Infra-Red Spectroscopy

Is the C=O bond stronger in an ester (1735cm^-1 ) or in a ketone (1715cm^-1) and why?

Anyone help please?

It is stronger in an ester, because the bond only absorbs the IR radiation of a higher wavenumber.

Therefore this indicates that the bond is stronger.

As to why the bond is stronger ... I'm not sure ...

Reply 2

Kiesha

Hmm me neither. Does the fact that it absorbs a higher wavelength mean that it requires more energy to break the bond so it must be stronger ...or something?

Reply 3

Fly.

Kiesha

Is the C=O bond stronger in an ester (1735cm^-1 ) or in a ketone (1715cm^-1) and why?

Anyone help please?

The bond is stronger in an ester, reflected in the greater frequency of absorption. In a ketone the carbonyl group is adjacent to a methyl group and so there is some degree of electron donation from the C-H sigma bonds. In an ester, the adjacent oxygen atom is actually slightly electron withdrawing, although there will be some donation from the lone pair. A carbonyl group can be considered as a resonant structure as indicated:

C=O <---> (+)C-0(-) (all charges are only partial)

An electron donating group will stabilise the positive charge on the carbon atom and so will favour the structure on the right, while an electron withdrawing group will favour the structure on the left. The nature of the group adjacent to the carbon atom determines the position of equilibrium. Another way of looking at it is to note that anything that donates electrons into a carbonyl group will donate them preferentially in the pi-antibonding molecular orbital and will thus weaken the double bond.

The structure on the right resonates at a lower frequency because it has greater single bond character, and frequency is proportional to the square root of bond strength, which correlates roughly with bond order, hence is greater for a double bond.

Reply 4

junners

That's actually very interesting. I'd never have thought that, but it makes a lot of sense.

Oh well, Chemistry, is insha'Allah over for me!

Did you learn this during the A-level course, Fly.?