Chapter 7 Part 1: Understanding Sn1 and Sn2

Question 1

What is an alkyl halide? vinyl halide? aryl halide?

Answer 1

alkyl - halide bonded to sp3 carbon
vinyl - halide bonded to c=c
aryl - halide bonded to benzene ring

Question 2

What degree halide is RCH2-X? R2CH-X? R3C-X?

Answer 2

One R group - 1st degree
Two R groups - 2nd degree
Three R groups - 3rd degree

Question 3

How does the size of the R group/ size of the halide effect boiling and melting points?

Answer 3

as the size of the R group increases, surface area increases and BP&MP increase.
as the size of X increases it becomes more polarizable, BP&MP increase.

Question 4

What kind of reactions do alkyl halides undergo with nucleophiles?

Answer 4

Substitution. The X in C-X is replaced by the nucleophile.

Question 5

What kind of reactions do alkyl halides undergo with bronstead lowery bases?

Answer 5

Elimination. The X from C-X and a Hydrogen from a beta carbon are removed and a pi bond is formed.

Question 6

How many steps are in the Sn2 Mechanism

Answer 6

1

Question 7

In the Sn2 mechanism is the alkyl halide in the rate eqation? is the nucleophile? How do their concentrations affect the rate?

Answer 7

Sn2 = One Step = All reactants in rate equation:

rate=k[alkyl halide][:Nu-]

Changing the concentration of either reactant has a mirroring effect on the rate.

Question 8

What is meant by backside attack? how does this affect the stereochemistry of the product? Which mechanism does this apply to?

Answer 8

:Nu approaches from the side opposite of the halide. This results in an inversion of the product. Applies to all Sn2 reactions

Question 9

What degree halides can undergo Sn2 reactions? Rank from slowest to fastest reaction.

Answer 9

Methyl Halides (CH3-X), 1st and 2nd degree alkyl halides.
Slow——>Fast
2nd 1st CH3-X
—————————————————————–>
^Both occur very fast^

Question 10

What is steric hindrance?

Answer 10

A decrease in reaction speed because of bulky groups around the reaction site.

Question 11

How does steric hindrance of the alkyl halide affect the Sn2 mechanism?

Answer 11

Bulky R groups make the backside attack by the :Nu more difficult. This decreases the speed of the reaction

Question 12

Does the Sn2 mechanism use 1st or 2nd order kinetics?

Answer 12

2nd order kinetics

rate=k[RX][:Nu-]

Question 13

Does the Sn1 mechanism use 1st or 2nd order kinetics?

Answer 13

1st order kinetics.

rate=k[RX]

Question 14

What is the first step of the Sn1 mechanism? What does this produce?

Answer 14

Heterolysis (breaking) of the bond between C and X. Produces a carbocation.

    C---/---X creates C(+) & X:(-)

Question 15

What is a racemic mixture?

Answer 15

An equal mixture of two enantiomers produced by the Sn1 mechanism.

Question 16

Which side of the stereogenic carbon does the nucleophile attack in the Sn1 mechanism.

Answer 16

Attacks the front or backside resulting in a racemic mixture

Question 17

Which alkyl halides react via the Sn1 mechanism?

Rank from slowest to fastest

Answer 17

2nd and 3rd degree halides.

Slower Fast
————————————————————–>
2ºR-X 3ºR-X

Question 18

Carbons with 3 R groups produce a ______ stable carbocation than Carbons with 1 R group. Why?

Answer 18

R3C-X produces a more stable carbocation than RCH2-X.

R groups are more polarizable than H atoms, so the (+) charge of the carbocation can be better dispersed.

Question 19

As the stability of the carbocation increases, the rate of the Sn1 reaction________

Answer 19

increases