RI 2024 Tut 14 RX Self Check Ans

-1- 2024 Tut 14 RX Self-Check Questions – Suggested Answers 1a Br Br BrBr A B C D 1b A B C D 2-bromobutane 1-bromobutane 2-bromo-2-methylpropane 1-bromo-2-methylpropane 2o bromoalkane 1o bromoalkane 3o bromoalkane 1o bromoalkane 1c B > D > A > C 2 Halogenoalkane SN1 or SN2 Explanation CH3CH2Cl SN2 CH3CH2Cl is a primary halogenoalkane. There is little steric hindrance around the electron- deficient carbon atom for backside attack by the nucleophile. Hence S N2 mechanism is favoured. (CH3)3CCl SN1 (CH3)3CCl is a tertiary halogenoalkane and hence able to form a tertiary carbocation which is stabilised by electronic effect. Hence SN1 mechanism is favoured. (+)−CH3CHBrCH2CH3 (Product does not rotate plane polarised light) SN1 Reactant is optically active but product solution is a racemic mixture which is optically inactive. Hence the reaction has occurred with racemisation. Thus, the mechanism is SN1. The reaction involves the formation of a carbocation with a trigonal planar geometry around the positively charged carbon. The OH - nucleophile attacks the positively charged carbon from both sides of the trigonal plane with equal likelihood, giving rise to equal amounts of a pair of enantiomers as products. CH3CHClCH2CH3 (Reaction is overall 2nd order) SN2 Overall second order ⇒ the reaction proceeds via SN2 mechanism as one CH3CHClCH2CH3 molecule and one OH- ion are involved in the one-step mechanism. C6H5CHClCH3 (Rate of reaction is independent of [NaOH]) SN1 Rate is independent of [NaOH] ⇒ NaOH is not involved in rate determining step, hence the mechanism is SN1. Note: The C6H5CH(CH3)+ carbocation is resonance-stabilised by dispersal of positive charge due to delocalisation of π−electrons to the positively charged carbon. Answers to Tutorial 14 MCQ: