Page 318 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Scheme 3.1. Classification of Substituent Groups 299
Substituent Resonance Polar SECTION 3.4
Electronic Substituent
Alkyl
Effects on Reaction
CH , C H , (CH ) C, etc ERG ERG (small) Intermediates
3 3
3
2 5
Vinyl and aryl
CH, C H , etc ERG or EWG EWG (small)
CH 2 6 5
Ethynyl
HC C, etc ERG or EWG EWG
Alkoxy, Acyloxy, Amino,
Acylamido
RO, RCO ,R N,RCONH, etc ERG EWG
2
2
Carbonyl
HC O, RC O, ROC O, EWG EWG
R NC O, etc.
2
Halogens
F, Cl, Br, I ERG EWG
Polyhaloalkyl
, CCl , etc. EWG EWG
CF 3 3
Other Nitrogen Groups
+
CN, NO 2 , R N , etc. EWG EWG
3
Sulfur Groups
RS ERG EWG
RSO ERG EWG
RSO 2 EWG EWG
Silyl Groups
R Si ERG ERG
3

higher electronegativity. Moreover, the resonance and polar effects both weaken going
down the periodic table. Resonance weakens because of longer bonds and poorer
orbital overlap with carbon, whereas the polar effect decreases as the electronegativity
diminishes. As a result, the halogens often exhibit unusual trends resulting from a
changing balance between the resonance and polar effects. Halogen substituent effects
are also affected by polarizability, which increases from fluorine to iodine. When
halogen is placed on a carbon, as in trifluoromethyl or trichloromethyl, the groups
are EWG by both polar and resonance mechanisms. The resonance component is
associated with orbitals. The sulfur substituents RS, RSO, and RSO show a shift
∗
2
from being resonance donors to strong polar acceptors, as the sulfur unshared pairs
are involved in bonding to oxygen. Trialkylsilyl groups are slightly better ERGs than
alkyl, since the silicon is less electronegative than carbon. Trialkylsilyl groups are very
good electron donors by hyperconjugation.
. .
Si C C
. .
C C X

silicon σ−donation halogen σ* acceptor