chapter 9 nucleophilic substitution & -elimination

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9-1 pter 9 Nucleophilic Substitution & -Eliminat 1. Nucleophilic Aliphatic Substitution 2. Solvents for Nucleophilic Substitution Reactions 3. Mechanisms of Nucleophilic aliphatic substitution 4. Evidence of S n 1 / S n 2 Mechanisms 5. Analysis of some Nucleophilic Substitution Rx’s I C H H 3 C D NC C H H 3 C D (-) :C N: (-) I 6. -Elimination 7. -Elimination mechanism 8. Evidence for E1 and E2 9. Substitution vs Elimination C C Br H R CH 2 C H R Br + t Bu O H H H t Bu O H + +

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6.  -Elimination 7.  -Elimination mechanism 8. Evidence for E1 and E2 9. Substitution vs Elimination. Chapter 9 Nucleophilic Substitution &  -Elimination. 1. Nucleophilic Aliphatic Substitution 2. Solvents for Nucleophilic Substitution Reactions - PowerPoint PPT Presentation

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No Slide Title5. Analysis of some Nucleophilic Substitution Rx’s
6. -Elimination
9. Substitution vs Elimination
What is nucleophilic substitution, how does it differ from free radical substitution, what is beta-elimination?
9-*
substitution
-elimination
4
What are the similarities and differences in Nu sub and 1,2-elimination?
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Product(s)
Leaving group - stable with pair of e’s, weak B:
9 Nucleophilic Substitution
Conditions - solvent, temperature, etc
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Rx:
NON-POLAR
dielectric
constant
≥ 15
≤ 5
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APROTIC
One simultaneous breaking & making; [SN2]
Other, break then make bonds stepwise; [SN1]
2 limiting mechanisms for substitution (SN2, SN1)
5
9-*
sp2
9-*
Structure of Nu: have on mechanism/rate?
Leaving group have on rate?
What effect does the:
The role of the solvent?
When or why:
Does rearrangement occur?
9-*
Kinetics/Nucleophilicity
Nucleophiles are also bases
4
rate = k[(CH3)3CBr]
k - rate constant
rate = k[ CH3Br ][ -OH ]
SN1 or SN2 with a 2o RX is on nucleophile
nucleopilicity
moderate
strong
weak
strong
bases
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Protic
Aprotic
The greater the the solvent’s dielectric constant, the better ions of opposite charge are separated.
Polar and Nonpolar Solvents
but poorly solvate anions, e.g.:
Solvents effects on Nu:-
the greater its Nucleophilicity
2
9-*
4
C
CH
3
CH
3
H
3
C
Cl
5
CH
3
H
3
C
CH
3
but substituent priorities changed
2
governed by steric factors
governed by electronic factors
1o allylic
SN1
rx
6
same
- less basic or more stable with e(-)s
better leaving gp.
8
Nu-R
+ :X
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good leaving gp.
special cases
Which of the given substrates would undergo SN2 substitution? Product(s)? Reason?
strong bases: (-)OH, (-)OCH3; (-)NH2 even stronger!
not leaving groups
6
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Alkyl Halide
SN2
SN1
11
9-*
molecule (HCl, HBr, HI, or HOH) is eliminated.
-Elimination
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E1: break bond, then make bond unimolecular, rate [ R-X ]
2 limiting mechanisms for -elimination rxs
-Elimination
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(more substituted) .
E2
9-*
E1
E2
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E1 mechanism
2
t.s.
SM
products
Stereoselectivity of E2
E2 most favorable (lowest Ea) - H and X are anti and coplanar
1
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Stereoselectivity of E2
E2 most favorable (lowest Ea) - H and X are anti and coplanar
X
E1: major product is the more stable alkene
E2: the major product is usually the more stable alkene, but on orientation of H and X
*also SN1 products
Stereochemistry of E2
The only anti-coplanar H to X arrangement forces formation of less stable olefin
(2.)
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E2
SN vs E
Many nucleophiles are also strong bases (OH- and RO-), thus SN and E reactions often compete.
9-*