Hydrolysis of Esters-Acid-Mechanism

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|THE MECHANISM FOR THE ACID CATALYSED HYDROLYSIS OF ESTERS | | | |This page looks in detail at the mechanism for the hydrolysis of esters in the presence of a dilute acid | |(such as hydrochloric acid or sulphuric acid) acting as the catalyst. It uses ethyl ethanoate as a typical| |ester. | | | |The mechanism for the hydrolysis of ethyl ethanoate | |A reminder of the facts | |Ethyl ethanoate is heated under reflux with a dilute acid such as dilute hydrochloric acid or dilute | |sulphuric acid. The ester reacts with the water present to produce ethanoic acid and ethanol. | |Because the reaction is reversible, an equilibrium mixture is produced containing all four of the | |substances in the equation. In order to get as much hydrolysis as possible, a large excess of water can be| |used. The dilute acid provides both the acid catalyst and the water. | |[pic][pic] | | | |The mechanism | | |[pic] | | |Warning!  This is a fairly complex mechanism, and is definitley NOT required for any UK A level | | |(or equivalent) syllabus. I have included it in case it is of use to my many non-UK visitors. | | |[pic] | |All the steps in the mechanism below are shown as one-way reactions because it makes the mechanism look | |less confusing. The reverse reaction is actually done sufficiently differently that it affects the way the| |mechanism is written. You will find a link to the esterification reaction further down the page if you are| |interested. | | |[pic] | | |Note:  The explanation assumes that you know about the use of curly arrows in organic reaction | | |mechanisms. If you aren't happy about these follow this link before you go any further. (To be | | |honest, if you are that unsure about the conventions used in reaction mechanisms, you probably | | |shouldn't be reading this page anyway - you will find it distinctly scary!) | | |Use the BACK button on your browser to return to this page. | | |[pic] | |Step 1 | |The actual catalyst in this case is the hydroxonium ion, H3O+, present in all solutions of acids in water.| | | |In the first step, the ester takes a proton (a hydrogen ion) from the hydroxonium ion. The proton becomes | |attached to one of the lone pairs on the oxygen which is double-bonded to the carbon. | |[pic]...
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