Chem 008X Table of pKa Values

A set of vocabulary flashcards detailing approximate pKa values and structural rationales for common functional groups in organic chemistry.

Sulfuric acid ($18\,M$)

$pKa \approx -10$. When added to water, it instantly reacts to form $H_3O^+$ and $HSO_4^-$. It should never be used as a base.

Hydrochloric acid ($12\,M$)

$pKa \approx -7$. The conjugate base ($Cl^-$) is almost never used to deprotonate anything in organic chemistry, and the acid should never be used as a base.

Doubly-protonated alcohol

$pKa \approx -2$. This species should never be used as a base.

Hydronium ion

$pKa \approx -1.7$. This is essentially the same as a doubly-protonated alcohol and should never be used as a base.

Carboxylic acid

$pKa \approx 5$. Deprotonation leads to a resonance-stabilized carboxylate with negative charge delocalized across two electronegative oxygens, making it about $11$ orders of magnitude more acidic than an alcohol.

Pyridinium (protonated pyridine)

$pKa \approx 5$.

Protonated arylamine (protonated aniline)

$pKa \approx 5$. Deprotonation leads to a lone pair on nitrogen that is resonance stabilized with the adjacent aromatic ring, making it more acidic than a protonated alkylamine ($pKa \approx 10$).

Protonated alkylamine

$pKa \approx 10$.

Phenol

$pKa \approx 10$. Deprotonation leads to a lone pair on oxygen that is resonance stabilized with the adjacent aromatic ring, making it more acidic than alcohols ($pKa \approx 16$).

Water

$pKa \approx 15.7$. This is essentially the same as an alcohol.

Alcohol

$pKa \approx 16\text{ to }19$. Alcohols lack resonance options but are more acidic than amines or alkanes because oxygen is more electronegative than $N$ or $C$.

Alkyne

$pKa \approx 25$. Deprotonation leads to a negative charge on an $sp$-hybridized carbon; since $sp$ atoms are lower in energy than $sp^2$ or $sp^3$, alkynes are more acidic than alkenes and alkanes.

Amine

$pKa \approx 35$. The $N^-$ anion is called an "amide" and is less stable than the conjugate base of an alcohol due to electronegativity differences. If $R = \text{arene}$, the $pKa$ drops to about $30$.

Alkene

$pKa \approx 40$. Deprotonation leads to a negative charge on an $sp^2$-hybridized carbon. These are effectively not acidic.

Alkane

$pKa \approx 50$. Deprotonation leads to a negative charge on an $sp^3$-hybridized carbon, which is higher in energy than both $sp$ and $sp^2$ atoms. These are effectively not acidic.