Slides 2 through 9 lead the students to the molecular orbital argument for backside displacement in S N 2 reactions.

• Slides 2 through 9 lead the students to the molecular orbital argument for backside displacement in SN2 reactions.

SN2 Mechanism

• backside attack•  2 explanations

– less hindrance to approach of nucleophile from behind C-L bond than from front

– MO theory

SN2 Mechanism

E

Nu filled nonbondingMO (n), HOMO

R-L empty antibondingMO s*, LUMO

SN2 Mechanism

 -         Is there good overlap?a. Yes b. Noc. This interaction is orthogonal. 

LRNu

HOMO (n) LUMO * of R-L bond

SN2 Mechanism

- Is this a stabilizing interaction?

 a. Yes b. No c. Neither 

LRNu

HOMO (n) LUMO * of R-L bond

SN2 Mechanism

          - Is this a stabilizing interaction? a. Yes b. No c. Neither 

Nu R L

filled n filled of R-L bond

SN2 Mechanism

• Front side attack

Nu

R L

antibondingbonding

empty * of R-L

filled n of Nu

SN2 Mechanism

-         Is there good overlap?

a. Yes

b. No

c. This interaction is orthogonal.

SN2 Mechanism

Is this a stabilizing interaction?

a. Yes

b. No

SN2 Mechanism

partial bond, charge is dispersed

  - 2 partial bonds are weaker than 1 full bond- nonbonded interactions are greater

Would you expect the intermediate structure to be higher or lower in energy than iodoethane and bromoethane?

 a. higher b. lower c. approximately equal

I + C

H3C

HH

Br C

CH3

HH

BrI

I C

H

H

CH3

+ Br

Potential Energy Diagram of Reaction

• The height of the TS above reactants is the amount of energy that is required for a molecule of starting material to be transformed into product. What is this specifically known as?

a. Kinetic energyb. Free energyc. Reaction energyd. Reactant energy e. Activation Energy 

• The following question addresses the effect of contact ion pairs and solvent separated ion pairs on the stereochemistry of SN1 reactions.

• Which enantiomer is more likely to form?a. Path a b. Path b. c. No preference 

C

CH3

ClPhH

C CH3

Ph

HOH2

a

b

a

b

O

C

CH3

HH

PhH

C

O

CH3PhH

H H

Cl-

• Slides 15 through 18 review the effect of substrate structure on nucleophilic reactions.

1. Effect of Substrate

Rate of SN2 reaction (from left to right)

a. increasing b. decreasing c. ~ same

H3C X H3C CH2

X H3C CH

CH3

X C

CH3

H3C

CH3

X

Why? 

a. electron donating ability of alkyl groups

b. electron withdrawing ability of alkyl groups

c. steric hindrance blocking approach of nucleophiles

d. leaving group ability increasing 

In the same series …

Ease of carbocation formation from left to right ?

a. increasing b. decreasing

c. ~ same

Rate of SN1 reaction from left to right?

a. increasing b. decreasing

c. ~ same 

• Slides 20 through 23 ensure that students understand potential energy diagrams in relation to solvent effects.

Solvents affect the energy difference between reactants and TS. It affects the height of the energy barrier.

• The dashed line represents how solvent affected the reaction. Would the reaction proceed faster or slower in this solvent?

PE

Extent of Reaction

• Would the reaction proceed faster or slower in this solvent?

a. faster b. slower c. ~ same

The activation energy is smaller. When that is true, the reaction rate is faster.

PE

Extent of Reaction

• Now the dashed line represents how another solvent affected the reaction.

PE

Extent of Reaction

• Would the reaction proceed faster or slower in this solvent?

a. faster b. slower c. ~ same 

PE

Extent of Reaction

What is the approximate hybridization of the central C atom of the transition state?

a. s b. sp c. sp2

d. sp3 e. p

CH3

C

BrH3C(H2C)5

HI- + C

CH3

I Br

(CH2)5CH3H

CH3

C

I(CH2)5CH3

H + Br-

What is the approximate electronic geometry around the central C atom?

a. Linear b. trigonal planar c. pyramidal

d. tetrahedral e. trigonal bipyramidal

CH3

C

BrH3C(H2C)5

HI- + C

CH3

I Br

(CH2)5CH3H

CH3

C

I(CH2)5CH3

H + Br-

What are the partial charges on the I and Br atoms respectively?

a. d+ and d+ b. d- and d+

c. d+ and d- d. d- and d-

e. There should be no partial charges.

CH3

C

BrH3C(H2C)5

HI- + C

CH3

I Br

(CH2)5CH3H

CH3

C

I(CH2)5CH3

H + Br-

What type of substrate is 2-bromooctane?

a. primary b. secondaryc. tertiary d. quaternary

e. methyl

CH3

C

BrH3C(H2C)5

HI- + C

CH3

I Br

(CH2)5CH3H

CH3

C

I(CH2)5CH3

H + Br-

Predict the product(s) of the following SN2 reaction.H

H

Br + NaSCH3

acetone

H

H

SCH3

H

H

SCH3

H

H

SCH3

H

H

SCH3

H

H

NaSCH3

H

H

NaSCH3

a. b.

c.

d. e.

Predict the product(s) of the following SN1 reaction.

CH3IH3C

CH3OH

CH3H3COH3C

CH3H3CH3CO

CH3H3COH3C

CH3H3CH3CO

CH3H3CHO

CH3HOH3C

CH3HOH3CO

a. b. c.

d. e.

What atom will bond to the indicated C atom in the final isolated product?

a. S b. N c. Cl d. O e. H

OH

SOCl2

N

Enter the #’s of the reagents in the proper sequence that will synthesize the D labeled compound from 3-methyl-1-butanol.

OH

SOCl2 N

from

D

1 D2O 2 Raney Ni, EtOH 3 Mg, Et2O 4 Cl2

5 6 H2O 7 -OD 8 HCl

531

Enter the #’s of the reagents in the proper sequence that will synthesize the D labelled compound from 3-methyl-1butanol.

SOCl2 N

from

1 D2O 2 Raney Ni, EtOH 3 Mg, Et2O(Li, THF)

4 Cl2 5

6 H2O 7 -OD 8 HCl

D

9 NaNH2, NH3 10 CH3CH2O-, CH3CH2OH,

11 H2SO4, H2O 12 Na 13 CH3CH2Br 14 PBr3

831

Predict the major product(s) of the following reaction.

I CH3

+ C N

NC CH3

a

NC CH3

b

c a+b

CH3

acetone

d

CH3 CH3

+e

Predict the major product of the following reaction.

a b

d

Br

+O

C

CH3

O

c

O- OH

O

C

O

CH3

Enter the #’s of the reagents in the proper sequence that will synthesize hex-3-yne from but-1-yne

SOCl2 N1 D2O 2 Raney Ni, EtOH 3 Mg, Et2O(Li, THF)

4 Cl2 5

6 H2O 7 -OD 8 HCl 9 NaNH2, NH3 10 CH3CH2O-, CH3CH2OH,

11 H2SO4, H2O 12 Na 13 CH3CH2Br 14 PBr3

H3C CH2 C C H

9 NaNH2, NH3

H3C CH2 C C13 CH3CH2Br

H3C CH2 C CH2C CH3

Which of the anions would be the strongest nucleophile in ethanol? 

a. F-

b. Cl-

c. Br-

d. I-

e. They are equally reactive.

Which of the following would be predicted to be the most stable 

carbocation? 

CH2

H2C

CH

CH3H3C

CH2

H2C

CH2

CH2H3C

H3C

CH

CH

CH3

CH3

H3C

H2C

C

CH3

CH3

H3C

H2C

HC

CH2

CH3

a. b. c.

d. e.

Which of the following substrates would be predicted to react fastest by the SN2 reaction?

CH2

H2C

HC

CH3H3C

CH2

H2C

CH2

H2CH3C

H3C

H2C

HC

H2C

CH3

a. b. c.

d. e.

Cl

Cl H3C

H2C

C

CH3

H3CCl

H3C

C

H2C

H3CCH3

Cl Cl

Which of the following substrates would be predicted to react fastest by the SN1 reaction?

CH2

H2C

HC

CH3H3C

CH2

H2C

CH2

H2CH3C

H3C

H2C

HC

H2C

CH3

a. b. c.

d. e.

Cl

Cl H3C

H2C

C

CH3

H3CCl

H3C

C

H2C

H3CCH3

Cl Cl

Based on inductive effects, which of the following anilines would be the least basic?

NH2

F

NH2

CH3

NH2

Cl

NH2

CH2Cl

NH2

A B C D E

Which of the following anilines would be the most nucleophiic?

NH2

F

NH2

CH3

NH2

Cl

NH2

CH2Cl

NH2

A B C D E