December 4 - Substitution and Elimination 2

Question 1

E1

What does it stand for?
What are the reactants?
What happens in it?

Answer 1

Unimolecular (1) elimination (E)

Between an alkyl halide and a base

In the first step, the alkyl halide dissociates, forming a carbocation. In the second step, the base removes a proton from the β-carbon and the electrons it shared with that carbon make a double bond between the β-carbon and the carbocation

8.7, p.283

Question 2

E2

What does it stand for?
What are the reactants?
What happens in it?

Answer 2

Bimolecular (2) elimination (E)

Between an alkyl halde and a base

The base removes a proton from the β-carbon, and the electrons it shared with that carbon make a double bond. Then the halogen leaves

8.7, p. 282

Question 3

Rate law for E2

Answer 3

k[alkyl halide][base]

Depends on the concentration of both

Question 4

Rate law for E1

Answer 4

k[alkyl halide]

Only depends on the concentration of the alkyl halide

Question 5

β-carbon

Answer 5

A carbon that is adjacent to the carbon bonded to the halogen

Question 6

Which alkyl halides undergo E1 and E2 reactions?

Answer 6

Only tertiary alkyl halides undergo E1

Primary, secondary, and tertiary alkyl halides undergo E2

Question 7

How to tell the stability of an alkene

Answer 7

The greater the number of alkyl substituents, the more stable the alkene

Question 8

Overall rate law

And interpretation of it

Answer 8

rate = k1[alkyl halide] + k2[alkyl halide][nucleophile] + k1[alkyl halide] + k2[alkyl halide][base]

SN2 and E2 reactions are favored by a high concentration of nucleophile/strong base

SN1 and E1 reactions are favored by a poor nucleophile/weak base

The sum of the rate laws for SN1, SN2, E1, and E2 reactions respectively

Question 9

Relative reactivities of alkyl halides in SN2 and E2 reactions

Which alkyl halides undergo either substitution or elimination?

Answer 9

SN2: 1° > 2° > 3°
E2: 3° > 2° > 1°

Primary alkyl halides mostly undergo substitution (SN2)
Secondary alkyl halides undergo both substitution and elimination
Tertiary alkyl halides only undergo elimination (E2)

Question 10

Relative reactivities of alkyl halides in SN1 and E1 reactions

Answer 10

Only tertiary alkyl halides can undergo SN1/E1

Because it’s a 2-step process, alkyl halides undergo both substitution and elimination

Question 11

Selectivity for E2 and E1

The exception for elimination reactions

Answer 11

Regioselectivity - which constitutional isomer is formed?

Stereoselectivity - which stereoisomer is formed?

Eliminations tend to be selective for the most stable alkene product but the EXCEPTION is with bulky bases