Elimination

• What are the three factors that influence alkene stability?

  1. Degree of substitution (more highly substituted=more stable)
  2. Stereochemistry: trans > cis b/c of reduced steric interactions
  3. Conjugated alkenes are more stable than isolated alkenes
• 3 bonding changes occur during eliminations:

  • a bond breaks H to C
  • A new pi bond forms C to C
  • a bond breaks C to LG
• E2?

  A concerted process, with a rate of reaction = k[base][R-LG].

  • base attacks H and breaks H-C bond causing the formation of a pi C-C bond and the C-LG bond to break.
• E1?

  C-Lg bond breaks first to form intermediate (carbocation), and then loss of proton to form pi bond between C-C+
• reaction order of E1?

  Tertiary>Secondary>Primary> methyl to stabilize carbocation
• reaction order of E2?

  Tertiary, secondary, primary, methyl because 2 sp3 atoms will be converted into sp2 atoms...
• Eliminations often favour trans-product over the cis-product for stability
• Zaitsev's rule?

  Preference for elimination to give the more highly substituted (more stable) alkene which is known as Zaitsev's product
• Increasing the size (sterics) of the base tends to increase the yield of the Hofmann (anti-zaitsev) product.
• Better leaving group will increase yield of Zaitsev product
• Zaitsev's rule also says that the carbon more likely to be deprotonated is the carbon with few hydrogens to begin with
• A sterically hindered base cannot effectively react with the tertiary H, so it will instead deprotonate the primary carbon leading to the formation of the anti-Zaitsev or hofmann product
• The higher the heat of combustion, the higher energy that the product is and thus the less stable the product is
• More alkyl groups on the c=c double unit make it more stable because alkyl groups are weak electron donors that can stabilize sp2C...
• In E2 reactions, there is a preference for the H-C and C-LG bonds to be coplanar and 180 degrees from each pther to help position the sigma bond about to break in the correct orientation to make a pi bond. AKA being in antiperiplanar conformation
• Synperiplanar arrangments where the angle between the H-C bond and C-LG is 0o are also known, usually in systems that are either inflexible rings or intramolecular eliminations
• In cyclic systems, the LG has to be axial in order to be anti to the C-H bond for E2
• strong nucleophiles but weak bases: CN, N3, Cl, Br I, SH, SR (all negatively charged ions)