Reaction and Mechanism:

The given compound ( )-2,3-Dibromobutane is treated with zinc dust in ethanol. This reaction is a dehalogenation reaction and is known as the dehalogenation of vicinal dihalides.

The mechanism of this reaction involves the formation of a carbanion intermediate. The zinc dust donates electrons to one of the bromine atoms forming a negatively charged bromine ion and a zinc bromide. The negatively charged bromine ion then removes a proton from the β-carbon atom, leading to the formation of a carbanion intermediate.

The carbanion intermediate then undergoes elimination of a bromine ion to form a double bond. Since the carbanion intermediate is stabilized by the electron-withdrawing effects of the two bromine atoms, the elimination of a bromine ion is favored over the elimination of a hydrogen ion.

Therefore, the major product formed is cis-2-butene.

Explanation:

The product formed in the reaction is cis-2-butene. This is because the carbanion intermediate formed during the reaction is stabilized by the electron-withdrawing effects of the two bromine atoms. This stabilization makes the elimination of a bromine ion more favorable than the elimination of a hydrogen ion.

The elimination of a bromine ion leads to the formation of a double bond between the β-carbon and the α-carbon atoms. Since the two bromine atoms are on the same side of the molecule, the double bond formed is cis. Therefore, the major product formed in the reaction is cis-2-butene.

Conclusion:

In conclusion, the dehalogenation of ( )-2,3-Dibromobutane with zinc dust in ethanol leads to the formation of cis-2-butene as the major product. This reaction involves the formation of a carbanion intermediate, which undergoes elimination of a bromine ion to form a double bond between the β-carbon and the α-carbon atoms.