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Organic Chemistry

19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation

Organic Chemistry19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation

19.7 • Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation

Addition of Hydride Reagents: Reduction

We saw in Section 17.4 that the most common method for preparing alcohols, both in the laboratory and in living organisms, is by the reduction of carbonyl compounds. Aldehydes are reduced with sodium borohydride (NaBH4) to give primary alcohols, and ketones are reduced similarly to give secondary alcohols.

First reaction: Carbonyl in aldehyde gets reduced to primary alcohol using sodium borohydride in ethanol. Second reaction: Carbonyl in ketone gets reduced to secondary alcohol using the same reagents.

Carbonyl reduction occurs by a typical nucleophilic addition mechanism under basic conditions, as shown earlier in Figure 19.5a. Although the details of carbonyl-group reductions are complex, LiAlH4 and NaBH4 act as if they were donors of hydride ion nucleophile, :H, and the initially formed alkoxide ion intermediate is then protonated by addition of aqueous acid. The reaction is effectively irreversible because the reverse process would require expulsion of a very poor leaving group.

Hydride from sodium borohydride adds to the carbonyl of ketone forming an alkoxide intermediate where O carries a negative charge. Further hydrolysis of the intermediate gives secondary alcohol and water.
Figure 19.6 MECHANISM
Mechanism of the Grignard reaction. Complexation of the carbonyl oxygen with the Lewis acid Mg2+ and subsequent nucleophilic addition of a carbanion to an aldehyde or ketone is followed by protonation of the alkoxide intermediate to yield an alcohol.
Three-step formation of alcohol from carbonyl and Grignard. Lewis acid-base complex forms between the carbonyl oxygen and magnesium; R attacks carbonyl; tetrahedral intermediate undergoes hydrolysis to give alcohol.

Addition of Grignard Reagents, RMgX

Just as aldehydes and ketones undergo nucleophilic addition with hydride ion to give alcohols, they undergo a similar addition with Grignard reagent nucleophiles, R: +MgX. Aldehydes give secondary alcohols on reaction with Grignard reagents in ether solution, and ketones give tertiary alcohols.

The conversion of aldehyde to secondary alcohol happens using R double prime M g X and hydronium. Conversion of ketone to tertiary alcohol happens using the same reagents.

As shown in Figure 19.6, a Grignard reaction begins with an acid–base complexation of Mg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R: then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water or dilute aqueous acid in a separate step yields the neutral alcohol. Like reduction, Grignard additions are effectively irreversible because a carbanion is too poor a leaving group to be expelled in a reversal step.

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