Enolates
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The Enolate
Protons that are on a carbon atom adjacent to a carbonyl, the alpha carbon atom, are quite acidic. This is because once the molecule is deprotonated, the resulting carbanion can be stabilized through resonance forms. The major resonance form is the one with the negative charge on the oxygen atom. That is more stable than the negative charge on the carbon atom. This negative species is called an enolate (pronounced een-ohl-ate) ion.
An enol is a compound with a carbon-carbon double bond and an alcohol group on that double bond. An enolate is an enol that is deprotonated making it have a negative charge. The negative charge is why it has the “ate” on the name.
Even though the major form of the enolate is the one with the negative charge on the oxygen atom, I think it is beneficial to keep in mind the minor version, with the negative charge on the carbon atom, for the reactions in this chapter.
A good base like hydroxide (HO-) is used to remove the alpha hydrogen atom. A bigger, bulkier base is also often used. The base of choice is one named LDA, lithium diisopropyl amide. This is a lithium salt of a negative nitrogen atom with two big, bulky isopropyl groups attached to it.
LDA, lithium diisopropyl amide
Enolate as nucleophile
Enolates are good nucleophiles. Once the negative carbon atom is formed, the enolate can attack halogens like Br2 or Cl2. The alpha carbon atom can be halogenated in this way.
Enolates can also perform SN2 reactions and attack alkyl halides.
1. What are the products of the following reactions?
a) acetone + LDA followed by chloromethane
b) cyclohexanone + LDA followed by Br2
c)
Answers
1.
a)
b)
c)
