Addition Reactions Followed by Water Loss Acetal Formation

9 Addition Reactions Followed by Water Loss Acetal Formation 

If aldehydes and ketones form hydrates in water (H—OH), shouldn t there be a similar reaction in the presence of alcohols (R—OH) Indeed there should be, and there is. However, the reactions are shghdy more compUcated than simple hydration, at least under acidic conditions. In base, the two reactions are entirely analogous, so let s look at that reaction first. 

FIGURE 16.37 The reaction of a carbonyl group with hydroxide ion in water parallels the reaction of a carbonyl with an aUcoxide ion in alcohol. In water, a cw-diol (or hydrate) is formed in alcohol, the analogous product is called a hemiacetal. 

Hydration and hemiacetal formation are comparable, so it is no surprise that the discussion in Section 16.7 on equilibrium effects is quite directly transferable to this reaction. TUdehydes, for example, are largely in their hemiacetal forms in basic alcoholic solutions (in acid the reaction goes further, as we will see), whereas ketones are not. 

FIGURE 16.38 Cychc hemiacetals are often more stable than their open-chain counterparts. 

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Addition Reactions Followed by Water Loss Acetal Formation Hydration reaction in acid... 

Anhydrides may be prepared by coupling two carboxylic acids under acidic conditions. If ethanoic acid (acetic acid, 21) is heated with HCl, protonation to give an oxocarbenium ion is followed by reaction with a second equivalent of acetic acid to give a tetrahedral intermediate. This reaction is the usual acid-catalyzed acyl addition mechanism. Protonation of the OH unit leads to loss of water and formation of the anhydride. Each step in this process is reversible and steps must be taken to drive the equilibrium (see Chapter 7, Section 7.10, for a discussion of equilibria) toward the anhydride product by removing the water by-product (see Chapter 18, Section 18.6.3). Remember that such techniques are an application of Le Chatelier s principle (discussed in Section 18.3). Even when this is done, isolation of pure anhydrides by this method can be difficult. Unreacted acid may contaminate the product and atternpts to remove the acid with aqueous base may induce hydrolysis of the anhydride. 

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