carboxylic acids the functional group of carboxylic acids...
TRANSCRIPT
The functional group of carboxylic acids consists of a C=O with
to the same carbon.
Structure of Carboxyl Carbon is
120°. O-H eclipsed with C=O, to get overlap of
pair on oxygen.
Carboxyl group is usually written
bonded to -COOH. Aromatic acids have an aryl group.
chain aliphatic acids. Almost all naturally occurring
acids (-CO2H).
Carboxylic acid groups are always terminal groups with a carbonyl carbon
also bound to a hydroxyl group.
For example:
Formic acid is the principal ingredient and stinging agent of most ant bites.
Benzoic acid is found in many plants: for example, the Voodoo lily uses it to
attract insects needed for
(fat) and (after reaction with NaOH) is one of the
lye soap as well as modern soaps (sodium tallowa
ingredient labels of soaps, it is a mixture of several similar sodium salts of
carboxylic acids, one of which is sodium stearate).
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Carboxylic Acids
The functional group of carboxylic acids consists of a C=O with -OH bonded
Carbon is sp2 hybridized. Bond angles are close to
H eclipsed with C=O, to get overlap of π orbital with orbital of lone
Carboxyl group is usually written -COOH. Aliphatic acids have an alkyl group
omatic acids have an aryl group. Fatty acids are long
Almost all naturally occurring acids are carboxylic
Carboxylic acid groups are always terminal groups with a carbonyl carbon
hydroxyl group.
Formic acid is the principal ingredient and stinging agent of most ant bites.
n many plants: for example, the Voodoo lily uses it to
attract insects needed for pollination. Stearic acid is found in beef tallow
(fat) and (after reaction with NaOH) is one of the active components in old
lye soap as well as modern soaps (sodium tallowate appears on the
ingredient labels of soaps, it is a mixture of several similar sodium salts of
of which is sodium stearate).
OH bonded
Bond angles are close to
orbital with orbital of lone
Aliphatic acids have an alkyl group
Fatty acids are long-
acids are carboxylic
Carboxylic acid groups are always terminal groups with a carbonyl carbon
Formic acid is the principal ingredient and stinging agent of most ant bites.
n many plants: for example, the Voodoo lily uses it to
pollination. Stearic acid is found in beef tallow
active components in old
te appears on the
ingredient labels of soaps, it is a mixture of several similar sodium salts of
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Dicarboxylic acids have 2 -CO2H groups and tricarboxylic acids have 3 -CO2H
groups (citric acid has 3 -CO2H groups).
Nomenclature
1. Common Names
Many aliphatic acids have historical names.
Some common names
HCO2H formic acid L. formica ant
CH3CO2H acetic acid L. acetum vinegar
CH3CH2CO2H propionic acid G. “first salt”
CH3CH2CH2CO2H butyric acid L. butyrum butter
CH3CH2CH2CH2CO2H valeric acid L. valerans
Positions of substituents on the chain are labeled with Greek letters
α-chlorobutyric acid
CH3CH2CHC
Cl
OH
O
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IUPAC Names
1-Remove -e from alkane (or alkene) name, add -oic acid.The carbon of the
carboxyl group is #1.
HCOOH methanoic acid
CH3CO2H ethanoic acid
CH3CH2CO2H propanoic acid
CH3
CH3CHCOOH 2-methylpropanoic acid
Br
CH3CH2CHCO2H 2-bromobutanoic acid
2-chlorobutanoic acid
2-Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids.
3-Aromatic acids are named as benzoic acids.
2-isopropylcyclopentanecarboxylic acid o-hydroxybenzoic acid
(Salicylic acid)
CH3CH2CHC
Cl
OH
O
C O O H
C H (C H 3 ) 2
COOH
OH
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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 drugs, preservatives.
• Adipic acid used to make nylon 66.
• Phthalic acid used to make polyesters
Physical properties
Carboxylic acids hydrogen bond to themselves to form a dimer:
• Carboxylic acids also form hydrogen bonds to water molecules:
Since carboxylic acids can form more than one set of hydrogen bonds, their
boiling points are usually higher than those of other molecules of the same
molecular weight (MW). Low-MW carboxylic acids are generally liquids at
room temp. (often, they are somewhat oily); higher- MW carboxylic acids
are generally waxy solids. Carboxylic acids with 12 to 20 carbon atoms are
often referred to as fatty acids, since they are found in the triglycerides in
fats and oils (more later). Short-chain carboxylic acids are also generally
more soluble in water than compounds of similar MW, since they can
hydrogen bond to more than one water molecule. As the number of
carbons in a carboxylic acid series becomes greater, the boiling point
increases and the solubility in water decrease. Many carboxylic acids that
are liquids at room temperature have characteristically sharp or unpleasant
odors.
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– Ethanoic acid/acetic acid is the main ingredient in vinegar.
– Butanoic acid is partially responsible for the odor of locker rooms and
unwashed socks.
– Hexanoic acid is responsible for the odor of Limburger cheese.
Like most acids, carboxylic acids tend to have a sour taste (e.g., vinegar,
citric acid, etc.)
Acidity of Carboxylic Acids Since all carboxylic acids are weak acids, all exist
in equilibrium in water. The generic equilibrium equation is:
Where [H3O+] ≡ [H
+]. (Recall that the water does not change concentration
and therefore is incorporated into the Ka constant.) Most simple carboxylic
acids have acid dissociation constants equal to about 10-5
. Such a small Ka
means that very little of the acid is ionized in water. Nonetheless, carboxylic
acids are much stronger acids than phenols (typical Ka = 10-10
).
Carboxylic acids react with bases, but only strong bases completely remove
the proton (hydrogen ion) from the acid group. The resulting carboxylate
ion is then a weak base. When a strong acid is added to a solution
containing carboxylate ions, the acid protonates it, regenerating the parent
carboxylic acid.
As we saw earlier, small carboxylic acids are quite water soluble. However,
these acids don’t normally reside in our bodies (except sometimes as food,
e.g. acetic acid (vinegar)). The carboxylic acids that are found in human
tissue are typically fatty acids. These are acids with long carbon chains that
cause them to have very low solubility. When a proton is removed from a
fatty acid, the resulting carboxylate ion will always have a significantly
higher solubility (even if in absolute terms the solubility is still low).
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Preparation of acids
1- From the oxidation of alkylbenzenes
2- from the oxidation of the alkenes and alkynes
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3- from the oxidation of aldehydes
4- From the oxidation of the primary alcohols
5- from hydrolysis of nitriles
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6- From the carboxylation of the Grignards
Carboxylic acids, reactions:
1. as acids
2. conversion into functional derivatives
a) � acid chlorides
b) � esters
c) � amides
d) � anhydrides
3. reduction
4. alpha-halogenation
5. EAS
1. as acids:
a) with active metals
RCO2H + Na � RCO2-Na
+ + H2(g)
b) with bases
RCO2H + NaOH � RCO2-Na
+ + H2O
c) Relative acid strength?
CH4 < NH3 < HC≡CH < ROH < HOH < H
d) quantitative
HA + H2O � H
Ka = [H3O+] [A
-] / [HA]
2. Conversion into functional derivatives:
The group bonded to the acyl carbon
a -OH, carboxylic acid
b -Cl, acid chloride
c -OR’, ester
d -NH2, amide
e- OCOR acid
These interconvert via nucleophilic acyl substitution.
Esterification
• Acid + alcohol yields ester + water.
• Acid catalyzed for weak nucleophile.
• All steps are reversible.
• Reaction reaches equilibrium.
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CH < ROH < HOH < H2CO3 < RCO2H < HF
H3O+ + A
- ionization in water
] / [HA]
Conversion into functional derivatives:
The group bonded to the acyl carbon determines the class of compound:
OH, carboxylic acid
Cl, acid chloride
OR’, ester
, amide
acid anhydride
These interconvert via nucleophilic acyl substitution.
yields ester + water.
Acid catalyzed for weak nucleophile.
All steps are reversible.
Reaction reaches equilibrium.
determines the class of compound:
These interconvert via nucleophilic acyl substitution.
Amides
are synthesized by first reacting a carboxylic acid
The product, an ammonium carboxylate salt, when heated
Acid Chlorides
• An activated form of the carboxylic acid.
• Chloride is a good leaving group, so undergoes acyl
substitution easily.
• To synthesize acid chlorides use thionyl
chloride or phosphorous pentachloride
Anhydrides
Removing water from a carboxylic acid can
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are synthesized by first reacting a carboxylic acid with ammonia, NH
The product, an ammonium carboxylate salt, when heated releases water:
An activated form of the carboxylic acid.
Chloride is a good leaving group, so undergoes acyl
substitution easily.
To synthesize acid chlorides use thionyl chloride,
or phosphorous pentachloride with the acid.
Removing water from a carboxylic acid can make an anhydride.
with ammonia, NH3.
releases water:
Chloride is a good leaving group, so undergoes acyl
chloride, oxalyl
3- Reduc!on to 1°°°° Alcohols
• Use strong reducing agent, LiAlH
• Borane, BH3 in THF, reduces carboxylic acid to alcohol, but does not
reduce ketone
4. Alpha-halogenations
RCH2COOH + X
X2 = Cl2, Br2
5. EAS: (-COOH is deactivating and
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Alcohols
Use strong reducing agent, LiAlH4.
in THF, reduces carboxylic acid to alcohol, but does not
halogenations: (Hell-Volhard-Zelinsky reaction)
COOH + X2, P � RCHCOOH + HX
X
α-haloacid
COOH is deactivating and meta- directing)
in THF, reduces carboxylic acid to alcohol, but does not
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Phenols Ar-OH
Phenols are compounds with an –OH group attached to an aromatic
carbon. Although they share the same functional group with
alcohols, where the –OH group is attached to an aliphatic carbon, the
chemistry of phenols is very different from that of alcohols.
Nomenclature.
Phenols are usually named as substituted phenols. The
methylphenols are given the special name, cresols. Some other
phenols are named as hydroxy compounds.
Physical properties
Phenols are polar and can hydrogen bond. Phenols are water
insoluble. Phenols are stronger acids than water and will
dissolve in 5% NaOH. Phenols are weaker acids than carbonic acid
and do not dissolve in 5% NaHCO3.
Intramolecular hydrogen bonding is possible in some ortho-
substituted phenols. This intramolecular hydrogen bonding reduces
water solubility and increases volatility. Thus, o-nitrophenol is steam
distillable while the isomeric p-nitrophenol is not.
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Reactions:
alcohols phenols
1. HX NR
2. PX3 NR
3. dehydration NR
4. as acids phenols are more acidic
5. ester formation similar
6. oxidation NR
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Phenols, reactions:
1. as acids
2. ester formation
3. ether formation
4. EAS
a) nitration f) nitrosation
b) sulfonation g) coupling with diaz. salts
c) halogenation h) Kolbe
d) Friedel-Crafts alkylation i) Reimer-Tiemann
e) Friedel-Crafts acylation
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We use the ionization of acids in water to measure acid strength (Ka):
HBase + H2O H3O+ + Base
-
Ka = [H3O+ ][ Base ] / [ HBase]
ROH Ka ~ 10-16
- 10-18
ArOH Ka ~ 10-10
Why are phenols more acidic than alcohols?
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Resonance stabilization of the phenoxide ion, lowers the PE of the products
of the ionization, decreases the ΔH, shifts the equal farther to the right,
makes phenol more acidic than an alcohol
Effect of substituent groups on acid strength?
Electron withdrawing groups will decrease the negative charge in the
phenoxide, lowering the PE, decreasing the ΔH, shifting the equal farther to
the right, stronger acid.
Electron donating groups will increase the negative charge in the
phenoxide, increasing the PE, increasing the ΔH, shifting the equilibrium to
the left, weaker acid.
Number the following acids in decreasing order of acid strength (let # 1 =
most acidic, etc.)
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At low temperature the reaction is non-reversible and the lower Eact ortho-
product is formed (rate control).
At high temperature the reaction is reversible and the more stable para-
product is formed (kinetic control).
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f) nitrosation
g) coupling with diazonium salts
(EAS with the weak electrophile diazonium)
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h) Kolbe reaction (carbonation)
i) Reimer-Tiemann reaction
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Amines
Structure, Naming, and Physical Properties
Amines are one of the six nitrogen-containing functional groups we use
(amide, nitrile, imine, azide, and nitro are the others.)
Structure
An amine has an sp3-hybridized nitrogen with three bonds and a lone pair
of electrons. As a result of the sp3-hybridization, bond angles around the
nitrogen are approximately 109.5º (typically a liSle smaller, such as 107º).
Amines exhibit an unusual inversion at room temperature, so we will not
observe chirality at the nitrogen of a neutral amine.
Besides ammonia, which is the simplest amine, substituted amines are
categorized as:
Primary if they have one bond from nitrogen to carbon (H2NR)
Secondary if they have two bonds from nitrogen to carbon (HNR2), and
Tertiary if they have three bonds from nitrogen to carbon (NR3).
Naming
• Official IUPAC name is based on analogous alcohol. The “-ol” ending gets
replaced by “-amine.”
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Two “common” naming systems:
1) “alkyl” precedes the word “amine”
2) “amino” (-NH2) substituent on a parent alkane.
• For secondary and tertiary amines, the longest continuous carbon chain
acts as the parent, and an “N” locant is used to describe the position of the
extra alkyl groups, which are listed in alphabetical order at the beginning of
the name. (In the common system, simply list the alkyl groups before the
word “amine.”)
• “Aniline” is the IUPAC and common name for the aromatic amine
C6H5NH2, which is used a parent compound for other aromatic amines.
• Ammonium salts are tetrasubstituted, cationic versions of amines:
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Physical Properties
Amines are slightly basic. This because they have a lone pair of electrons to
donate to a proton. This same feature makes them nucleophiles.
Typical amines have Kb values = 10-3
to 10-4
Water solubility: Amines are mostly insoluble in water; however, those with
fewer than ~5 carbon atoms are water soluble. (With fewer than ~5 carbon
atoms, the polarity of the N-part of the molecule dominates over the non-
polar hydrocarbon part of the molecule.) Boiling point/melting point.
Hydrogen bonding is possible in amines that contain NH or NH2 groups. An
NH2 exhibits stronger hydrogen bonding than an NH. However, when
comparing amines to alcohols, an OH group exhibits stronger hydrogen
bonding than an NH2, since oxygen is more electronegative than nitrogen.
Preparation of amines
1-Reduction of Nitriles
Nitriles may be reduced with lithium aluminum hydride to generate primary
amines.
RNH2 + H OH RNH3 + OH
Kb = [RNH3 ] [OH ]
[RNH2]
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2- Reduction of Amides
Amides may be reduced with LiAlH4 to generate subsVtuted amines.
3-Reduction of Nitro Compounds
Nitro groups may be reduced by H2 and palladium on carbon to generate
amines. This reaction is particularly useful for aromatic systems because of
the ease of installing a nitro group selectively.
4-The Hofmann Rearrangement
The Hofmann rearrangement allows us to shorten a chain by one carbon by
the conversion of an amide to a primary amine. The overall conversion and
the mechanism are given below.
5-The Gabriel Amine Synthesis
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This phthalimide derivative may then be hydrolyzed with aqueous base to
generate phthalic acid and the desired primary amine. Alternatively, the
phthalimide may be heated with hydrazine to generate the free amine.