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My textbook says that Clemmensen Reduction cannot take place in the presence of an acid-sensitive group and the Wolf Kishner reaction won't happen in the presence of a base sensitive group, citing $\ce{-OH}$ (alcohol) and $\ce{-X}$ (halogens) as respective examples.

However, I read from somewhere else that:

Acid sensitive groups are groups which are electron donors or have spare electrons, like $\ce{NH3}$. Base sensitive groups are groups, which are in need of electrons or have empty orbitals, like $\ce{BH3}$.

This seems incorrect to me as Halogens are clearly not in need of electrons. I can see why alcohols would be acid-sensitive but why would Halogens be base-sensitive?

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  • $\begingroup$ Why do you think alcohols are acid-sensitive? By which I mean, what is going to happen to an alcohol in acidic medium? Then apply the same reasoning to 'halogens', as you call them, in basic medium, and you will have your answer. $\endgroup$ Commented Aug 30, 2020 at 14:14
  • $\begingroup$ A proton can attack an OH group so that the resulting group can leave as H2O. This happens because the proton requires electrons and the alcohol has extra lone pairs of electrons. In the second case, both the Halogen and attacking base have a surplus of electrons, so why would there a reaction? $\endgroup$
    – John Tony
    Commented Aug 30, 2020 at 14:30
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    $\begingroup$ What textbook is this from? $\endgroup$ Commented Aug 30, 2020 at 14:32
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    $\begingroup$ What if you are a tertiary alcohol under Clemmensen conditions? The term "sensitive" means some group also reacts under conditions designed for some other group. If you want to effect a Wolff-Kishner, be aware of E2 elimination elsewhere in the molecule. If you choose to do an acid hydrolysis of an ester, be aware of the lability of acetals under these conditions. $\endgroup$
    – user55119
    Commented Aug 30, 2020 at 14:35
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    $\begingroup$ Halogens can undergo substitution reactions under W-K conditions. The hydrazine will quite happily displace the halogen, indeed OH- can displace or eliminate the halogen too. $\endgroup$
    – Waylander
    Commented Aug 30, 2020 at 15:07

2 Answers 2

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Alcohols show -I effect and are electron pair accpetors. They will be base sensitive. But halogens ACCEPT a pair of electrons to achieve stability and are also base sensitive.

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If a grp on addition of H+/acid it changes then it is called acid sensitive like OH grp on addition to H+ gives H2O+ hense acid sensitive and Cl grp is an EWG and withdraws e- from nearby grps due to -I effect and hense base sensitive and it withdraws e- from base(like OH-).

Explanation2:OH has -I effect and also +R effect and +R dominates over -I hense it acts as EDG and is acid sensitive while in Cl -I dominates and is base sensitive

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  • $\begingroup$ You shouldn't use abbreviations in you comments. It does not add to the readability of your answer. $\endgroup$
    – PAEP
    Commented Jan 23 at 14:53

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