In an E2 elimination, which statement correctly describes the regioselectivity under basic vs acidic conditions?

In an E2 elimination, which statement correctly describes the regioselectivity under basic vs acidic conditions?

• A. Basic conditions favor attack at the less substituted carbon; acidic conditions favor attack at the more substituted carbon.
• B. Basic conditions favor attack at the more substituted carbon; acidic favors less substituted.
• C. Both conditions always attack the more substituted carbon.
• D. Site of attack is independent of substitution.

Answer: (A)

In E2, the base that removes the β-hydrogen controls which carbon is involved. A bulky base is hindered by substituents, so it preferentially abstracts a proton from the less substituted β-carbon where there’s less steric clash. That concerted step with the leaving group then gives the less substituted alkene—the Hofmann product. If the environment is acidic, the leaving group is better activated and the more substituted alkene is thermodynamically more stable, so elimination tends to favor the more substituted double bond. The need for an anti-periplanar arrangement still guides which hydrogen can be removed, but the steric/electronic context shifts the preference. Thus basic conditions lead to attack at the less substituted carbon, while acidic conditions favor attack at the more substituted carbon.

In E2, the base that removes the β-hydrogen controls which carbon is involved. A bulky base is hindered by substituents, so it preferentially abstracts a proton from the less substituted β-carbon where there’s less steric clash. That concerted step with the leaving group then gives the less substituted alkene—the Hofmann product. If the environment is acidic, the leaving group is better activated and the more substituted alkene is thermodynamically more stable, so elimination tends to favor the more substituted double bond. The need for an anti-periplanar arrangement still guides which hydrogen can be removed, but the steric/electronic context shifts the preference. Thus basic conditions lead to attack at the less substituted carbon, while acidic conditions favor attack at the more substituted carbon.