Chapter 20 Carboxylic Acids - psau.edu.sa · Common Names Many aliphatic acids have historical...
Transcript of Chapter 20 Carboxylic Acids - psau.edu.sa · Common Names Many aliphatic acids have historical...
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Copyright © 2010 Pearson Education, Inc.
Organic Chemistry, 7th Edition L. G. Wade, Jr.
Carboxylic Acids
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Introduction
▪ The functional group of carboxylic acids
consists of a C═O with —OH bonded to
the same carbon.
▪ Carboxyl group is usually written —COOH.
▪ Aliphatic acids have an alkyl group bonded
to —COOH.
▪ Aromatic acids have an aryl group.
▪ Fatty acids are long-chain aliphatic acids.
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Common Names
▪ Many aliphatic acids have historical names.
▪ Positions of substituents on the chain are
labeled with Greek letters starting at the
carbon attached to the carboxylic carbon.
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IUPAC Names
▪ Remove the final -e from alkane name, add
the ending -oic acid.
▪ The carbon of the carboxyl group is #1.
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Unsaturated Acids
▪ Remove the final -e from alkene name, add
the ending -oic acid.
▪ Stereochemistry is specified.
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Aromatic Acids
▪ Aromatic acids are named as derivatives of benzoic acid.
▪ Ortho-, meta- and para- prefixes are used to specify the location of a second substituent.
▪ Numbers are used to specify locations when more than 2 substituents are present.
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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
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Structure of Formic Acid
▪ The sp2 hybrid carbonyl carbon atom is planar, with nearly trigonal bond angles.
▪ The O—H bond also lies in this plane, eclipsed with the C═O bond.
▪ The sp3 oxygen has a C—O—H angle of 106°.
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Resonance Structures of Formic Acid
▪ Carbon is sp2 hybridized.
▪ Bond angles are close to 120.
▪ O—H eclipsed with C═O, to get overlap of
orbital with orbital of lone pair on oxygen.
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Boiling Points
▪ Higher boiling points than similar alcohols,
due to the formation of a hydrogen-bonded
dimer.
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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
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Solubility
▪ Water solubility decreases with the length of the
carbon chain.
▪ With up to 4 carbons, acid is miscible in water.
▪ Very soluble in alcohols.
▪ Also soluble in relatively nonpolar solvents like
chloroform because the hydrogen bonds of the
dimer are not disrupted by the nonpolar solvent.
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Acidity of Carboxylic Acids
▪ A carboxylic acid may dissociate in water to give a proton and a carboxylate ion.
▪ The equilibrium constant Ka for this reaction is called the acid-dissociation constant.
▪ The acid will be mostly dissociated if the pH of the solution is higher than the pKa of the acid.
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Energy Diagram of Carboxylic
Acids and Alcohols
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Acetate Ion Structure
▪ Each oxygen atom bears half of the negative charge.
▪ The delocalization of the negative charge over the
two oxygens makes the acetate ion more stable than
an alkoxide ion.
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Substituent Effects on Acidity
• The magnitude of a substituent effect depends on its distance from
the carboxyl group.
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Aromatic Carboxylic Acids
▪ Electron-withdrawing groups enhance the acid strength and electron-donating groups decrease the acid strength.
▪ Effects are strongest for substituents in the ortho and para positions.
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Deprotonation of Carboxylic Acids
▪ The hydroxide ion deprotonates the acid to
form the carboxylate salt.
▪ Adding a strong acid, like HCl, regenerates
the carboxylic acid.
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Deprotonation of Carboxylic Acids
▪ The hydroxide ion deprotonates the acid to
form the acid salt.
▪ Adding a mineral acid regenerates the
carboxylic acid.
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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
CH3CH2CHCH2COO-K
+
Cl
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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.
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Hydrolysis of Fats and Oils
• The basic hydrolysis of fat and oils produces soap
(this reaction is known as saponification).
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Some Important Acids
▪ Acetic acid is in vinegar and other
foods, used industrially as solvent,
catalyst, and reagent for synthesis.
▪ Fatty acids from fats and oils.
▪ Benzoic acid in found in drugs and
preservatives.
▪ Adipic acid used to make nylon 66.
▪ Phthalic acid used to make polyesters.
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Synthesis Review
▪ Oxidation of primary alcohols and
aldehydes with chromic acid.
▪ Cleavage of an alkene with hot KMnO4
produces a carboxylic acid if there is a
hydrogen on the double-bonded carbon.
▪ Alkyl benzene oxidized to benzoic acid
by hot KMnO4 or hot chromic acid.
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Oxidation of Primary Alcohol to
Carboxylic Acids
▪ Primary alcohols and aldehydes are commonly oxidized to acids by chromic acid (H2CrO4 formed from Na2Cr2O7 and H2SO4).
▪ Potassium permanganate is occasionally used, but the yields are often lower.
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Cleavage of Alkenes Using KMnO4
▪ Warm, concentrated permanganate solutions oxidize
the glycols, cleaving the central C═C bond.
▪ Depending on the substitution of the original double
bond, ketones or acids may result.
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Alkyne Cleavage Using Ozone or
KMnO4
▪ With alkynes, either ozonolysis or a vigorous
permanganate oxidation cleaves the triple
bond to give carboxylic acids.
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Side Chain Oxidation of
Alkylbenzenes
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Conversion of Grignards to
Carboxylic Acids
▪ Grignard reagent react with CO2 to produce, after protonation, a
carboxylic acid.
▪ This reaction is sometimes called “CO2 insertion” and it
increases the number of carbons in the molecule by one.
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Hydrolysis of Nitriles
CH2Br CH2CNNaCN
acetoneH+, H2O
CH2CO2H
▪ Basic or acidic hydrolysis of a nitrile (—CN)
produces a carboxylic acid.
▪ The overall reaction, starting from the alkyl
halide, adds an extra carbon to the molecule.
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Acid Derivatives
▪ The group bonded to the acyl carbon
determines the class of compound:
▪ —OH, carboxylic acid
▪ —Cl, acid chloride
▪ —OR’, ester
▪ —NH2, amide
▪ These interconvert via nucleophilic acyl
substitution.
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Nucleophilic Acyl Substitution
▪ Carboxylic acids react by nucleophilic acyl
substitution, where one nucleophile replaces
another on the acyl (C═O) carbon atom.
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Fischer Esterification
▪ Reaction of a carboxylic acid with an alcohol under acidic
conditions produces an ester.
▪ Reaction is an equilibrium, the yield of ester is not high.
▪ To drive the equilibrium to the formations of products use a
large excess of alcohol.
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Fischer Esterification Mechanism
▪ Step 1:▪ The carbonyl oxygen is protonated to activate the carbon
toward nucleophilic attack.
▪ The alcohol attacks the carbonyl carbon.
▪ Deprotonation of the intermediate produces the ester hydrate.
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Fischer Esterification Mechanism
▪ Step 2:
▪ Protonation of one of the hydroxide creates a good leaving
group.
▪ Water leaves.
▪ Deprotonation of the intermediate produces the ester.
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Ethyl orthoformate hydrolyzes easily in dilute acid to give formic acid and three equivalents of
ethanol. Propose a mechanism for the hydrolysis of ethyl orthoformate.
Solved Problem 1
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Esterification Using
Diazomethane)Mechanism(
▪ Carboxylic acids are converted to their methyl esters very simply by adding an ether solution of diazomethane.
▪ The reaction usually produces quantitative yields of ester.
▪ Diazomethane is very toxic, explosive. Dissolve in ether.
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LiAlH4 or BH3 Reduction of
Carboxylic Acids
▪ LiAlH4 reduces carboxylic acids to primary alcohols.
▪ The intermediate aldehyde reacts faster with the reducing agent than the carboxylic acid.
▪ BH3•THF (or B2H6) can also reduce the carboxylic acid to the alcohol
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Conversion of Carboxylic Acids to
Ketones
▪ A general method of making ketones involves the reaction of a carboxylic acid with two equivalents of an organolithium reagent.
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Mechanism of Ketone Formation
▪ The first equivalent of organolithium acts as a base, deprotonating the carboxylic acid.
▪ The second equivalent adds to the carbonyl.
▪ Hydrolysis forms the hydrate of the ketone, which converts to the ketone.
R C
O
OH 2 R' Li
R C
OLi
OLi
R'
H3O+
R C
OH
OH
R'
R C
O
R' + H2O
dianion hydrate of ketone ketone
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Synthesis of Acid Chlorides
▪ The best reagent for converting carboxylic acids to
acid chlorides are thionyl chloride (SOCl2) and oxalyl
chloride (COCl2) because they form gaseous by-
products that do not contaminate the product.
▪ Thionyl chloride reaction produces SO2 while the
oxalyl chloride reaction produces HCl, CO, and CO2
(all gaseous).
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Mechanism of Acid Chloride
FormationStep 1
Step 2
Step 3
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Amide Synthesis
▪ Ammonia and amines react with acid chlorides to give amides
▪ NaOH, pyridine, or a second equivalent of amine is used to neutralize the HCl produced to prevent protonation of the amine.