chapter 3: conformational, steric, and introduction ... · chapter 3: conformational, steric, and...
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Chapter 3: Conformational, Steric, and Stereoelectronic EffectsIntroduction
Strain energy: excess energy resulted from nonideal geometry or bonding arrangementSteric strain: strain attributed to van der Waals repulsion between two neighboring groups Torsional strain: strain resulted from nonoptimal rotational alignment
(dihedral or torsional angle)
Angle strain: strain resulted from nonideal bond angle
YX
XY
Θ
3. 1 Conformations of Acyclic Molecules
1. F. Weinhold, Nature 2001, 411, 539-541.2. F. M. Bicjemhaupt; E. J. Baerends Angew. Chem. Int. Ed. 2003, 42, 4183-41883. F. Weinhold Angew. Chem. Int. Ed. 2003, 42, 4188-4194
1.4
3.4
1.4
+
4.0
6.0
6.0
6.0
4.0
4.0
3. 2 Conformations of aldehydes and olefins
OCHHH
H
OC
H
H
H H
3. 4 Conformations of cyclohexane Derivatives
11
11 11
11
2. Substituted cyclohexanes
: Free energy difference (- ∆G, kcal/mol) between equatorial andaxial substituent on a cyclohexane ring
E. Heteroatom-substituted 6-membered ring
3. 5 The Anomeric Effect
3. 6 Conformational Effects on Reactivitya) Hydrolysis rate of ester : Scheme 3.2 in page 157
trans
trans
cis
cis
∆Gg
∆Gt
b) Oxidation of alcohol : Scheme 3.2 in page 157
trans
trans
cis
cis
∆Gg
∆Gt
kcis/ktrans = 3.23
∆Gg < ∆Gt
Therefore Eatrans < ∆Eacis
∆Gg > ∆Gt
Therefore Eatrans > ∆Eacis
c) E2 elimination of neomentyl chloride and mentyl chloride
k1/k2 = 200
3. 7 Rate of CyclizationIn forming small rings, ring strain decreases the rate and it can offset advantages in entropy
Ring strains are a combination ofangle strain and torsional strain
3. 8 Rules for Ring Closure: Baldwin’s rule
Exo Endo
Ring sizesp sp2 sp3
dig trig tetsp sp2
dig trig
3
4
5
6
7
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