Acid catalysis 2053312 211865084 2008-05-12T13:27:30Z 129.11.77.197 /* Use in synthesis */ In '''acid catalysis''' and '''base catalysis''' a [[chemical reaction]] is [[catalysis|catalyzed]] by an [[acid]] or a [[base (chemistry)|base]]. The acid is often the [[hydrogen ion|proton]] and the base is often a [[hydroxyl]] ion. Typical reactions catalysed by proton transfer are [[esterfication]]s and [[aldol reaction]]s. In these reactions the [[conjugate acid]] of the [[carbonyl]] group is a better [[electrophile]] than the neutral carbonyl group itself. Catalysis by either acid or base can occur in two different ways: '''specific catalysis''' and '''general catalysis'''. == Use in synthesis == Acid catalysis is mainly used for [[Organic chemistry|organic]] chemical reactions. There are many possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as [[sulphuric acid]] (H<sub>2</sub>SO<sub>4</sub>) can be used. Usually this is done to create a more likely [[leaving group]], such as converting an OH group to a H<sub>2</sub>O<sup>+</sup> group. With carbonyl compounds such as [[ester]]s, synthesis and hydrolysis go through a tetrahedral transition state, where the central carbon has an oxygen, an alcohol group, and the original alkyl group. Strong acids protonate the carbonyl, which makes the oxygen positively charged, so that it can easily receive the double bond electrons when the alcohol attacks the carbonyl carbon. This enables ester synthesis and hydrolysis. The reaction is an equilibrium between the ester and its cleavage to carboxylic acid and alcohol. On the contrary, strong bases deprotonate the attacking alcohol or amine, which also promotes the reaction. However, bases also deprotonate the acid, which is irreversible. Therefore, in a strongly basic, aqueous environment, esters only hydrolyze. == Kinetics == === Specific catalysis === In specific acid catalysis taking place in solvent S , the [[reaction rate]] is proportional to the [[concentration]] of the protonated solvent molecules SH<sup>+</sup>. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the [[chemical equilibrium]] between solvent S and AH in favor of the SH<sup>+</sup> species. S + AH &rarr; SH<sup>+</sup> + A<sup>-</sup> For example in an aqueous [[buffer solution]] the reaction rate for reactants R depends on the [[pH]] of the system but not on the [[concentration]]s of different acids. <math>\ rate= -\frac{d[R_1]}{dt} = k[SH^+][R_1][R_2]</math> This type of [[chemical kinetics]] is observed when reactant '''R'''<sub>'''1'''</sub> in a fast equilibrium with its conjugate base '''R'''<sub>'''1'''</sub>'''H'''<sup>'''+'''</sup> which proceeds to react slowly with '''R'''<sub>'''2'''</sub> to the reaction product for example in the acid catalysed [[aldol reaction]]. === General catalysis === In general acid catalysis all species capable of donating protons contribute to [[reaction rate]] acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example [[diazonium coupling]] reactions. <math>\ rate= -\frac{d[R_1]}{dt} = k_1[SH^+][R_1][R_2] + k_2[AH^1][R_1][R_2] + k_3[AH^2][R_1][R_2] + ... </math> When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst. [[Category:catalysts]] [[Category:chemical kinetics]]