Retention or Elimination?

📌 CONCEPT: An E1 reaction is a type of elimination reaction that involves the removal of a leaving group from a secondary halide to form an alkene, with the alkyl group attached to the carbon bearing the leaving group being retained.

📐 RULE / FORMULA: The E1 reaction follows the carbocation mechanism, where the leaving group is removed first, forming a carbocation intermediate, and then the alkene is formed by the attack of a nucleophile on the carbocation.

💡 WORKED EXAMPLE: Consider the reaction of C2H5Br: the bromine is removed first to form a tertiary carbocation intermediate, and then a nucleophile attacks the carbocation to form an alkene. For example, if C2H5Br reacts with OH- (hydroxide ion), the product is C2H5=CH2 (ethene) and HBr.

⚠️ COMMON MISTAKE: Students often confuse the E1 reaction with the E2 reaction, where the alkyl group is removed along with the leaving group, but in an E1 reaction, the alkyl group is retained.