18.4 Comparison with Water

Both alcohols and water contain a hydroxyl ($-OH$) group. Due to the high electronegativity of oxygen, the $O-H$ bond in both molecules is polar, which allows them to act as very weak acids by donating a proton ($H^{+}$).

$R-\stackrel{\delta -}{O}-\stackrel{\delta +}{H}$ $H-\stackrel{\delta -}{O}-\stackrel{\delta +}{H}$

However, the acidity of alcohols is influenced by the alkyl group ($R$) attached to the oxygen atom.

Effect of the Alkyl Group (+I Effect)

Alkyl groups are electron-donating (positive inductive effect, $+I$ effect). This has a significant effect on the polarity of the $O-H$ bond:

• The alkyl group pushes electron density towards the oxygen atom.
• This increased electron density on oxygen reduces the polarity of the $O-H$ bond.
• A less polar $O-H$ bond is harder to break, meaning the proton ($H^{+}$) is released less easily.
• The resulting alkoxide ion ($R{O}^{-}$) is destabilized by the $+I$ effect, making it a stronger base and the alcohol a weaker acid.

As a result, alcohols are generally weaker acids than water. The conjugate base of an alcohol is called an alkoxide ion ($R{O}^{-}$).

Order of Acidity Among Alcohols

The order of acidity among alcohols is:

$C{H}_{3}OH>1^{\circ }>2^{\circ }>3^{\circ }$

As the number of alkyl groups increases, the $+I$ effect increases, making the $O-H$ bond less polar and the alkoxide ion less stable (more basic), thus decreasing acidity.

Comparison: Phenol vs. Ethanol

Phenol ($C_6{H}_{5}OH$) is significantly more acidic than ethanol ($C_2{H}_{5}OH$):

• The phenoxide ion ($C_6{H}_5{O}^{-}$) is stabilized by resonance with the benzene ring — the negative charge is delocalized over the ring.
• The ethoxide ion ($C_2{H}_5{O}^{-}$) is destabilized by the $+I$ effect of the ethyl group.
• Therefore: Phenol > Water > Methanol > Ethanol > tert-Butanol in terms of acidity.

Reaction with Active Metals

Like water, alcohols react with active metals such as sodium ($Na$) or potassium ($K$) to release hydrogen gas and form an alkoxide. This reaction demonstrates their acidic nature.

General Reaction for an Alcohol:

$2R-OH+2Na\to 2R-O^{-}N{a}^{+}+H_2\uparrow$

Where $R-O^{-}N{a}^{+}$ is a sodium alkoxide.

Specific Example — Ethanol with Sodium:

$2C{H}_{3}C{H}_{2}OH+2Na\to 2C{H}_{3}C{H}_2{O}^{-}N{a}^{+}+H_2\uparrow$

Reaction for Water (for comparison):

$2H_{2}O+2Na\to 2NaOH+H_2\uparrow$

Note: Water reacts more vigorously than ethanol with sodium, confirming that water is a stronger acid than ethanol.