JEE Chemistry Hydrocarbons Notes Set 04

Alkanes at a Glance

Alkanes are saturated hydrocarbons with only C-C and C-H sigma bonds and general formula \( \ce{CnH2n+2} \). Every carbon is sp3 hybridised with bond angle about 109.5 degrees. They are called paraffins (little affinity) because they resist acids, bases and common oxidising and reducing agents.

Methods of Preparation

Methane is also obtained from aluminium carbide, \( \ce{Al4C3 + 12 H2O -> 4 Al(OH)3 + 3 CH4} \), and by the Sabatier reaction of \( \ce{CO} \) with \( \ce{H2} \) over nickel. Industrially it comes from natural gas and from bacterial decay of cellulose (marsh gas).

Physical Properties

1. The first four members are gases, \( \ce{C5} \) to \( \ce{C17} \) are liquids and higher members are waxy solids.
2. They are insoluble in water but dissolve in non-polar solvents, and are lighter than water.
3. Boiling points rise by about 20 to 30 degrees C per added carbon because van der Waals forces grow with surface area.
4. Branching lowers the boiling point because the molecule is more compact: n-pentane (36 degrees C), isopentane (28), neopentane (9.5).
5. Melting points do not follow the same smooth trend: symmetrical molecules pack better in the crystal. Neopentane melts much higher than n-pentane.

Halogenation: a Free Radical Chain

1. Initiation: light or heat splits the halogen, \( \ce{Cl2 -> 2 Cl^.} \)
2. Propagation: \( \ce{Cl^. + CH4 -> HCl + CH3^.} \) and \( \ce{CH3^. + Cl2 -> CH3Cl + Cl^.} \)
3. Termination: radicals combine, for example \( \ce{CH3^. + CH3^. -> C2H6} \), which is why a little ethane appears in the product.
4. Further substitution gives \( \ce{CH2Cl2} \), \( \ce{CHCl3} \) and \( \ce{CCl4} \); a large excess of the alkane favours the mono derivative.

Reactivity of the halogens is F2 > Cl2 > Br2 > I2. Fluorination is explosive, and iodination is reversible because HI reduces the iodoalkane, so it works only if an oxidising agent such as iodic acid removes HI. The ease of replacement of hydrogen follows the stability of the radical formed: tertiary > secondary > primary > methyl.

Counting products: the number of monochloro isomers equals the number of different kinds of hydrogen. Neopentane gives one, propane two, n-butane two, isobutane two, 2,3-dimethylbutane two and n-hexane three.

Other Reactions

Methane and Ethane: Quick Facts

Going Up and Down the Series

1. Ascent: \( \ce{CH3I + 2 Na + ICH3 -> CH3CH3 + 2 NaI} \); ethane gives ethyl chloride in light, and Wurtz coupling of ethyl chloride gives butane.
2. Descent: decarboxylation removes one carbon. Ethane is converted to ethanol, then to ethanal and acetic acid, and soda lime on sodium acetate gives methane.
3. Alkane to alkene: halogenate in light, then heat with alcoholic KOH.