Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Alkane
1) Introduction
2) Structural formula
3) Isomerism
4) Conformation
5) Nomenclature
6) Methods of preparation
7) Physical Properties
8) Chemical Properties
9) Uses
---------------------------------------------------------------------------------------------------------------------------------Alkanes
Introduction
Alkanes are aliphatic saturated hydrocarbons. They are represented by the general formula
CnH2n+2 where ‘n’ is the no. of carbon atoms.
They contain C-C single bonds. They are also called as paraffin’s (means less affinity) because
they do not undergo chemical reactions easily.
Structural formula
Structural formula of a compound gives the exact arrangement of atoms various elements in a
molecule. It can be represented in the different ways.
E.g. Methane CH4
Condensed formula
In this – (dash) representing covalent bonds are omitted and identical groups attach to ‘C’ atoms
indicated by subscript.
E.g. Hexane C6H14
1
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Bond line formula
A molecule can be represented by lines representing C-C bond in zig - zag manner. In this
terminal denotes – CH3 group and line junction indicates – CH2 group.
E.g. n-hexane C6H14
Classification of Alkanes
Isomerism
The compounds having same molecular formula but different structural formulae are called
isomers of each other and phenomenon is called as isomerism.
First three alkanes do not show isomerism. From butane and onwards alkanes show isomerism.
Alkanes show chain isomerism which is due to the difference in structure of ‘C’ chain i.e. they
contain different ‘C ’skeleton. Isomers have different physical and chemical properties.
eg. Pentane C5H12.
It shows three isomers
2
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Types of ‘C’ atoms
There are four types of ‘C’ atoms

Primary (10) carbon atom: It is the carbon atom
which is attached to one ‘C’ atom.

Secondary (20) carbon atom: It is the ‘C’ atom which is attached to 2 more ‘C’ atoms.

Tertiary (30) carbon atom: It is the ‘C’ atom

Quaternary (40) carbon atom: It is the ‘C’ atom which is attached to 4 more ‘C’ atoms.
which is attached to 3 more ‘C’ atoms.
E.g.
In the above alkane there are 5 primary ‘C’ atoms (10) , one secondary atom (20), one tertiary ‘C’
atom (30) and one quaternary ‘C’ atom (40). The ‘H’ atom attached to 10 ‘C’ atom is called as
primary H atom and attached to 20 ‘C’ atom is called as secondary hydrogen atom.
Confirmations of Ethane
1.
Conformation:
The phenomenon of different spatial arrangements (three dimensional arrangement in space)
of atoms that can be converted into one – another by free rotation of atoms about C– C single
bonds is called conformation and the compounds are called conformers or rotamers.
These conformational isomers interconvert rapidly and cannot be isolated easily.
2.
Conformations of ethane:
3
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Ethane (C2H6) has two carbon atoms joined by single covalent bond. Each carbon atom has three
hydrogen atoms. Considering free rotation of one of the carbon atom around the C – C bond axis,
infinite number of spatial arrangements of hydrogen atoms (attached to one carbon atom with
respect to the hydrogen atoms attached to another carbon atom) are possible. These are called
conformational isomers.
As they all have nearly the same energy, they can change from one form to another freely.
Thus two extreme arrangements are considered:
viz. staggered and eclipsed conformations.
a.
Staggered conformation: In this arrangement hydrogen atoms attached to two carbon
atoms are as far apart as possible.
b.
Eclipsed conformation: In this arrangement hydrogen atoms attached to two carbon
atoms are as close as possible.
The intermediate conformations during rotation are called skew conformation.
3.
Representation of conformation:
This conformation is represented by the following two projections
i.
Sawhorse projection of ethane: In this representation, C – C bond is viewed from an
oblique angle which indicates spatial arrangements by showing all C – H bonds.
ii.
Newmann projection of ethane: In this representation, C – C bond is viewed directly
end – on and represents two carbon atoms by a circle.
In this, bonds attached to the front carbon atom are shown by lines drawn from the centre of a
circle and bonds attached to rear carbon atom are shown by lines drawn from the edge of the
circle.
4
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Alkyl Group
An alkyl group is a free radical obtained by homolytic fission of C – H bond in alkane.
Alkyl groups are classified as follows:
1.
Primary alkyl group: In this alkyl group, valency
on the primary carbon atom is
available.
Example: methyl radical
2.
1o
CH
3
Secondary alkyl group: In this alkyl group
valency of a secondary carbon atom is
available.
2o
Example: isopropyl radical,
3.
CH3  CH  CH 3
|
Tertiary alkyl group: In this alkyl group, valency
Example: tert – butyl radical,
 CH3 3
of a tertiary carbon atom is available.
3o
C
Nomenclature of alkanes
1.
Common/ Trivial names –
The first four members from methane (CH4) to butane (C4H10) are named by common names. But
from pentane and onwards named depending upon the no. of ‘C’ atoms. The prefix of greek
number like pent (5) , hex (6) etc. are used before – ane.
Prefix like n, iso, sec, neo are used to indicate isomers.
5
Mrs. Rashmi R.Patwardhan
2.
M.No: 9860144335
IUPAC nomenclature of alkanes –
The rules of IUPAC nomenclature which you
have studied in chapter 12 are applied. Common
names of first 4 alkanes are used in IUPAC nomenclature. All normal chain alkanes are
considered as parent alkanes.
E.g.
1.
2.
3.
4.
5.
6.
6
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Methods of preparation of alkanes
1.
From unsaturated hydrocarbons by
hydrogenation
Alkenes and alkynes are unsaturated hydrocarbons. When mixture of alkenes/
and H2(g) passed over Raney Ni
alkynes
catalyst at 473K to 573K form corresponding alkane by
addition reaction.
e.g.
CH 2 = CH 2 + H 2 ¾ Raney
¾ D¾Ni¾
® CH 3 - CH 3
Ethene
Ethane
CH º CH + 2H2 ¾ Raney
¾ D¾Ni¾
® CH3 - CH3
Ethyne
2.
Ethane
Decarboxylation of Na – salt of fatty acids
When anhydrous Na- salt of fatty acid is heated with soda lime (mixture of NaOH + CaO)
forms alkane by decarboxylation. Alkane containing one atom less than acid is obtained.
e.g.
C2 H 5 COONa + NaOH ¾ CaO
¾D ¾® C2 H 6 + Na 2 CO3
sodium propionate
3.
ethane
From alkyl halides by reduction
Alkyl halides when treated with reducing agent like Zn – Cu couple and alcohol form
corresponding alkane.
e.g.
CH3 Br
Methyl bromide
- Cu
+ 2H ¾ Zn
¾
¾¾
® CH 4 + HBr
alcohol
D
methane
- Cu
C2 H5 Br + 2H ¾ Zn
¾
¾¾
® C2 H6 + HBr
alcohol
Ethyl bromide
4.
D
Ethane
Wurtz synthesis
When alkyl halide is treated with sodium metal in the presence of dry ether as solvent
gives higher alkanes.
e.g.
Dry
C2 H5 Br + 2Na + Br C2 H5 ¾ ether
¾
¾
® C4 H10 + 2NaBr
Ethyl bromide
Butane
7
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Chemical properties of alkane
1.
Halogenation
Alkanes react with halogens like chlorine or bromine to form alkyl halide in presence of
diffused sunlight or U.V. light or by heating.
e.g.
Chlorination of methane:
In this reaction one by one H atoms replaced by Cl atoms to form mixture of products. Alkyl
halide is obtained by limiting supply of chlorine.
CH 4
methane
light
+ Cl2 ¾ U.V.
¾¾
D ¾®
CH3 Cl
+ HCl
Methyl chloride
light
CH3 Cl + Cl2 ¾ U.V.
¾¾
D ¾®
CH 2 Cl2
+ HCl
Methylene dichloride
light
CH 2 Cl2 + Cl2 ¾ U.V.
¾ ¾
D ¾®
light
CHCl3 + Cl2 ¾ U.V.
¾¾
D ¾®
CHCl3 + HCl
Chloroform
CCl4
+ HCl
Carbon tetrachloride
Free radical mechanism
Methane reacts with chlorine to form methyl chloride in the presence of ultraviolet light.
Chlorination of methane is a chain reaction which takes place by free radical mechanism in
the following stages.
1.
Chain initiation:
Chlorine molecule absorbs energy from photons
homolytically to give free
and
CI
–
CI
bond
breaks
radicals of chlorine.
.
hv
Cl  Cl 
 2 Cl
2.
Chain propagation:
i. Chlorine free radical being very reactive abstracts a hydrogen atom from methane to
form methyl free radical and hydrogen chloride.
.
CI
.
 CH4  CH3  HCI
ii. Methyl free radical thus formed attacks a chlorine molecule to give methyl chloride and
a chlorine free radical.
.
.
CH3  CI – CI  CH3 – CI  CI
The chlorine free radical attacks methane (step i). The steps (i) and (ii) are repeated many times
and a chain of reactions is set up. The overall reaction is,
CH4 + CI – CI → CH3 – CI + HCI
3.
Chain termination:
8
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
When all the reactants are consumed, the reaction stops and the chain is terminated by the
combination of free radicals.
i.
ii.
iii.
Methyl free radical combines with chlorine free radical to form methyl chloride.
.
.
CH3  CI  CH3 – CI
Two methyl free radicals combine to form ethane
.
CH3 
CH3  CH3 – CH3
Two chlorine free radicals combine to form a molecule of chlorine.
.
CI
4.
.
.
 CI  CI2
Chlorination of methane also gives a mixture of chloroderivatives of methane.
hv
CH4  Cl2 
CH3  Cl  HCl
Methyl chloride
hv
CH3  Cl  Cl2 
CH2Cl2  HCl
Dichloromethane
hv
CH2Cl2  Cl2 
CHCl3  HCl
Chloroform
hv
CHCl3  Cl2 
CCl4  HCl
Carbon tetrachloride
---------------------------------------------------------------------------------------------------------------------------------Bromination : carried out in presence of AlBr3
3
C2 H6 + Br2 ¾ AlBr
¾D¾®
C2 H5 Br + HBr
Ethyl bromide
---------------------------------------------------------------------------------------------------------------------------------2.
Nitration of alkanes :
In this reaction one H atom of alkane is replaced by nitro ( - NO2 ) group.
e.g.
C2 H 6 +
Ethane
HNO3
Conc.nitricacid
698 k
¾ 423
¾ to¾
¾ ® C2 H5 NO2 + H 2 O
D
nitroethane
---------------------------------------------------------------------------------------------------------------------------------3.
Pyrolysis of alkanes:
The thermal decomposition of alkanes in absence of air to give lower alkanes, alkenes and H2 is
called as pyrolysis. It takes place as follows.
9
Mrs. Rashmi R.Patwardhan
i.
M.No: 9860144335
Dehydrogenation : It involves breaking of C - H bond in alkanes to form alkenes by
dehydrogenation.
e.g.
CH 3 - CH 3 ¾ D¾® CH 2 = CH 2 + H 2
ethene
ethane
ii.
Cracking : It involves breaking of C - C bonds and C - H bonds to form lower alkanes and
alkenes.
CH3 - CH 2 - CH3 ¾ D¾® CH 2 = CH2 + CH4
ethene
Pr opane
iii.
Methane
Aromatization : Alkanes containing more than 5 ‘C’ atoms get cyclised to benzene
and it’s homologues on heating under 10 to 20 atm. at 773K in presence of Cr2O3,
---------------------------------------------------------------------------------------------------------------------------------4.
Combustion
Alkane when heated in air, combine with oxygen to give CO2(g) and water vapour. It is
an
exothermic reaction because large amount of heat is evolved.
e.g.
CH 4 (g) + 2 O 2 (g)
¾ D¾® CO2 (g) + 2 H 2 O (g) + heat
Methane
Uses of Alkane
1) As fuels e.g. LPG, CNG, Petrol, Diesel etc.
2) Liquid alkanes used as solvents.
3) C17 to C20 as lubricants.
4) C21 to C30 as a lubricant base for preparation of cosmetics and candles.
5) As a source of Hydrogen.
6) Incomplete combustion gives carbon black for manufacture of printing ink, polishes, black
pigments etc.
10
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Multiple Choice Questions
1.
2.
Which of the following is aliphatic saturated hydrocarbon ?
a. C6H6
b. C6H10
c. C6H12
d. C6H14
Which of the following conversion is an example of
substitution reaction?
a. Acetylene → Ethane
b. Propene → Propane
c. Sodium acetate → Methane
d. Ethane → Chloroethane
3.
4.
5.
6.
Which of the following alkanes are not prepared
a. Methane
b. Ethane
c. Propane
d. Butane
by
Which of the following alkane is liquid at room temperature?
a. C4H10
b. C10H22
c. C22H46
d. C46H94.
Which of the following represents the condensed formula of neohexane?
a. (CH3)2CH(CH2)2CH3
b. (CH3)3CCH(CH3)2
c. (CH3)2CHCH(CH3)2
d. (CH3)3 CCH2CH3
An alkane is obtained by decarboxylation of sodium
propionate. Same alkane can be
obtained by –
a. Catlytic hydrogenation of propene.
b. Action of Na – metal on iodomethane.
c. Reduction of 1 – chloropropane.
d. Reduction of bromomethane.
8.
Wurtz reaction?
a.
2,3 – dimethyl – 2,3 – Dimethyl butane.
b.
2 – Ethyl – 2, 3, 3 Trimethyl pentane.
c.
3,3,4,4 – Tetramethyl hexane.
d.
2,3,3,3 – Tetra methyl hexane.
In the structure, the no. of primary carbon atoms are
a. 5
b. 14
c. 15
d. 20
11
Mrs. Rashmi R.Patwardhan
9.
M.No: 9860144335
Which of the following alkanes contain
(CH3)2)CH – group
a. 2,2 – Dimethyl pentane
b. 3,3 – Dimethyl pentane
c. 2,3 – Dimethyl pentane
d. 3,4 – Dimethyl pentane
10.
The formula of Isobutyl is
a.
b.
c.
d.
11.
12.
The order of reactivity of halogens is
a. Cl2 > Br2 > I2 > F2
b. Br2 > I2 > F2 > Cl2
c. I2 > F2 > Cl2 > Br2
d. F2 > Cl2 > Br2 > I2
Paraffin wax contains alkanes ranging from
a. C5 to C9
b. C10 to C15
c. C16 to C20
13.
d. C21 to C30
A student named an alkane as 2 – ethyl propane,
the correct IUPAC name of alkane is
a. 2 – Methyl butane
b. 2,2 – dimethyl butane
c. 2,3 – dimethyl butane
d. 2,3 dimethyl pentane
14.
Number of chain isomers of pentane are
a. 2
15.
c. 4
d. 5
Carbon atom in methane undergoes-hybridization
a. sp3
16.
b. 3
b. sp2
c. sp
In CH4 all C - H bond angles are
a. 1070 28’
b. 1090 28’
c. 1040 28’
d. 1200
12
d. dsp2
Mrs. Rashmi R.Patwardhan
17.
M.No: 9860144335
The molecular formula of an alkane containing 21
‘ C ’ atoms is -
18.
19.
a. C21H44
b. C21H42
c. C21H40
d. None of these
Molecular wt. of an alkane is 142. Its mol. formulais a. C9H20
b. C10H22
c. C11H24
d. C12H26
Identify ‘C’ in the following reactions
AlBr3
CaO
C2 H5 COONa ¾ NaOH,
¾¾
¾® 'A ' ¾ Br¾2 /D¾
¾® 'B' ¾ Na
¾ metal
¾¾
® 'C' + NaBr.
D
Dry ether
20.
a. Ethyl bromide
b. Ethane
c. n - butane
d. Propane
Complete oxidation of 1 mole of an alkane forms 1
a. C2H6
mole of carbon dioxide. The alkane is
b. C3H8
c. CH4
d. C4H10
---------------------------------------------------------------------------------------------------------------------------------Ans1: (d) C6H14,
General formula of aliphatic saturated hydrocarbon (alkane) is CnH2n + 2
Ans2: (d) Ethane → Chloroethane 1 H atom of ethane is replaced by Cl atom.
Ans3: (a) Methane
Ans4: (b) C10H22
C1 to C4 – gases, C5 to C17 – liquids,
C18 onwards – solids.
Ans5: (d) (CH3)3 CCH2CH3
St. formula of neohexane is
(d) is correct.
Ans6: (b) Action of Na – metal on iodomethane.
Ans7: (c)
3,3,4,4 – Tetramethyl hexane.
Ans8: (c) 15
In above structure first find out primary ‘C’ atoms and then count H atoms on them.
13
Mrs. Rashmi R.Patwardhan
Ans9: (c) 2,3 Dimethyl pentane
Ans10:
(a)
Ans11:
(d) F2 > Cl2 > Br2 > I2
Ans12:
(d) C21 to C30
Ans13:
(a) 2 - methyl butane
Ans14:
(b) 3
Ans15:
(a) SP3
Ans16:
(b) 1090 28’
Ans17:
(a) C21H44
Ans18:
(b) C10H22
M.No: 9860144335
Ans (c) is correct.
Mol. wt. is algebraic sum of at. wt. of all atoms in a
molecule. At. wt. of C = 12, H = 1.
General formula of alkane is CnH2n+ 2
Mol. wt. of alkane = n × 12 + 2n × 1 + 2 = 14n + 2
Given Mol. wt. is 142 ∴ 14 n + 2 = 142 ∵ n = 10
∴ Mol. formula = C10H22
Ans19.
(c) n - butane
Write all the reactions
1. decarboxylation
2. bromination
3. Wurtz reaction
Ans20:
(c) CH4
CH 4 (g) + 2 O 2 (g) ¾ D¾® CO 2 (g) + H 2 O (g) + heat
----------------------------------------------------------------------------------------------------------------------------------
14
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
Intext Question
Q.1
1.
Write all possible st. formulae of alkane containing 6 ‘C’ atoms.
2.
Classify them as straight chain and branched
3.
Indicate 1º, 2º, 3º, 4º, ‘C’ atoms.
chain alkanes.
Ans:
1.
CH3 – CH2 – CH2 – CH2 – CH2 – CH3
Straight chain alkane.
2.
3.
4.
5.
______________________________________________________________________________
Q.2. Write the chemical reactions that take place on heating mixture of methyl iodide and ethyl
iodide in presence of Na and dry ether solvent.
Ans: i.
ii.
iii.
dry
CH3 I  2Na  I C2H5 
 CH3  C2H5  2NaI
ether
Propane
Mixture of ethane, butane, Propane is obtained.
______________________________________________________________________________
15
Mrs. Rashmi R.Patwardhan
Q.3
M.No: 9860144335
How will you convert.
i.
Methane into ethane
ii.
Methyl bromide into ethyl bromide.
iii.
Acetylene into nitro ethane?
Ans:
i) Methane into ethane
dry ether
2CH3Cl  2Na 
 C2H6  2NaCl  wurtz reaction 
ethane
ii) Methyl bromide into ethyl bromide.
iii) Acetylene into nitro ethane?
Ni
CH  CH  2H2 
 CH3  CH3 Reduction 

Acetylene
C2H6 
ethane
ethane
HNO3
423 to 678 k

 C2H5NO2  H2O

conc. nitric acid
nitro ethane
------------------------------------------------------------------------------------------------------------------------------Q.4 Identify compound A, B and C in the following reaction and rewrite complete equations.
NaOH / CaO
Na
2
3  C 
A 
B 
n  Butane


dry ether
Br / AlBr
Ans:
O
||
CaO
CH3CH2  C  Na  NaOH 
 C2H6  Na2CO3

ethane
B 
Sodium propionate
A
3  C H Br  HBr
C2H6  Br2 
2 5

AlBr
C
dry
2C2H5Br  2Na 
 C4H10  2NaBr
ether
n-Butane
______________________________________________________________________________
16
Mrs. Rashmi R.Patwardhan
M.No: 9860144335
17