Which reducing agent can venice double bond between carbon?
Answers
Oxidation and reduction reactions are the most complex of all reactions in organic chemistry and they are dependent on so many factors. I have tried to keep a balance here between what have been/maybe asked in IIT/AIIMS exam. The results which are not explained here are better understood by factors related to different solvents and sometimes easily explained by MOT.
Generally, neither LiAlH4 nor NaBH4 is able to reduce an isolated C=C But——
Cinnamaldehyde, C6H5-CH=CH-CHO when added slowly to an excess of the LiAlH4 dissolved in ether (normal addition), gives C6H5CH2CH2-OH
whereas in “inverse addition method” where LiAlH4/ether is added slowly to Cinnamaldehyde, so that LiAlH4 is never in excess, the product is cinnamyl alcohol, C6H5-CH=CH-CH2OH.
The exact process and products of reduction of conjugated carbonyl compounds are dependent on the nature of reductant.
In case of conjugated aldehyde or ketone, the nucleophilic attack takes place at either the β-carbon (conjugate addition) or at the carbonyl carbon (direct addition).
According to the the vinylic β-carbon is a "soft" electrophilic center and prefer to react with "soft" nucleophiles whereas the carbonyl carbon is a "hard" electrophilic center and hence prefers to react with "hard" nucleophiles.
NaBH4 is a rather soft nucleophile and thus it prefers in some cases to react with 4-position or reduces conjugated >C=C< (called 1,4-addition) followed by reduction of aldehyde or ketone into the alcohol by NaBH4 in a second step.
The extent of reduction to saturated alcohol is usually greater with NaBH4 than with LiAlH4.
Source: Organic chemistry by Solomons [P -869]
The reduction of conjugated carbonyl compounds by NaBH4 gives a substantial amount of fully saturated alcohols. However check
(p-1070)
Source; Organic Chemistry By Jonathan Clayden, Nick Greeves, Stuart Warren
The conjugate reduction of enones is easier than that for enal as aldehyde is “softer” electrophile than the ketone.
LiAlH4 is a rather hard nucleophile and thus it attacks on more electrophilic site and reduces carbonyl group (1,2-addition).
LiAlH4 gives more 1,2- attack than LiBH4 which, in turn, is more reactive for direct attack than NaBH4.
>C=C< double bonds are attacked by LiAlH4 if there is extensively conjugated phenyl group attached to the β carbon of carbonyl compound, and further if LiAlH4 is taken in excess for example, PhCH=CHCOOH or allyl alcohols etc. . Also,
However, with LiAlH4, the proportion of 1,2- adduct (direct addition on C=O only) decreases and that of 1,4- addition increases as the Al is attached to more electron-withdrawing groups or gets sterically hindered thereby decreasing the hydride donation capacity.
Source:
Extra reading:
Source; Organic Chemistry By Jonathan Clayden, Nick Greeves, Stuart Warren
Note 1-
Note 2-
Note 3-
12.8k Views · · Answer requested by
Can LiAlH4 & NaBH4 reduce double bond present in conjugation with ether?
Can carbon form five bonds in any case?
Is a double bond between carbon and oxygen also an olefinic bond?
Why does a bond questionnaire not exist between two carbon atoms?
Does the number of bonds between two carbons affect the degree of that carbon?
1.9k Views · · Answer requested by
LiAlH4 reduces a C-C double bond which is in conjugation i.e. resonance.
Eg:- Styrene is reduced to ethyl benzene on reduction with also reduces C-C double bond of Cinnamaldehyde because it is in conjugation with benzene ring.
Styrene:
Cinnamaldehyde:
But 1-phenylprop-2-ene is not reduced by LiAlH4.
Here, one thing is to be noted that LiAlH4 does not reduces aromatic double bond even though it is in conjugation. Thus, the benzene ring remains unaffected from reaction by LiAlH4.
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