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1
Introduction to carboxylic acids
2
Learning Objectives
• Identify the general structure of carboxylic acids
• Give the IUPAC and the common names• Explain the physical properties of the
carboxylic acids ; e.g solubility , boiling point, etc
• Give examples and uses of carboxylic acids
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Functional Groups
4
R = Alkyl Groups
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Carboxylic Acids
A class of organic compounds containing at least one carboxyl group
Introduction
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R C
O
OH
R = alkyl group Aliphatic acid
R = aryl group aromatic acid
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(1) The IUPAC names of carboxylic acids
Replace the ‘e’ with ‘oic acid’ at the end of the name of the hydrocarbon
Naming Carboxylic Acids
H H
H C C H
H Hethane
H O
H C C O H
Hethan oic acid
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• Number substituents from the carboxyl carbon 1.
CH3 O | ║
CH3—CH—CH2—C—OH 4 3 2 1
3-methylbutanoic acid
IUPAC Names
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The common names of simple carboxylic acids HCOOH formic acid, CH3—COOH acetic acid
• Locate substituents using greek letters , , γ for the carbon atoms adjacent to the carboxyl carbon.
CH3
γ α | CH3—CH—CH2—COOH
α-methylbutryic acid ( IUPAC ; 2-methylbutanoic acid )
(2) Common Names
CH3CH2CH2CHCH2COOH
Ph
-phenylcaproic acid ε δ γ α
Common and IUPAC Names of Some Carboxylic Acids
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(3) Naming Cyclic Acids
Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids
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Br
O
HO
3-bromo-cyclohexane carboxylic acid
O
OH
1-cyclopentene carboxylic acid
COOH
Cyclohexanecarboxylic acid
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(4) Naming Aromatic Acids
OHC
O
3-chlorobenzoic acid 4-aminobenzoic acid m-chlorobenzoic acid p-aminobenzoic acid
Locates substituent by assigning 1 to the carbon attached to the carboxyl group. Prefixes ortho, meta, and para for 2 substituents:
ortho 1, 2 locationmeta 1, 3 location para 1, 4 location
Benzoic acid
o-hydroxybenzoic acid2-hydroxybenzoic acid
(salicylic acid)
OHC
O
OHC
O
ClNH2
COOH
OH
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Dicarboxylic Acids• Aliphatic diacids are usually called by their
common names.• For IUPAC name, number the chain from the
end closest to a substituent.
3-bromohexanedioic acid-bromoadipic acid
HOOCCH2CHCH2CH2COOH
Br
Some examples of dicarboxylic acid
Common name IUPAC name(Historical name) alkanedioic acid
HOOC-COOH
HOOCCH2COOH
HOOC(CH2)2COOH
oxalic acid
malonic acid
succinic acid
ethanedioic acid
propanedioic acid
butanedioic acid
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COOH
COOH
phthalic acidO-phthalic acid
COOHHOOC 1,3-benzenedicarboxylic acid
HOOC
COOH
1,4-benzenedicarboxylic acid
Common name IUPAC name
1,2-benzenedicarboxylic acid
isophthalic acidm-phthalic acid
terephthalic acidp-phthalic acid
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PHYSICAL PROPERTIES
carboxylic acids are soluble in organic solvents ; e.g alcohols they are also soluble in water due to hydrogen bonding
small ones dissolve readily in cold water ( up to 4 carbons) as mass increases, the solubility decreases
benzoic acid is fairly insoluble in cold, but soluble in hot water
Solubility
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The boiling points of carboxylic acids• Are higher than alcohols, ketones, and aldehydes of similar
mass.
• Are high because they form dimers in which hydrogen bonds form between the polar groups in the two carboxyl groups.
O H—O || |
CH3—C C—CH3
| ||
O—H O
A dimer of acetic acid
Boiling Points
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Melting Points• Aliphatic acids with more than 8 carbons are solids at
room temperature.• Double bonds (especially cis) lower the melting
point. The following acids all have 18 carbons:– Stearic acid (saturated): 72C
– Oleic acid (one cis double bond): 16C
– Linoleic acid (two cis double bonds): -5C
CO2H
CO2H
CO2H
Polarity of Carboxylic Acids
Polarity isCarboxylic acids are strongly polar because they have
two polar groups hydroxyl (OH) and carbonyl (C=O)
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Acidity of Carboxylic Acids
• A carboxylic acid may dissociate in water to give a proton and a carboxylate ion (known as conjugate base).
• The equilibrium constant Ka for this reaction is called the acid-dissociation constant, The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5)
• The acid will be mostly dissociated if the pH of the solution is higher than the pKa of the acid.
• are weak acids compared to inorganic acids
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Acidity of Carboxylic Acids Carboxylic acids are significantly more acidic than water or alcohols.
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4 -electrons delocalizedover three p-prbitals
C-O bond length of a carboxylates are the same
This greater acidity of carboxylic acids is attributed to greater stabilization of carboxylate ion by:
a. Inductive effect of the C=O group
b. Resonance stabilization of the carboxylate ion
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Electron delocalization stabilizes carbonyl group
Carbon is sp2 hybridized.
Bond angles are close to 120.
RC
OH
O••
•• ••
•• RC
OH
O••
•• ••
••+••
– RC
OH
O••
••
••
+
••
–
Electron delocalization; resonance
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Electronegative substituents increase acidity
X CH2C-OH
O X Ka pKa
HF
Cl
1.8 x 10-5 4.72.5 x 10-3 2.6
1.4 x 10-3 2.9
Substituent Effects on Acidity
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Substituent Effects on Acidity
• Multiple electronegative substituents have synergistic effects on acidity
• Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid
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The magnitude of a substituent effect depends on its distance from the carboxyl group.
Substituent Effects on Acidity
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Aromatic Carboxylic Acids
Electron-withdrawing groups (e.g NO2) enhance the acid strength while electron-donating groups ( e.g alkyl ) decrease the acid strength.
Effects are strongest for substituents in the ortho and para positions.
Substituent Effects
• An electronegative group will drive the ionization equilibrium toward dissociation, increasing acidity
• An electron-withdrawing group (-NO2) increases acidity by stabilizing the carboxylate anion, and an electron-donating (activating) group (OCH3) decreases acidity by destabilizing the carboxylate anion
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Substituent Effects in Substituted Benzoic Acids
29
So:• Carboxylic acids are weak acids, average
pH = 5• Are Polar because of ( the high
electronegative O)• Soluble in water• Have high boiling and melting points
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Learning Check
Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.
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Solution
Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.
O O
CH3CH2COH + H2O CH3CH2CO– + H3O+
carboxylic acid carboxylate ion
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Naming Carboxylic Acid Salts
• First name the cation.• Then name the anion by replacing the
-ic acid with -ate.
potassium 3-chloropentanoate
C H3 C H2 C HC H2 C O O- K
+
C l
Neutralization of Carboxylic Acids
Carboxylic acid salts • are a product of the neutralization of a carboxylic acid with a
strong base
CH3—COOH + NaOH CH3—COO– Na+ + H2Oacetic acid sodium acetate
(carboxylic acid salt)• are used as :• preservatives and flavor enhancers
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Properties of Acid Salts
• Usually solids with no odor.
• Carboxylate salts of Na+, K+, Li+, and NH4+
are soluble in water.• Soap is the soluble sodium salt of a long
chain fatty acid.• Salts can be formed by the reaction of an
acid with NaHCO3, releasing CO2.
36
Uses of Organic Acids
Alpha hydroxy acids Acetic acid
skin care products. in vinegar as preservative and flavourings
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Uses of Organic Acids
Making of drugs, dyes, paints, insecticides, plastics.
Making of esters.
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Limes, lemons
Citric acid
Formic acid
Insects
Other Important Organic Acids
Malic acid
Apples and pears
Lactic acid
Sour milkTartaric acid
Grape juice
Oxalic acid
Spinch
39
VITAMIN C (Ascorbic
acid)