Water, acyl addition alkenes

With respect to this sequence of observed reactions, experimental evidence shows that 21 reacts with the base (NaOEt) to form an enolate anion, and the nucleophilic carbon atom of that enolate anion attacks the carbonyl of a second aldehyde to give the alkoxide of 22. This is a normal acyl addition reaction, and the nucleophile is the a-carbon of the enolate anion. Treatment of this initial alkoxide product with aqueous acid under mild conditions simply generates alcohol 22, as with all other acyl addition reactions (see Chapter 18). Product 22 is called an alilol or an aldolate. The reaction of an aldehyde or a ketone with a base generates an aldol product. Vigorous acid hydrolysis led to protonation of the OH unit in 22 by the strong acid (to form an oxonium ion), which eliminated a molecule of water (dehydration) to give the alkene unit in 23. 

The reaction is also called hydrocarboxylation. According to a later modification, the alkene first reacts with carbon monoxide in the presence of the acid to form an acyl cation, which then is hydrolyzed with water to give the carboxylic acid.97 The advantage of this two-step synthesis is that it requires only medium pressure (100 atm). Aqueous HF (85-95%) gave good results in the carboxylation of alkenes and cycloalkenes.98 Phosphoric acid is also effective in the carboxylation of terminal alkenes and isobutylene, but it causes substantial oligomerization as well.99 100 Neocarboxylic acids are manufactured industrially with this process (see Section 7.2.4). The addition may also be performed with formic acid as the source of CO (Koch-Haaf reaction).101 102 The mechanism involves carbocation formation via protonation of the alkene97 103 [Eq. (7.10)]. It then reacts with carbon monoxide... 

Titanium tetrachloride is a moisture-sensitive, highly flammable liquid reacting violently with water (34). It is a strong Lewis acid capable of promoting Diels-Alder reactions (35) and induces the addition of silyl enol ethers and allyl silanes to carbonyl compounds and derivatives (34r-36). It is a less commonly used catalyst in Friedel-Crafts reactions but very useful for the acylation of activated alkenes and in the Fries rearrangement. 

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