A carboxylic acid is characterized by the presence of the carboxyl group -COOH. The chemical reactivity of carboxylic acids is dominated by the very positive carbon, and the resonance stabilization that is possible should the group lose a proton. These two factors contribute both to acidity and to the group's dominant chemical reaction: nucleophilic substitution.

Preparation

1) from alkenes

       R-CH=CHR + KMnO₄ + OH^- + Heat----> 2RCOOH

2) from ROH

       RCH₂OH + OXIDIZING AGENT ----> RCOOH

Aliphatic carboxylic acids are formed from primary alcohols or aldehydes by reflux with potassium dichromate (VI) acidified with sulphuric acid.

3) from toluene etc.

       toluene + KMnO₄ ----> benzoic acid

Alkyl benzenes (methyl benzene, ethyl benzene, etc) react with potassium manganate (VII) to form benzoic acid. All alkyl benzenes give the same product, because all but one alkyl carbon is lost.

No acidification is needed. The reaction is refluxed and generates KOH. The benzoic acid is worked up by adding a proton source (such as HCl).

4) from methyl ketones

       RCOCH₃ + NaOH + I-I ----> RCOO^- + CHI₃

5) from Grignard reagents

       RMgX + O=C=O ----> RCOOMgX 
       RCOOMgX + HOH ----> RCOOH + MgX(OH)

Properties

Nomenclature

The systematic IUPAC nomenclature for carboxylic acids requires the longest carbon chain of the molecule to be identified and the -e of alkane name to be replaced with -oic acid.

The traditional names of many carboxylic acids are still in common use.

**Nomenclature of carboxylic acids**
formula	IUPAC name	traditional name
HCOOH	methanoic acid	formic acid
CH₃-COOH	ethanoic acid	acetic acid
CH₃CH₂-COOH	propanoic acid	propionic acid
CH₂=CH-COOH	propenoic acid	acrylic acid
CF₃-COOH	trifluoroethanoic acid

The systematic approach for naming dicarboxylic acids (alkanes with carboxylic acids on either end) is the same as for carboxylic acids, except that the suffix is -dioic acid. Common name Nomenclature of dicarboxylic acids is aided by the acronym OMSGAP (Om's Gap), where each letter stands for the first letter of the first seven names for each dicarboxylic acid, starting from the simplest.

**Nomenclature of dicarboxylic acids**
formula	IUPAC name	traditional name
HOOCCOOH	Ethanedioic acid	Oxalic acid
HOOCCH₂-COOH	propanedioic acid	Malonic acid
HOOCCH₂CH₂-COOH	butanedioic acid	Succinic acid
HOOCCH₂CH₂CH₂-COOH	pentanedioic acid	Glutaric acid
HOOCCH₂CH₂CH₂CH₂-COOH	hexanedioic acid	Adipic acid
HOOCCH₂CH₂CH₂CH₂CH₂-COOH	heptanedioic acid	Pimelic acid

Acidity

Most carboxylic acids are weak acids. To quantify the acidities we need to know the pKa values: The pH above which the acids start showing mostly acidic behaviour: Ethanoic acid: 4.8 Phenol: 10.0 Ethanol: 15.9 Water: 15.7

**Acidity of carboxylic acids in water**
acid	formula	pK_a
methanoic acid	H-COOH	3.75
ethanoic acid	CH₃-COOH	4.75
propanoic acid	CH₃CH₂-COOH	4.87
propenoic acid	CH2=CH-COOH	4.25
benzoic acid	C₆H₅-COOH	4.19
trifluoroethanoic acid	CF₃-COOH	0.3
phenol	C₆H₅-OH	10.0
ethanol	CH₃CH₂-OH	15.9
water	H₂O	15.7

Data from CRC Handbook of Chemistry & Physics, 64th edition, 1984 D-167-8 Except http://en.wikipedia.org/wiki/Trifluoroacetic_acid

Clearly, the carboxylic acids are remarkably acidic for organic molecules. Somehow, the release of the H⁺ ion is favoured by the structure. Two arguments: The O-H bond is polarised by the removal of electrons to the carbonyl oxygen. The ion is stabilised by resonance: the carbonyl oxygen can accept the charge from the other oxygen. The acid strength of carboxylic acid are strongly modulated by the moiety attached to the carboxyl. Electron-donor moiety decrease the acid strength, whereas strong electron-withdrawing groups increase it.

Reactions

Acid Chloride Formation

Carboxylic acids are converted to acid chlorides by a range of reagents: SOCl₂, PCl₅ or PCl₃ are the usual reagents. Other products are HCl & SO₂, HCl & POCl₃ and H₃PO₃ respectively. The conditions must be dry, as water will hydrolyse the acid chloride in a vigorous reaction. Hydrolysis forms the original carboxylic acid.

CH₃COOH + SOCl₂ → CH₃COCl + HCl + SO₂

C₆H₅COOH + PCl₅ → C₆H₅COCl + HCl + POCl₃

3 CH₃CH₂COOH + PCl₃ → 3 CH₃CH₂COCl + H₃PO₃

Esterification

Alcohols will react with acid chlorides or carboxylic acids to form esters. This reaction is catalyzed by acidic or basic conditions. See alcohol notes.

C₆H₅COCl + CH₃CH₂OH → C₆H₅COOCH₂CH₃ + HCl

With carboxylic acids, the condensation reaction is an unfavourable equilibrium, promoted by using non-aqueous solvent (if any) and a dehydrating agent such as sulfuric acid (non-nucleophilic), catalyzing the reaction).

CH₃COOH + CH₃CH₂CH₂OH = CH₃COOCH₂CH₂CH₃ + H₂O

Ethanoic Acid reacts with Propanol to form Propyl Ethanoate

Reversing the reaction is simply a matter of refluxing the ester with plenty of aqueous acid. This hydrolysis produces the carboxylic acid and the alcohol.

C₆H₅COOCH₃ + H2O → C₆H₅COOH + CH₃OH

Alternatively, the reflux is done with aqueous alkali. The salt of the carboxylic acid is produced. This latter process is called 'saponification' because when fats are hydrolysed in this way, their salts are useful as soap.

Anhydrides

See acid anhydride.

Amides

Conceptually, an amide is formed by reacting an acid (an electrophile) with an amine compound (a nucleophile), releasing water.

RCOOH + H₂NR' → RCONHR' + H₂O

However, the acid-base reaction is much faster, which yields the non-electrophilic carboxylate and the non-nucleophilic ammonium, and no further reaction takes place.

RCOOH + H₂NR' → RCOO^- + H₃NR'⁺

To get around this, a variety of coupling reagents have been developed that first react with the acid or carboxylate to form an active acyl compound, which is basic enough to deprotonate an ammonium and electrophilic enough to react with the free base of the amine. A common coupling agent is dicyclohexylcarbodiimide, or DCC, which is very toxic.

Acid Decarboxylation

On heating with sodalime (NaOH/CaO solid mix) carboxylic acids lose their –COOH group and produce a small alkane plus sodium carbonate:

CH₃CH₂COOH + 2 NaOH →CH₃CH₃ + Na₂CO₃ + H₂O

Note how a carbon is lost from the main chain. The product of the reaction may be easier to identify than the original acid, helping us to find the structure.

Ethanoic anhydride

Industrially, ethanoic anhydride is used as a less costly and reactive alternative to ethanoyl chloride. It forms esters and can be hydrolysed in very similar ways, but yields a second ethanoic acid molecule, not HCl The structure is formed from two ethanoic acid molecules…

Polyester

Polyester can be made by reacting a diol (ethane-1,2-diol) with a dicarboxylic acids (benzene-1,4-dicarboxylic acid). n HO-CH₂CH₂-OH + n HOOC-C₆H₄-COOH → (-O-CH₂CH₂-O-OC-C₆H₄-CO-)n + n H₂O Polyester makes reasonable fibres, it is quite inflexible so it does not crease easily; but for clothing it is usually combined with cotton for comfort. The plastic is not light-sensitive, so it is often used for net curtains. Film, bottles and other moulded products are made from polyester.

Distinguishing carboxylic acids from phenols

Although carboxylic acids are acidic, they can be distinguished from phenol because: Only carboxylic acids will react with carbonates and hydrogencarbonates to form CO2

2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + H₂O + CO₂

C₆H₅COOH + NaHCO₃ → C₆H₅COONa + H₂O + CO₂

Some phenols react with FeCl₃ solution, giving a characteristic purple colour.

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Organic Chemistry/Carboxylic acids

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