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Material Type: Notes; Class: Organic Chemistry; Subject: Chemistry and Biochemistry; University: University of Southern Mississippi; Term: Unknown 1989;
Typology: Study notes
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k^1
k^2
A^
B^
C
.
k^1
k^2
slower
faster
Case A: k
< k 1
Step 1 (A
B) is slower
than step 2 (B
The free energy of activation forstep 1 is larger leading to a smallerrate constant
The overall rate of Ais controlled by step 1 whichis the rate-determing step.
slow step
Note: A chemical intermediate, such as B, is an energy minimum in the free energydiagram. It persists for a finite time. A transition state is an energy maximum that istransitory, existing for perhaps the time of a bond vibration, ~
-12^ s
t-butyl carbocationa high energy intermediate
3
3
3
3
+^
nucleophile
+^ :
t-butyloxonium ion
3
3
3
3
3
+^
O + Cl 2
CH^ -C^3^ key intermediate
CH
3 CH
3
O + Cl 2
(CH
)^ COH 33
1
+O (^3)
)^ COH 33
A General Scheme
chiral achiral (plane of symmetry)
chiral
3 R^2
3 R^2
Example: The Hydrolysis of (S)-3-Bromo-3-methylhexane(S)-3-bromo-3-methylhexane
acetone
(50%)3-methyl-3-hexanol
Racemic Form
Br
3
3 CH
3
3
CH
3
via S
achiral There is an equal probability ofattack by H
O at each face. 2
SolvolysisA nucleophilic substitution reaction where the solvent is thenucleophile is called solvolysis.When water is the solvent, the term hydrolysis is used.Methanolysis means methanol is the nucleophile/solvent.
hydrolysismethanolysisethanolysisacetolysis
Factors Influencing the Rates of the
SN
1 and S
2 ReactionsN
An Example: Incorporation of isotopic bromine in RBr
R = CH
3
CH
CH 3
2
CH
CH 3 CH
3
CH
CH C 3
3 CH
3
CH
-C-CH 3
2 CH
3 CH
3
.
This study shows that methyl bromide reacts 25,000times faster than tert-butyl bromide, and 5 million timesfaster than neopentyl bromide, in this S
2 reactionN
relativerate
76
(1.0)
faster
The Steric Effect
.
This slowdown in the rate of the S
2 reaction with increasingN
alkyl substitution around the reaction center is attributed to asteric effect, increasing steric hindrance in approaching thereaction center by the incoming nucleophile
Nu:
C^
X H H H
ethyl
Nu:
-^
3 H H
isopropyl Nu:
-^
3 H CH
3
tert-butyl
Nu:
-^
3 CH
3 CH^3
increasing steric hindrance
The Structure of CarbocationsCarbocations are trivalent carbonspecies with a positive charge.
R^3
+C
.
Carbocations are locally planarwith the three valences in a plane
.
R
Relative Stabilities of CarbocationsMany experiments indicatethe stability order ofcarbocations is:
o^3
o^2
o^1
methyl
^
^
most stable
least stable
+^
R-H
R+
cation
+^
+^
+^
increasing stability
3
2
3
3
3
Hyperconjugation
.
Interaction of the sigma electrons in the C-H bond of the methyl groupwith the empty p-orbital of the carbocation disperses and stabilizes thepositive charge
free energy
reaction coordinate
According to the Hammond-Leffler Postulate, since S
1 processes areN
highly endergonic, the transition state comes late along the reactioncoordinate and resembles the carbocation state. Factors that stabilizecarbocations will stabilize the TS in an S
1 reaction leading to a faster rate.N
resembles product state TS
resemblesreactant state
reactant state
product state
highly exergonic step reactant state
product state highly endergonic step