2, 2-dimethyl-3-oxobutyric acid undergoes a reaction with bromine in the presence of heat to produce 3-bromo-3-methyl-2-butanone. This reaction involves the formation of an unstable intermediate, which is 2-hydroxy-3-methylbutene.

Explanation:

1. Reaction Overview:
- The reaction involves the addition of bromine to the double bond present in 2, 2-dimethyl-3-oxobutyric acid.
- This addition reaction leads to the formation of an unstable intermediate, which further undergoes a rearrangement to give the final product, 3-bromo-3-methyl-2-butanone.
- The mechanism of this reaction proceeds through the formation of a carbocation intermediate, which is subsequently attacked by bromine.

2. Formation of the Unstable Intermediate:
- The initial step of the reaction involves the addition of bromine to the double bond present in 2, 2-dimethyl-3-oxobutyric acid.
- This addition leads to the formation of a bromonium ion intermediate, which is a cyclic structure with a positive charge on one of the bromine atoms and a partial positive charge on the other bromine atom.
- The bromonium ion intermediate is highly unstable due to the positive charge on one of the bromine atoms, and it undergoes a rearrangement to stabilize itself.

3. Rearrangement to Stabilize the Intermediate:
- The unstable bromonium ion intermediate undergoes a rearrangement to stabilize itself and form a more stable intermediate.
- This rearrangement involves the migration of a hydrogen atom from the methyl group adjacent to the carbonyl group to the carbon atom bearing the positive charge.
- The resulting intermediate is 2-hydroxy-3-methylbutene, which is stabilized by the presence of a double bond and an oxygen atom.
- The stabilization of the intermediate is achieved by delocalization of the positive charge through resonance with the adjacent double bond and oxygen atom.

4. Formation of the Final Product:
- The stabilized intermediate, 2-hydroxy-3-methylbutene, can react further with bromine to yield the final product, 3-bromo-3-methyl-2-butanone.
- This reaction involves the addition of bromine to the double bond present in 2-hydroxy-3-methylbutene, resulting in the substitution of the hydrogen atom with a bromine atom.
- The resulting product, 3-bromo-3-methyl-2-butanone, is a ketone compound with a bromine atom attached to the carbon adjacent to the carbonyl group.

In conclusion, the unstable intermediate involved in the reaction between 2, 2-dimethyl-3-oxobutyric acid and bromine is 2-hydroxy-3-methylbutene. This intermediate is formed through the rearrangement of the initial bromonium ion intermediate and is subsequently converted into the final product, 3-bromo-3-methyl-2-butanone.