Enolates

[NA19]

Can someone explain this reaction? The answer says that LDA is a strong, hindered base, not a nucleophile. How do you tell?

---

Members don't see this ad.

 

[Schenker]

Hindered bases usually have bulky R groups that sterically crowd a lone pair of electrons. In the case of LDA, two isopropyl groups are attached to a nitrogen; one of the lone pairs on the nitrogen is coordintaed to a lithium ion, and the other lone pair on the nitrogen is basic. That is, the two isopropyl groups crowd the basic lone pair, making the lone pair unable to attack most electrophiles. This lone pair, however, is still able to attack very small "electrophiles" like the H+ ion.

Many of the same hindered bases are used over and over again in different chemical reactions. Many of these are secondary or tertiary amines with bulky R groups, like LDA. Keep an eye out for them when you learn new reactions, or try and find a list of them. On the MCAT, if you are asked a question that requires you to know if a base is hindered, there is a good chance that that base is something you would have seen before.

LDA is definitely a base that appears over and over again in your organic chemistry textbook, or in your review books.

As for the reaction, you must know that LDA is a hindered base. That means that all it will do is abstract a proton from the reactant. Thus, look for the proton on the reactant that has the lowest pKa, i.e. the proton that is easiest to abstract. In other words, which carbanion from the parent compound is most stable? The most stable carbanion is the one that is alpha to the carbonyl, since a lone pair of electrons on the alpha carbon can delocalize into the pi orbitals of the carbonyl. There are two possible carbanions that are alpha to the carbonyl: a secondary carbanion and a tertiary carbanion. The secondary carbanion is more stable than the tertiary.

The answer shown is just a resonance form of the secondary carbanion.

 

[monkeyvokes]

Hindered bases usually have bulky R groups that sterically crowd a lone pair of electrons. In the case of LDA, two isopropyl groups are attached to a nitrogen; one of the lone pairs on the nitrogen is coordintaed to a lithium ion, and the other lone pair on the nitrogen is basic. That is, the two isopropyl groups crowd the basic lone pair, making the lone pair unable to attack most electrophiles. This lone pair, however, is still able to attack very small "electrophiles" like the H+ ion.

Many of the same hindered bases are used over and over again in different chemical reactions. Many of these are secondary or tertiary amines with bulky R groups, like LDA. Keep an eye out for them when you learn new reactions, or try and find a list of them. On the MCAT, if you are asked a question that requires you to know if a base is hindered, there is a good chance that that base is something you would have seen before.

LDA is definitely a base that appears over and over again in your organic chemistry textbook, or in your review books.

As for the reaction, you must know that LDA is a hindered base. That means that all it will do is abstract a proton from the reactant. Thus, look for the proton on the reactant that has the lowest pKa, i.e. the proton that is easiest to abstract. In other words, which carbanion from the parent compound is most stable? The most stable carbanion is the one that is alpha to the carbonyl, since a lone pair of electrons on the alpha carbon can delocalize into the pi orbitals of the carbonyl. There are two possible carbanions that are alpha to the carbonyl: a secondary carbanion and a tertiary carbanion. The secondary carbanion is more stable than the tertiary.

The answer shown is just a resonance form of the secondary carbanion.

what? I don't believe this involves carbocation stability. Tertiary are more stable anyway aren't they?
I think the reason your double bond goes to the less hindered carbon is because of the temp, it's the kinetic product (not the thermodynamic product). It's the product that forms easier because there is less steric hindrance, but it isn't the most stable product. The more stable product just requires a bit more energy to form so (and so you can avoid it with low temps)

 

[tuhtles]

LDA is definitely a base that appears over and over again in your organic chemistry textbook, or in your review books.

Yuppp. 👍
OP, I think for the MCAT you should just know LDA is a strong base and can deprotonate an alpha hydrogen to make an enolate.
LDAs are good for making quaternary alpha carbon esters (probably dont need to know this part)

I don't believe this involves carbocation stability. Tertiary are more stable anyway aren't they?

Tertiary carbocations are more stable than secondary carbocations.
However, it's the opposite for carboanions. Secondary carboanions are more stable than tertiary ones because secondary carboanions have less methyl groups and therefore less electron density (since methyl groups are electron donating) to destabilize the negative charge of the carboanion.

 

[monkeyvokes]

Tertiary carbocations are more stable than secondary carbocations.
However, it's the opposite for carboanions. Secondary carboanions are more stable than tertiary ones because secondary carboanions have less methyl groups and therefore less electron density (since methyl groups are electron donating) to destabilize the negative charge of the carboanion.

ohh haha read that wrong, nice