- Hydrocarbon - Those organic compounds which contain only carbon and hydrogen atoms are known as hydrocarbons.
- Alkanes are the saturated non-polar hydrocarbon having the general formula CnH2n+2.
General Method of Preparation
➢ By Catalytic Reduction of Alkenes and Alkynes
- Hydrogenation → Addition of H2 to unsaturated bond.
- Hydrogenation is of two kinds:
(a) Heterogeneous: It is two-phase hydrogenation. The catalyst is finely divided metal like Ni, Pt or Pd and a solution of alkene.
(b) Homogeneous: It is one phase hydrogenation both catalyst and alkenes are solution. In this hydrogenation catalyst are the organic complex of a transition metal like Rh or Ir.
- R - C º ≡ C - R' R - CH2 - CH2 - R'
- R - CH = CH - R' R - CH2 - CH2 - R'
- Hydrogenation is exothermic, qualitative and during the hydrogenation, total heat evolved to hydrogenate one mole of an unsaturated compound is called heat of hydrogenation. The heat of hydrogenation is the measurement of the stability of isomeric alkenes.
Stability of alkene
➢ From Alkyl Halide
1. From Organometallic Compound
(a) By Wurtz Reaction:
- 2R - X + 2Na R → R + 2NaX
- R - X + R' - X R - R, R- R', R' - R'
- Mechanism: Two mechanisms are suggested
(i) Ionic mechanism:
2Na 2Na+ + 2e-
(ii) Free radical mechanism:
- The alkyl halide should be 1º or 2º. With 3º R - X, SN2 and free radical coupling are not possible due to steric hindrance so in that case elimination or disproportion is possible.
- In the ionic mechanism, alkyl sodium gives strong base as well as a nucleophile which gives SN2 with R - X. Ether should be dry otherwise, if moisture is present then forms R - H instead of R - R with H2O.
(b) By Grignard Reaction:
(c) By Corey-House Alkane Synthesis:
R2CuLi is the source of
R2 CuLi do not reacts with -NO2, - CN, > C = O etc.
- Example:, if C is CH3 - CH2 - (CH2)5 - CH3, then what is Y.
Ans. CH3 - (CH2)6 - Br
(d) By Frankland's Reagent:
- R - X Zn R - X R - R + Zn X2
2. By Reduction of Alkyl Halides
(a) With Metal-Acid:
Reducing agent: Zn / acid, Zn - Cu / H2O or Zn - Cu acid, Zn - Cu / C2H5OH, Na - Hg / acid, Al - Hg / H2O etc.
(b) With Metal Hydrides:
- TPH (Ph3SnH) : It reduces 1º, 2º & 3º R - X
R - X R - H
➢ By Red P & HI
- Red P & HI is strong reducing agent.
- R - COOH R - CH3
- R - CH3
- R - CH3
- R - X R - H
- R - OH R - H + H2O
➢ By Soda-Lime
- Fatty acids are a good source of hydrocarbon, correction, heating of sodium salt of carboxylic acid (R - COONa) with soda lime (NaOH - CaO) gives hydrocarbon, which is known as decarboxylation (e.g. replacement of - COOH group by -H).
- Decarboxylation also takes place on heating only, when the compound is gem dicarboxylic acid or there is a keto group or double bond on ß carbon.
- Example:, What are A and B?
Ans. A is
Question. Try this:
Write the structure of A and mention its stereochemistry?
➢ By Kolbe's Electrolysis
- 2RCOOK + 2HOH R-R + 2CO2 + H2 + 2KOH
- Example: 2CH3 - COOK + 2H2O CH3-CH3 + 2CO2 + H2 + 2KOH.
If n is the number of carbon atoms in the salt of carboxylic acid, the alkane formed has 2(n - 1) carbon atoms.
➢ Reduction of Aldehydes and Ketones
1. By Clemenson's reduction (with Zn - Hg / conc. HCl):
- R - CHO RCH3 H2O
- RCH2R' H2O
CH3 - CHO CH3CH3 H2O
- Clemenson's reduction is not used for a compound that has an acid-sensitive group.
2. By Wolf-Kishner reduction (with NH2NH2 / KOH):
- Wolf-Kishner reduction is not used for compounds that have base sensitive groups.
Physical Properties of Alkanes
- Alkanes are colourless and odourless.
- They possess weak Vander Waal's force of attraction.
- Alkanes having 1-4 carbon atoms are gases, then from 5-17 carbon atoms are liquid and alkanes having 18 or more carbon atom are solid at 298K.
- Structure of alkanes: In alkanes, all the carbon atoms are sp3 hybridised which mean that they form four sigma bonds with either carbon or hydrogen atoms. Their general formula is CnH2n+2.Structure of Methane
- Melting and Boiling point: Shorter chain alkanes have low melting and boiling points but as the number of carbon atoms in the chain increase melting and boiling point rise.
(a) Boiling Point: It increases with the increasing molecular weight as the Vander Waal's force increases with the increasing molecular weight. Straight chain alkanes have a higher boiling point than their structural isomer.
(b) Melting Point: It also increases with increasing molecular weight because it is difficult to break the intermolecular forces of attraction between higher alkanes as they generally solids. Even-numbered alkanes have a better packing in the solid phase than the odd ones as they form a well-organised structure which is difficult to break hence even-numbered alkanes have a higher melting point than odd-numbered ones.
(a) Alkanes are generally nonpolar molecules because of the covalent bonds between C-C and C-H and also because of the very small difference between electronegativities of carbon and hydrogen.
(b) We know that polar molecules are soluble in polar solvents and nonpolar molecules are soluble in nonpolar solvent, generally, so this implies alkanes are insoluble in water or hydrophobic in nature.
(c) When a non-polar alkane is added to a polar solvent, the water molecule are attracted to each other and alkane molecules do not attract each other.
(d) In organic solvents, they are soluble because the energy required to overcome existing Vander Waal's forces and to generate new Vander Waal's forces is quite comparable.
- Alkanes have a lower density than water, they float on water. Density increases with an increase in molecular mass.
- Apart from weak Vander Waal's forces, London forces, Dispersion forces, weak intermolecular forces act between the molecules of alkanes.
Chemical Properties of Alkanes
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 the evolution of heat. Therefore, alkanes are used as fuels.
- CnH2n+2 + [(3n+1)/2] O2 → n CO2 + (n+1) H2O; ΔH = -ve
- Example: CH4 + 2 O2 → CO2 + 2 H2O; ΔH = -ve
2. Incomplete oxidation:
- Incomplete oxidation of alkanes in limited supply of air gives carbon black and carbon monoxide.
- 2 CH4 + 3 O2 → 2 CO + 4 H2O
- CH4 + O2 → C (carbon black) + 2 H2O
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 photoirradiation, 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 differences 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,
CH4 + I2 → CH3I + HI
5 HI + HIO3 → 3 I2 + H2O
Iodides are more conveniently prepared by treating the chloro- or bromo- derivative with sodium iodide in methanol or acetone solution.
i.e. 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)
Mechanism of methane chlorination:
(a) Initiation Step:
Cl : Cl —→ 2Cl· ; ΔH = +243 kJ mol-1
The required enthalpy comes from ultraviolet (UV) light or heat.
(b) Propagation Step:
(i) H3C : H + Cl· → H3C· + H : Cl ; ΔH = -4 kJ mol-1 (rate determining)
(ii) H3C· + Cl : Cl → H3C : Cl + Cl· ; ΔH = -96 kJ mol-1
(iii) The sum of the two propagation steps in the overall reaction,
CH4 + Cl2 → CH3Cl + HCl; ΔH= -100 kJ mol-1
(c) Terminating Step:
In propagation steps, the same free radical intermediates (here Cl· and H3C·) were 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.
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 products:
(i) Probability Factor: This factor is based on the number of each kind of H atom in the molecule.
Example: In CH3CH2CH2CH3, there are six equivalent 1° H’s and four equivalent 2° H’s. The ratio of abstracting a 1° H is 6 to 4, or 3 to 2.
(ii) Reactivity of H· : The order of reactivity of H is 3° > 2° > 1°.
(iii) 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 single tertiary hydrogen leads to the formation of tert-butyl chloride.
The probability of favorable 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 the abstraction of tertiary hydrogen than for the abstraction of primary hydrogen.
The rate of abstraction of hydrogen atoms is always found to follow the sequence 3º > 2º > 1º.
Example: At room temperature, the relative rate per hydrogen atom is 5.0 : 3.8 : 1.0. Using these values we can predict quite well the ratio of isomeric chlorination products from a given alkane.
In spite of these differences in reactivity, chlorination rarely yields a great excess of any single isomer.
The same sequence of reactivity, 3° > 2° > 1°, is found in bromination, but with enormously larger reactivity ratios.
Example: At 127 °C, 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 127 ºC,
Hence, tert-butyl bromide happens to be the exclusive product (over 99%).
- Replacement of H atom of alkane by -NO2 group is known as nitration. Nitration of an alkane is made by heating vapours of alkanes and HNO3 at about 400 ºC 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 the 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:
HONO2HO· + ·NO2
C3H7-H + HO· → C3H7· + H2O
C3H7· + ·NO2 → C3H7NO2
- The process of conversion of one isomer into other is known as isomerization. Straight chain alkanes on heating with AICI3 + HCI at about 200 ºC and 35 atm pressure are converted into branched-chain alkanes.
- The process of conversion of an aliphatic compound into an 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 a catalyst.
- Alkanes are dehydrogenated on heating in presence of a catalyst to produce corresponding alkenes.
- The decomposition of a compound on heating in absence of air is known as pyrolysis. The phenomenon of pyrolysis of an alkane is also known as cracking.
- Alkane vapours on passing through a red hot metal tube in absence of air decompose to simpler hydrocarbons.
- The product formed during cracking depends upon:
(a) Nature of alkane
(b) Temperature and Pressure
(c) Presence or absence of a catalyst
- The ease of cracking in alkanes increases with an increase in molecular weight and branching in an alkane.
- The fission of C-C bonds produces alkanes and alkenes whereas fission of C-H bonds produces alkene and hydrogen.
- The presence of Cr2O3, V2O2, MoO3 catalyses C-H bond fission and the presence of SiO2, AI2O3, ZnO catalyses C-C bond fission.
- The no. of products obtained during cracking increases with an increase in molecular weight of alkane undergoing cracking.
- Cracking has an important role in the petroleum industry. Higher alkanes are converted into lower ones (petrol C6 to C11) by cracking.