Dual reaction channels for solvolyses of acyl chlorides in alcohol–water mixtures

Abstract

Rate constants are reported for solvolyses at 0°C of trimethylacetyl chloride (3) in 90-30% V/V acetonitrile-, acetone-, ethanol-and methanol-water mixtures, of adamantane-1 -carbony1 chloride (4) in 90-60% acetone-, ethanol-and methanol-water mixtures, and of cyclopropanecarbonyl chloride (5) in 9040% acetone-water. Quantitative product data (acid and ester) are also reported for solvolyses of trimethylacetyl chloride in 98-20% ethanol-and methanol-water mixtures. Product selectivities (S) show maxima in 90-95% alcohol-water mixtures, similar to those, reported previously for solvolyses of p-chlorobenzoyl chloride (1 a), benzoyl chloride (1 b), 2,4.6trimethylbenzenesulfonyl chloride (2a) and 4-methoxy-2.6-dimethylbenzenesulfonyl chloride (2b).Using rate-rate profiles, logarithms of rate constants are dissected into the competing reaction channels. Given the sharp maxima in S that occur at different alcohol compositions for different substrates, and the link to 'breaks' in rate-rate profiles, the results are consistent with competing mechanisms having different rate-limiting steps. The competing mechanisms involve two broad reaction channels: (1) nucleophilic attack by one molecule of solvent assisted by a second molecule of solvent acting as a general base catalyst-consequently, in aqueous alcohols there are four mechanistic combinations operating simultaneously within this one reaction channel; (2) nucleophilic attack by solvent occurs via a carbocationic reaction within the sN2-SNl mechanistic spectrum, involving for example, a solvent-separated ion pair intermediate (SJ ) or a concerted nucleophilic attack (S,2).Solvolyses of acid chlorides in alcohol-water mixtures give ester and acid products, and product selectivities (S) can be defined using eqn. (I), which relates the molar product ratio to the molar ratios of the two components of the solvent. S Values reach sharp maxima for solvolyses of pchlorobenzoyl chloride (la) in 5040% ethanol-or methanolwater and of benzoyl chloride (lb) in 80% alcohol-water.'" Extending this work to acyl halides, we now report rate and selectivity data for solvolyses of trimethylacetyl chloride (3), and we strengthen the previously proposed link between l a Z = C I b Z = H Paper 3/01 880F

Kinetic and Theoretical Consideration of 3,4- and 3,5-Dimethoxybenzoyl Chlorides Solvolyses

“…Substrates of 3 and 5 were reported that they are mainly followed by unimolecular pathway (ionization) due to the resonance effect of paramethoxy substituent. [6][7][8][9]15 The l/m ratio of 1 in all range of solvents was 0.47, it is the similar value of 3 and 5, which means that the solvolysis reaction pathway of 1 containing para-methoxy substituent, is consistent with the results of 3 and 5. In case of 2, the l/m ratio was 2.59 in 17 nucleophilic solvents and was 0.64 in 8 electrophilic solvents, respectively.…”

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

“…S N 1 like pathways, with the dominance of ionization of chloride ion, were considered to occur for compounds bearing very strong e‐d groups in strongly polar protic solvents 17–19. Competing S N 2 and S N 1 mechanisms were proposed for hydrolyses with increasing contribution of the S N 1 pathway, when the water content of the solvent mixtures increased 12, 20–25. The mechanism of the hydrolyses of acid chlorides bearing strong e‐w groups was considered to change for an Ad‐E pathway with rate determining addition step 12, 16, 25–28.…”

Mechanistic Studies of the Solvolyses of Carbamoyl Chlorides and Related Reactions

“…For example, Bentley has made extensive use of this technique in establishing (or excluding) dual reaction channels in the solvolyses of acyl chlorides (RCOCl), and also sulfonyl chlorides (RSO 2 Cl), in alcohol-water mixtures. These solvolyses can lead to either an ester (attack by alcohol) or an acid (attack by water) [36,37,38,39]. …”