Alkanes
are the saturated hydrocarbons containing carbon and hydrogen only. The
molecules of the hydrocarbons are characterized by the presence of single
covalent linkages between carbon and carbon, and carbon and hydrogen. All the four
valencies of carbon atoms are thus completely satisfied and they can take up no
more hydrogen or any other atoms. Thus being fully saturated in respect of
chemical combination, these are called 'paraffins' (Latin: parum àNO, affinsà
affinity).
Alkanes
are very stable compounds and do not react with the reagents like potassium
permanganate, alkalis and even with the stronger acids under the ordinary
conditions because bond fission between C and H is low as:
1. C - C
and C-H are strong 'PI' bond.
2. Low electronegativity difference
between C and H
General methods of
preparation
1.
Decarboxylation Of Fatty Acids
Cao
When sodium salts of fatty acids (RCOONa) are heated with
soda-lime (NaOH+CaO), a molecule of
carbon dioxide is lost and an alkane is obtained. This is called
DECARBOXYLATION. The alkane obtained contains one carbon atom less than the
fatty acids.
R
COONa + NaO H
RH + Na2CO3
Cao
Eg: CH3 COONa + NaO H
CH4 + Na2CO3
Sod.acetate Methane
2.
Reduction Of Alkyl Halides With Nascent
Hydrogen
Alkyl Halides
on treatment with Zn/HCl or Zn/NaOH or Zn-Cu couple on alcohol or LiAlH4
in ether , are reduced to alkanes.
RX + 2H RH + HX
Alkyl Halide
Alkane
Eg: CH3I
+ 2H CH4 + HI
Methyl Iodide
Methane
3.
Hydrogenation Of Unsaturated
HydroCarbons
When the mixture of alkene or alkyne is passed over
I) CnH2n + H2 CnHn+2
Alkene Alkane
CH2=CH2 +
H2 CH3.CH3
Ethene Ethane
II) CnH2n-2
+ 2H2 CnH2n+2
Alkyne Alkane
Ethyne Ethane
However methane cannot be prepared by this method.
4.
By the Reduction of Alcohols
Reduction of alcohols may be carried out by heating them
with hydroiodic acid and phosphorous at 150oC.
ROH + 2HI
RH + H2O +I2
Alcohol Alkane
CH3CH2OH
+ 2HI CH3CH3 + H2O
+ I2
Ethyl Alcohol Ethane
5.
By the reduction of Aldehydes and
Ketones
Like alcohols , their reduction may also be carried
heating with conc. Hydroiodic acid at 150oC , under pressure in the
presence of Red Phosphorus.
RCHO + 4HI RCH3
+ H2O + 2I2
Aldehyde Alkane
CH3CHO
+ 4HI CH3CH3 + H2O
+2I2
Acetaldehyde Ethane
RCOR
+ 4HI RCH2R
+ H2O + 2I2
Ketone
Alkane
CH3CHOCH3
+ 4HI CH3CH2CH3
+ H20 +2I2
Acetone Propane
Fatty acids when reacted with HI in the presence of red phosphorus at 150oC
in a sealed tube yield alkanes.
RCOOH +
6HI RCH3 + 2H2O + 3I2
Acid Alkane
CH3COOH
+ 6HI CH3CH3
+ 2H2O + 3I2
Acetic acid Ethane
7. From
Grignard Reagents
Grignard's reagents is Alkyl Magnesium halide which is
obtained by treating alkyl halide with Magnesium metal in the presence of
carbon tetra chloride as solvent.
When Grignard's reagent is boiled with water it undergoes
hydrolysis to give alkane.
R-MgX + H-OH RH+ Mg(OH)X
boil
CH3MgBr + HOH CH4 + Mg(OH)Br
1. The
first four hydrocarbons methane ,ethane , propane and butane are colourless and
odourless gases. The next eleven members of the series (C5H12
TO C15H32) are volatile liquids and the rest of them are
solids.
2. Their
boiling points and specific gravities rise with increasing molecular masses.
Branched chain hydrocarbons have lower boiling points than corresponding
straight chain ones.
3. Their
melting points go on incresing with increase in their molecular masses
4. They are
insoluble in water but soluble in organic solvents like alcohol , ether ,
acetone etc.
Undoubtedly
alkanes are relatively inert chemically . They do not react with strong acids ,
alkalis and oxidising agents under the ordinary conditions. But modern
researches have shown that they gives many reactions under the drastic
conditions.The general reactions of the alkanes are given below:
a. Halogenation
The replacement of one or more
hydrogen atoms by halogens is called the halogenation. These reactions occur
slowly in darkness but more rapidly in sunlight.
Chlorine and bromine react with
alkanes with considerable ease in the presence of sunlight or by the use of a
catalyst like iron fillings.
When the hydrogen of latter are replaced in turn to form
four different halogen derivatives.
CH4 +
Cl2
CH3Cl +
HCl
Methyl
chloride
CH3Cl +
Cl2
CH2Cl2
+ HCl
Methylene
Chloride
CH2Cl2
+ Cl2
CHCl3 + HCl
Chloroform
CHCl3 + Cl2
CCl4 + HCl
Carbon
Tetra chloride
Iodine does not react with alkanes as the HI produced
during iodination reduces the alkyl iodide back to the hydrocarbon.
RH +
I2 RI
+ HI
To make the
reaction move forward , it is carried out in the presence of an oxidising
agents like conc. Nitric acid or
iodic acid which oxidises the hydroiodic acid to iodine.
5HI +
HIO3
3I2 + 3H2O
b. Nitration
The replacement of hydrogen of alkane by nitro group
RH + HO-NO2 RNO2 +
H2O
Nitro
alkane
CH3CH3
+ HO-NO2 CH3CH2NO2 +
H2O
Ethane Nitro
ethane
c. Sulphonation
It is a process in which a hydrogen atom of an alkane is
C6H11-H + HO-SO3H
C6H11SO3H +
H2O
The lower gaseous hydrocarbons are more stable and
dissolve in sulphuric acid and are not sulphonated.
d. Oxidation
I) Normal
alkanes are not oxidised by potassium permanganate but it readily oxidises a tertiary hydrogen atoms to a
hydroxyl group. Thus isobutane is oxidised to a tertiary butanol.
(CH3)3CH
+[O] (CH3)3C.OH
Isobutane
Tertiary Butanol
II) Lower hydrocarbons
when heated with limited supply of air at 350 –500oC, yield the
corresponding aldehydes .thus
CH4
+ O2 HCHO +
H2O
Formaldehyde
Higher alkane on slow oxidation yield long chain of fatty acids.
CH3(CH2)9CH3 +
O2 CH3(CH2)9.COOH
Undecane
Undecanoic
III) When burnt in air or oxygen, alkanes are
completely oxidised to carbon dioxide and water.
CH4 +
2O2 CO2 +
2H2O