Substitution And Elimination Reactions & Related Concepts

Question 1

Why do carbocation rearrangements occur?

Answer 1

They occur to produce a more stable carbocation. For example, a secondary carbocation will become a tertiary carbocation through a hydride shift.

Question 2

(Weak/strong) bases make good leaving groups.

Answer 2

Weak

Question 3

A good prediction is that a good leaving group is also a ___________.

Answer 3

good nucleophile

Question 4

How do sterically hindered substrates affect the type of substitution reaction that occurs?

Answer 4

Sterically hindered substrates undergo SN1 reactions; they don’t undergo SN2 reactions.

Question 5

SN2 reactions are compatible with (polar protic/polar aprotic) solvents.

Answer 5

Polar aprotic solvents

Question 6

SN1 reactions are compatible with (polar protic/polar aprotic) solvents.

Answer 6

Polar protic solvents

Question 7

What are the stereochemical outcomes of a SN2 reaction?

Answer 7

An SN2 reaction is stereospecific because the nucleophile attacks from the back, this causes the product to change R/S configuration from the original reactant.

Question 8

What are the stereochemical outcomes of an SN1 reaction?

Answer 8

Racemic mixture of enantiomers given that the initial reactant has a stereo enter (chiral carbon)

Question 9

What does it mean when a SN2 reaction is said to be “second order”?

Answer 9

It means that the rate of the reaction is dependent upon both the type of substrate and the type of nucleophile used.

Question 10

True or false:
For an SN1 reaction, the rate of the reaction depends on the substrate (alkyl halide) and the nucleophile used.

Answer 10

False, an SN1 reaction is first order which means that reaction rate will only be dependent on the substrate used.

Question 11

Which reactant will have a faster rate when undergoing an SN1 reaction?
1. CH3C(CH3)2CH2Br
2. CH3CHCH3CH2Br

Answer 11

1. Because it is more sterically hindered and go undergo a methyl shift via carbocation rearrangement to form a more stable tertiary carbocation.

Question 12

Name, examples of polar protic solvents and polar aprotic solvents.

Answer 12

Polar protic - water, alcohols (any solvent with OH), IPA, ethanol, methanol
Polar aprotic - DMSO, acetone, DMF, THF, ACN

Question 13

How do inductive atoms (such as F or Cl) on adjacent carbons affect stability of the carbocation?

Answer 13

Inductive atoms, such as fluorine or chlorine are very electronegative which means that they withdraw electrons from the nearby electrophile, making the subsequent carbocation that forms less stable.

Question 14

True or false:
Sterically hindered substrates (even hinderance/substituents present on the beta carbon) next to the alkyl halide affect the reaction rate of an SN2 reaction.

Answer 14

True. For example, methyl groups that are present on the beta carbon next to the bromine affect the reaction rate of an SN2 reaction.

Question 15

How does the type of nucleophile affect the rate of an SN1 reaction?

Answer 15

Nucleophile choice doesn’t affect SN1 reactions because they are first order and only dependent on type of substrate used. Therefore, a sterically hindered or non-hindered nucleophile has the same effect on rate (doesn’t change).

Question 16

If you are reacting a primary alkyl bromide with a sterically hindered nucleophile, how does this affect the rate of the reaction?

Answer 16

A sterically hindered nucleophile for an SN2 reaction causes a decrease in rate of the reaction.

Question 17

Substrates that undergo SN1:

Answer 17

Secondary and tertiary alkyl halides, allylic alkyl halide (resonance forming)

Question 18

Substrates that undergo SN2

Answer 18

Primary alkyl halides

Question 19

What are the factors that determine the strength/reactivity of a nucleophile?

Answer 19

1. Negative charge
2. Higher electronegativity
3. Bigger size
4. No resonance
5. No steric hindrance

Question 20

Why are alkyl fluorides not used for SN2 reactions?

Answer 20

Because the C—F bond is to strong, so it doesn’t make a good leaving group.

Question 21

What is the rate limiting step of SN1 reaction?

Answer 21

Formation of carbocation intermediate (1st step)

Question 22

What is the rate limiting step of SN2 reaction?

Answer 22

Nucleophilic attack

Question 23

Name the nucleophiles used in SN1 reactions. Are they weak or strong nucleophiles?

Answer 23

H2O, MeOH, and EtOH
WEAK

Question 24

Name the nucleophiles used in SN2 reactions

Answer 24

Strong nucleophiles ONLY; I-, Br-, Cl-, OH-, EtO-, MeO-

Question 25

What makes a good leaving group? What is the best LG out of all halogens?

Answer 25

They must be able to stabilize the negative charge when the nucleophile attacks. Therefore, out of all of the halogen LGs, Iodide is the best one because it has the largest size (atomic radius) and can stabilize the negative charge best.