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Solutions for Carboxylic Acids
46 (A more complete discussion of acidity and electronic effects can be found in Appendix 2 in this 
manual.) A few words about the two types of electronic effects: induction and resonance. Inductive effects 
are a result of polarized σ bonds, usually because of electronegative atom substituents. Resonance effects 
work through π systems, requiring overlap of p orbitals to delocalize electrons.
All substituents have an inductive effect compared to hydrogen (the reference). Many groups also have a 
resonance effect; all that is required to have a resonance effect is that the atom or group have at least one p 
orbital for overlap.
The most interesting groups have both inductive and resonance effects. In such groups, how can we tell 
the direction of electron movement, that is, whether a group is electron-donating or electron-withdrawing? 
And do the resonance and inductive effects reinforce or conflict with each other? We can never "turn off" 
an inductive effect from a resonance effect; that is, any time a substituent is expressing its resonance effect, 
it is also expressing its inductive effect. We can minimize a group's inductive effect by moving it farther 
away; inductive effects decrease with distance. The other side of the coin is more accessible to the 
experimenter: we can "turn off" a resonance effect in order to isolate an inductive effect. We can do this 
by interrupting a conjugated π system by inserting an sp3-hybridized atom, or by making resonance overlap 
impossible for steric reasons (steric inhibition of resonance).
These three problems are examples of separating inductive effects from resonance effects.
HC
H E EH
C
HC
H E
CH2 C
O
O H O HC
O
CH2H
The greater acidity of phenylacetic acid shows that the phenyl substituent is electron-withdrawing, thereby 
stabilizing the product carboxylate's negative charge. Does this contradict what was said above? Yes and 
no. What is different is that, since there is no p-orbital overlap between the phenyl group and the carboxyl 
group because of the CH2 group in between, the increased acidity must be from a pure inductive effect. This 
structure isolates the inductive effect (which can't be "turned off") from the resonance effect of the phenyl 
group.
We can conclude three things: (1) phenyl is electron-withdrawing by induction; (2) phenyl is (in this case) 
electron-donating by resonance; (3) for phenyl, the resonance effect is stronger than the inductive effect 
(since it is an ortho,para-director).
is a stronger acid than
plus other
resonance forms
(a) and (b) In electrophilic aromatic substitution (EAS), the phenyl substituent is an ortho,para-director 
because it can stabilize the intermediate from electrophilic attack at the ortho and para positions. The 
phenyl substituent is electron-donating by resonance.
BUT:
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