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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