Chapter 10: Elimination Reactions

Question 1

What is the mechanism for this reaction?

Answer 1

E2 elimination reaction
Base extracts H+ atom from Beta carbon and the C-H bond breaks to form a new double bond. Leaving group also departs to accommodate the formation of the new double bond.

Question 2

The rate of E2 elimination reactions are dependent on what?

Answer 2

Concentrations of the base and substrate

Question 3

What is an alpha carbon?

Answer 3

Carbon directly attached to a leaving group

Question 4

What is a beta hydrogen atom?

Answer 4

hydrogen atoms attached to carbon atoms directly adjacent to the alpha carbon (the double bond will form between the alpha carbon and beta hydrogen)

Question 5

What is a regiochemical outcome?

Answer 5

Important for E2 reactions
The region of the molecule the reaction takes place (where the double bond forms)

Question 6

What is the favored regiochemical outcome?

Answer 6

The more substituted alkene (Zaitsev) because they are more stable

Question 7

What is the minor or less substituted alkene called?

Answer 7

Hoffman

Question 8

When would a less substituted alkene be favored in an E2 reaction?

Answer 8

when a sterically hindered base is used (bulky)

Question 9

Answer 9

Sterically hindered (bulky) base
Will favor a less substituted alkene

Question 10

Answer 10

Sterically hindered (bulky) base
Will favor a less substituted alkene

Question 11

What is a stereochemical outcome?

Answer 11

Important for E2 elimination reactions that have two identical beta positions
The formation of the double bond will produce that same result so regiochemistry is not the issue here

Question 12

What stereoisomer is favored and will be the major product in elimination reactions?

Answer 12

Trans (more stable than Cis)

Question 13

When is an E2 reaction said to be stereoselective?

Answer 13

When the substrate has two identical beta positions

Question 14

When is an E2 reaction said to be stereospecific?

Answer 14

When the beta position only contains one proton
The LG and beta proton must be antiperiplanar

Question 15

How do you know if the LG and beta H are antiperiplanar?

Answer 15

Draw newman projection and rotate the molecule

Question 16

What is happening in this reaction?

Answer 16

E2 reaction that is stereospecific because there is only one major beta proton so only one product is formed.
The LG and beta H were antiperiplanar prior to being eliminated, giving us the product shown

Question 17

Answer 17

How to determine major products with a cyclohexane
Need to look for antiperiplanar arrangement of LG and beta H atoms in the chair conformations

Question 18

Answer 18

Notice that the only antiperiplanar arrangement of LG and beta H atom is in the less favored chair conformation. This means the reaction will be very slow.

Question 19

What is the mechanism for this reaction?

Answer 19

E1 elimination reaction
The LG departs forming a carbocation
Base extracts H+ from beta carbon and the C-H bond breaks to form a new double bond

Question 20

What determines the rate of E1 reactions?

Answer 20

The concentration of substrate
Increasing the concentration of the base will not effect the rate of E1 reactions

Question 21

What is slower E1 or E2 reactions?

Answer 21

E1

Question 22

What regiochemical outcome is favored in elimination reactions?

Answer 22

The more substituted alkene (Zaitsev)

Question 23

What is the favored stereochemical outcome in elimination reactions?

Answer 23

Trans/E isomer

Question 24

How do you determine if a reaction will be substitution, elimination, or both? Will they be SN1, SN2, E1, E2, or a combination of both?

Answer 24

Look at the function of the reagent
Strong or weak nucleophile?
Strong or weak base?
The substrate will also matter (primary, secondary, or tertiary)

Question 25

Weak or Strong nucleophiles?
Weak or strong bases?

Answer 25

Strong nucleophiles, weak bases
Will favor substitution reactions (need to look at the substrate to determine SN1 or SN2)

Question 26

Weak or Strong nucleophiles?
Weak or strong bases?

Answer 26

Strong bases, weak nucleophiles
Will favor E2 elimination reactions
Won’t result in E1 reactions because they are not dependent on the base like E2

Question 27

Weak or Strong nucleophiles?
Weak or strong bases?

Answer 27

Strong nucleophiles, strong bases
Will be a mixture of elimination and substitution reactions
the substrate (primary, secondary, tertiary) will determine if the reaction favors E2 or SN2

Question 28

What mechanism will be favored with a strong nucleophile/strong base and a PRIMARY substrate?

Answer 28

SN2 favored unless a bulky base like t-BuOK is used. In that case, E2 will be favored because of steric hinderance

Question 28

What mechanism will be favored with a strong nucleophile/strong base and a SECONDARY substrate?

Answer 28

E2 favored because it is not sensitive to steric considerations like SN2

Question 28

What mechanism will be favored with a strong nucleophile/strong base and a TERTIARY substrate?

Answer 28

E2 will only occur because SN2 is too sterically hindered by the tertiary substrate

Question 29

Weak or Strong nucleophiles?
Weak or strong bases?

Answer 29

weak nucleophiles, weak bases
Typically will only cause SN1 and E1 reactions with tertiary substrates

Question 30

What mechanism will occur with a weak nucleophile/weak base and a PRIMARY substrate?

Answer 30

SN2 and E2
Reaction is very slow and typically won’t occur

Question 31

What mechanism will occur with a weak nucleophile/weak base and a SECONDARY substrate?

Answer 31

SN2, SN1, E2, E1
Not practical because these reactions are very slow and too many products formed
A secondary alcohol treated with sulfuric acid and heat will undergo an E1 reaction

Question 32

What mechanism will occur with a weak nucleophile/weak base and a TERTIARY substrate?

Answer 32

SN1 and E1 equally