Question

The molecules of the alkene family are represented by a general formula C_nH_2n. Answer the following:
(i) What do n and 2n signify?
(ii) What is the name of alkene when n = 4?
(iii) What is the molecular formula of the alkene, if there are six hydrogen atoms in it?
(iv) Write the molecular and structural formula of the first member of alkene family.
(v) Write the molecular formulae of lower and higher homologues of an alkene which contains four carbon atoms.

Answer 1

(i) The molecules of the alkene family are represented by a general formula C_nH_2n where n and 2n signify the number of carbon and hydrogen atoms, respectively.

(ii) When n = 4, the alkene is butene (C₄H₂₍₄₎ = C₄H₈).

(iii) In an alkene, the number of carbon atoms is always half the number of hydrogen atoms present. So, if there are 6 hydrogen atoms in the alkene then there must be 3 carbon atoms present. Hence, the molecular formula of alkene will be C₃H₆.

(iv)The first member of the alkene family is ethene.
Molecular formula is: C_​2H₄
The structural formula is:


(v)The alkene which contains four carbon atoms is butene (C₄H₈). There is always a difference of CH₂ between two successive members of a homologous series. Hence, the lower and higher structural homologues of butene (C₄H₈) are propene (C_​3H₆) and pentene (C₅H₁₀), respectively.