[science123]

I saw in one homework problem from Wade 6th Organic Chemistry textbook that we could selectively protect aldehydes when both ketones and aldehydes are present. What's the reason behind it?

[MissPhosgene]

 Alkyl substituents are weak electron donors, so the carbon at a ketone is less electrophilic than that of an aldehyde.

[science123]

Does it have to do with rate of the reaction? I know that ethylene glycol does protect ketones. Are you saying that if we stop the reaction as soon as aldehyde gets protected, then it's selective right? but if we let the reaction happen long enough then wouldn't both be protected?

[Doc Oc]

That would require an excess of the ethylene glycol, so you can control that aspect of the reaction through stoichiometry.  But the aldehyde will certainly react first, it is a much more active carbonyl than a ketone.