Properties
Alcohols are weak acids, even weaker than water. Ethanol has a pKa of 15.9 compared to
water’s pKa of 15.7. The larger the alcohol molecule, the weaker an acid it is.
On the other hand, alcohols are also weakly basic. This may seem to be contradictory--how
can a substance be both an acid and a base? However, substances exist that can be an
acid or a base depending on the circumstances. Such a compound is said to be amphoteric
or amphiprotic. As a Bronsted base, the oxygen atom in the -OH group can accept a
proton (hydrogen ion.) This results in a positively-charged species known as an oxonium
ion. Oxonium ions have the general formula ROH 2 +, where R is any alkyl group.
48.3 Alkoxides
WhenObecomesdeprotonated, theresultisanalkoxide. Alkoxidesareanions. Thenames
of alkoxides are based on the original molecule. (Ethanol=ethoxide, butanol=butoxide, etc.)
Alkoxides are good nucleophiles due to the negative charge on the oxygen atom.
48.3.1 Producing an alkoxide
R-OH -> H++ R-O-
In this equation, R-O-is the alkoxide produced and is the conjugate base of R-OH
Alcohols can be converted into alkoxides by reaction with a strong base (must be stronger
than OH-) or reaction with metallic sodium or potassium. Alkoxides themselves are basic.
The larger an alkoxide molecule is, the more basic it is.