CH9: Substitution Reactions

Question 1

What is the difference between the 1 and 2 in SN1/SN2?

Answer 1

The 1 stands for only 1 component involved in the RDS (can also be thought of as 1 arrow per 1 step).
The 2 stands for 2 components involved in RDs (can be thought of as 2 arrows in 1 step).

Question 2

What do SN1 and E1 both need?

Answer 2

A distinct, stable carbocation intermediate as well as a stable leaving group.

Question 3

Which mechanisms lose stereochemistry in the final product?

Answer 3

SN1 and E1

Question 4

What happens to the stereochemistry of SN2?

Answer 4

If the nucleophile is attacking the same carbon that has stereochemistry, then the configuration flips (wedges turn to dashes, vice versa) but ONLY on the carbon being attacked

Question 5

What 4 factors determine which mechanism the reaction proceeds through?

Answer 5

Electrophile/Substrate
Nucleophile
Leaving Group
Solvent (least important)

Question 6

What type of substrates will be SN2? SN1?

Answer 6

SN2: methyl, 1, 2
SN1: 2, 3

Question 7

Why will a 3* substrate NOT be SN2?

Answer 7

Due to the R-groups blocking the way for the nuc to attack.

Question 8

What are the exceptions to 1* substrates?

Answer 8

If they are resonance stabilized, they may be SN1.
Happens with allylic and benzylic carbocations.

Question 9

What does a strong nuc favor? a weak nuc? Why?

Answer 9

Strong nuc favors SN2 because it has to be able to attack AND kick off the L.G.
SN1 can use strong or weak, but typically favors a weak nuc because it is easy to attack the positive charge on a carbocation

Question 10

What makes a nuc strong?

Answer 10

A negative charge.
*Look out for charge balancers such as Na, Li, etc.

Question 11

What makes a nuc weak?

Answer 11

Lone pairs. Fluorine (F) is exception as very electronegative.

Question 12

What makes a leaving group good?

Answer 12

It is stable even without a negative charge.

Question 13

What makes a leaving group bad?

Answer 13

Unstable on its own. This includes L.G that are already negative before they leave.

Question 14

What are examples of good/strong leaving groups?

Answer 14

Halides (I, Br, Cl)
H2O+
Sulfonate Ions

Question 15

What are the 3 Sulfonate ions and their abbreviations?

Answer 15

Tosylate -OTs or -TS
Mesylate -OMS
Triflate - OTF

Question 16

What are examples of weak/bad leaving groups?

Answer 16

-OH , RO- , O- , NH2

Question 17

What is NOT a L.G.?

Answer 17

Any C-H only groups, and R-groups.

Question 18

What are the 2 types of solvents? What is their difference?

Answer 18

Polar Protic: Have an acidic proton/hydrogen
Polar Aprotic: NO acidic proton/hydrogen

Question 19

What are examples of polar protic solvents?

Answer 19

H2O , ROH
(It is protic if a hydrogen is bonded to anything other than a carbon)

Question 20

What are examples of polar aprotic solvents (5)?

Answer 20

Acetone, DMSO, DCM, DMF, THF

Question 21

Which mechanism prefers polar protic and why?

Answer 21

SN1 as solvent stabilizes leaving group and carbonation.
(Solvent for SN1 rxn is more important than the solvent for SN2, but protic slows down SN2 due to stabilizing nucleophile).

Question 22

Which mechanism prefers polar aprotic solvents and why?

Answer 22

SN2 as the solvent traps the counter ion on the nucleophile in a shell, creating a more exposed nucleophile. Additionally it stabilizes the transition state of SN2

Question 23

Define polar

Answer 23

Difference in electronegativity values.

Question 24

A 2* carbocation can be stabilized through…

Answer 24

Hydride and Methyl Shifts

Question 25

What mechanism will it be if you depronate the acidic nucleophile to turn an OH L.G. into a good L.G.

Answer 25

SN1 as it is now 2 steps.