Chp 6 Ionic Reaction Elimination

• Elimination reactions are a type of chemical reaction where several atoms either in pairs or groups are removed from a molecule.
• Dehydrohalogenation is used for the synthesis of alkenes and involves competition with a substitution reaction.
• Strong bases such as alkoxides favor elimination.
• The alkoxide bases are made from the corresponding alcohols.
• The E2 reaction involves concerted removal of the proton, formation of the double bond, and departure of the leaving group.
• Both alkyl halide and base concentrations affect the rate of the E2 reaction and therefore the reaction is 2nd order.
• The E1 reaction competes with the S N 1 reaction and likewise goes through a carbocation intermediate.
• Zaitsev’s rule states that base-induced elimination reactions generally give the more stable alkene product.
• The more highly substituted alkene product predominates in base-induced elimination reactions.
• Approach to carbon is sterically hindered and E2 elimination is favored in a secondary substrate.
• Potassium tert - butoxide is an extremely bulky base and is routinely used to favor E2 reaction.
• Increasing temperature favors elimination over substitution.
• In a cyclohexane ring, the eliminating substituents must be diaxial to be anti coplanar.
• Anti coplanar orientation is preferred because all atoms are staggered.
• Approach to carbon is extremely hindered and elimination predominates especially at high temperatures in a tertiary substrate.
• Large sterically hindered bases favor elimination because they cannot directly approach the carbon closely enough to react in a substitution.
• The Stereochemistry of E2 Reactions: All four atoms involved in the transition state must be in the same plane.
• If the base is small, S N 2 competes strongly because approach at carbon is unhindered.
• Hydrogen abstraction at the most substituted carbon is a key factor in base-induced elimination reactions.
• Alexander M. Zaitsev (1841 - 1910) was a Russian chemist who worked on organic compounds and predicted the product composition of an elimination reaction.
• Heats of hydrogenation of three butene isomers are used to predict the overall relative stabilities of alkenes.
• The greater the number of attached alkyl groups (i.e., the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability.
• Hofmann’s rule states that due to steric hindrance, the bulky base will have difficulty in approaching β-hydrogen from more substituted carbon.
• Formation of the less substituted alkene will be favored in base-induced elimination reactions.