Transcript Organic Chemistry II Introduction

Organic Chemistry II
Carboxylic Acids and Nitriles
Dr. Ralph C. Gatrone
Department of Chemistry and Physics
Virginia State University
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Chapter Objectives
• Nomenclature
• Properties
• Preparation
• Reactions
• Spectroscopy
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The Importance of Carboxylic Acids
(RCO2H)
• Abundant in nature from oxidation of aldehydes and alcohols in
metabolism
– Acetic acid (CH3CO2H) - vinegar
– Butanoic acid (CH3CH2CH2CO2H) – smell of rancid butter
– Caproic acid (CH3CH2CH2CH2CH2CO2H) - smell of sweaty gym
socks
– Long-chain aliphatic acids from the breakdown of fats
• Starting materials for acyl derivatives (esters, amides, and acid
chlorides)
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Nomenclature
• Carboxylic Acids, RCO2H
• If derived from open-chain alkanes, replace the
•
terminal -e of the alkane name with -oic acid
The carboxyl carbon atom is C1
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Nomenclature
• Compounds with CO2H bonded to a ring
are named using the suffix -carboxylic acid
• The CO2H carbon is not itself numbered in
this system
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Common Names
• Many common names in use
O
H
O
O
OH
Formic
H3C
OH
Acetic
Butyric
OH
Propionic
O
O
CH3CH2CH2
CH3CH2
OH
CH3CH2CH2CH2
O
OH CH CH CH CH CH
3
2
2
2
2
OH
Caproic
Valeric
O
CO2H
CO2H
OH
Acrylic
Crotonic
Pivalic
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Common Names
CO2 H
CO2 H
Benzoic
CO2 H
CH3
Toluic
HO2 C
CO2 H
CO2 H
Oxalic
CO2 H
OH
Salicylic
CO2 H
CO2 H
Malonic
Phthalic
CO2 H
CO2 H
Succinic
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Nitriles
•
•
•
•
•
Closely related to carboxylic acids
Named by adding -nitrile as a suffix to the alkane name,
Nitrile carbon is numbered 1
Complex nitriles are named as derivatives of carboxylic
acids.
Replace -ic acid or -oic acid ending with -onitrile
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Common Names
CN
CH3CN
Acetonitrile
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Benzonitrile
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Structural and Physical
Properties
• Carboxyl carbon sp2 hybridized
• Carboxylic acid groups are planar with C–C=O
and O=C–O bond angles of approximately 120°
• Carboxylic acids form hydrogen bonds, existing
as cyclic dimers held together by two hydrogen
bonds
• Strong hydrogen bonding causes much higher
boiling points than the corresponding alcohols
• Acetic acid (bp = 117.9oC)
•Spring,
Ethanol
(bp = 78.3oC)
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Hydrogen Bonding
O H O
R
R
O H
O
hydrogen bond
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Dissociation of Carboxylic Acids
• Carboxylic acids are proton donors toward weak
•
•
and strong bases
Producing metal carboxylate salts, RCO2 +M
Carboxylic acids with more than six carbons are
only slightly soluble in water, but their conjugate
base salts are water-soluble
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Acidity Constant and pKa
• Carboxylic acids transfer a proton to water to
•
•
•
give H3O+ and the carboxylate anion, RCO2
H3O+ is a much stronger acid
The acidity constant, Ka,, is about 10-5 for a
typical carboxylic acid
pKa ~ 5
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Acidity of Carboxylic Acids
• Carboxylic acids – stronger acids than ROH
• The pKa of ethanol is ~16, compared to ~5 for acetic acid
• In an alkoxide ion, the negative charge is localized on oxygen while
in a carboxylate ion the negative charge is delocalized over two
equivalent oxygen atoms, giving resonance stabilization
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Resonance
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Henderson-Hasselbalch Equation
• Covered in General Chemistry
• Be certain you know Section 15.3
• You must know how to use the equation
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Substituent Effects on Acidity
• Electron withdrawing groups increase the acidity
• Promote formation of the carboxylate ion
• Electron donating groups decrease the acidity
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Preparation of Carboxylic Acids
• Oxidation of a substituted alkylbenzene with
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KMnO4 or Na2Cr2O7 gives a substituted benzoic
acid
1° and 2° alkyl groups can be oxidized, but
tertiary groups do not react
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From Alkenes
• Oxidative cleavage of an alkene with
KMnO4 gives a carboxylic acid if the alkene
has at least one vinylic hydrogen
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From Alcohols
• Oxidation of a primary alcohol or an aldehyde
with CrO3 in aqueous acid
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Hydrolysis of Nitriles
• Hot acid or base yields carboxylic acids
• Conversion of an alkyl halide to a nitrile (with
•
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cyanide ion) followed by hydrolysis produces a
carboxylic acid with one more carbon
RBr  RCN  RCO2H
Best with primary halides because elimination
reactions occur with secondary or tertiary alkyl
halides
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Carboxylation of Grignard Reagents
• Grignard reagents react with dry CO2 to yield a
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carboxylic acid after acid hydrolysis
Limited to alkyl halides that can form Grignard
reagents
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Reactions of Carboxylic Acids
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Reactions of Carboxylic Acids
• Carboxylic acids transfer a proton to a base
to give carboxylic anions, which are good
nucleophiles in SN2 reactions
• Like ketones, carboxylic acids undergo
addition of nucleophiles to the carbonyl
group
• In addition, carboxylic acids undergo other
reactions characteristic of neither alcohols
nor ketones
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Reduction of Carboxylic Acids
• Reduced by LiAlH4 to yield primary alcohols
• The reaction is difficult
• Generally requires heating in THF
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Reduction with Borane
• Borane in tetrahydrofuran (BH3/THF) converts
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carboxylic acids to primary alcohols selectively
Preferable to LiAlH4 because of its relative ease,
safety, and specificity
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Chemistry of Nitriles
• Nitriles and carboxylic acids both have a carbon
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atom with three bonds to an electronegative
atom, and both contain a  bond
Both are electrophilic centers
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Preparation of Nitriles
• Primary alkyl halides + cyanide
• SN2 reaction – same constraints
KCN
R X
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RCN
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Preparation of Nitriles
• Primary amides RCONH2 react with SOCl2 or
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POCl3 (or other dehydrating agents)
Not limited by steric hindrance or side
reactions (as is the reaction of alkyl halides
with NaCN)
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Aromatic Nitriles
• Conversion of anilines into diazonium salt
• Followed by CuCN/KCN
NH2
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HONO
N2 + CuCN/KCN
CN
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Reactions of Nitriles
• RCN is strongly polarized
• Electrophilic carbon atom
• Attacked by nucleophiles to yield sp2-hybridized imine
anions
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Hydrolysis of Nitriles
• Hydrolyzed by acid or base to the carboxylic acid
• Ammonia or an amine is other product
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Reduction of Nitriles
• Reduction of a nitrile with LiAlH4 gives a
primary amine
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Nitriles with Organometallic Reagents
• Grignard reagents add to give an intermediate
imine anion that is hydrolyzed by addition of
water to yield a ketone
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Infrared Spectroscopy
• Carboxylic Acids
– OH and C=O absorptions
– OH is very broad over 2500 – 3300 cm-1
– C=O absorbs over 1710 – 1760 cm-1
• See next spectrum
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Infrared Spectroscopy
• Nitriles intense C-N triple bond absorption
• 2250cm-1 for saturated compounds
• 2230cm-1 for aromatic and conjugated
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•
compounds
Highly diagnostic for nitriles
See next spectrum
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NMR Spectroscopy
• Carboxyl carbons resonate between 165 – 185d
• Nitrile carbons resonate between 115 – 130d
• CO2H resonates near 12d
• Proton is exchangeable with Deuterium
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