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CARBOXYLIC ACIDS

STRUCTURE

They are made of a carbonyl group (C=O) and a hydroxyl group (OH)bonded to the same carbon.Carboxyl carbon is sp2 hybridized. Bond angles are close to 120. Often represented as RCOOH, RCO2H, or ArCOOH.

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NOMENCLATURE

A. Common NamesMany aliphatic acids have historical names (based on Latin).

Positions of substituents on the chain are labelled with Greek letters.

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B. IUPAC Find longest chain with the –COOH group as the parent.Drop the –e in parent name, replace with –oic acid.The carbon of the carboxyl group is numbered carbon 1.

methanoic acid propanoic acid 2,3-dibromo-3-phenylbutanoic acid

2-hydroxypropanoic acid pentanedioic acid

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C. Cyclic AcidsCompounds with –CO2H bonded to a ring are named as alkane carboxylic acids.Aromatic acids are named as benzenecarboxylic acid (IUPAC) or as benzoic acids (common).

cyclohexanecarboxylic acid benzenecarboxylic acidbenzoic acid

2-cyclohexenecarboxylic acid

3-chloro-2-nitrobenzenecarboxylic acid3-chloro-2-nitrobenzoic acid

2-chlorobenzenecarboxylic acid2-chlorobenzoic acid

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D. Acid SaltsName the cation, then name the anion by replacing the –oic acid with –oate.

sodium ethanoate or sodium acetate

ammonium butanoate or ammonium butyrate

magnesium propanoate or magnesium propionate

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PHYSICAL PROPERTIES

Carboxylic acids have polar C–O and O–H bonds. They formintermolecular hydrogen bonds, existing as cyclic dimers heldtogether by two hydrogen bonds.

Carboxylic acids have higher boiling points and melting points than other compounds of comparable molar mass.

Water solubility decreases with the length of the carbon chain. Up to 4 carbons, acid is miscible in water.

Also soluble in relatively nonpolar solvents like chloroform because it dissolves as a dimer.

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ACIDITY OF CARBOXYLIC ACIDS

Carboxylic acids have mildly acid protons.

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They are the most acidic class of compounds that contain only C, H and O. They are weak acids when compared to strong ones (HCl, H2SO4).

Ka pKa

H2SO4 ∞ −10

HCl ∞ −7

HCOOH 1.8 x 10−4 3.75

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The acidity of carboxylic acids is due to stability of the conjugate base, which is resonance-stabilized.

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The presence of electron-withdrawing substituents has an impact on acidity of carboxylic acids.

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PREPARATION OF CARBOXYLIC ACIDS

A. Oxidative Methods 1° alcohols can be oxidized to a carboxylic acid using chromic acid (H2CrO4), which can be formed either from chromium trioxide (CrO3) or from sodium dichromate (Na2Cr2O7) in aqueous acidic solution.

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Aldehydes are particularly sensitive to oxidation. A variety of oxidizing agents can be used, including CrO3, Na2Cr2O7, K2Cr2O7, and KMnO4.

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They can also oxidized selectively in the presence of other functional groups using silver(I) oxide in aqueous ammonium hydroxide (Tollens reagent).

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Alkylbenzenes can be converted to aromatic carboxylic acids using dichromate or permanganate reagents, R group must have at least one H atom on C atom attached to the ring.

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B. Addition of Carbon Dioxide to a Grignard Reagent

CO2 (g) can be passed through the Grignard solution, or CO2 (s) can be slowly added to flask containing Grignard reagent.

How would you prepare benzoic acid from benzene?

?

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C. Hydrolysis of Nitriles

Acidic or basic hydrolysis of a nitrile (cyano compound) produces a carboxylic acid.

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An example

How can you use this synthesis to make 2-phenylethanoic acid from toluene?

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Br2

lightNaCN

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REACTIONS OF CARBOXYLIC ACIDS

A. With Bases

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B. Reduction Reduction of carboxylic acids to 1° alcohols using lithium aluminum hydride – not the weaker sodium borohydride.

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C. Hell-Volhard-Zelinsky (HVZ) reaction Alpha-halogenation of the carboxylic acid using elemental phosphorus (or PCl3) as a catalyst; halogen may be Cl2 or Br2.

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Synthetically useful, halogen can then be replaced in a nucleophilic substitution reaction.

CH3CH2CH2COOH CHCH3CH2

Br

COOH

CHCH3CH2

Br

COOH

CHCH3CH2

OH

COOH CHCH3CH2

I

COOH CHCH3CH2

NH2

COOH

Br2

P

1. OH -, heat2. H 3O+

aq KI

aq NH 3

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D. Electrophilic Aromatic Substitution EAS on benzoic acid occurs more slowly than on benzene; -COOH being a deactivating group and a meta director.

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E. Conversion to Functional Group Derivatives Important part of carboxylic acid chemistry (next...).

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A. Mass spectrometry Parent peak is normally easy to identify.Common fragmentation patterns are α-cleavage of the carboxyl group to give [COOH]+.

SPECTROSCOPY

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B. IR spectroscopy C=O stretch at 1710-1760 cm−1 (free monomer: 1760 cm−1; dimer: 1710 cm−1).O-H stretch at 2500-3300 cm−1 (broad).

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C. UV-Visible spectroscopy Weak absorption (200 nm), in the absence of any other active functional groups.

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D. Proton NMR The CO2H proton absorbs as singlet near 12 δ.The CO2H proton can be replaced by deuterium (with D2O), causing signal to disappear.The CO2H group affects the chemical shift value of neighbouring H atoms.

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E. Carbon NMR Carboxyl carbon atoms absorb at 165-185 δ (strongly deshielded absorption).δ value higher for carbonyl carbon of aldehyde or ketone.