The Student Room Group

Why are aldehydes more reactive than ketones

Like what does electron density mean and stuff
Original post by Roxibox123
Like what does electron density mean and stuff


Good question.

Electron density is more or less an idea of how many electrons are in a given region.

Electronegative elements draw electrons close to themselves and tend to be electron-dense, whilst less electronegative elements tend not to be.

In the case of aldehydes and ketones, they have a C=O bond in their structure, in which the oxygen is considerably more electronegative than the carbon, so the oxygen has a higher electron density than the carbon and is weakly positive. You could say it’s δ+ C=O δ-.

The key difference between an aldehyde and a ketone is exactly what is bound to the C=O group. If the C=O is sandwiched between carbon atoms, it is a ketone. If the C=O has at least 1 hydrogen bound to it, it’s an aldehyde.

You should know that alkyl groups (a term which usually refers to the rest of the carbon chain) are electron-donating. The more of them on the carbon in the C=O, the less positive the carbon is.

This therefore makes the carbon in the C=O in ketones less attractive to nucleophiles that may form a new bond with it, compared to aldehydes.
What do you mean by less positive
And why is that
And why is o more electron negative
Also do u tutor ?
Original post by Roxibox123
What do you mean by less positive
And why is that
And why is o more electron negative

What I mean by ‘less positive’ is the carbon should be very weakly positive, but due to the adjacent carbon atoms donating negative electrons to it, the positive charge is less strong than it could be.

Oxygen is a smaller atom than carbon (as you go from left to right along a period of the periodic table, the atoms get smaller), and it has more protons. This makes the nucleus of an oxygen atom more able to draw the electrons closer to itself, so it’s more electronegative
(edited 1 year ago)
Original post by Roxibox123
Also do u tutor ?


No. I help out when I can, but that’s not tuition, really
Reply 6
Original post by TypicalNerd
Good question.

Electron density is more or less an idea of how many electrons are in a given region.

Electronegative elements draw electrons close to themselves and tend to be electron-dense, whilst less electronegative elements tend not to be.

In the case of aldehydes and ketones, they have a C=O bond in their structure, in which the oxygen is considerably more electronegative than the carbon, so the oxygen has a higher electron density than the carbon and is weakly positive. You could say it’s δ+ C=O δ-.

The key difference between an aldehyde and a ketone is exactly what is bound to the C=O group. If the C=O is sandwiched between carbon atoms, it is a ketone. If the C=O has at least 1 hydrogen bound to it, it’s an aldehyde.

You should know that alkyl groups (a term which usually refers to the rest of the carbon chain) are electron-donating. The more of them on the carbon in the C=O, the less positive the carbon is.

This therefore makes the carbon in the C=O in ketones less attractive to nucleophiles that may form a new bond with it, compared to aldehydes.

When you say that alkyl groups are electron donating, is this the same principle as the positive inductive effect or are they different?
Also good luck tomorrow if you're with AQA
Could you possibly draw out what U mean like visually I don't rlly get it
Original post by Q_LDN
When you say that alkyl groups are electron donating, is this the same principle as the positive inductive effect or are they different?
Also good luck tomorrow if you're with AQA

That’s exactly it. Well done.

I’m not doing AQA. I’m actually doing Edexcel. Good luck nonetheless.

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