Beta Elimination

Question 1

What bases promote ß-Elimintaion?

Answer 1

OH^-, OR^-, NH₂^- acetylide anions (RC≡C^-)

Question 2

What is Zaitsev’s Rule?

Answer 2

A rule stating that the major product of a Beta-Elimintaion reaction is the most stable alkene; that is, it is the alkene with the greatest number of substituents on the carbon-carbon double bond.

Question 3

Define E1.

Answer 3

A unimolecular Beta-Elimination reaction. Only one species is involved in the rate-determining step, that forms a carbocation.

Question 4

Define E2.

Answer 4

A bimolecular Beta-Elimination reaction; both the haloalkane and the base are involved in the transition state for the rate-determining step.

Question 5

What are the kinetics for E1 reactions?

Answer 5

First order Rate=k[RX]

Question 6

What are the kinetics for E2 reactions?

Answer 6

Second order Rate= k[RX][Base]

Question 7

What is the Regioselectivity seen in E1 reactions?

Answer 7

After the carbocation is formed in the rate-determining step of an E1 reaction, it may lose a hydrogen to complete Beta-Elimination, or it may rearrange to a more stable carbocation and then lose a hydrogen.

Question 8

What is the Regioselectivity seen in E2 reactions?

Answer 8

The transition state of lowest energy is commonly that leading to the most highly substituted alkene. For similar reasons, trans double bonds predominate over cis double bonds in the products when either is possible. Note that E2 elimination at a 2nd degree Carbon predominates over SN2 reaction with the strongly basic alkoxide ions.

Question 9

Describe the reactions seen in methyl halides (CH₃X).

Answer 9

SN2 is the only reaction observed. SN1 reactions of methyl halides are never observer. The methyl cation is so unstable that it is not observed in common solvents

Question 10

Describe the reactions seen with Primary Halides (RCH₂X).

Answer 10

S_N2 - The main reaction with good nucleophiles/weak bases such as I^- and CH₃COO^-.

E2 - The main reaction with strong, bulky bases such as (CH₃)₃CO^-.

S_N1/E1 - Primary cations are rarely formed in solution, and therefore,S_N1 and E1 reactions of primary halides are unlikely.

Question 11

Describe the reactions seen with Secondary Halides (R₂CHX).

Answer 11

S_N2 - The main reaction with bases/nucleophile where pKa of the conjugate acid is 11 or less, as in I^- and CH₃COO^-.

E2 - The main reaction with bases/nucleophiles where the pKa of the conjugate acid is 11 or greater, as in OH^- and CH₃CH₂O^-.

S_N1/E1 - Common in reactions with weak nucleophiles in polar protic solvents, such as water, methanol, and ethanol.

Question 12

Describe the reactions seen with Tertiary Halides (R₃CX).

Answer 12

E2 - Main reaction with strong bases such as HO^- and RO^-.

S_N1/E1 - Main reactions with poor nucleophiles/weak bases of the solvent is polar protic.

S_N2 - SN2 reactions of tertiary halides never observed because of the extreme crowding around the 3º carbon.