Posted by: Mark Foreman | February 2, 2013

THe right and the wrong way to reduce an aldehyde

Dear reader,

As promised we are going to consider how the mechanism of carbonyl reduction works. Rather than just showing you the right mechanism, I am going to try to show you why one mechanism is right and the other is wrong.

If we start with a very simple aldehyde (ethanal, acetaldehyde) we could react it with borohydride to reduce it.

because of resonance a partial positive charge exists on the carbonyl carbon, as shown below.

The two resonance forms of the aldehyde, the one on the left hand side is the major form while the one on the right is a minor resonance form

The two resonance forms of the aldehyde, the one on the left hand side is the major form while the one on the right is a minor resonance form

One person recently asked why can we not get the negative charge on the boron of the borohydride to make a bond to the carbonyl carbon.

red1

This pair of curved arrows are wrong

This is wrong, it is not possible to have a five coordinate boron, also the negative charge does not exist as an isolated charge on the boron. The borohydride ion has a total charge of -1, but this charge is spread out over all the atoms. The boron is more electronegative than the hydrogens so the boron is a bad choice for the start of the arrow which attacks the carbonyl carbon. Instead we should use an electron pair in a B-H bond to attack the carbonyl carbon.

The right arrows for the reduction of the aldehyde by the borohydride anion

The right arrows for the reduction of the aldehyde by the borohydride anion

The borane (BH3) is a very strong lewis acid and will very quickly react with the ethanol which is commonly used as the solvent for borohydride reductions to form a complex, this complex will deprotonate to form a new anion which will then act as a reducing agent. In this way one mole of borohydride can react with more than one mole of aldehyde. Below is the reaction which converts the borane into a new reducing agent.

Conversion of borane into a new anion by ethanol

Conversion of borane into a new anion by ethanol

The carbonyl oxygen which was converted into an alkoxide will be aquire a proton from the ethanol, thus converting its self into an alcohol.

 

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Responses

  1. Reblogged this on Organic Reactions in Medchem.


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