Transcript 2 Properties Carboxylic Acids GOB Structures

16.2 Properties of Carboxylic Acids
Carboxylate salts are often
used as preservatives and
flavor enhancers in soups
and seasonings.
Sodium propionate, a
preservative, is added to
cheeses, bread, and other
bakery items to inhibit the
spoilage of the food by
microorganisms.
Learning Goal Describe the boiling points, solubilities,
dissociation, and neutralization of carboxylic acids.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Polarity of Carboxylic Acids
Carboxylic acids are strongly polar because they have two
polar groups:
• a hydroxyl group (—OH) and
• a carbonyl group (C = O).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Boiling Points of Carboxylic Acids
• The polar carboxyl groups allow carboxylic acids to form
multiple hydrogen bonds with other carboxylic acid
molecules.
• This effect of hydrogen bonds gives carboxylic acids higher
boiling points than alcohols, ketones, and aldehydes of
similar molar mass.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Carboxylic Acids, Dimers
• The higher boiling points
of carboxylic acids can
also be attributed to the
formation of dimers
between two carboxylic
acids.
• Two hydrogen bonds form
between their carboxyl
groups, which effectively
double the mass of the
molecule, increasing the
temperature required to
reach the boiling point.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Match each of the compounds 2-butanol, pentane, and
propanoic acid with the boiling points 141°C, 100°C, and 36°C.
(They have about the same molar mass.)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Match each of the compounds 2-butanol, pentane, and
propanoic acid with the boiling points 141°C, 100°C, and 36°C.
(They have about the same molar mass.)
Boiling points increase due to
• increased formation of hydrogen bonds.
• increased dipole–dipole attractions.
• increasing molar mass.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Match each of the compounds 2-butanol, pentane, and
propanoic acid with the boiling points 141°C, 100°C, and 36°C.
(They have about the same molar mass.)
• Pentane, an alkane, cannot form hydrogen bonds. It has the
lowest boiling point, 36°C.
• 2-Butanol, an alcohol, can form hydrogen bonds, so it has a
higher boiling point, 100°C.
• Propanoic acid, a carboxylic acid, can form dimers
increasing the effective molar mass. It therefore has the
highest boiling point, 141°C.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solubility in Water
Carboxylic acids
• form hydrogen bonds with
many water molecules.
• with one to five carbon
atoms are very soluble in
water.
• are less soluble as the
number of carbons increases.
Acetic acid forms hydrogen bonds with
water molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Acidity of Carboxylic Acids
Carboxylic acids
• are weak acids.
• dissociate in water to produce carboxylate ions and
hydronium ions.
• can lose a proton because two oxygen atoms in a
carboxylate ion stabilize negative charge.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Properties of Carboxylic Acids
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the balanced equation for the ionization of butanoic acid
in water and identify the carboxylate ion.
butanoic acid
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Write the balanced equation for the ionization of butanoic acid
in water and identify the carboxylate ion.
+ H2O
+ H3O+
carboxylate ion
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Neutralization of Carboxylic Acids
Carboxylate salts are a product of the neutralization of a
carboxylic acid with a strong base such as NaOH or KOH.
The carboxylate ion is named by replacing the ic acid ending of
the acid name with ate.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Neutralization of Carboxylic Acids
Carboxylate salts
• are ionic compounds with strong attractions between
positively charged metal ions such as Li+, Na+, and K+ and
the negatively charged carboxylate ion.
• are solids at room temperature.
• have high melting points.
• are usually soluble in water.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Carboxylic Acids, Preservatives
• Sodium propionate, a preservative, is added to bread,
cheeses, and bakery items to inhibit the spoilage of the food
by microorganisms.
• Sodium benzoate is added to juices, margarine, relishes,
salads, and jams to inhibit the growth of mold and bacteria.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Carboxylic Acids, Preservatives
• Monosodium glutamate (MSG) is added to meats, fish,
vegetables, and bakery items to enhance flavor, although it
may cause headaches in some people.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the equation for the reaction of propanoic acid
with NaOH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Write the equation for the reaction of propanoic acid
with NaOH.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Carboxylic Acids in Metabolism
At the pH of the aqueous environment in the cells,
• the carboxylic acids are dissociated.
• it is actually the carboxylate ions that take part in the
reactions of the citric acid cycle.
For example, in water, succinic acid is in equilibrium with its
carboxylate ion, succinate.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Carboxylic Acids in Metabolism
Carboxylates are part of the metabolic processes within our
cells. For example,
• during glycolysis, a molecule of glucose is broken down
into two molecules of pyruvate, the carboxylate salt of
pyruvic acid.
• during strenuous exercise when oxygen levels are low
(anaerobic), pyruvate is reduced to give lactic acid or the
lactate ion.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Carboxylic Acids in Metabolism
In the citric acid cycle (Krebs cycle),
• di- and tricarboxylates are oxidized and
decarboxylated (loss of CO2) to produce
energy for cells.
• citrate, with six carbons, is oxidized to a
five-carbon
α-ketoglutarate:
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health:
Carboxylic Acids in Metabolism
The citric acid cycle continues as
• α-ketoglutarate loses CO2 to give a four-carbon succinate.
• a series of reactions then converts succinate to oxaloacetate.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.