Why Trichloroacetic Acid Is Far Stronger Than Acetic Acid

Swap one hydrogen in acetic acid for a chlorine atom and the acid becomes much stronger; attach methyl groups to a carbocation and it becomes more stable. These changes come from how groups push or pull electrons through a molecule — the electronic effects that explain acidity, basicity and the course of organic reactions.

This lesson covers the inductive, resonance and hyperconjugation effects, and how to identify electron-donating and electron-withdrawing groups.

The inductive effect is a permanent shift of sigma electrons caused by a more or less electronegative group, the resonance effect is the delocalisation of pi electrons or lone pairs through a conjugated system, and hyperconjugation is the delocalisation of C-H sigma electrons into a neighbouring empty p orbital or pi bond — and all three stabilise charges by spreading them out.

Inductive effect:

- Passes along sigma bonds and fades quickly, becoming negligible beyond about three carbons
- -I groups pull electrons: $-\mathrm{N}{\mathrm{O}}_2>-\mathrm{CN}>-\mathrm{COOH}>-\mathrm{F}>-\mathrm{Cl}>-\mathrm{Br}>-\mathrm{I}>-\mathrm{OH}$
- +I groups push electrons: alkyl groups, with $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{C}$ strongest and $\mathrm{C}{\mathrm{H}}_3$ weakest

Worked example — the chloroacetic acids. Approximate $\mathrm{p}{K}_a$ values fall as chlorine atoms are added: acetic acid 4.7, chloroacetic acid 2.9, dichloroacetic acid 1.3 and trichloroacetic acid 0.7. Each chlorine pulls electron density away from the carboxylate ion and stabilises it. A drop of 4.0 in $\mathrm{p}{K}_a$ means trichloroacetic acid is about $10^{4.0}=10000$ times stronger.

Resonance effect:

- +R groups donate lone pairs into a pi system: $-\mathrm{OH}$, $-\mathrm{OR}$, $-\mathrm{N}{\mathrm{H}}_2$ and the halogens
- -R groups withdraw pi electrons: $-\mathrm{N}{\mathrm{O}}_2$, $-\mathrm{CHO}$, $-\mathrm{COOH}$ and $-\mathrm{CN}$
- Phenol, with a $\mathrm{p}{K}_a$ of about 10, is far more acidic than ethanol, because the phenoxide ion spreads its negative charge over the benzene ring

Hyperconjugation:

- C-H bonds on a carbon next to a carbocation or double bond donate sigma electrons into it
- More alpha hydrogens mean more hyperconjugation: the tert-butyl cation has 9, the isopropyl cation 6, the ethyl cation 3 and the methyl cation none
- Carbocation stability: tertiary > secondary > primary > methyl
- Alkene stability rises with the number of alkyl groups on the double bond

An everyday example. Dermatology clinics use trichloroacetic acid for chemical skin peels, a job the far weaker acetic acid in kitchen vinegar could never do.

The substance. Resonance usually outweighs the inductive effect — except for the halogens, whose strong -I effect beats their +R effect.

An electron-donating group pushes electron density into the rest of the molecule by a +I, +R or hyperconjugation effect, while an electron-withdrawing group pulls electron density away by a -I or -R effect — so to classify a group, weigh its inductive and resonance effects together.

Electron-donating groups:

- Alkyl groups — +I and hyperconjugation
- $-\mathrm{OH}$, $-\mathrm{OR}$, $-\mathrm{N}{\mathrm{H}}_2$ and $-\mathrm{NHR}$ — +R outweighs -I when attached to a benzene ring or double bond
- $-{\mathrm{O}}^{-}$ — strongly donating, because of its negative charge

Electron-withdrawing groups:

- $-\mathrm{N}{\mathrm{O}}_2$, $-\mathrm{CN}$, $-\mathrm{CHO}$, $-\mathrm{COR}$ and $-\mathrm{COOH}$ — both -I and -R
- $-\mathrm{C}{\mathrm{F}}_3$ and $-\mathrm{N}{\mathrm{R}}_3^{+}$ — strong -I, with no lone pair to donate
- Halogens — -I outweighs +R, so they withdraw electrons overall

What the groups do:

- Acidity — withdrawing groups stabilise the anion and raise acid strength; donating groups lower it
- Basicity — donating groups make amines more basic by pushing electrons onto nitrogen; withdrawing groups make them less basic
- Stability — donating groups stabilise carbocations, while withdrawing groups stabilise carbanions
- Benzene ring — donating groups activate it towards electrophilic substitution; withdrawing groups deactivate it

Worked example — nitrophenol. Phenol has a $\mathrm{p}{K}_a$ of about 10.0, while 4-nitrophenol has about 7.1. The nitro group withdraws electrons by -I and -R, stabilising the phenoxide ion. A difference of 2.9 units makes 4-nitrophenol about $10^{2.9}\approx 800$ times more acidic.

Classifying step by step:

- Does the atom attached to the ring or chain carry a lone pair? If so, it can donate by +R
- Is that atom more electronegative than carbon? If so, it withdraws by -I
- Compare the two: for oxygen and nitrogen, +R wins; for the halogens, -I wins

An everyday example. Paracetamol tablets sold at every Indian chemist contain a benzene ring carrying an $-\mathrm{OH}$ group and an $-\mathrm{NHCOC}{\mathrm{H}}_3$ group, both electron-donating on the ring.

The substance. Halogens are a boundary case — they withdraw electrons and deactivate the ring, yet their lone pairs still direct incoming groups to the ortho and para positions.