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Why is it a requirement that the ester should have 2 alpha hydrogens in this reaction although we actually need just one for the reaction to occur?enter image description here

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Great question!

The Claisen reaction happens under equilibrium conditions, and the addition of the ester enolate is highly reversible. Your mechanism omits the very important end of this reaction, which is that the β-keto ester product is highly acidic and easily deprotonated to yield the corresponding enolate.

The enolate formation drives the reaction forward according to Le Châtelier's principle. Very importantly, your question about the α-hydrogen. The deprotonation of the β-keto ester is not possible without two hydrogens on the original ester that will serve as the nucleophile.

Claisen

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    $\begingroup$ This photo could use cropping... Also it's better to post drawings of reactions made with software. $\endgroup$ – Mithoron Mar 18 '17 at 21:49
  • $\begingroup$ I don't have a good reaction editor anymore. I looked at Marvin sketch but their free license doesn't seem to apply to me. Suggestions? $\endgroup$ – Zhe Mar 19 '17 at 0:13
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    $\begingroup$ Chemsketch, ISIS Draw... $\endgroup$ – Mithoron Mar 19 '17 at 16:09
  • $\begingroup$ @Zhe I took the liberty to redraw the reaction scheme with chemfig; in case you find it unacceptable, feel free to revert my edit. If you are interested, the $\mathrm{\LaTeX}$ code can be found at Pastebin. $\endgroup$ – andselisk Mar 28 at 2:25
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    $\begingroup$ @andselisk You'll get no objections from me. Thanks! $\endgroup$ – Zhe Mar 29 at 1:03
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Two hydrogen are use , because if only one H is used then again the product form having acidic H which is abstracted by ethoxide ion then this ethoxide ion convert in Ethanol and final product is obtained.

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