH  = CHR

  • From b bromo ether [Boord synthesis]

Br     O C2 H5

|     |                                                     Br

 

R CHCH

|

R¢

¾¾Zn ® RCH  = CHR¢ + 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

 

H

C = C

H

 

CH3

CH3

 

H

C = C

CH3

 

H

 

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

 

H

 

 

(5)  Chemical properties

H

C = C

H

Propene

m = 0.35 D

CH3 – CH2

 

H

H

C = C

H

Bute 1-ene

m = 0.37 D

 

  • Francis experiment : According to Francis electrophile first attacks on olefinic

CH2 = CH2 + Br – Br  ¾¾CC¾l4  ®  CH2 – CH2

|          |

Br         Br

      ¾¾Na¾Cl ®  CH2 – CH2 + CH2 – CH2

 

|          |

Br         Br

|           |

Br         Cl

 

H     H                                            H  H

|    |                           |   |

 

  • Reaction with hydrogen :

RC = CR + H2  ¾¾Ni ® RCCR

 

|   |

H  H

  • Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis
2                                         2             2 3                                             2             3

RCH  = CH   ¾¾H¾BH¾2  ®(RCH    CH  )  B ¾¾H+ ¾/ HO ® RCH    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

| 6CH3CH2CH2C =

CH3

|

CH2  ¾¾B2H¾6  ® 2[CH3  – (CH2 )2  – C

|

H

CH2 ]3 B ¾¾Ag NO¾3 Na¾O¾H  ®

 

CH3

|

CH3  – CH2  – CH2  – C

|

H

CH3

|

CH2CH2C

|

H

 

CH2 – CH2 – CH3

 

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

 

R CH = CH2

¾¾N¾a ® R

+e

C H

C H 2

¾¾Et¾O¾H ® RCHCH3

¾¾N¾a ® R –  –

C H

+e

CH3

¾¾Et.¾O¾H ® RCH2

CH3

 

  • Halogenation

 

CH3CH = CH2

  • Cl2

¾¾500¾o¾C ® ClCH   CH = CH

2

2

  • 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 + |

  • CO

 

N – Br

CH2                      CH2CH = CH2+ |

  • CO

N H

 

 

Propene

CH2CO

NBS

|

Br

Allyl bromide

CH2CO

Succinimide

 

  • In presence of polar medium alkene form vicinal dihalide with

H   H                                                       H  H

|   |                                 |   |

RC = CH + XX ¾¾CC¾l4  ® RCCH

|   |

X  X

Vicinal dihalide

 

Reactivity of halogen is

F2 >

Cl2  >

Br2

  • I 2

 

 

 

 

 

  • Reaction with HX [Hydrohalogenation]

H

|

 

C = C

 

alkene

+ HX ¾¾®

C C

|

X

Alkyl halide

 

According to markownikoff’s rule and kharasch effect.

 

 

H   H

|   |

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

|   |

Br  H

H   H                            H   H

|   |                 |   |

(Based on F.R.M.) CH3  – CH  = CH 2  + HBr ¾¾Pero¾xi¾de ® CH3  – CCH + CH3  – CCH

 

|   |

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

 

4

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®

NO

|

C C

|

Cl

 

( NOCl is called Tillden reagent)

 

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

 

NO

|      H

C C                  

C C = NOH ;

 

C = C

C

  • NOCl ¾¾®

C  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

|   |                                               |    |

RC = CH + [O] + HOH ¾¾Alk¾KM¾nO¾4  ® RCCH

 

OH

|   |

HO OH

 

glycol

H      H                                                O

|    |                             ||

 

With acidic

KMnO4

: RC  = CH + [O] ¾¾acid¾ic ® RCOH + CO2  + H 2 O

KMnO4

 

 

 

  • Hydroxylation

 

  • Using per oxy acid :

CH3

|

H C

||

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 :

 

H

||+ OsO4

 

R

Trans

+  ¾¾I  ®

H              OH

HO                H

R

(±)

 

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

RCH  = CHR ¾¾C6 H¾5 C¾OH ® RCHCHR ¾¾ H¾2¾O ® RCHCHR

 

or CH 3 CO3 H

|     |

OH  OH

O

[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

¾¾O¾3  ®

I

O

C      C

O       O

Ozonide

¾¾H2O¾/ H¾+  / ¾Zn ® ZnO +

II

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

 

H

|

CH3                                        H

|                            |

 

CH3C = O + O = C

CH3 ¾¾® CH3C = C

|

  • CH3

 

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 CCH  = CH 2  + (CHCOO)2 Hg ¾¾®(CH3 )3 CCHCH 2 – Hg ¾¾NaB¾H/ Na¾O¾H ®(CH 3 )3 CCHCH 3

 

|

OCOCH3

THF

 

3, 3-

|

OH

Dimethyl 2butanol

 

 

 

 

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

O                                          ||

 

  • By O2 / Ag

: CH2

= CH2

  • 1O

2   2

¾¾A¾g ® CH2

  • CH2

C – O – O – H

|

O

 

  • Epoxidation by performic acid or perbenzoic acid :

 

O

||

CH2  = CH2 ¾¾¾¾¾¾® CH2  – CH 2

O

 

 

  • Hydroboration

CH3  – CH = CH2  ¾¾H¾CO¾O¾¾H ® CH3  – CHCH2

 

3R CH = CH2

  • BH3

¾¾®(RCH2CH

2 )3

B ¾¾H2O¾2 / O¾H¾-  ® RCH

  • CH2OH + B(OH)3

 

Tri alkyl borane

2

(Anti markownikoff’s rule)

 

 

H      H

|   |

  • Hydroformylation : RCH  = CH 2  + CO + H 2  ¾¾CoH¾(C¾O¾)4  ® RCCH

 

 

 

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

RCH  = CH 2  + CO + H 2 O ¾¾CoH¾(C¾O¾)4  ® RCH 2  – CH 2

|

COOH

|    |

H     C = O

|

H

 

  • Addition of formaldehyde

 

Å                                               Å

H C = O +       ¾¾®[H C =

¬¾®

ÅOH] ¾¾RC¾H =¾CH¾2  ® R – Å

  • CH
  • CH
  • OH

HOH

 

2                  H                    2

OH           HC

R – CH

CH2

CH2

C H             2             2

Å

  • H +

 

¬¾HC¾HO¾/ H ¾ RCHCH2  – CH2

 

O                      O

CH2

|              |

OH                   OH

 

  • Polymerisation

Cyclic acetal

1, 3-diol

 

ê

H      H                                      é  H      H      H      Hù

|
ú

|    |                       ê   |    |    |      ú

C  = C ¾¾Trac¾e O¾2 +C¾ata¾ly¾st ®ê- CCCC ú

 

1500o / high pressure

|    |

H      H

ê

|

ê          |    |

ëê  H     H      H

ú

|

ú

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     

CH3CH2CH = 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 180o .
  • Geometry of carbon is
  • C C triple bond length is 120 Å
  • C H bond length is 108 Å
  • CC triple bond energy is 190 Kcal / mol .
  • C H bond energy is 38 Kcal / mol .

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