Chapter 2: ELECTRONIC & STERIC EFFECTS pps

The more electronegative the X, the stronger the –I effect... The more electropositive the Z, the stronger the +I effect... O is more electronegative than CElectrons move through π-bond

ORGANIC CHEMISTRY

Dr Nam T S Phan Faculty of Chemical Engineering

HCMC University of Technology

Office: room 211, B2 Building Phone: 8647256 ext 5681 Email: ptsnam@hcmut.edu.vn

INDUCTIVE EFFECTS (I)

The more electronegative the X, the stronger the –I

effect

The more electropositive the Z, the stronger

the +I effect

K a .10 5

CH 3 CH 2 CH 2 COOH 1.5

CH 3 CH 2 CH(Cl)COOH 139

CH CH(Cl)CH COOH 8.9 Strong -I

multiple bonds

O is more electronegative than C

Electrons move through π-bond network towards C=O

The conjugated system is polarized

C=O has negative conjugation / mesomeric effect

(-C / -M) on the conjugated system

-C groups generally contain an electronegative atom (s)

or / and a π-bond (s):

CHO, C(O)R, COOH, COOR, NO 2 , CN, aromatics, alkenes

Cl, Br, OH, OR, SH, SR, NH 2 , NHR, NR 2 , aromatics, alkenes

+C groups generally contain a lone pair of electrons

or a π-bond (s):

Aromatics or alkenes can be both +C and-C

Through a period in a periodic table

Through a group in a periodic table

+C

H O

INDUCTIVE vs CONJUGATION

EFFECTS

• C effects are generally stronger than I

effects

• C effects can be effective over much

provided that conjugation is present

• I effects are determined by distance, C

effects are determined by relative

positions

HYPERCONJUGATION EFFECTS (H)

Electron density from Cα-H flows into the vacant p orbital

(in carbocation / C=C / C≡C) because orbitals can partially

overlap

Hyperconjugation effects (H)

H C

H H

•H effect of -CH 3 is stronger than H effect of -CH 2

-•H effect is generally stronger than I effect

Electron-donating ability of -CH 3 is stronger

STERIC EFFECTS

by space-filling properties of those parts of a

molecule attached at / near the reacting site

atoms / groups at / near the reacting site hinders / retards a reaction

the formation of the required transition state

Steric hindrance

Steric hindrance

ACIDITY & BASICITY

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If –C groups are introduced at ortho- & para

position on phenol rings:

+ The anion (-O - ) can be further stabilized by

delocalization through the conjugated system as the negative charge can be spread onto the -C

groups

+ The O-H bond is more polarized as electron

density on –OH can be spread onto the -C groups

Acidity of phenols is generally increased

If –I groups are introduced on phenol rings, the effect will depend on the distance:

+ The closer the –I group is to the negative

charge (-O - ), the greater the stabilizing effect is

+ The closer the –I group is to the –OH, the O-H bond is more polarized

Acidity of phenols is generally increased

Note: there might be ortho-effects

Benzoic acid derivatives

pKa

Position on benzene ring

Ortho- Meta-

STABILITY OF CARBOCATIONS+H & +I

Allylic & benzylic carbocations

Allylic & benzylic carbocations are generally stable

due to the electron delocalization (+C effects)

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Not all allylic & benzylic carbocations have the

same stability

Relative stability of carbocations

STABILITY OF RADICALS

STABILITY OF CARBANIONS