Lesson 1 of 13 · 7 min

Classification and the alkane family

NCERT §9.1, §9.2

The cylinder in your kitchen holds liquefied petroleum gas. Open the knob and what hisses out is a mixture of molecules made of just two elements, carbon and hydrogen.

The story this chapter follows: The gas cylinder in the kitchen

An LPG cylinder holds hydrocarbons, largely propane and butane, kept liquid under pressure. This chapter follows butane from its skeleton and boiling point to the flame on the stove, then moves to the double and triple bonds of alkenes and alkynes and the special ring of benzene.

The lesson in notes

In short

A hydrocarbon contains only two elements, carbon and hydrogen. LPG, CNG and LNG, and petrol, diesel and kerosene (from fractional distillation of petroleum), are all mixtures of hydrocarbons used as fuels.

Hydrocarbons also feed industry: polythene, polypropene and polystyrene are made from them, higher members serve as paint solvents, and many dyes and drugs start from them.

By the kind of carbon–carbon bond present they fall into three groups: saturated (only single bonds; open-chain alkanes and ring-shaped cycloalkanes), unsaturated (at least one double or triple bond) and aromatic (a special class of ring compounds).

Alkanes are saturated open-chain hydrocarbons with general formula CₙH₂ₙ₊₂. Methane, CH₄, heads the series; it occurs in coal mines and marshy places.

Replacing one H of methane by –CH₃ gives ethane, C₂H₆; repeating the step gives C₃H₈, C₄H₁₀ and so on, each member differing by CH₂.

The old name paraffins (Latin parum, little; affinis, affinity) reflects how unreactive alkanes are towards acids, bases and most reagents under ordinary conditions.

Methane is tetrahedral by VSEPR theory, with every H–C–H angle 109.5°. In alkanes the C–C bond is 154 pm and the C–H bond 112 pm; both are σ bonds from sp³ orbitals of carbon (with 1s of H for C–H).