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I had the following question at an exam:

What is the fastest reacting compound towards nucleophilic addition with Grignard reagent?

(a) Acyl chloride
(b) Aldehyde
(c) Ester

It is obvious that ester would be the slowest, but among acyl chloride and aldehyde, acyl chloride would have a highly electrophilic, but a bit sterically hindered carbon atom and the resonance will also decrease the electrophilicity, whereas the aldehyde will have a less sterically hindered and moderately electrophilic carbon atom.

Which one will prevail and how to judge?

What would happen if instead acyl chloride there were acyl bromide?

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  • $\begingroup$ IMO,the approach at the Burgi-Dunitz angle will be having quite less steric hindrance both in the case of (a) and (b) (although there will be relatuve differences). But the increased electrophilicity of the acyl chloride(in MO terms it essentially means that the LUMO has gone down in the energy profile) means that the reaction rate should be faster for the acyl chloride as compared to the aldehyde(assuming all the three options to be approximately homologous with similar substituents) $\endgroup$ – Yusuf Hasan Jun 9 '19 at 3:52
  • $\begingroup$ @OscarLanzi Gringard being a nucleophile attacks the Carbon of the Carbonyl , which on hydrolysis yield a 2° Alcohol (3° in case of Ketones) $\endgroup$ – RandomAspirant Jun 9 '19 at 7:25
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    $\begingroup$ Oh , Oops , Sorry , Nucleophilic Addition, My bad , so sorry.. $\endgroup$ – RandomAspirant Jun 9 '19 at 8:47
  • $\begingroup$ Tech note: please avoid unnecessary edits that lower the quality of your post, namely in written English there is no need for a white space before punctuation marks (with rare exceptions). For your case with ? see Is it ever correct to have a space before a question or exclamation mark?. $\endgroup$ – andselisk♦ Jun 9 '19 at 8:51
  • $\begingroup$ @andselisk Non Native English Speaker , I will takes a note of that $\endgroup$ – RandomAspirant Jun 9 '19 at 8:57
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The answer will be (a) Acyl chloride.

The acyl carbon in an acyl chloride is connected to the chlorine atom. While you have acknowledged this fact, you have missed the point that the +M (resonance) effect of Chlorine is overcome by the -I (inductive) effect. Thus, the electrophilicity of the acyl carbon is actually greater than the electrophilicity of the carbonyl carbon in an aldehyde. This jump in electrophilicity is so significant that the difference in steric hindrance is neglected.

In fact, this the major reason why acyl chlorides are considered to be one of the best electrophiles in all of organic chemistry.

If Acyl bromides were used, then they would have an even greater electrophilicity and it would be the answer to this question.

Hope this answers your question!

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