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
R COONa + NaO H RH + Na2CO3
Sod. Salt of acid Alkane
Eg: CH3 COONa + NaO H CH4 + Na2CO3
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
When the mixture of alkene or alkyne is passed over heated Nickel around temperature of 250- 300oC, alkanes are formed.
I) CnH2n + H2 CnHn+2
CH2=CH2 + H2 CH3.CH3
II) CnH2n-2 + 2H2 CnH2n+2
CHºCH + 2H2 CH3.CH3
However methane cannot be prepared by this method.
Reduction of alcohols may be carried out by heating them with hydroiodic acid and phosphorous at 150oC.
ROH + 2HI RH + H2O +I2
CH3CH2OH + 2HI CH3CH3 + H2O + I2
Ethyl Alcohol Ethane
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
CH3CHO + 4HI CH3CH3 + H2O +2I2
RCOR + 4HI RCH2R + H2O + 2I2
CH3CHOCH3 + 4HI CH3CH2CH3 + H20 +2I2
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
CH3COOH + 6HI CH3CH3 + 2H2O + 3I2
Acetic acid Ethane
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.
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:
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
CH3Cl + Cl2 CH2Cl2 + HCl
CH2Cl2 + Cl2 CHCl3 + HCl
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
The replacement of hydrogen of alkane by nitro group (NO2 Group) is known as Nitration . Nitration occurs in vapour Phase. Alkanes may be converted into nitrodervatives by heating the hydrocarbons with the vapours of the nitric acid at the temperature of about 450oC.
RH + HO-NO2 RNO2 + H2O
CH3CH3 + HO-NO2 CH3CH2NO2 + H2O
Ethane Nitro ethane
It is a process in which a hydrogen atom of an alkane is replaced by a sulphonic acid group, S03H. Alkanes from hexane onwards when treated with fuming sulphuric acid, undergo sulphonation yielding sulphonic acid. Thus:
C6H11-H + HO-SO3H C6H11SO3H + H2O
The lower gaseous hydrocarbons are more stable and dissolve in sulphuric acid and are not sulphonated.
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
Higher alkane on slow oxidation yield long chain of fatty acids.
CH3(CH2)9CH3 + O2 CH3(CH2)9.COOH
Undecane Undecanoic acid
III) When burnt in air or oxygen, alkanes are completely oxidised to carbon dioxide and water.
CH4 + 2O2 CO2 + 2H2O