Chapter 19 Aliphatic Hydrocarbons Part 1 by TEACHING CARE Online coaching and tuition classes
“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”.
Aliphatic or open chain Alicyclic or closed chain or cyclic Aromatic or arenes
Cyclo alkanes Cyclo alkenes
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.
- 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
Ca + 2C ® CaC2
Mg + 2C ® MgC2
4 Al + 3C ® Al4 C3
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
3[CH º CH] ¾¾Poly¾me¾risat¾io¾n ® C6 H6 ;
CH3 – CH = CH – CH3 ¾¾Isom¾eris¾atio¾n ® CH3 – CH 2 – CH = CH 2
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 – 150o
30 – 70o
70 – 120o
120 – 150o
150 – 250o
250 – 400o
|C1 – C4||Fuel gases: refrigerants; production of|
|carbon black, hydrogen; synthesis of|
|Crude naphtha on||C5 – C10|
|(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.|
|(i) Gas oil|
|(ii) Fuel oil|
|(iii) Diesel oil|
|Residual oil on||C17 – C40|
|fractionation by vacuum|
|(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)|
- 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 ®
RSSR + PbS + 2NaOH
- 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 ¾¾Fe2¾O¾3 ® Mix. Of hydrocarbons or crude oil
450 – 500o C
- Fischer- tropsch process : The overall yield of this method is slightly higher than Bergius
H O + C ¾¾120¾0o¾C ® CO + H
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 — C — CH2 — CH — 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
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
CH3CH2CH2CH2CH3 ¾¾AlC¾l3 ® CH3 CHCH2CH3
AlCl3 at 200o C .
Pentane (Octane number = 62)
Isopentane (Octane number = 90)
- Alkylation: CH3 CH + CH 2 = CCH3 ¾¾H2S¾O¾4 ® CH3 CCH 2 CHCH3
(Octane number = 100)
- Aromatisation : CH3 (CH2 )5 CH3 ¾¾Pt /¾Al2¾O¾3 ® + 4 H2
The octane no. of petrol can thus be improved.
- 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
Cetane no. = 0
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 :
, C2 H6 ,
- Structure : (i) Every carbon atom is
sp 3 hybridized.
- The bond length between carbon-
carbon and carbon-hydrogen are
- Bond angle in alkanes are tetrahedral angles having a value of 5o (109o.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
Note : ®Methane is not prepared by this method
- Birch reduction :
- From alkyl halide
R – CH = CH 2 ¾¾1. N¾a / N¾H¾3 ® R – CH 2 – CH3
- 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 +
¾¾C¾u ®(CH3 CH 2 O)2 Zn+ 2H
- Wurtz reaction : R
RX + 2H ¾¾® RH + HX
R¢ ¾¾Dry¾eth¾er ® R – R+ 2NaX
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 ¾–¾Cl ® CH3 – CH 2 – CH 2 – CH3
Note : ® Reaction is suitable for odd number of Alkanes.
- From Grignard reagent
- By action of acidic ‘H’ :
- HOH ¾¾® RH + Mg(OH)X
Alkyl magnesium halide
- 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 ® R – H+ Na2 CO3
Note : ® NaOH and CaO is in the ratio of 3 : 1.
- Kolbe’s synthesis : R – C– O–
R – C– O–
- Na +
At anode [Oxidation] : 2R – C – O– + 2e – ¾¾® 2R – C– O
2 R+ 2CO2
2 R ¾¾® R – R (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
- By reduction of alcohols, aldehyde, ketones or acid derivatives
+ 2HI ¾¾Red¾P ® CH4 + H2O + I2 ;
CH3CHO+ 4 HI ¾¾Red¾P ® C2 H6 + H2O + 2I2
Methanol (Methyl alcohol)
CH3COCH3 + 4 HI ¾¾Red¾P ® CH3CH2CH3 + H2O + 2I 2 ;
CH3 – C– Cl+ 6HI ¾¾Red¾P ® CH3 – CH 3 + H 2 O + HCl + 3I 2
Acetyl chloride (Ethanoyl chloride)
CH3 – C– NH 2 + 6HI ¾¾Red¾P ® CH3 – CH3 + H 2 O + NH 3 + 3I 2
Note : ® Aldehyde and ketones when reduced with amalgamated zinc and conc. HCl also yield alkanes.
Clemmensen reduction : CH3 CHO+ 2H 2 ¾¾Zn–¾H¾g ® CH3 – CH3 + H 2 O
CH3COCH3 + 2H2 ¾¾Zn–¾H¾g ® CH3CH2CH3 + H2O
Note : ® Aldehydes and ketones
(> C = O)
can be reduced to hydrocarbon in presence of excess of
hydrazine and sodium alkoxide on heating.
Wolff–kishner reduction : C = O ¾¾H2 N¾NH¾2 ®
C = NNH2 ¾¾C2 H¾5 O¾N¾a ®
R¢ – H 2 O R¢
- Hydroboration of alkenes
- On treatment with acetic acid
R – CH = CH2 ¾¾B2H¾6 ®(R – CH2 – CH2 )3 B ¾¾CH¾3CO¾O¾H ® R – CH2 – CH3
- Coupling of alkyl boranes by means of silver nitrate
6[R – CH = CH2
(4) Physical Properties
] ¾¾2B2¾H¾6 ®[2R – CH2
B ¾¾AgN¾O3¾25o¾C ® 3[RCH
- Physical state : Alkanes are colourless, odourless and
C1 – C4 C5 – C17
Liquid state [Except neo pentane] [gas]
& above Solid like waxes
- Density : Alkanes are lighter than
- Solubility : Insoluble in water, soluble in organic solvents, solubility µ 1
- 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 + Cl – Cl ¾¾u.v.¾lig¾ht ® CH 2 – Cl2 ¾¾u.v.¾lig¾ht ® CHCl3 ¾¾–H¾Cl ® CCl 4
HNO3 / HIO3 / HgO
– HCl –HCl
- Reaction based on free radical mechanism
- Nitration :
R – H+ HONO2 ¾¾Hi¾gh ® R – NO2 + H2O
Nitrating mixture : (i) (Con. HNO3 + Con. H 2 SO4 ) at 250o C
- (HNO3 vapour at
- Sulphonation : Free radical mechanism
400o – 500o C).
R – H + HOSO3 H ¾¾S¾O3¾® R – SO3 H + H2O Prolonged heating
Note : ® Lower alkanes particularly methane, ethane, do not give this reaction.
- Complete Oxidation or combustion : C H
+ æ 3n + 1 öO
+ (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
2 ÷ 2 2 2
CH4 + O2 ¾¾® C +2H2O
- Catalytic Oxidation :
CH4 + [O] ¾¾Cu–¾tu¾be ® 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.
¾¾O¾2 ® CH
3 100 -160 o C 3
- Chemical oxidation : (CH3 )3 CH ¾¾KM¾nO¾4 ®(CH3 )3 .C.OH
Isobutane Tertiary butyl alcohol
- Thermal decomposition or cracking or pyrolysis or fragmentation
¾¾100¾0o¾C ® C + 2H
; C2 H
¾¾500¾o¾C ® CH
6 Cr2O3 + Al2O3 2
C3 H8 ¾¾® C2 H4 + CH4 or C3 H6 + H2
Note : ® This reaction is of great importance to petroleum industry.
- Isomerisation : CH CH CH CH
¾¾AlC¾l3 +¾H¾Cl ® CH
¾¾AlC¾l3 +¾H¾Cl ®
- Aromatisation :
3 2 2
3 200o C, 35atm
2-Methyl pentane heat
2,3 Dimethyl butane
¾¾Cr O¾/¾Al O¾®
2 3 2 3
600o C / 15 atm
- Step up reaction
¾¾Cr2O¾3 /¾Al2O¾3 ®
Methyl cyclo Hexane
- Reaction with CH2 N2 : R – CH 2 – H + CH 2 N 2 ¾¾hv ® R – CH 2 – CH 2 – H
- Reaction with CHCl3 / NaOH
: R – CH 2
- H ¾¾CH¾Cl3 ¾/ OH¾- ® R – CH
: CCl2 2
- Reaction with CH2 = C :
R – CH 2 – H ¾¾CH¾2 =C¾/¾D ® R – CH 2 – CH3
:CH2 / –CO
- HCN formation :
2CH4 ¾¾N2 ¾/ ele¾ctric¾a¾rc ® 2HCN + 3H 2
or CH4 + NH3 ¾¾Al2O¾3 ® HCN + 3H2
- 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
- Synthesis : q By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at 1200oC, methane is
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 .
- 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
- Methods of preparation
- Laboratory method of preparation :
C2 H5 I + 2H ¾¾Zn–¾Cu¾cou¾p¾le ® C2 H6 + HI
- Industrial method of preparation : CH2 = CH2 + H2 ¾¾Ni ® CH3 – CH3
(iii) Physical properties
- 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.
(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
Heat with Na in ether
- Butane from ethane :
C2 H6 ¾¾C¾l2 ® C2 H5 Cl ¾¾Wu¾rtz re¾acti¾¾on ® C2 H5 – C2 H5
UV Ethyl chloride Heat with Na in ether
Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion.
Ethane to methane
C2 H6 ¾¾C¾l2 ® C2 H5Cl ¾¾Aq.¾KO¾H ® C2 H5OH ¾¾[¾O] ® CH3CHO ¾¾[¾O] ® CH3COOH ¾¾NaO¾H ® CH3COONa ¾¾NaO¾H /¾Ca¾O ® CH4
UV Ethyl chloride
Higher ¾¾C¾l2 ® Alkyl ¾¾A¾q. ® Alcohol ¾¾[¾O] ® Aldehyde ¾¾[¾O] ® Acid ¾¾NaO¾H ® Sodium salt of ¾¾NaO¾H /¾Ca¾O ® Lower alkane
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
- 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
- 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
CH = CH2
and CH3 – C – H
H – C – CH3
Trans – 2-butene
CH2 – CH2
- Optical Isomerism :
H – C – CH3
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 ¾¾Lind¾lar’¾s Ca¾taly¾st ® R – C = C – H
Note : ® Poison’s catalyst such as alkene.
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
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 ¾¾D ® R – C = C – H + ZnX 2
| | 300o C
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
Br Br I I
| | | |
- By action of NaI on vic dihalide :
C – C
C = C
- From alcohols [Laboratory method] : CH3CH2OH ¾¾H2 S¾O4¾or H¾3 P¾O¾4 ® CH2 = CH2 + H2O
- Kolbe’s reaction :
+ 2H2O ¾¾Elec¾trol¾y¾sis ®||
+ 2CO2 + H2 + 2KOH
- From esters [Pyrolysis of ester] : CH3 – CO – O
H ¾¾Gla¾ss w¾ool 4¾50¾o ® CH3 – COOH
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)
- Action of copper alkyl on vinyl chloride :
H2C = CHCl ¾¾CuR¾2 ® H2C = CHR
- 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
¾¾Zn ® R – CH = CH – R¢ + Zn
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®
- 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.
C = C
C = C
m = 0
m = 0.25 D
Thus symmetrical and unsymmetricals cis alkene are polar and hence have finite dipole moments
(5) Chemical properties
C = C
m = 0.35 D
CH3 – CH2
C = C
m = 0.37 D
- Francis experiment : According to Francis electrophile first attacks on olefinic
CH2 = CH2 + Br – Br ¾¾CC¾l4 ® CH2 – CH2
¾¾Na¾Cl ® CH2 – CH2 + CH2 – CH2
H H H H
| | | |
- Reaction with hydrogen :
R – C = C– R + H2 ¾¾Ni ® R – C – C– R
- Reduction of alkene via hydroboration : Alkene can be converted into alkane by protolysis
RCH = CH ¾¾H–¾BH¾2 ®(R – CH – CH ) B ¾¾H+ ¾/ H2¾O ® 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
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
| 6CH3 – CH2 – CH2 – C =
CH2 ¾¾B2H¾6 ® 2[CH3 – (CH2 )2 – C –
CH2 ]3 B ¾¾Ag /¾NO¾3 Na¾O¾H ®
CH3 – CH2 – CH2 – C –
CH2 – CH2 – C –
CH2 – CH2 – CH3
- Birch reduction : This reaction is believed to proceed via anionic free radical
R – CH = CH2
¾¾N¾a ® R –
C H 2
¾¾Et–¾O–¾H ® R – CH – CH3
¾¾N¾a ® R – –
¾¾Et.–¾O–¾H ® R – CH2
CH3CH = CH2
¾¾500¾o¾C ® ClCH – CH = CH
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
CH2 – CO
CH2 – CO
- In presence of polar medium alkene form vicinal dihalide with
H H H H
| | | |
R – C = C– H + X – X ¾¾CC¾l4 ® R – C – C– H
Reactivity of halogen is
- I 2
- Reaction with HX [Hydrohalogenation]
C = C
+ HX ¾¾®
C – C
According to markownikoff’s rule and kharasch effect.
CH3 – CH = CH 2 + HBr ¾¾® CH3 – C – C– H Markownikoff rule
H H H H
| | | |
(Based on F.R.M.) CH3 – CH = CH 2 + HBr ¾¾Pero¾xi¾de ® CH3 – C – C– H + CH3 – C – C– H
- 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.
C – C ⇌
C – C = NOH ;
C = C
- NOCl ¾¾®
C – C
C | | C
- 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 ¾¾Alk¾KM¾nO¾4 ® R – C – C– H
H H O
| | ||
: R – C = C – H + [O] ¾¾acid¾ic ® R – C– O – H + CO2 + H 2 O
- Using per oxy acid :
H – C
H – C
¾¾H2O¾2 , H¾CO¾O¾H ®
H – C – OH
OH – C – H
- Hydroxylation by OsO4 :
+ ¾¾I ®
Note : ® If per benzoic acid or peroxy acetic acid is used then oxirane are formed.
R – CH = CH – R ¾¾C6 H¾5 C¾O3¾H ® R – CH – CH – R ¾¾– H¾2¾O ® R – CH – CH– R
or CH 3 CO3 H
- Combustion : Cn
- 3n O
- nH 2 O
They burn with luminous flame and form explosive mixture with air or oxygen.
C = C
¾¾H2O¾/ H¾+ / ¾Zn ® 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.
CH3 – C = O + O = C – CH3 ¾¾® CH3 – CH = CH – CH3
CH3 – C = O + O = C
– CH3 ¾¾® CH3 – C = C
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 ¾¾NaB¾H4¾/ Na¾O¾H ®(CH 3 )3 C – CH – CH 3
Dimethyl – 2–butanol
CH = CH2 BH3
Antimarkowni – koff rule
(CH2 – CH2)3 B
Mercuration markowni – koff rule
CH CH3 ⇌
CH2 –CH3 OH
Less stable Carbocation
More stable carbocation
- By O2 / Ag
¾¾A¾g ® CH2
C – O – O – H
- Epoxidation by performic acid or perbenzoic acid :
CH2 = CH2 ¾¾¾¾¾¾® CH2 – CH 2
CH3 – CH = CH2 ¾¾H–¾C–O¾–O¾–¾H ® CH3 – CH – CH2
3R – CH = CH2
¾¾®(R – CH2 – CH
B ¾¾H2O¾2 / O¾H¾- ® R – CH
- CH2 – OH + B(OH)3
Tri alkyl borane
(Anti markownikoff’s rule)
- Hydroformylation : R – CH = CH 2 + CO + H 2 ¾¾CoH¾(C¾O¾)4 ® R – C – C – H
Note : ® If CO + H2O is taken then respective acid is formed.
R – CH = CH 2 + CO + H 2 O ¾¾CoH¾(C¾O¾)4 ® R – CH 2 – CH 2
H C = O
- Addition of formaldehyde
H C = O + ¾¾®[H C =
Å – OH] ¾¾R–C¾H =¾CH¾2 ® R – Å
2 H 2
OH H2 C
R – CH
C H 2 2
- H +
¬¾HC¾HO¾/ H ¾ R – CH – CH2 – CH2
H H é H H H Hù
| | ê | | | ú
C = C ¾¾Trac¾e O¾2 +C¾ata¾ly¾st ®ê- C – C – C – C ú
1500o / high pressure
ê | |
ëê H H H
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.
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
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
- 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
- C – C triple bond energy is 190 Kcal / mol .
- C – H bond energy is 38 Kcal / mol .