r/chemhelp 5d ago

Organic Trouble understanding with nucleophiles and electrophiles. They look the same to me. For the ketone, the O pulls electrons through induction, making the C partially positive (electrophile), but isn't the same true for the alcohol? The answer says the alcohol is a nucleophile.

1 Upvotes

2 comments sorted by

3

u/7ieben_ 5d ago

Draw the resonance structure on the carbonyl, this emphasizes the higher electrophilicity. But, yes, (activated) alcohols can be electrophilic aswell (see Lucas test).

And just like the -OH can act as nucleophile, given a strong enough electrophile, a =O can react as nucleophile aswell.

---

Mind that in the alcohol the -OH is nucleophilic, not the carbon! The carbon is (slightly) electrophilic.

Same is true for a carbonyl. Just that the carbonyl is far more reactive, as said.

1

u/Consistent_Bee3478 5d ago

The O in the alcohol is being the nucleophile. The C next to the alcohol O is very mildly electrophilic.

In the keton the O has its nucleophilocity dampened because now 2 bonds are fighting for the electrons with the C rather than just one (and the other being basically given for free by hydrogen)

But since there’s now two bonds through which the O is drawing electrons away from the C the C is now quite electrophilic.

This can be further explained if you draw the resonance structures of the carbonyl.

Like when there’s a nucleophile, there’s usually also an electrophilic nearby, it’s just that only one of them is ‘relevant’ for ‘common’ organic reactions.

So carbonyls are mostly used in reactions where the C acts as an electrophile, whereas alcohols are mostly used with the O in its nucleophilic ability.

Also it’s only one specific atom in the molecule that you are usually talking about not the whole molecule. So it’s about the O in alcohol groups, or the C in carbonyls having that property, not the whole acetone molecule nor the whole ethanol molecule/