1 chapter 19 carboxylic acids. 2 carboxylic acid structure carboxylic acids are compounds containing...
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Chapter 19Carboxylic acids
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Carboxylic Acid Structure
• Carboxylic acids are compounds containing a carboxy group (COOH).
• The structure of carboxylic acids is often abbreviated as RCOOH or RCO2H, but keep in mind that the central carbon atom of the functional group is doubly bonded to one oxygen atom and singly bonded to another.
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• The C–O single bond of a carboxylic acid is shorter than the C–O bond of an alcohol.
• This can be explained by looking at the hybridization of the respective carbon atoms.
• Because oxygen is more electronegative than either carbon or hydrogen, the C–O and O–H bonds are polar.
Bonds in Carboxylic Acids
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R O
OH
R O
OH
Protonate at carbonyl oxygen!!!!
R O
OH
HBr
R O
OH
H
Br
R O
OH
H
lp on "sp3" oxygen are not HOMO
R O
OH
HBr
R O
OH
H
NO!
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• In the IUPAC system, carboxylic acids are identified by a suffix added to the parent name of the longest chain with different endings being used depending on whether the carboxy group is bonded to a chain or a ring.
• If the COOH is bonded to a chain, find the longest chain containing the COOH, and change the “e” ending of the parent alkane to the suffix “oic acid”.
• If the COOH is bonded to a ring, name the ring and add the words “carboxylic acid”.
• Number the carbon chain or ring to put the COOH group at C1, but omit this number from the name.
• Apply all the other usual rules of nomenclature.
Naming Carboxylic Acids
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• Greek letters are used to designate the location of substituents in common names.
• The carbon adjacent to the COOH is called the α carbon, followed by the β carbon, followed by the γ carbon, the δ carbon and so forth down the chain.
• The last carbon in the chain is sometimes called the ω carbon.
• The α carbon in the common system is numbered C2 in the IUPAC system.
Common Naming for Carboxylic Acids
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• Compounds containing two carboxy groups are called diacids.
• Diacids are named using the suffix-dioic acid.
• Metal salts of carboxylate anions are formed from carboxylic acids in many reactions.
• To name the metal salt of a carboxylate anion, put three parts together:
Naming Diacids and Salts
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Figure 19.2
Naming Metal Salts of Carboxylate Anions
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• Carboxylic acids exhibit dipole–dipole interactions because they have polar C–O and O–H bonds.
• They also exhibit intermolecular hydrogen bonding.• Carboxylic acids often exist as dimers held together by
two intermolecular hydrogen bonds.Figure 19.3
Two molecules of acetic acid(CH3COOH) held together by
two hydrogen bonds
Hydrogen Bonds Cause Dimers
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• Carboxylic acids have very characteristic IR and NMR absorptions.
• In the IR:
• The C=O group absorbs at ~1710 cm−1.
• The O–H absorption occurs from 2500–3500 cm−1.
• In the 1H NMR:
• The O–H proton absorbs between 10–12 ppm.
• The α protons absorb between 2–2.5 ppm.
• In the 13C NMR, the C=O appears at 170–210 ppm.
Spectroscopy of Carboxylic Acids
H3C O
OH
H3C O
OH
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Figure 19.4
IR Spectrum of Butanoic Acid
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Figure 19.5
1H NMR Spectra of Propanoic Acid
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13C NMR Spectra of Propanoic Acid
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• Acetic acid is the sour component of vinegar.
• The air oxidation of ethanol to acetic acid is the process that makes “bad” wine taste sour.
• Acetic acid is an industrial starting material for polymers used in paints and adhesives.
Acetic Acid
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• Butanoic acid is an oxidation product that contributes to the disagreeable smell of body odor.
• Its common name, butyric acid, is derived from the Latin word butyrum, meaning “butter,” because butyric acid gives rancid butter its peculiar odor and taste.
Butanoic Acid
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•Oxalic acid, lactic acid and citric acid are simple carboxylic acids that are quite prevalent in nature.
Carboxylic Acids in Nature
OH
O
OH
O OHO
HO
citric acid
preservative, citric acid cyclic metabolite
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Carboxylic acids in plastics
O
OH
acrylic acid
monomer
O
HO
O
OH
adipic acid
Monomer Nylon 6,6
CO2H
CO2H
terephthalic acidMonomer for Kevlar and PET
CO2H
CO2H
phthalic acid
precursor to plasticizers
O
HONH2
6-aminohexanoic acid
monomer for Nylon 6
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O
OH
(RS)-2-(4-(2-methylpropyl)phenyl)propanoic acid
Ibuprofen (Advil, Motrin)
O
OH
O(+)-(S)-2-(6-methoxynaphthalen-2-yl)propanoic acid
Naproxen (Aleve)
O OH
2-Propylpentanoic acid
Valproic acidanticonvulsantSchizophrenia, epilepsybipolar disorder
Drugs with carboxylic acid groups
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• Salts of carboxylic acids are commonly used as preservatives.
• Sodium benzoate is a fungal growth inhibitor and is also used as a preservative in soft drinks.
• Potassium sorbate is an additive that prolongs the shelf-life of baked goods and other foods.
Interesting Salts of Carboxylic Acids
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• Aspirin (acetylsalicylic acid) is a synthetic carboxylic acid, similar in structure to salicin, a naturally occurring compound isolated from willow bark, and salicylic acid, found in meadowsweet.
Aspirin and Related Compounds
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Saturated Fatty acids
O
OH12
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
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stearic acid
Octadecanoic acid
O
OH12
3
4
5
6
7
8
9
10
11
12
13
14
15
16
palmitic acid
hexadecanoic acid
O
OH12
3
4
5
6
7
8
9
10
11
12
13
14
myristic acid
tetradecanoic acid
Nutmeg (Myristica fragrans) butter is 75% myristic acid
O
OH12
3
4
5
6
7
8
9
10
11
12
lauric acid
dodecanoic acid
milk, palm kernal, coconut oils
palm, palm kernal and coconut oilsuet, tallow, lard, meat
also inmilk and milk products
necessary for membranes & storing energy (9 kcal/g)too much: cardiovascular disease & more
from suet
O OO
O
O
R R
O
R
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Napalm A
napalm = aluminum salts of palmitic acid and naphthenic acid added to make gels from gasoline
O
O Al3+
O
O
O
O
O
O Al3+
O
O
O
O
naphthenic
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Mono-unsaturated Fatty acids
(9Z)-Octadec-9-enoic acid
O
OH
Oleic acid
from triglycerides in olive oil
O OO
O
O
Otriglyceride
omega-9
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omega-6 fatty acids
cis, cis-9,12-Octadecadienoic acid
O
OH1
2
3
4
5
6
linon = Greek for flax
linoleic acid
A diet only deficient in linoleate causes mild skin scaling, hair loss, and poor wound healing
Along with oleic acid, linoleic acid is released by cockroaches upon death which has the effect of preventing other roaches from entering the area. This is similar to the mechanism found in ants and bees, in which oleic acid is released upon death
omega 6 unsaturated Fatty acids
alpha-linoleic acid
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omega 3 unsaturated Fatty acids
O
OH1
2
3
omega-3 fatty acids
timnodonic acid.
(5Z,8Z,11Z,14Z,17Z)-5,8,11,14,17-icosapentaenoic acid
EPA
O
O
(4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoic acid
DHA (primary membrane structural fatty acid in brain phospholipids)
essential fatty acids
O
OH
linolenic acid (ALA)(9Z,12Z,15Z)-9,12,15-Octadecatrienoic acid
1
2
3
12 3α
not stable to high temperatures or UV
linolenic acid cures into polymer used in making linoleum
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• Aspirin lessens pain and decreases inflammation because it prevents the synthesis of prostaglandins, the compounds responsible for both of these physiological responses.
• Aspirin inactivates cyclooxygenase, an enzyme that converts arachidonic acid to PGG2, an unstable precursor of PGF2α and other prostaglandins.
Arachidonic Acid and Prostaglandins
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[1] Oxidation of 1° alcohols
[2] Oxidation of alkyl benzenes
Preparation of Carboxylic Acids
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[3] Oxidative cleavage of alkynes
Preparation of Carboxylic Acids
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Preparation of fatty acids: hydrolysis of esters and phospholipids
O OO
O
O
R R
O
R
triglyceride esters
> 3H2O
H+ or OH-
HO OHOH
OH
O
R
membrane amphiphile or cooking oil
glycerin or glycerol
+ 3
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• The most important reactive feature of a carboxylic acid is its polar O–H bond, which is readily cleaved with base.
Acid–Base Reactions of Carboxylic Acids
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• The nonbonded electron pairs on oxygen create electron-rich sites that can be protonated by strong acids (H–A).
• Protonation occurs at the carbonyl oxygen because the resulting conjugate acid is resonance stabilized (Possibility [1]).
• The product of protonation at the OH group (Possibility [2]) cannot be resonance stabilized.
Protonation of Carboxylic Acids
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• The polar C–O bonds make the carboxy carbon electrophilic.
• Thus, carboxylic acids react with nucleophiles.
• Nucleophilic attack occurs at an sp2 hybridized carbon atom, so it results in the cleavage of the bond as well.
Reaction with Nucleophiles and Bases
• Carboxylic acids are strong organic acids, and as such, readily react with BrØnsted–Lowry bases to form carboxylate anions.
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• An acid can be deprotonated by a base that has a conjugate acid with a higher pKa.
• Because the pKa values of many carboxylic acids are ~5, bases that have conjugate acids with pKa values higher than 5 are strong enough to deprotonate them.
Deprotonation of Carboxylic Acids
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• Carboxylic acids are relatively strong acids because deprotonation forms a resonance-stabilized conjugate base—a carboxylate anion.
• The acetate anion has two C–O bonds of equal length (1.27 Å) and intermediate between the length of a C–O single bond (1.36 Å) and C=O (1.21 Å).
Resonance of Carboxylate Anions
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• Resonance stabilization accounts for why carboxylic acids are more acidic than other compounds with O–H bonds—namely alcohols and phenols.
• To understand the relative acidity of ethanol, phenol, and acetic acid, we must compare the stability of their conjugate bases and use the following rule:
• Anything that stabilizes a conjugate base A:− makes the starting acid H–A more acidic.
Acidity of Common Organic Acids
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• Ethoxide, the conjugate base of ethanol, bears a negative charge on the O atom, but there are no additional factors to further stabilize the anion.
• Because ethoxide is less stable than acetate, ethanol is a weaker acid than acetic acid.
Acidity of Ethanol
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• Phenoxide, the conjugate base of phenol, is more stable than ethoxide, but less stable than acetate because acetate has two electronegative O atoms upon which to delocalize the negative charge, whereas phenoxide has only one.
Acidity of Phenol
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• Structures 2–4 have the negative charge on a C atom, a less electronegative element than O, making them less stable than structures 1 and 5.
• Structures 1 and 5 have intact aromatic rings, whereas structures 2–4 do not, making structures 1 and 5 more stable.
Figure 19.6
Energies of Resonance Structures for Phenoxide
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• Resonance stabilization of the conjugate base is important in determining acidity, the absolute number of resonance structures alone is not what is important!
Figure 19.7
Relationship Between Acidity and Stability of Conjugate Base
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Inductive Effects in Carboxylic Acids
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Inductive Effects in Carboxylic Acids
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• Substituents on a benzene ring either donate or withdraw electron density, depending on the balance of their inductive and resonance effects.
• These same effects also determine the acidity of substituted benzoic acids.
[1] Electron-donor groups destabilize a conjugate base, making an acid less acidic—the conjugate base is destabilized because electron density is being donated to a negatively charged carboxylate anion.
Substituent Effects in Aromatic Carboxylic Acids
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[2] Electron-withdrawing groups stabilize a conjugate base, making an acid more acidic. The conjugate base is stabilized because electron density is removed from the negatively charged carboxylate anion.
Substituent Effects in Aromatic Carboxylic Acids
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Figure 19.8How common substituents
affect the reactivity of abenzene ring towards
electrophiles and the acidity ofsubstituted benzoic acids
Common Substituent Effects
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• Extraction is a technique that permits the separation of mixtures of compounds based on acid–base principles, and solubility differences.
• When two immiscible liquids come into contact, two layers are formed.
• The two layers can be separated using a piece of glassware called a separatory funnel.
Figure 19.9
Organic Extraction
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• Suppose you had a mixture of benzoic acid and NaCl.
• Benzoic acid will be soluble in an organic solvent such as CH2Cl2, and NaCl will dissolve in water.
• Since CH2Cl2 and H2O are immiscible, you now have two layers.
• The more dense bottom layer (CH2Cl2) will contain the benzoic acid, and the less dense top layer (H2O) will contain the NaCl.
• If the mixture of liquids is transferred to a separatory funnel, the two layers can be separated by simply draining away the bottom layer.
• Once the two layers are separated, the solvents can be evaporated away, leaving behind pure benzoic acid and NaCl.
Separating Benzoic Acid and Salt
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How could a mixture of benzoic acid and cyclohexanol be separated?
• Both compounds are organic, and as a result, both are soluble in an organic solvent such as CH2Cl2, and insoluble in H2O.
• If a mixture of benzoic acid and cyclohexanol was added to a separatory funnel with CH2Cl2 and water, both would dissolve in the CH2Cl2 layer, and the two compounds would not be separated from one another.
Separating Benzoic Acid and Cyclohexanol
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• Since there is a significant enough difference in acidity between benzoic acid and cyclohexanol, extraction can be used to separate the two compounds.
• When benzoic acid is treated with NaOH, it is deprotonated, forming sodium benzoate, an ionic, water soluble salt.
• The much weaker acid, cyclohexanol, does not react with NaOH and remains in the organic solvent.
Separating Benzoic Acid and Cyclohexanol
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Figure 19.10
Separation of Benzoic Acid and Cyclohexanol
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• Sulfonic acids have the general structure RSO3H.
• The most widely used sulfonic acid is p-toluenesulfonic acid (TsOH).
• Sulfonic acids are very strong acids because their conjugate bases (sulfonate anions) are resonance stabilized, and all the resonance structures delocalize negative charge on oxygen.
Sulfonic Acids
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• Amino acids contain two functional groups—an amine group (NH2) and a carboxy group (COOH).
• Amino acids are the building blocks of proteins.
• The simplest amino acid, glycine, has R=H. When R is any other group, the α carbon is a stereogenic center.
Amino Acids
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• An amino acid is both an acid and a base.
• Amino acids are never uncharged neutral molecules, they exist as salts, so they have very high melting points and are very water soluble.
Zwitterion Character of Amino Acids
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• An amino acid can exist in three different forms depending on pH.
pH and Amino Acid Structure
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• Alanine exists in three different forms depending on the pH of the solution in which it is dissolved.
• As the pH of a solution is gradually increased from 2 to 10, the following process occurs:
Summary of Acid–Base Reactions
Figure 19.11
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• Because a protonated amino acid has at least two different protons that can be removed, a pKa value is reported for each of these protons.
• The pH at which the amino acid exists primarily in its neutral form is called its isoelectric point, abbreviate as pI.
• Generally, the isoelectric point is the average of both pKa values of an amino acid:
Isoelectric Point