Question 5 of 5: Constitutional Isomers of C 7 H 16 ; Classifying Three Alkene Reactions

Question 5: Constitutional Isomers of C₇H₁₆; Classifying Three Alkene Reactions (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Five constitutional isomers of C₇H₁₆ (heptane). C₇H₁₆ has nine constitutional isomers in total; any five distinct skeletons answer the question. A representative, unambiguous set:

Each retains the same molecular formula C₇H₁₆ (zero degrees of unsaturation, an acyclic saturated hydrocarbon) while differing in how the seven carbons are branched — a straight chain, a mono-branched chain (two ways, depending where the methyl sits), a di-branched chain, and a chain built around a fully substituted quaternary-adjacent carbon.

(b) Classifying the three reactions.

1. (i) Addition. H₂ adds across propene's C=C, converting one π bond into two new C–H σ bonds and increasing the degree of saturation (propene → propane): $\boxed{\text{addition}}$.
2. (ii) Elimination. Acid-catalysed dehydration removes H and OH from adjacent carbons of 1-propanol to form propene's C=C and release water — the reverse sense of (i), decreasing saturation: $\boxed{\text{elimination}}$.
3. (iii) Substitution. Hydroxide displaces chloride at the sp³ carbon of chloroethane (an S_N2-type nucleophilic substitution); no C=C is created or consumed, one leaving group is simply replaced by one nucleophile: $\boxed{\text{substitution}}$.