# 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 + KMnO`~`4`~` + OH`^`-`^` + Heat----> 2RCOOH`\
`       `

2\) from ROH

`       RCH`~`2`~`OH + 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 + KMnO`~`4`~` ----> 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

`       RCOCH`~`3`~` + NaOH + I-I ----> RCOO`^`-`^` + CHI`~`3`~

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.

  formula           IUPAC name               traditional name
  ----------------- ------------------------ ------------------
  HCOOH             methanoic acid           formic acid
  CH~3~-COOH        ethanoic acid            acetic acid
  CH~3~CH~2~-COOH   propanoic acid           propionic acid
  CH~2~=CH-COOH     propenoic acid           acrylic acid
  CF~3~-COOH        trifluoroethanoic acid   
                                             

  : **Nomenclature of carboxylic acids**

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.

  formula                              IUPAC name          traditional name
  ------------------------------------ ------------------- ------------------
  HOOCCOOH                             Ethanedioic acid    Oxalic acid
  HOOCCH~2~-COOH                       propanedioic acid   Malonic acid
  HOOCCH~2~CH~2~-COOH                  butanedioic acid    Succinic acid
  HOOCCH~2~CH~2~CH~2~-COOH             pentanedioic acid   Glutaric acid
  HOOCCH~2~CH~2~CH~2~CH~2~-COOH        hexanedioic acid    Adipic acid
  HOOCCH~2~CH~2~CH~2~CH~2~CH~2~-COOH   heptanedioic acid   Pimelic acid
                                                           

  : **Nomenclature of dicarboxylic acids**

## 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

  acid                     formula           *pK*~a~
  ------------------------ ----------------- ---------
  methanoic acid           H-COOH            3.75
  ethanoic acid            CH~3~-COOH        4.75
  propanoic acid           CH~3~CH~2~-COOH   4.87
  propenoic acid           CH2=CH-COOH       4.25
  benzoic acid             C~6~H~5~-COOH     4.19
  trifluoroethanoic acid   CF~3~-COOH        0.3
  phenol                   C~6~H~5~-OH       10.0
  ethanol                  CH~3~CH~2~-OH     15.9
  water                    H~2~O             15.7

  : **Acidity of carboxylic acids in water**

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:
SOCl~2~, PCl~5~ or PCl~3~ are the usual reagents. Other products are HCl
& SO~2~, HCl & POCl~3~ and H~3~PO~3~ respectively. The conditions must
be dry, as water will hydrolyse the acid chloride in a vigorous
reaction. Hydrolysis forms the original carboxylic acid.

CH~3~COOH + SOCl~2~ → CH~3~COCl + HCl + SO~2~

C~6~H~5~COOH + PCl~5~ → C~6~H~5~COCl + HCl + POCl~3~

3 CH~3~CH~2~COOH + PCl~3~ → 3 CH~3~CH~2~COCl + H~3~PO~3~

## 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.

C~6~H~5~COCl + CH~3~CH~2~OH → C~6~H~5~COOCH~2~CH~3~ + 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).

CH~3~COOH + CH~3~CH~2~CH~2~OH = CH~3~COOCH~2~CH~2~CH~3~ + H~2~O

!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.

C~6~H~5~COOCH~3~ + H2O → C~6~H~5~COOH + CH~3~OH

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 + H~2~NR\' → RCONHR\' + H~2~O

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 + H~2~NR\' → RCOO^-^ + H~3~NR\'^+^

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:

CH~3~CH~2~COOH + 2 NaOH →CH~3~CH~3~ + Na~2~CO~3~ + H~2~O

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-CH~2~CH~2~-OH +
n HOOC-C~6~H~4~-COOH → (-O-CH~2~CH~2~-O-OC-C~6~H~4~-CO-)n + n H~2~O
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 CH~3~COOH + Na~2~CO~3~ → 2 CH~3~COONa + H~2~O + CO~2~

C~6~H~5~COOH + NaHCO~3~ → C~6~H~5~COONa + H~2~O + CO~2~

Some phenols react with FeCl~3~ solution, giving a characteristic purple
colour.

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