NaBH4 cannot reduce these compounds into alcohols but LiAlH4 can. Sources I have read say that this is because NaBH4 is less reactive than LiAlH4 and that is fine, only my intuition tells me that a carbonyl carbon on a carboxylic acid/ester should be MORE electrophilic than a carbonyl carbon on an aldehyde/ketone due to oxygen's electron-withdrawing effects. Why is it that a carbonyl carbon on an aldehyde/ketone is MORE electrophilic than a carbonyl carbon of a carboxylic acid/ester? Gracias.
-
Hello, fellow o-chemer!
So you would think that a carboxylic acid would be more electrophilic than, say, a ketone because there is more than one oxygen attached to the carbonyl carbon to withdraw electron density inductively, right?
Well, usually it isn't quite that simple and there are more things going on. True, both oxygen atoms inductively withdraw electron density, but they also both donate it by resonance as well. Let's say that our carbonyl oxygen is feeling greedy and pulls the pi-bond electrons up to be its own lone pairs (as happens in an important resonance contributor) to gain a -1 charge, and forcing the carbon to gain a +1 charge. Immediately, the other oxygen of the carboxilic acid's -OH group can donate a lone pair of its own to gain an unfavorable, but possible, +1 charge and restore the carbon's neutrality. These are all resonance structures that contribute to the overall average charge on each of the atoms involved.
Now, let's say that a ketone's carbonyl oxygen sees this and decides, "hey, I want to be greedy too!" as it takes the pi bonded elections to become its lone pair and gains a charge as -1 and forces the carbonyl carbon to adopt a +1 charge, as happened before. Now, there is no other oxygen with a lone pair to come to the rescue with another lone pair to collapse down into a pi bond to the carbonyl carbon, so the carbon is stuck with a +1 charge.
So you would think that a carboxylic acid would be more electrophilic than, say, a ketone because there is more than one oxygen attached to the carbonyl carbon to withdraw electron density inductively, right?
Well, usually it isn't quite that simple and there are more things going on. True, both oxygen atoms inductively withdraw electron density, but they also both donate it by resonance as well. Let's say that our carbonyl oxygen is feeling greedy and pulls the pi-bond electrons up to be its own lone pairs (as happens in an important resonance contributor) to gain a -1 charge, and forcing the carbon to gain a +1 charge. Immediately, the other oxygen of the carboxilic acid's -OH group can donate a lone pair of its own to gain an unfavorable, but possible, +1 charge and restore the carbon's neutrality. These are all resonance structures that contribute to the overall average charge on each of the atoms involved.
Now, let's say that a ketone's carbonyl oxygen sees this and decides, "hey, I want to be greedy too!" as it takes the pi bonded elections to become its lone pair and gains a charge as -1 and forces the carbonyl carbon to adopt a +1 charge, as happened before. Now, there is no other oxygen with a lone pair to come to the rescue with another lone pair to collapse down into a pi bond to the carbonyl carbon, so the carbon is stuck with a +1 charge.
12
keywords: NaBH,borohydride,esters,sodium,or,cant,Why,acids,carboxylic,reduce,Why cant sodium borohydride (NaBH4) reduce carboxylic acids or esters