S N 2 Mechanism
41
• Slides 2 through 9 lead the students to the molecular orbital argument for backside displacement in S N 2 reactions.
description
Slides 2 through 9 lead the students to the molecular orbital argument for backside displacement in S N 2 reactions. S N 2 Mechanism. backside attack 2 explanations less hindrance to approach of nucleophile from behind C-L bond than from front MO theory. S N 2 Mechanism. S N 2 Mechanism. - PowerPoint PPT Presentation
Transcript of S N 2 Mechanism
• 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. Yesb. Noc. This interaction is orthogonal.
SN2 Mechanism
Is this a stabilizing interaction?
a. Yesb. 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
HH
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
H OH2
a
b
a
b
O
CCH3
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 groupsb. electron withdrawing ability of alkyl
groupsc. steric hindrance blocking approach of
nucleophilesd. 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. ~ sameThe 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
CBrH3C(H2C)5
HI- + C
CH3
I Br
(CH2)5CH3H
CH3
CI (CH2)5CH3
H + Br-
What is the approximate electronic geometry around the central C atom?
a. Linear b. trigonal planar c. pyramidald. tetrahedral e. trigonal bipyramidal
CH3
CBrH3C(H2C)5
HI- + C
CH3
I Br
(CH2)5CH3H
CH3
CI (CH2)5CH3
H + Br-
What are the partial charges on the I and Br atoms respectively?
a. + and + b. - and +
c. + and - d. - and -
e. There should be no partial charges.
CH3
CBrH3C(H2C)5
HI- + C
CH3
I Br
(CH2)5CH3H
CH3
CI (CH2)5CH3
H + Br-
What type of substrate is 2-bromooctane?
a. primary b. secondaryc. tertiary d. quaternarye. methyl
CH3
CBrH3C(H2C)5
HI- + C
CH3
I Br
(CH2)5CH3H
CH3
CI (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
acetoned
CH3 CH3
+e
Predict the major product of the following reaction.
a b
d
Br
+O
CCH3
O
c
O- OH
OC
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 H9 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
CH3H3CCH2
H2C
CH2
CH2H3C
H3CCH
CH
CH3
CH3
H3C
H2C
CCH3
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
CH3H3CCH2
H2C
CH2
H2CH3C
H3C
H2C
HC
H2C
CH3
a. b. c.
d. e.
Cl
Cl H3C
H2C
C
CH3
H3CCl
H3CC
H2C
H3CCH3
Cl Cl
Which of the following substrates would be predicted to react fastest by the SN1 reaction?
CH2
H2C
HC
CH3H3CCH2
H2C
CH2
H2CH3C
H3C
H2C
HC
H2C
CH3
a. b. c.
d. e.
Cl
Cl H3C
H2C
C
CH3
H3CCl
H3CC
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
