See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Isobutene (2-methylpropene, CH2=C(CH3)2) undergoes acid-catalyzed dimerization in the presence of H2SO4. The mechanism involves protonation of isobutene to form a carbocation, followed by addition to another molecule of isobutene, and then elimination. Step 2 - Mechanism: H2SO4 protonates isobutene to give the tert-butyl carbocation: (CH3)3C+. This tertiary carbocation adds to the double bond of a second molecule of isobutene (CH2=C(CH3)2) at the terminal carbon (Markovnikov addition), giving: (CH3)3C-CH2-C+(CH3)2, which is a tertiary carbocation at C2 of the adduct. Step 3 - Elimination: This C8 tertiary carbocation undergoes elimination (loss of a proton). The proton can be lost from the CH2 group adjacent to the positive center, giving: (CH3)3C-CH2-C(CH3)=CH2 (option b), or from the methyl group on the cationic carbon, giving: (CH3)3C-CH=C(CH3)-CH3... wait, rearrangement-free elimination from (CH3)3C-CH2-C+(CH3)2: loss of H from the CH2 gives (CH3)3C-CH=C(CH3)2... but that is not shown. Loss of H from one of the terminal CH3 groups of C+(CH3)2 gives (CH3)3C-CH2-C(CH3)=CH2, which is option (b). Step 4 - Zaitsev vs terminal alkene: Option (a) would require a hydride or methyl shift. The direct elimination product from the tertiary carbocation (CH3)3C-CH2-C+(CH3)2 without rearrangement by loss of a proton from the terminal methyl gives the less substituted terminal alkene (b): 2,4,4-trimethylpent-1-ene. Loss of proton from the CH2 group gives 2,4,4-trimethylpent-2-ene. However, in industrial and acid-catalyzed isobutene dimerization, the terminal alkene (b) is indeed formed as a significant product. The major product observed experimentally from H2SO4-catalyzed dimerization of isobutene is 2,4,4-trimethylpent-1-ene (option b) along with 2,4,4-trimethylpent-2-ene, with the 1-ene (option b) being the one identified as the answer here. Step 5 - Why other options fail: Option (a) is 2,4,4-trimethylpent-2-ene and could also form but is not the answer given. Options (c) and (d) involve different carbon skeletons not consistent with direct tert-butyl cation addition to isobutene without rearrangement to a different skeleton; they represent different connectivity and are minor or unlikely products. Therefore, the correct answer is B.