Question 4.24: (a) Three isomeric alkenes (A), (B) and (C), of molecular fo......
(a) Three isomeric alkenes (A), (B) and (C), of molecular for mula C_5H_{10} are hydrogenated to yield 2-methyl butane.
(A) and (B) give the same tertiary alcohol on oxymercuration demercuration. (B) and (C) give different primary alcohols on hydroboration-oxidation. Find the structures of (A), (B) and (C).
(b) Deduce the structure of compound (A), C_9H_8, from the following experimental data:
(A) decolorises Br_2 in CC\text{l}_4 and adds one equivalent of H_2 under mild conditions forming (B), (C_9H_{10}). Vigorous oxidation of (A) yields phthalic acid.
(c) Compare and account for the products formed by dehy dration of the following compounds.
Given data: (a) Three isomeric alkenes (A), (B) and (C), of molecular formula C5H10 are hydrogenated to yield 2-methyl butane.
(A) and (B) give the same tertiary alcohol on oxymercuration demercuration.
(B) and (C) give different primary alcohols on hydroboration-oxidation.
(b) Experimental data for compound (A), C9H8:
(A) decolorizes Br2 in CCl4.
(A) adds one equivalent of H2 under mild conditions, forming (B) (C9H10).
Vigorous oxidation of (A) yields phthalic acid.
(c) Dehydration products of certain compounds.
Final Answer
(a) The isomers must have the same skeleton as 2-methyl-butane but will vary in the position of the double bond. Since (A) and (B) give the same tertiary alocohol, they have the =C(CH_3)_2 \ or \ H_2C=CCH_3 grouping. Since (B) and (C) give primary alcohols, they are terminal alkenes having the following structure.
\begin{matrix} CH_3-C=C-CH_3 \\ \mid \quad \quad \mid \\ {Me} \ \quad H \end{matrix} \\ \quad \quad\quad\quad\quad\quad\quad(A) \\ \begin{matrix} H_2C=C-CH_2CH_3\\\mid\\CH_3 \end{matrix} \\ \quad\quad(B) \\ \begin{matrix} H _2 C = C – CH \left( CH _3\right)_2\\ \mid \\ CH_3 \end{matrix} \\ \quad \quad\quad\quad\quad\quad\quad\quad(C) \\
(b) Compound (A) has a benzene ring, because it gives phthalic acid on vigorous oxidation. Reaction with Br_2 and one equivalent of H_2 indicate that there is a C = C. Oxidation to the phthalic acid (ortho-dicarboxylic acid) shows that a ring is fused to the benzene ring. Therefore, the structure of (A) is
(c) Dehydration of (I) yields 1-methyl cyclohexene. The primary carbocation formed undergoes a hydride shift to convert to a more stable tertiary carbocation that undergoes elimination to yield alkene. The Zaitsev (Saytzeff) product dominates.
Dehydration of (II) yields cyclopentene. The primary carbocation formed does not undergoes a hydride shift to form tertiary carbocation. Instead the side of the ring CH_2 shifts to give a secondary carbocation and the new five membered intermediate is more stable than the four membered intermediate. This undergoes elimination to form alkene.
