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| [[File:Fischer esterification mechanism.svg|thumb|400px|In acid-catalyzed [[Fischer esterification]], the [[proton]] binds to oxygens and functions as a [[Lewis acid]] to activate the ester [[carbonyl]] (top row) as an [[electrophile]], and converts the [[hydroxyl]] into the good [[leaving group]] water (bottom left). Both lower the kinetic barrier and speed up the attainment of [[chemical equilibrium]].]]
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| 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 [[hydroxide]] ion. Typical reactions catalyzed 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'''.
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| ==Use in synthesis==
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| 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 [[sulfuric 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, which can then be eliminated as water (H<sub>2</sub>O). Acids specifically used for acid catalysis include [[hydrofluoric acid]] (in the [[Alkylation#Oil refining|alkylation process]]), [[phosphoric acid]], [[toluenesulfonic acid]], [[polystyrene sulfonate]], [[heteropoly acid]]s, [[zeolite]]s and [[graphene oxide]].
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| 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.
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| ===Solid acid catalysts===
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| In industrial scale chemistry, many processes are catalysed by "solid acids." As [[heterogeneous catalysts]], solid acids do not dissolve in the reaction medium. Well known examples include [[zeolite]]s, [[alumina]], and various other metal [[oxide]]s. Such acids are used in [[cracking]]. A particularly large scale application is [[alkylation]], e.g. the combination of benzene and ethylene to give [[ethylbenzene]].<ref>Michael Röper, Eugen Gehrer, Thomas Narbeshuber, Wolfgang Siegel "Acylation and Alkylation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000. {{DOI|10.1002/14356007.a01_185}}</ref> Many alkyl[[amine]]s are prepared by amination of alcohols.
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| [[File:Zeolite-ZSM-5-3D-vdW.png|thumb|right|Zeolite, ZSM-5 is widely used as a solid acid catalyst.]]
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| ==Kinetics==
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| ===Specific catalysis===
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| 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>.<ref>
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| {{Cite web
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| |url=http://www.iupac.org/goldbook/S05796.pdf
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| |title=IUPAC Compendium of Chemical Terminology, 2nd Edition (1997)
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| |publisher=www.iupac.org
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| |accessdate=2009-11-22
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| |last=
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| |first=
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| }}
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| </ref> 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.
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| :S + AH → SH<sup>+</sup> + A<sup>-</sup>
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| 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.
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| :<math> rate= -\frac{d[R 1]}{dt} = k[SH^+] [R 1] [R 2]</math>
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| This type of [[chemical kinetics]] is observed when reactant '''R'''<sub>'''1'''</sub> is in a fast equilibrium with its conjugate acid '''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]].
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| ===General catalysis===
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| In general acid catalysis all species capable of donating protons contribute to [[reaction rate]] acceleration.<ref>
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| {{Cite web
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| |url=http://www.iupac.org/goldbook/G02609.pdf
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| |title=IUPAC Compendium of Chemical Terminology, 2nd Edition (1997)
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| |publisher=www.iupac.org
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| |accessdate=2009-11-22
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| |last=
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| |first=
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| }}
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| </ref> The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example [[diazonium coupling]] reactions.
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| <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>
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| 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.
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| ==References==
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| {{Reflist}}
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| {{DEFAULTSORT:Acid Catalysis}}
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| [[Category:Chemical kinetics]]
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| [[Category:Catalysis]]
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Screen Printer Anton Fant from Dawson Creek, has interests which include 4 wheeling, new launching property developers in singapore and history. Is a travel freak and recently traveled to Fort and Shalamar Gardens in Lahore.