Acidity of Alcohols
The O-H bond of an alcohol is the most acidic position in the molecule. At pKa roughly 16 to 18, alcohols are about as acidic as water - far too weak to react with ordinary bases like hydroxide, but strong enough to be deprotonated by alkali metals (Na, K) and strong bases (NaH, NaNH₂). The conjugate base, an alkoxide (RO⁻), is the species that does most of the interesting chemistry in this chapter.
Understanding alcohol acidity is important for two reasons: (1) it tells you which base is strong enough to generate an alkoxide, and (2) it sets up the contrast with phenols, which are six pKa units more acidic and have a completely different reactivity profile.
Ethanol vs. Water
Ethanol (pKa 15.9) and water (pKa 15.7) are almost equally acidic. This makes sense: both have an O-H bond, and the conjugate bases (ethoxide and hydroxide) are both oxygen-centered anions. Ethoxide is slightly less stable than hydroxide because the alkyl group of ethanol donates electron density to the alkoxide oxygen via induction, which destabilizes the negative charge.
Larger alkyl groups destabilize the alkoxide more (more inductive donation), so tert-butanol (pKa 18) is less acidic than ethanol, which is less acidic than methanol (pKa 15.5). The trend is methanol > ethanol > isopropanol > tert-butanol in acidity.
Substituent Effects on Alcohol Acidity
Electron-withdrawing groups (EWGs) nearby stabilize the alkoxide by dispersing the negative charge via induction:
| Alcohol | pKa |
|---|---|
| Methanol (CH₃OH) | 15.5 |
| 2,2,2-Trifluoroethanol (CF₃CH₂OH) | 12.5 |
| Hexafluoroisopropanol ((CF₃)₂CHOH) | 9.3 |
Three fluorines on the adjacent carbon lower the pKa by 3 units (a 1000-fold increase in Ka). Six fluorines lower it by another 3 units. This is purely inductive - electron density gets pulled toward the fluorines, away from the alkoxide oxygen, stabilizing the anion.
Electron-donating groups (alkyl groups, for the most part) raise pKa. Fluorinated alcohols like hexafluoroisopropanol are so acidic they approach the pKa of phenols.
Generating Alkoxides
To use an alcohol as a nucleophile or base in a reaction, you often need to deprotonate it to form the alkoxide. Common methods:
- Alkali metals (Na, K): dissolve sodium metal in ethanol and you get sodium ethoxide plus H₂ gas. Widely used in lab.
- Sodium hydride (NaH): deprotonates almost any alcohol cleanly; byproduct is H₂. Good for all alcohols.
- Potassium hydride (KH): even more reactive than NaH, used when NaH is too slow.
- Grignard reagents (R-MgX): deprotonate alcohols immediately - which is why you must keep Grignards away from protic solvents.
Hydroxide (NaOH, KOH) is roughly a 50-50 mix with alcohols at equilibrium (pKa of water ≈ pKa of alcohol), so it is not effective for fully deprotonating alcohols to alkoxides. Use NaH, NaK, or Na metal for a clean alkoxide.
Phenols: A Different Animal
Phenol (C₆H₅OH) is about a million times more acidic than ethanol (pKa 10 vs. 16). Same OH group, same oxygen, same alkyl/aryl framework at first glance. So why the huge difference?
Resonance. The phenoxide anion delocalizes the negative charge into the aromatic ring. Three resonance contributors place partial negative charge on the ortho and para positions of the ring. The negative charge is spread across four atoms instead of concentrated on one oxygen, which is much more stable.
Substituents on the phenol ring modulate acidity in predictable ways:
| Phenol | pKa |
|---|---|
| Phenol | 10.0 |
| 4-nitrophenol (-NO₂ para) | 7.2 |
| 2,4-dinitrophenol | 4.1 |
| 2,4,6-trinitrophenol (picric acid) | 0.4 |
| 4-methylphenol (4-methyl) | 10.3 |
| 4-methoxyphenol | 10.2 |
Electron-withdrawing groups (especially at ortho/para via resonance stabilization of the anion) dramatically increase acidity. Picric acid is stronger than acetic acid. Electron-donating groups (methyl, methoxy) slightly decrease acidity by destabilizing the anion.
Section 5.13 covers phenols in much more depth.
Practical Consequence: Phenols React with NaOH, Alcohols Do Not
Because phenols are more acidic than water (pKa 10 vs. 15.7), they ARE fully deprotonated by NaOH. This gives a simple experimental test: add NaOH to an unknown compound. If it dissolves (because a soluble sodium salt forms), the compound is probably a phenol or a carboxylic acid. If it does not dissolve, it is probably an alcohol (or a non-polar compound).