Video Transcript
Which of the following correctly
compares carboxylic acids with alcohols? (A) Alcohols are more acidic than
carboxylic acids because alcohols can form dimers between their molecules. (B) Carboxylic acids are more
acidic than alcohols because carboxylate anions are stabilized by resonance, while
alkoxides are not. (C) Carboxylic acids are more
acidic than alcohols because carboxylic acids can form dimers between their
molecules. Or (D) alcohols are more acidic
than carboxylic acids because alkoxides are stabilized by resonance, while
carboxylate anions are not.
Upon taking a closer look at all of
the answer choices, the phrase “more acidic” is used in every answer, followed by a
reasoning for stating which is more acidic, carboxylic acids or alcohols. Since our focus in comparing
carboxylic acids and alcohols will pertain to acid strength, let’s first recall an
acid is defined as a molecule with the ability to ionize and donate a proton in
solution. The stronger the acid, the greater
its ability to ionize and the more protons donated in solution. A weaker acid will exhibit lower
ionization and donate fewer protons in solution.
Analyzing answer choices (B) and
(D), they both indicate the stronger acid generates a conjugate base that is
stabilized through resonance. As it turns out, this general
concept is correct. In an acid–base reaction, donation
of a proton from an acid to a base transforms the acid into the conjugate base. The greater the stability of the
conjugate base, the more easily the proton is donated to generate the conjugate base
in solution and, therefore, results in greater acid strength.
To determine which organic molecule
class is more acidic due to resonant stability of the conjugate base, let’s first
recall that the chemical structure of a carboxylic acid contains a carboxyl group,
which is a carbonyl group bonded to a hydroxy group, while an alcohol only contains
a hydroxy group. In order to determine which
conjugate base, the carboxylate anions mentioned in answer choice (B) or the
alkoxides mentioned in answer choice (D), equates to a stronger acid, let’s use
these general chemical structures drawn here to compare the two.
Upon donation of their acidic
proton to a base, the conjugate base of a carboxylic acid is the negatively charged
carboxylate anion and the negatively charged alkoxides for an alcohol. Now that we have generated a
conjugate base for each molecule type, let’s determine which one demonstrates
resonance.
Recalling chemistry, resonance
refers to the delocalization of electrons within molecules. When the molecule demonstrates
resonance, it means its chemical structure is actually made up of multiple chemical
structures, where the only thing that should change between resonant structures is
where the electrons are located.
These chemical structures that only
differ in electron locations are called resonant structures. Between the carboxylate anion and
the alkoxide, the carboxylate anion is more stable due to the resonance displayed as
a result of the delocalization of electrons and spread of the negative charge across
the carboxylic group. The alkoxide, on the other hand,
does not exhibit resonance because the negative charge cannot be spread across the
carbon-hydrogen bonds and the electrons cannot be delocalized.
Generally, carboxylic acids are
more acidic than their corresponding alcohol due to the increased stability of the
carboxylate anion as a result of the resonance character it possesses. Due to this information, we can
eliminate choice (D) because the statement that correctly compares carboxylic acids
and alcohols is answer choice (B).
Even though we have already
determined the correct answer for this question, let’s take a moment to understand
why the remaining two answer choices cannot be the correct answer. A common theme among answer choices
(A) and (C) is their focus on the formation of dimers as the reason for generating
more acidic molecules. In general, carboxylic acids can
form dimers, while alcohols cannot. This is because two carboxylic
acids can form two hydrogen bonds between their molecules, with each hydrogen bond
occurring between the carbonyl oxygen atom of one carboxylic acid molecule and the
hydroxy group hydrogen atom of the second carboxylic acid molecule.
On the other hand, each alcohol
molecule generally only contains one hydroxy functional group and, therefore, has
the ability to form only one hydrogen bond between their molecules. Because carboxylic acids form more
hydrogen bonds, they have higher boiling points than their corresponding
alcohols. While this reasoning explains why
carboxylic acids in general have higher boiling points than their corresponding
alcohols, stability of the conjugate base through resonance explains why carboxylic
acids are more acidic than their corresponding alcohols. This is why answer choices (A) and
(C) cannot be the correct answer for this question.
Therefore, the answer choice that
correctly compares carboxylic acids and alcohols is answer choice (B).
