Question

You are given the following molecular formulae of some hydrocarbons:
C₅H₈; C₇H₁₄; C₆H₆; C₅H₁₀; C₇H₁₂; C₆H₁₂
(a) Which formula represent cyclohexane as well as hexene?
(b) Which formula represents benzene?
(c) Which three formulae represent open chain unsaturated hydrocarbons having double bonds?
(d) Which two formulae represent unsaturated hydrocarbons having triple bonds?
(e) Which three formulae can represent cyclic hydrocarbons?

Answer 1

(a) C₆H₁₂ is the molecular formula which represents cyclohexane as well as hexene. The molecular formula for both hexene and cyclohexane is the same in which there are 6 carbon atoms. However, their structure differs, as in a cyclic compound, the carbon atoms form closed chains while hexene has an open chain structure.

(b) C₆H₆ represents is the molecular formula of benzene. Benzene is an unsaturated cyclic hydrocarbon made up of 6 carbon atoms and 6 hydrogen atoms.

(c) C₇H_14, C₅H_{10 ,}C₆H₁₂ are the molecular formulae for the open chain unsaturated hydrocarbons having double bonds, as they correspond to the general formula of alkenes, i.e. C_nH_2n,​ where n is the number of carbon atoms in its one molecule.

(d) C₅H₈ and C₇H₁₂ are the molecular formulae for the unsaturated hydrocarbons comprising triple bonds, as they correspond to the general formula of alkynes, i.e. C_nH_2n-2, where n is the number of carbon atoms in its one molecule.

(e) C₇H₁₄, C₅H_10, C₆H₁₂ are the three molecular formulae which represent cyclic hydrocarbons, as they correspond to the general formula C_nH_2n, where n is the number of carbon atoms in its one molecule. The closed chains of cyclic hydrocarbons contain as many carbon atoms as denoted by their molecular formulae. Therefore, a cyclic hydrocarbon containing 7 carbon atoms will be a closed cycloheptane, a cyclic hydrocarbon with 5 carbon atoms will form a pentagonal ring called cyclopentane and a cyclic hydrocarbon with 6 carbon atoms will form a cyclohexane.