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Alkanes

The form a . Like all homologous series, the alkanes:

  • have the same
  • differ by CH2 in from neighbouring
  • show a gradual variation in for example which increase as the alkanes get longer
  • have similar (flammability decreases as the alkanes get longer)

General formula

The general formula for the alkanes is CnH2n+2 (where n is the number of carbon atoms in the molecule).

Worked example

Decane is an alkane. Its contain 10 carbon . Predict the molecular formula of decane and explain your answer.

The formula will be C10H22.

This is because n = 10.

So, 2n + 2 = (2 脳 10) + 2 = 20 + 2 = 22.

Question

Hexane is an alkane. Its molecules contain six carbon atoms. Predict the molecular formula of hexane.

Structures

The table shows four alkanes, their molecular formulae, displayed formulae and structural formulae as well as a drawing of their ball-and-stick constructional models.

Table of four alkanes with their molecular, structural and displayed formula, and their ball and stick model.

Unlike the displayed formulae, the ball-and-stick models show that the are not really at angles of 90掳.

Question

The alkane pentane has the molecular formula C5H12.

Write down the displayed and structural formula of pentane.

The alkanes are saturated hydrocarbons:

  • , because they are compounds containing hydrogen and carbon only
  • , because their carbon atoms are joined by C-C single bonds