Organic chemistry teaching

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.

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.

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…

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