After completing this section, you should be able to
- identify the two possible ways in which a given enolate anion could conceivably react with an electrophile.
- write an equation to illustrate the haloform reaction.
- identify the products formed from the reaction of a given methyl ketone with a halogen and excess base.
- identify the methyl ketone, the reagents, or both, needed to obtain a specified carboxylic acid through a haloform reaction.
Make certain that you can define, and use in context, the key terms below.
- haloform reaction
Because the negative charge on an enolate ion is delocalized, there are two reactive sites and therefore two potential products. The α‑substituted product is much more common.
A “haloform” is any compound of the type CHX3, where X = Cl, Br or I. Of these three compounds, chloroform is the most common.
The haloform reaction describedin the reading is usually carried out with iodine. This reaction is called the “iodoform test,” and is one of the reactions carried out in the laboratory as a simple qualitative test for methyl ketones.
General Reaction of Enolates
The Ambident Character of Enolate Anions
Since the negative charge of an enolate anion is delocalized over the alpha-carbon and the oxygen, electrophiles may bond to either atom. Reactants having two or more reactive sites are called ambident, so this term is properly applied to enolate anions. Modestly electrophilic reactants such as alkyl halides are not sufficiently reactive to combine with neutral enol tautomers, but the increased nucleophilicity of the enolate anion conjugate base permits such reactions to take place. Because alkylations are usually irreversible, their products should reflect the inherent (kinetic) reactivity of the different nucleophilic sites.
If an alkyl halide undergoes an SN2 reaction at the carbon atom of an enolate anion the product is an alkylated aldehyde or ketone. On the other hand, if the SN2 reaction occurs at oxygen the product is an ether derivative of the enol tautomer; such compounds are stable in the absence of acid and may be isolated and characterized. These alkylations (shown above) are irreversible under the conditions normally used for SN2 reactions, so the product composition should provide a measure of the relative rates of substitution at carbon versus oxygen. It has been found that this competition is sensitive to a number of factors, including negative charge density, solvation, cation coordination and product stability.
The Haloform Reaction
Methyl ketones typically undergo halogenation three times to give a trihalo ketone due to the increased reactivity of the halogenated product as discussed above. This trihalomethyl group is an effective leaving group due to the three electron withdrawing halogens and can be cleaved by a hydroxide anion to effect the haloform reaction. The product of this reaction is a carboxylate and a haloform molecule (CHCl3, CHBr3, CHI3). Overall the haloform reaction represents an effective method for the conversion of methyl ketones to carboxylic acids. Typically, this reaction is performed using iodine because the subsequent iodoform (CHI3) is a bright yellow precipitate which is easily filtered off.
Example: The Haloform Reaction
1) Formation of the trihalo species
2) Nulceophilic attack on the carbonyl carbon
3) Removal of the leaving group