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I know that the amount of substance of Grignard reagent used is 5 moles. But the answer given is 2. Is this anything to do with 'equivalents'?

image of the exercise

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    $\begingroup$ What is the acetaldehyde even doing in this reaction? $\endgroup$ – SendersReagent Apr 12 '16 at 9:57
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    $\begingroup$ The trouble here is that the number used is ambiguous as the grignard is doing two different things: three equivalents deprotonate active alcohols (why not protect them with something else?) and two react to appear in the product. Maybe the question his only interested in the equivalents that get into the product? $\endgroup$ – matt_black Apr 12 '16 at 10:45
  • $\begingroup$ @matt_black If there are no side reactions, the reason not to protect them is because PhMgBr is a relatively cheap, commercially available reagent. Protection could waste precious material. $\endgroup$ – SendersReagent Apr 12 '16 at 11:20
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    $\begingroup$ @SendersReagent as finally alcoholic and amino groups are as it is (i.e. not reacted with grignard as base) it must be ensured that finally no PhMgBr is left. So first acetaldehyde uses all left grignard, and then H+ protonates acidand amino group. {Therefore , this information gives us a hint that grignard reagent is in excess.} $\endgroup$ – BEWARB Apr 12 '16 at 16:04
  • $\begingroup$ Acetaldehyde seems like a much harder way to go than acetone. Maybe they were worried about making an acetylide hemiaminal ether if they use acetone. $\endgroup$ – SendersReagent Apr 12 '16 at 17:02
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No you are right it should be five. there are three dissociative protons thus one from the secondary amine, and two from the hydroxy groups. These must be reacted first then you need two to react with the ester. The question was made by a human and the book answer of 2 is probably just an error from such.

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    $\begingroup$ You only use two equivalents of the Grignard reagent. The other three go to waste. I think it's a question of semantics rather than an error of a human. $\endgroup$ – Martin - マーチン Apr 12 '16 at 9:03
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    $\begingroup$ @Martin-マーチン I see how you might think that, but if you break down the question it asks for the value of $x$. If $x$ is not greater than or equal to 5, there reaction will not proceed to completion. $\endgroup$ – A.K. Apr 12 '16 at 13:26
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    $\begingroup$ It's bad question as deprotonation of those groups would lead to side reactions $\endgroup$ – Mithoron Apr 12 '16 at 16:25

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