[davidenarb]

Hi all,

In the formation of the acetal, for many simple aldehydes, the equilibrium favors formation of the acetal; however, for most ketones, the equilibrium favors reactants rather than products.

Why is that?

[kriggy]

Probably because of inductive effects of alkyl groups which reduce the electrophility of carbonyl carbon

[davidenarb]

Probably because of inductive effects of alkyl groups which reduce the electrophility of carbonyl carbon

Very logical explanation! Thank you

[clarkstill]

Probably because of inductive effects of alkyl groups which reduce the electrophility of carbonyl carbon

Isn't that a kinetic argument?  I think its probably more likely sterics in the acetal product: the quaternary ketal is much more hindered.

[davidenarb]

Probably because of inductive effects of alkyl groups which reduce the electrophility of carbonyl carbon

Isn't that a kinetic argument?  I think its probably more likely sterics in the acetal product: the quaternary ketal is much more hindered.

I think that you are right because in the acetal formation is acid-catalyzed, so the power of an electrophile explains actually the ability on how a weak nucleophile can attack the carbonyl group.

In the textbook, it says that the position of the equilibrium depends on how the nucleophile is able to function as a leaving group as well.

if it function only as a nucleophile ans cannot function a leaving group, so the equilibrium so greatly favors the products; however, if the of the nucleophile can function as a leaving group, then the equilibirum is established.

In the acetal formation, whether it is with aldehyde or ketone,  the nuclophile is ROH (the same nucleophile), so how can we explain this :

"In the formation of the acetal, for many simple aldehydes, the equilibrium favors formation of the acetal; however, for most ketones, the equilibrium favors reactants rather than products."

[rwiew]

Exactly as said above, the electrophilicity is a kinetic argument and the situation here is thermodynamically controlled. The most important factor when comparing the two sides of the equlibrium is that when forming the acetal you break a C=O bond and form two C-O bonds instead. Aldehydes have a weaker C=O bond than ketones - this is due to higher stabilization of that bond in ketones by hyperconjugation from the second alkyl group (sigma(C-H) - pi*(C=O) overlap) - this is where a higher electrophilicity of aldehydes comes from as well, aldehydes also form more stable acetals (hence the C-O bonds formed should be stronger) due to lower steric hindrance than in ketones. Overall these two factors make the equilibrium for aldehydes more favourable towards the acetal side.