E2 reactions

Question 1

basics of E2 reactions

Answer 1

• beta-elimination: loss of electron-donating group (often H)
  pushes electrons to make alkene
• good LG on alpha carbon is kicked out

Question 2

dehydrohalogenation

Answer 2

specific type of E2 reaction with H and halogen eliminated!

Question 3

best bases for E2

Answer 3

big, bulky bases are best, because they are less likely to act as nucleophiles!

Question 4

stereochemistry in E2 reactions

Answer 4

• concerted, back-side attack on beta carbon
• H must be anticoplanar to LG
• often mixture of products based on which type of beta carbon is attacked

Question 5

Zaitsev’s rule

Answer 5

in alcohol dehydrations (but also in E2 reactions) the most stable alkene is formed preferentially!

Question 6

what makes alkenes more stable?

Answer 6

1. more alkyl groups: dipoles increase stability
2. trans alkenes are more stable than cis because there’s no steric hindrance!

Question 7

stability of cycloalkenes

Answer 7

• alkenes in small rings are very unstable due to angle strain!
• cycloalkane chairs have an altered form; positions are now “pseudoaxial” and “pseudoequatorial”
• trans cycloalkenes are very unstable and basically impossible if less than cyclooctene

Question 8

making cis vs trans alkenes during E2 reactions

Answer 8

• alpha and beta carbon bond must be rotated to make different stereoisomer alkenes
• cis alkenes have more steric interactions in transition state, making formation less favorable

Question 9

axial vs equatorial LG in E2 reactions

Answer 9

• axial LG has anticoplanar H for backside attack
• equatorial LG has no anticoplanar H!!

Question 10

sterics of Sn2 vs E2 reactions

Answer 10

• SN2: less substituted LG is best
• E2: less substituted beta carbon is best; also more substituted LG!

Question 11

nucleophiles vs bases for Sn2 vs E2 reactions

Answer 11

• Sn2 reactions need good nucleophiles
• E2 reactions need good bases

Question 12

solvents for Sn2 and E2 reactions

Answer 12

• both want moderately polar, aprotic solvents
• nucleophile and electrophile matter more than the specific solvent!

Question 13

determining if Sn2 or E2 will happen

Answer 13

• E2 dominates: strong base; more steric hindrance on alkyl halide and/or base
• Sn2 dominates: strong nucleophile but weak base; minimal steric hindrance on alkyl halide and base

Question 14

good nucleophiles that are weak bases (for Sn2 reactions)

Answer 14

• thiolates (HS-, RS-)
• cyanides (CN-)
• azide (N3 -)
• halides (Cl-, Br-, I-)