Section 1.17 Organic Acids and Bases; pKaand pH 43A protonatedcompound is a compound that has gained an additional proton. A pro-
tonated alcohol or a protonated carboxylic acid is a very strong acid. For example,
protonated methanol has a of protonated ethanol has a of and
protonated acetic acid has a of
An amine can behave as an acid and donate a proton, or as a base and accept a proton.
Compounds with groups are amines.
Amines, however, have such high values that they rarely behave as acids. Ammo-
nia also has a high
Amines are much more likely to act as bases. In fact, amines are the most common or-
ganic bases. Instead of talking about the strength of a base in terms of its value, it
is easier to talk about the strength of its conjugate acid as indicated by its value,
remembering that the stronger the acid, the weaker is its conjugate base. For example,
protonated methylamine is a stronger acid than protonated ethylamine, which means
that methylamine is a weaker base than ethylamine. Notice that the values of pro-
tonated amines are about 10 to 11.
It is important to know the approximate values of the various classes of com-
pounds we have discussed. An easy way to remember them is in units of five, as shown
in Table 1.8. (R is used when the particular carboxylic acid or amine is not specified.)
pKaprotonated methylamine
pKa = 10.7protonated ethylamine
pKa = 11.0CH 3 NH 3+
CH 3 CH 2 NH 3+pKapKapKbmethylamine
pKa = 40CH 3 NH 2
ammonia
pKa = 36NH 3pKa.pKaCH 3 NH 2 CH 3 NH + H 2 O
an acid+ HO−++−CH 3 NH 2 CH 3 NH 3 H 2 O
a base+ H 3 O+NH 2protonated methanol
pKa = −2.5protonated ethanol
pKa = −2.4protonated acetic acid
pKa = −6.1CH 3 OH+
ΗCH 3 CH 2 OH+
Η CH 3C
OH+OHpKa -6.1.pKa -2.5, pKa -2.4,Table 1.8 Approximate pKa ValuespKa0pKa~5 pKa~10 pKa~15RNH 3+
ROHH 2 OROH 2+R+OHH 3 O+
protonated
watera protonated
carboxylic acida protonated
alcohol a carboxylic
acida protonated
aminean alcoholwater
C
OHROC
OH