Organic Chemistry/Carboxylic acids

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.

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.

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

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 &rarr; CH3COCl + HCl + SO2

C6H5COOH + PCl5 &rarr; C6H5COCl + HCl + POCl3

3 CH3CH2COOH + PCl3 &rarr; 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 &rarr; 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



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 &rarr; 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' &rarr; 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' &rarr; 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 &rarr;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 &rarr; (-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 &rarr; 2 CH3COONa + H2O + CO2

C6H5COOH + NaHCO3 &rarr; C6H5COONa + H2O + CO2

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

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