See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: This reaction involves acid-catalyzed alkylation (the Koch or more precisely the acid-catalyzed addition/alkylation reaction). Under HF (a strong acid catalyst) at 273K, an alkene undergoes protonation to form a carbocation, which then reacts with an alkane (isobutane) via hydride transfer and subsequent combination. Step 1: Identify the reactants. - Alkene: 2-methylpropene (isobutylene), CH3-C(CH3)=CH2 (2-methylpropene, C4H8) - Alkane: (CH3)2CHCH3 = isobutane (C4H10) - Together: C4H8 + C4H10 = C8H18, consistent with the product formula. Step 2: Mechanism of HF-catalyzed alkylation. - HF protonates 2-methylpropene (isobutylene) following Markovnikov's rule: H+ adds to CH2, giving the tertiary carbocation (CH3)3C+ (tert-butyl cation). - The tert-butyl carbocation abstracts a hydride (H-) from isobutane (CH3)2CHCH3 at the tertiary C-H bond, generating a new tert-butyl carbocation (CH3)3C+ and the product (CH3)4C... - Actually more precisely: (CH3)3C+ + (CH3)2CHCH3 --> (CH3)3CH + (CH3)3C+ - The newly formed tert-butyl cation then combines with isobutylene or the chain continues. - The net result of isobutane + isobutylene alkylation under acid is the classic refinery alkylation product: 2,2,4-trimethylpentane (isooctane). Step 3: Determine the structure of the product. - 2,2,4-trimethylpentane has the structure: (CH3)3C-CH2-CH(CH3)-CH3 - Molecular formula: C8H18. This is the well-known 'isooctane' produced industrially by alkylation of isobutane with isobutylene using HF or H2SO4 catalyst. Step 4: Why other options fail. - (a) 2,2,3-trimethylpentane: This would require a different carbocation rearrangement pathway; not the primary product. - (c) 2,2-dimethylhexane: This is C8H18 but would require a different carbon skeleton not consistent with C4+C4 alkylation of isobutane and isobutylene. - (d) n-octane: This is a straight-chain C8H18; not formed under these conditions with branched reactants. The major product of HF-catalyzed alkylation of isobutane with 2-methylpropene is 2,2,4-trimethylpentane. Therefore, the correct answer is B.