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A2 Optical Isomerism Question.

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    • Thread Starter

    Saw this in a multiple choice and didn't really get it.

    "One optically active isomer of 2-chlorobutane reacts with hydroxide ions to form butan-2-ol. The organic product is mixture of enantiomers because:
    A) Butan-2-ol contains a chiral carbon atom.
    B) The reaction is a nucleophilic substitution.
    C) 2-chlorobutane forms a carbocation intermediate.
    D) 2-chlorobutane forms a five-bonded transition state."

    Now I don't get it because obviously it's a primary halogenoalkane and those undergo SN2 reactions which only give one optical isomer of the alchohol. So how come there's a mixture here and how come the answer's C?

    Is the went under SN2 it would only produce one of the optical isomers. But this case it has produced a racemic mixture so it has undergone SN1 reaction producing a carbocation intermediate which produces the pair of optical isomers.

    "This is because the carbocation formed in the rate determining step (b) (i), has a trigonal planar arrangement of C-R bonds around the C+ carbon, so the hydroxide ion (or water) can attack each side of the carbocation with equal probability, giving equal amounts of each possible optical isomer. The two optically activities cancel each other out, so zero rotation of plane polarised light."

    Website: http://www.docbrown.info/page07/isomerism2a.htm (case study 2b.5)
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