(Woods) Chem-131 Lec-15 09-4 Carboxyllic acid 1
Carboxylic Acid Derivatives
Esters, Acid Anhydrides, Amides, and Acid Halides
• The oxidation state of carbon in a carboxylic acid is the highest of the
organic compounds of carbon:
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Nomenclature
Common Names: Similar to corresponding Aldehydes.
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•Carboxylic acids have higher secondary forces than alcohols, aldehydes,
or ketones, and consequently have higher boiling and melting points.
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Physical Properties
• Both the carbonyl group and the hydroxyl group in a carboxylic acid are
polar and capable of hydrogen bonding.
• The presence of these two groups allows carboxylic acids to form
hydrogen bonded dimers under most conditions:
Physical Properties
• Higher solubility in water than alcohol: three
H-bond interactions with water.
• Solubility decreases more slowly than that of
alcohols as the number of carbons increases.
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Physical Properties - Acidity
• Carboxylic acids are weak acids that ionized to form carboxylate ion
and hydronium ion in water:
• The extent of ionization is less than 1-2%.
Physical Properties - Acidity
• Carboxylic acids are weaker acids than the strong acids (HCl, HBr, HI,
H2SO4, HNO3, and HClO4), but stronger acids than phenols, and much
stronger than alcohols.
• Due to the greater electron withdrawing
power of the carbonyl group,
• Compare to a benzene ring, which allows
it to spread the negative charge more.
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Physical Properties - Acidity
• Carboxylic acids are acidic enough to turn
blue litmus paper red
• Distinguishes them from alcohols, but not
phenols which also turn blue litmus paper red.
• Sodium hydroxide and sodium bicarbonate react with carboxylic acids to
form a carboxylate salt:
Physical Properties - Acidity
• Carboxylic acid in water has a pH ≈ 3: only a small percentage of the acid
exits as ionized molecules (pH = -log[H3O+]).
• In alkaline media, the fraction of ionized molecules increases.
• Physiological Fluids are maintained near neutral pH by the
bicarbonate-carbonate buffer system:
Blood pH is 7.4 and the pH in most cells ranges from 6.8 to 7.1.
At pH
H = 7,
7 carboxylic
b li acids
id almost
l t entirely
ti l ionized
i i d (carboxylate).
( b l t )
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Carboxylate Salt
• Carboxylate Salt: Product of carboxylic acid and a strong base.
• The sodium salt of butanoic acid would be called sodium butanoate.
Carboxylate Salt
• Carboxylate salts are ionic and possess much higher boiling and melting
points than those of the corresponding carboxylic acids (ionic forces are
much stronger than secondary forces).
• All carboxylate salts are solids at room temperature.
Sodium formate: MP 253°C solid at room temperature
Formic acid: MP 8°C liquid at room temperature
• Carboxylate salts are much more soluble than their carboxylic acids.
• 6+ carbons are slightly soluble or insoluble in water.
• Sodium stearate (18 carbons) is water soluble.
• Stearic acid (18 carbons) is insoluble.
• Increased solubilities in neutral or basic environments because the acids
are converted into their carboxylate ions.
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Soaps
• Soaps: usually Na / K salts of long-chain carboxylic acids (fatty acids)
• Fatty acids are typically unbranched aliphatic chains of 12 to 20 carbon
atoms obtained from animal fats and plant oils.
• The fatty acid carboxylate salts are obtained from fats or oils by a
process called Saponification.
• An example of a soap is the sodium carboxylate salt of stearic acid,
CH3(CH2)16COO-Na+.
• Nonpolar dirt (eg. grease) does not
dissolve in water because it is
hydrophobic.
hydrophobic
• Amphipathic:
Hydrophilic (water-loving) and
Hydrophobic (water-hating) parts.
Soaps
• The hydrophobic tail of a soap molecule has an affinity for the greasy
dirt; the hydrophilic head, for water:
• The requirements for a good soap are an ionic group as the hydrophilic
head and a long hydrocarbon group as the hydrophobic tail.
• Stearic acid (C18): Not a soap, because no ionic head group.
Its carboxylate salt is a good soap, however.
• Sodium butyrate, CH3(CH2)2COO-Na+, Not a good soap because its
hydrocarbon tail is not long enough (not sufficiently hydrophobic).
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Esters
• Ester is formed when Acid is heated with an alcohol (R’ = aliphatic) or a
phenol (R’ = aromatic) in the presence of a strong acid catalyst.
• This process is called Esterification.
• Esterification is a reversible reaction. Removal of water shifts the
equilibrium to the right.
Esters
• Esters in organisms are synthesized by reactions catalyzed by enzymes.
• Esters can also be formed from the reaction of an alcohol (or phenol)
with a carboxylic acid anhydride or halide, rather than a carboxylic acid.
• Esters of carboxylic acids have pleasant odors and flavors:
• Pineapple ethyl butanoate
• Raspberry isobutyl methanoate
• Banana 3-methyl-butyl ethanoate
• Various esters are used to create the fragrances of perfumes.
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Aspirin
2-hydroxybenzoic acid (salicylic acid) + acetic anhydride
Î Aspirin (acetylsalicylic acid) an ester
[2-Acetyloxybenzoic acid]
Nomenclature of Esters
• Esters are derived from the names of the alkoxy or aryloxy (alcohol or
phenol) part and the acyl (carboxylic acid) part of the compound.
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Nomenclature of Esters
Physical Properties of Esters
• Much lower boiling and melting points than carboxylic acids.
No H-bond to itself.
• Propanoic acid 141°C
• Methyl acetate 57°C
• Less soluble in water than carboxylic acids.
Not as many H-bonds to water.
• Lower melting and boiling points than aldehydes.
• Methyl acetate 57°C
• Butanal 76°C
• Same water solubilities as aldehydes and ketones.
All three hydrogen bond to water equally well.
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Hydrolysis of Esters
• Acidic hydrolysis of an ester is the reverse of the acid catalyzed
formation of an ester. Both are Equilibrium Reactions.
• The equilibrium can be shifted to favor hydrolysis by carrying out the
reaction with a large excess of water.
• Basic hydrolysis with a strong base (NaOH or KOH). Not Reversible.
• Saponification: Manufacture soaps from animal fats and vegetable oils.
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Carboxylic Acid Anhydrides & Halides
• Anhydride: A compound that is formed by dehydration.
• Acid anhydrides are highly reactive with water.
Not found in biological systems
systems.
• Carboxylic acid anhydrides: Change the word acid to anhydride.
Carboxylic Acid Anhydrides & Halides
• Acid anhydrides react with alcohols (or phenols) to form esters:
• Ester synthesis from anhydrides or acid halides is more efficient than
from the carboxylic acid itself.
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Carboxylic Acid Anhydrides & Halides
• Acid anhydrides, as well as carboxylic acids, are often called acyl
transfer agents because they transfer the acyl group to the alcohol
(or phenol).
• Acyl transfer reactions are biologically important:
• Protein Synthesis occurs through an acyl transfer reaction.
• Coenzyme A thioesters participate in metabolic reactions through
acyl transfer reactions.
Phosphoric Acids
• Phosphoric acid Î Diphosphoric and Triphosphoric acid:
• Stronger than carboxylic acids.
• Each O-H group is acidic (polyprotic acids).
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Phosphoric Acid Anhydrides
• Di & Triphosphoric acids are anhydrides as well as acids:
• Esterification to form Phosphoric acid esters or Phosphate esters.
• Named in the same way as carboxylic esters:
• The name of the alcohol (or phenol) is followed by the name of the
acid with its ending changed from –ic to –ate.
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Phosphate esters are biologically important:
ATP, NADH and NAD+, phospholipids, DNA and RNA (nucleic acids),
and phosphate esters of carbohydrates.
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