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Electronegative atoms pull a conjugate base's charge along sigma bonds and strengthen the acid; the effect adds up with more such atoms, is larger for more electronegative ones, and fades quickly with distance.
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You will calculate how much a substituent lowers pKa, rank the chloroethanoic and haloethanoic acids, predict the effect of a substituent's position, and explain induction in an alcohol.
You know an acid is strong when its conjugate base's charge is spread out, and that resonance spreads it through pi bonds. Induction does the same job through sigma bonds.
The inductive effect is the pull (or push) an atom exerts on electron density through the sigma bonds of a chain. An electron-withdrawing group pulls; an electron-donating group, such as an alkyl group, pushes weakly.
Put a chlorine on ethanoic acid's CH3. In the conjugate base, ClCH2COO−, the chlorine draws electron density towards itself along the C–C and C–Cl sigma bonds, taking a little of the carboxylate's negative charge. A more spread charge is a more stable base, so the acid is stronger: pKa 2.86 against 4.76.
Two rules follow, and the measured values show both:
| Acid | pKa |
|---|---|
| ethanoic | 4.76 |
| chloroethanoic | 2.86 |
| dichloroethanoic | 1.29 |
| trichloroethanoic | 0.65 |
The effect adds up: each chlorine pulls more.
| Acid | pKa |
|---|---|
| 2-chlorobutanoic | 2.86 |
| 3-chlorobutanoic | 4.05 |
| 4-chlorobutanoic | 4.52 |
| butanoic | 4.82 |
The effect fades with distance: a chlorine three bonds further away barely matters.
Another way: steps
To predict an inductive effect:
Induction stabilizes any nearby charge, so it strengthens alcohols too: CF3CH2OH has a pKa of 12.4 against ethanol's 16, with no resonance at all. And the halogens rank as their electronegativities do — fluoroethanoic acid 2.59, chloro 2.86, bromo 2.90, iodo 3.18. Resonance, where it is possible, is usually the bigger effect; induction is the smaller, steadier one that fine-tunes pKa.
An inductive effect grows stronger with distance. It fades with every bond.
Induction is resonance. It travels through sigma bonds, not pi bonds.
One chlorine anywhere has the same effect. Position matters.
Only carboxylic acids feel induction. Any nearby charge does.
Fluorine, the most electronegative atom, sits one carbon from the carboxyl.
Close and strong.
It pulls charge off the carboxylate: pKa 2.59 against 4.76.
A stabilized base.
The drop is 2.17 units, a factor of about 150 in Ka.
Induction, measured.
Where is the chlorine, relative to the COOH?
Two carbons away.
Stronger or weaker than butanoic acid, and by roughly how much?
Stronger, but only by 0.77 units: 4.05 against 4.82.
Ethanoic acid has a pKa of 4.76; bromoethanoic acid, BrCH2COOH has a pKa of $2.9$. By how many units has the substitution lowered the pKa?
Answer:
Rank these acids from strongest to weakest: ethanoic acid, chloroethanoic acid, dichloroethanoic acid, trichloroethanoic acid.
Number the steps in order (write the number in the box):
Which is the strongest acid: 2-chlorobutanoic acid, 3-chlorobutanoic acid, or 4-chlorobutanoic acid?
Match each haloethanoic acid to its measured pKa: 2.59, 2.86, 2.90, 3.18.
| pKa 2.59 | pKa 2.86 | pKa 2.90 | pKa 3.18 | |
|---|---|---|---|---|
| fluoroethanoic acid | ||||
| chloroethanoic acid | ||||
| bromoethanoic acid | ||||
| iodoethanoic acid |
2,2,2-trifluoroethanol, CF3CH2OH, has a pKa of about 12.4; ethanol's is about 16. Why is the fluorinated alcohol more acidic, though its conjugate base has no resonance?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
2-chlorobutanoic acid has a pKa of $2.86$ and 3-chlorobutanoic acid a pKa of $4.05$. By how many units does moving the chlorine from carbon $2$ to carbon $3$ raise the pKa?
Moving the chlorine raises the pKa by a units.
You can predict an inductive effect from substituent and position. Tell someone why a chlorine far along the chain barely matters. Next: how the hybridization of the atom holding the charge changes acidity.
8. Your turn: 3-chlorobutanoic against butanoic acid, step 3