The two syntheses with different approaches are to be devised for m e s o − 2 , 3 − butanediol starting with acetylene (ethyne) and methane. Concept introduction: Terminal alkynes are deprotonated with sodium amide to produce alkynides. Alkynides are nucleophilic and therefore, undergo S N 2 reactions with alkyl halides to give alkyl substituted alkynes, which on hydrogenation with Lindlar’s catalyst ( Pd-CaCO 3 , quinoline) gives ( Z )- alkenes , and on reaction with sodium in liquid ammonia, gives ( E )-alkenes. ( Z )-Alkenes on reaction with either osmium tetroxide or with cold dilute potassium permanganate, give syn esters and then produce syn diols . If the product obtained has an internal mirror plane of symmetry, then it is a meso compound. ( E )-Alkenes on reaction with meta -chloroperbenzoic acid give epoxides . The epoxide can either be opened using a hydroxide nucleophile or by aqueous acidic work up (in this case the nucleophile is water). The addition of nucleophile happens in anti-manner for hydroxide and for acidic aqueous work up, there is probability of both syn and anti addition. If the product obtained has an internal mirror plane of symmetry, then it is a meso compound.

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Chapter 11, Problem 1LGP

Interpretation Introduction

Interpretation:

The two syntheses with different approaches are to be devised for meso−2,3−butanediol starting with acetylene (ethyne) and methane.

Concept introduction:

Terminal alkynes are deprotonated with sodium amide to produce alkynides. Alkynides are nucleophilic and therefore, undergo SN2 reactions with alkyl halides to give alkyl substituted alkynes, which on hydrogenation with Lindlar’s catalyst (Pd-CaCO3, quinoline) gives (Z)-alkenes, and on reaction with sodium in liquid ammonia, gives (E)-alkenes.

(Z)-Alkenes on reaction with either osmium tetroxide or with cold dilute potassium permanganate, give syn esters and then produce syn diols. If the product obtained has an internal mirror plane of symmetry, then it is a meso compound.

(E)-Alkenes on reaction with meta-chloroperbenzoic acid give epoxides. The epoxide can either be opened using a hydroxide nucleophile or by aqueous acidic work up (in this case the nucleophile is water). The addition of nucleophile happens in anti-manner for hydroxide and for acidic aqueous work up, there is probability of both syn and anti addition. If the product obtained has an internal mirror plane of symmetry, then it is a meso compound.

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