Alkanes are quite inert substances with a highly stable nature. Their inactiveness has been explained as:
In alkanes, all the C-C & C-H bonds being stronger sigma bonds and are not influenced by acids, alkalies, oxidants under ordinary conditions.
The C-C (completely non-polar) & C-H (weak polar) bonds in alkanes- are practically non-polar because of the small electronegativity difference in C (2.6) and H (2.1). Thus polar species i.e., electrophiles or nucleophiles are unable to attack these bonds under ordinary conditions.
In spite of a less reactive nature, alkanes show some characteristic reactions.
Oxidation Reactions of Alkane:
Oxidation of alkanes gives different products under different conditions.
1. Complete oxidation or combustion : Alkanes burn readily with non luminous flame in presence of air or oxygen to give CO2 & water along with evolution of heat. Therefore, alkanes are used as fuels.
CnH2n+2 + [(3n+1)/2]O2 → nCO2 + (n+1)H2O; ΔH = -ve
CH4 + 2O2 → CO2 + 2H2O; ΔH = -ve
2. Incomplete oxidation : Incomplete oxidation of alkanes in limited supply of air gives carbon black and carbon monoxide.
2CH4 + 3O2 → 2CO + 4H2O
CH4 + O2 → C + 2H2O
3. Catalytic oxidation :
Lower alkanes are easily converted to alcohols and aldehydes under controlled catalytic oxidation.
Higher alkanes on oxidation in presence of manganese acetate give fatty acids.
4. Chemical oxidation : Tertiary alkanes are oxidized to tertiary alcohols by KMnO4.
Substitution Reactions of Alkanes
Substitution in alkanes shows free radical mechanism. For mechanism see free radical substitution.Following substitution reactions in alkanes are noticed.
1. Halogenation of Alkanes :
Chlorination may be brought about by photo irradiation, heat or catalysts, and the extent of chlorination depends largely on the amount of chlorine used. A mixture of all possible isomeric monochlorides is obtained, but the isomers are formed in unequal amounts, due to difference in reactivity of primary, secondary and tertiary hydrogen atoms.
The order of ease of substitution is
Tertiary Hydrogen > Secondary Hydrogen > Primary Hydrogen
Chlorination of isobutane at 300 oC gives a mixture of two isomeric monochlorides.
The tertiary hydrogen is replaced by about 4.5 times as fast as primary hydrogen. Bromination is similar to chlorination, but not so vigorous. Iodination is reversible, but it may be carried out in the presence of an oxidising agent such as HIO3, HNO3 etc., which destroys the hydrogen iodide as it is formed and so drives the reaction to the right, e.g.
CH4 + I2 → CH3I + HI
5HI + HIO3 → 3I2 + H2O
Iodides are more conveniently prepared by treating the chloro or bromo derivative with sodium iodide in methanol or acetone solution. e.g:
RCl + NaI—→ RI + NaCl (in presence of acetone).
This reaction is possible because sodium iodide is soluble in methanol or acetone, whereas sodium chloride and sodium bromide are not. This reaction is known as Conant Finkelstein reaction.
Direct fluorination is usually explosive; special conditions are necessary for the preparation of the fluorine derivatives of the alkanes.
RH + X2 —→ RX + HX
(Reactivity of X2: F2 > Cl2 > Br2; I2 does not react)
The mechanism of methane chlorination is:
Cl : Cl —→ 2Cl· DH = + 243 KJ mol-1
The required enthalpy comes from ultraviolet (UV) light or heat.
i). H3C : H + Cl· → H3C· + H : Cl ΔH = - 4KJ mol-1 (rate determining)
ii). H3C· + Cl : Cl → H3C : Cl + Cl· ΔH = - 96 KJ mol-1
The sum of the two propagation steps in the overall reaction,
CH4 + Cl2 → CH3Cl + HCl ΔH= - 100 KJ mol-1
In propagation steps, the same free radical intermediates, here Cl· and H3C·, being formed and consumed. Chains terminate on those rare occasions when two free-radical intermediates form a covalent bond.
Cl· + Cl· → Cl2 ; H3C· + Cl· → CH3 : Cl
H3C· + ·CH3 → H3C : CH3
Inhibitors stop chain propagation by reacting with free radical intermediates, e.g.
In more complex alkanes, the abstraction of each different kind of H atom gives a different isomeric product. Three factors determine the relative yields of isomeric product.
Probability Factor: This factor is based on the number of each kind of H atom in the molecule. For example, in CH3CH2CH2CH3 there are six equivalent 1o H’s and four equivalent 2o H’s. The ratio of abstracting a 1oH are thus 6 to 4, or 3 to 2.
Reactivity of H· : The order of reactivity of H is 3o > 2o > 1o.
Reactivity of X· : The more reactive Cl· is less selective and more influenced by the probability factor. The less reactive Br· is more selective and less influenced by the probability factor, as summarized by the Reactivity-Selectivity Principle. If the attacking species is more reactive, it will be less selective, and the yields will be closer to those expected from the probability factor.
In the chlorination of isobutane abstraction of one of the nine primary hydrogens leads to the formation of isobutyl chlorides, whereas abstraction of a single tertiary hydrogen leads to the formation of tert-butyl chloride. The probability of favourable formation of isobutyl chloride is of the ratio 9:1. But the experimental results show the ratio roughly to be 2:1 or 9:4.5. Evidently, about 4.5 times as many collisions with the tertiary hydrogen are successful as collisions with the primary hydrogen. The Eact is less for abstraction of a tertiary hydrogen than for abstraction of a primary hydrogen.
The rate of abstraction of hydrogen atoms is always found to follow the sequence 3o > 2o > 1o. At room temperature, for example, the relative rate per hydrogen atom are 5.0:3.8:1.0. Using these values we can predict quite well the ratio of isomeric chlorination products from a given alkane. For example:
Inspite of these differences in reactivity, chlorination rarely yields a great excess of any single isomer.
The same sequence of reactivity, 3o > 2o > 1o, is found in bromination, but with enormously larger reactivity ratios. At 127oC, for example, the relative rates per hydrogen atom are 1600:82:1. Here, differences in reactivity are so marked as vastly to outweigh probability factors. Hence bromination gives selective product.
In bromination of isobutane at 127oC,
Hence, tert-butyl bromide happens to be the exclusive product (over 99%).
2. Nitration :
Replacement of H atom of alkane by -NO2 group is known as nitration. Nitration of alkane is made by heating vapours of alkanes and HNO3 at about 400oC to give nitroalkanes. This is also known as vapour phase nitration.
CH4(g) + HNO3(g) CH3NO2 + H2O
During nitration, C-C bonds of alkanes are also decomposed due to strong oxidant nature of HNO3 to produce all possible nitroalkanes.
The nitration of alkane also shows the order: T.H. > S.H. > P.H. > methane
The nitration of alkanes follows free-radical mechanism
HONO2 HO + NO2
C3H7-H + HO → C3H7 + H2O
C3H7 + NO2 → C3H7NO2
3. Sulphonation :
Replacement of H atom of alkane by -SO3H is known as sulphonation. Lower normal alkanes are not suphonated, but higher normal alkanes show sulphonation (hexane onwards) when heated with oleum (i.e., conc. H2SO4) at 400oC.
C6H14 + H2SO4 → C6H13SO3H + H2O
Lower members are sulphonated in vapour phase sulphonation. The reactivity order for sulphonation is T.H. > S.H. > P.H. Thus isobutene is easily sulphonated as it contains tertiary hydrogen atom.
Sulphonation of alkanes also follows free radical mechanism.
HOSO3H HO + SO3H
C3H13-H + OH → C6H13 + H2O
C3H13 + SO2H → C6H13SO3H
4. Isomerization :
The process of conversion of one isomer into other is known isomerization. Straight chain alkanes on heating with AICI3 + HCI at about 200oC and 35 atm pressure are converted into branched chain alkanes.
5. Aromatization :
The process of conversion of aliphatic compound into aromatic compound is known as aromatization. Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst.
6. Dehydrogenation : Alkanes are dehydrogenated on heating in presence of catalyst to produce corresponding alkenes.
7. Pyrolysis :
The decomposition of a compound on heating in absence of air is known as pyrolysis. The phenomenon of pyrolysis of alkane is also known as cracking. Alkane vapours on passing through red hot metal tube in absence of air decomposes to simpler hydrocarbons. The product formed during cracking depends upon:
(a) nature of alkane
(b) temperature and pressure
(c) presence or absence of catalyst
The ease of cracking in alkanes increases with increase in molecular weight and branching in alkane. Fission of C-C bonds produces alkanes and alkenes whereas fission of C-H bonds produces alkene and hydrogen.
Presence of Cr2O3, V2O2, MoO3 catalyses C-H bond fission and presence of SiO2, AI2O3, ZnO catalyses C-C bond fission.The no. of products obtained during cracking increases with increase in molecular weight of alkane undergoing cracking.
Cracking has an important role in petroleum industry. Higher alkanes are converted into lower one (petrol C6 to C11) by cracking.