carboxylic acid nomenclature. © 2013 pearson education, inc.chapter 202 introduction the...
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Carboxylic Acid Carboxylic Acid
NomenclatureNomenclature
© 2013 Pearson Education, Inc. Chapter 20 2
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.
Table 18.1
Systematic NameO
HCOH
O
CH3COH
O
CH3(CH2)16COH
methanoic acid
ethanoic acid
octadecanoic acid
Systematic IUPAC names replace "-e" ending of alkane with "oic acid"
Table 18.1
Systematic Name Common Name
methanoic acid formic acid
ethanoic acid acetic acid
octadecanoic acid stearic acid
Common names are based on natural origin rather than structure.
O
HCOH
O
CH3COH
O
CH3(CH2)16COH
Table 18.1
Systematic Name Common Name
2-hydroxypropanoic acid lactic acid
(Z)-9-octadecenoic acidor (Z)-octadec-9-enoic acid
oleic acid
O
CH3CHCOH
OH O
(CH2)7COH
C C
H H
CH3(CH2)7
© 2013 Pearson Education, Inc. Chapter 20 6
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
two substituents are present.
© 2013 Pearson Education, Inc. Chapter 20 7
Structure of the Carboxyl Group
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°.
© 2013 Pearson Education, Inc. Chapter 20 8
Boiling Points
Carboxylic acids boil at considerably higher temperatures than do alcohols, ketones, or aldehydes of similar molecular weights.
The high boiling points of carboxylic acids result from formation of a stable, hydrogen-bonded dimer.
Carboxylic acids are similar to alcohols in respect to their solubility in water.
They form hydrogen bonds to water.
Solubility in Water
H3CC
O H O
OHO
H
H
H
© 2013 Pearson Education, Inc. Chapter 20 10
Solubility
Water solubility decreases with the length of the carbon chain.
Acids with more than 10 carbon atoms are nearly insoluble 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.
Acidity of Carboxylic AcidsAcidity of Carboxylic Acids
Most carboxylic acids have a pMost carboxylic acids have a pKKaa close to 5. close to 5.
But carboxylic acids are far more acidic than alcohols.
Carboxylic Acids are Weak Acids
CH3COH
O
CH3CH2OH
pKa = 4.7 pKa = 16
© 2013 Pearson Education, Inc. Chapter 20 13
Energy Diagram of Carboxylic Acids and Alcohols
© 2013 Pearson Education, Inc. Chapter 20 14
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.
© 2013 Pearson Education, Inc. Chapter 20 15
Substituent Effects on Acidity
The magnitude of a substituent effect depends on its distance from the carboxyl group.
© 2013 Pearson Education, Inc. Chapter 20 16
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.
Hybridization Effect
pKa
4.2
4.3
1.8
COH
O
H2C CH COH
O
COH
O
HC C
sp2-hybridized carbon is more electron-withdrawing than sp3, and sp is more electron-withdrawing than sp2.
Salts of Carboxylic Acids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© 2013 Pearson Education, Inc. Chapter 20 19
Deprotonation of Carboxylic Acids
The hydroxide ion completely deprotonates the acid to form the carboxylate salt.
© 2013 Pearson Education, Inc. Chapter 20 20
Protonation of Carboxylic Acids Salts
Adding a strong acid, like HCl, regenerates the carboxylic acid.
© 2013 Pearson Education, Inc. Chapter 20 21
Naming Carboxylic Acid Salts
First name the cation. Then name the anion by replacing the
-ic acid with -ate.
© 2013 Pearson Education, Inc. Chapter 20 22
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.
© 2013 Pearson Education, Inc. Chapter 20 23
Basic Hydrolysis of Fats and Oils
• The basic hydrolysis of fat and oils produces soap (this reaction is known as saponification).
Unbranched carboxylic acids with 12-18 carbonsgive carboxylate salts that form micelles in water.
Micelles O
ONasodium stearate
(sodium octadecanoate)
CH3(CH2)16CO
O
Na+–
Micelles O
ONa
polarnonpolar
Micelles O
ONa
polarnonpolar
Sodium stearate has a polar end (the carboxylate end) and a nonpolar "tail“.
The polar end is hydrophilic ("water-loving”).
The nonpolar tail is hydrophobic ("water-hating”).
In water, many stearate ions cluster together to form spherical aggregates; carboxylate ions are on the outside and nonpolar tails on the inside.
Figure 18.5: A micelle
Micelles
The interior of the micelle is nonpolar and has the capacity to dissolve nonpolar substances.
Soaps clean because they form micelles, which are dispersed in water.
Grease (not ordinarily soluble in water) dissolves in the interior of the micelle and is washed away with the dispersed micelle.
Dicarboxylic AcidsDicarboxylic Acids
Dicarboxylic Acids
One carboxyl group acts as an electron-withdrawing group toward the other; effect decreases with increasing separation.
Oxalic acid
Malonic acid
Heptanedioic acid
1.2
2.8
4.3
COH
O
HOC
O pKa
HOCCH2COH
OO
HOC(CH2)5COH
O O
side-chain oxidation of alkylbenzenes (Section 11.12)
oxidation of primary alcohols (Section 15.9)
oxidation of aldehydes (Section 17.15)
Synthesis of Carboxylic Acids: Review
© 2013 Pearson Education, Inc. Chapter 20 32
Side Chain Oxidation of Alkylbenzenes
© 2013 Pearson Education, Inc. Chapter 20 33
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.
© 2013 Pearson Education, Inc. Chapter 18 34
Oxidation of Aldehydes
Aldehydes are easily oxidized to carboxylic acids.
© 2013 Pearson Education, Inc. Chapter 20 35
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.
© 2013 Pearson Education, Inc. Chapter 20 36
Alkyne Cleavage Using Ozone or KMnO4
With alkynes, either ozonolysis or a vigorous permanganate oxidation cleaves the triple bond to give carboxylic acids.
Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acidsby theby the
Carboxylation of Grignard ReagentsCarboxylation of Grignard Reagents
© 2013 Pearson Education, Inc. Chapter 20 38
Carboxylation of Grignard Reagents
Grignard reagents 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.
Example: Alkyl Halide
CH3CHCH2CH3
(76-86%)
1. Mg, diethyl ether
2. CO2
3. H3O+
CH3CHCH2CH3
Cl CO2H
2-methylbutanoic acid2-methylbutanoic acid
Example: Aryl Halide
(82%)
1. Mg, diethyl ether
2. CO2
3. H3O+
CH3
CO2HBr
CH3
Synthesis of Carboxylic AcidsSynthesis of Carboxylic Acidsby theby the
Preparation and Hydrolysis of NitrilesPreparation and Hydrolysis of Nitriles
© 2013 Pearson Education, Inc. Chapter 20 42
Hydrolysis of Nitriles
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.
A limitation is that the halide must be reactive toward substitution by SN2 mechanism.
Example
NaCN
DMSO(92%)
CH2Cl
CH2CN
(77%)
H2O
H2SO4
heat
CH2COH
O
Example: Dicarboxylic Acid
BrCH2CH2CH2Br
NaCN H2O
(77-86%)NCCH2CH2CH2CN
H2O, HCl heat
(83-85%)HOCCH2CH2CH2COH
OO
via Cyanohydrin
1. NaCN
2. H+
CH3CCH2CH2CH3
O
CH3CCH2CH2CH3
OH
CN
(60% from 2-pentanone)
H2O
HCl, heat
CH3CCH2CH2CH3
OH
CO2H
Reactions of Carboxylic Acids:Reactions of Carboxylic Acids:
A Review and a PreviewA Review and a Preview
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Reduction with LiAlH4 (Section 15.3)
Formation of acyl chlorides (Section 12.7)
Esterification (Section 15.8)
Reactions already discussed
Reactions of Carboxylic Acids
© 2013 Pearson Education, Inc. Chapter 20 48
LiAlH4 Reduction of Carboxylic Acids
LiAlH4 reduces carboxylic acids to primary alcohols.
© 2013 Pearson Education, Inc. Chapter 20 49
Synthesis of Acid Chlorides
The best reagents for converting carboxylic acids to acid chlorides are thionyl chloride (SOCl2) and oxalyl chloride (COCl2).
They form gaseous by-products that do not contaminate the product.
© 2013 Pearson Education, Inc. Chapter 20 50
Mechanism of Acid Chloride Formation
Step 1
Step 2
Step 3
© 2013 Pearson Education, Inc. Chapter 20 51
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 toward the formation of products, use a large
excess of alcohol.
© 2013 Pearson Education, Inc. Chapter 20 52
Mechanism of the Fischer Esterification
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.
© 2013 Pearson Education, Inc. Chapter 20 53
Mechanism of the Fischer Esterification (Continued)
Step 2: Protonation of one of the hydroxide groups creates a good
leaving group. Water leaves. Deprotonation of the intermediate produces the ester.
Protonation of carbonyl group activates carbonyl oxygen.
Nucleophilic addition of alcohol to carbonyl groupforms tetrahedral intermediate.
Elimination of water from tetrahedral intermediate restores carbonyl group.
Key Features of Mechanism
Intramolecular Ester Formation:Intramolecular Ester Formation:
LactonesLactones
Lactones are cyclic esters.
Formed by intramolecular esterification in acompound that contains a hydroxyl group anda carboxylic acid function
Lactones
Examples
IUPAC nomenclature: replace the -oic acid ending of the carboxylic acid by –olide.
Identify the oxygenated carbon by number.
HOCH2CH2CH2COH
O O
O+ H2O
4-hydroxybutanoic acid 4-butanolide
Examples
HOCH2CH2CH2COH
O O
O+ H2O
4-hydroxybutanoic acid 4-butanolide
HOCH2CH2CH2CH2COH
O O
O
+ H2O
5-hydroxypentanoic acid 5-pentanolide
Common names O
O
O
O
-butyrolactone -valerolactone
Ring size is designated by Greek letter corresponding to oxygenated carbon
A lactone has a five-membered ring.
A lactone has a six-membered ring.
Reactions designed to give hydroxy acids often yield the corresponding lactone, especially if theresulting ring is 5- or 6-membered.
Lactones
Example
5-hexanolide (78%)
O
H3C
O
CH3CCH2CH2CH2COH
OO
1. NaBH4
2. H2O, H+
via:via:
CHCH33CHCHCHCH22CHCH22CHCH22COHCOH
OOOOHH
Decarboxylation of Malonic AcidDecarboxylation of Malonic Acid
and Related Compoundsand Related Compounds
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylatereadily.
RH + CO2RCOH
O
But malonic acid does.
150°CCH3COH
O
+ CO2HOCCH2COH
O O
O
HO O
O
H H
H
Mechanism of Decarboxylation of Malonic Acid
This compound is the enol form of acetic acid.
O O
OHHO
H H
H
H
OH
HO+ C
O
O
One carboxyl group assists the loss of the other.
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
HOCCH3
O
These hydrogens play no role. H
H
OH
HO+ C
O
O
O
HO O
O
H H
H O O
OHHO
H H
R
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
HOCCHR'
O
Groups other than H may be present. R
R’
OH
HO+ C
O
O
O
HO O
O
R R’
H O O
OHHO
R R’
185°C
Decarboxylation is a general reactionfor 1,3-dicarboxylic acids
160°C
CO2H
CO2H
CO2H
H
(74%) CH(CO2H)2
(96-99%)
CH2CO2H
Mechanism of Decarboxylation of Malonic Acid
One carboxyl group assists the loss of the other.
This OH group plays no role. R
R’
OH
HO+ C
O
O
O
HO O
O
R R’
H O O
OHHO
R R’
R
HOCCHR'
O
Mechanism of Decarboxylation of Malonic Acid O O
OHR"
R R'
R
C
O
O
One carboxyl group assists the loss of the other.
Groups other than OH may be present.
R"CCHR'
O
R
O
O
O
R R'
H R"
R'
OH
+R"
Mechanism of Decarboxylation of Malonic Acid O O
OHR"
R R'
This kind of compoundis called a -keto acid.
R"CCHR'
O
R
Decarboxylation of a -keto acid gives a ketone.
Decarboxylation of a -Keto Acid
CCH3C
O
CH3
CH3
CO2H25°C
CO2
CCH3C
O
CH3
CH3
H
+
© 2013 Pearson Education, Inc. Chapter 20 72
Some Important Acids
Acetic acid is in vinegar and other foods, used industrially as a solvent, catalyst, and reagent for synthesis.
Fatty acids from fats and oils. Benzoic acid is found in drugs and
preservatives. Adipic acid is used to make nylon 66. Phthalic acid is used to make polyesters.
Spectroscopic Analysis ofSpectroscopic Analysis of
Carboxylic AcidsCarboxylic Acids
© 2013 Pearson Education, Inc. Chapter 20 74
IR Bands of Carboxylic Acids
There will be two features in the IR spectrum of a carboxylic acid: the intense carbonyl stretching absorption (1710 cm–1) and the OH absorption (2500–3500 cm–1).
Conjugation lowers the frequency of the C═O band.
© 2013 Pearson Education, Inc. Chapter 20 75
IR Spectroscopy
© 2013 Pearson Education, Inc. Chapter 20 76
NMR of Carboxylic Acids
Carboxylic acid protons are the most deshielded protons we have encountered, absorbing between 10 and 13.
The protons on the carbon atom absorb between 2.0 and 2.5.
© 2013 Pearson Education, Inc. Chapter 20 77
NMR Spectroscopy
13C NMR
Carbonyl carbon is at low field ( 160-185 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone ( 190-215 ppm).
Aliphatic carboxylic acids undergo a varietyof fragmentations.Aromatic carboxylic acids first form acylium ions,which then lose CO.
Mass Spectrometry
ArCOH
••O •
•
ArCOH
•+O •
•
ArC O ••
+Ar
+