Question

Butanone, when treated with methyl magnesium bromide and then dilute aqueous acid, yields 2-methyl-2-butanol.

a) Please explain how Grignard reagents work. For example, what role does the magnesium play, and why is carbon unusually nucleophilic?

(b) Please explain why the product not is a racemic mixture of enantiomers of 2-

methyl-2-butanol.

(c)What role does acid (H+) play in this reaction? [Hint: it is not a catalyst.]

Answer 1

What's striking about Grignard formation is that polarity is reversed in this reaction. In the alkyl halide, the carbon attached to the halogen has a partially positive charge, because carbon is further to the left than halogens in the periodic table. After magnesium insertion, this same carbon has a partial negative charge, because carbon is farther to the right in the periodic table than magnesium. This sort of reversal in reactivity is sometimes called "umpolung chemistry" so megnesim play a vital role in revesal of the polarity.

The nucleophilic bond donates to the carbon of the carbonyl, breaking the ?-bond, releasing a pair of electrons to the electronegative oxygen atom. Alternatively, you could still draw the bromine attached to a Mg+ at this point.

b) to become a assymmetric carbon it should have 4 different groups on same carbon.

in the present case there are two methyl groups on carbon after attck of the methyl grignard over stereogenic carbonyl center. results in non chiral center. so the handed ness of the molecule disapper based on the face distingushed attack of the nucleophile