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Carboxylic Acid Functional Group

The properties and reactivities of organic compounds are largely determined by the type of functional groups a particular molecule possess. The functional groups are groups of atoms that react in a particular manner more or less independently as per the conditions prevailing. The carboxylic acid functional group consist of a carbon atom double bonded to an oxygen atom and single bonded to an OH group.

The fourth valence on carbon could be attached anywhere on a carbon and any molecule that has a carboxylic acid functional group is expected to be an acid. The customary properties of a functional group and not a specific properties of an individual molecule we need a symbol to represent a generic carbon framework and the letter R is used for the purpose.

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Carboxylic Acid Functional Group Definition

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The carboxylic acid functional group consist of a carbonyl and hydroxyl group, both of which when taken individually are polar groups. The hydrogen of the –OH can hydrogen bond to either of the oxygen groups in another carboxyl function.

The amount and strength of a hydrogen bonding in the case of a carboxylic acid are greater than in the case of alcohols or phenols because of the greater acidity of the carboxylic acid and also due to the additional sites of bonding.

The effect of the strong intermolecular hydrogen bonding can be checked for several carboxylic acids while the hydrogen bonding is shown for strong affinity for water due to solubility of carboxylic acids with water. The lipophilic hydrocarbon length of chain results in the decrease in solubility.

Carboxylic Acid

The dissociation of a carboxylic acid which results in giving up the proton makes the group an acid and the strength of an acid depends upon completely on the concentration of protons in solution which depends upon the extent of dissociation.

In case a molecule has more than one functional group, any one of them take the priority for the last suffix and the order of functional group priorities which includes a carboxylic acid functional group along with others are given below:

Functional Group

The six functional groups from carboxylic acids to nitriles are closely related chemically. These molecules have carbon atom with three bonds to an electronegative atom, either with oxygen or nitrogen and the conversions are relatively easy.

The carboxylic acid group has the highest priority among the common functional groups where the –COOH substituent replaces a –CH3 group in an acyclic hydrocarbon with a suffix of –oic acid.

Carboxylic Acid Functional Group Reactions

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As the molecule of a carboxylic acid is comprised of an alkyl group and a carboxylic group, the genera chemical properties are also mainly due to these group as well. The main set of carboxylic acid functional group reactions are categorised as
  • Reactions owing to the presence of alkyl group
  • Reactions owing to the presence of carboxylic group
  • Reactions owing to the hydrogen replacement
  • Reactions due to the carboxylic acid group presence as a whole
  • Reactions due to the presence of –OH of the carboxylic acid
  • Reactions due to the presence of carbonyl (CO) part in carboxylic acid
Reactions due to the presence of alkyl group:

The reaction with either chlorine or bromine in the presence of phosphorus or iodine catalysts is mainly due to alkyl group. The aliphatic form of carboxylic acids can undergo halogenation.

$CH_{3}COOH + Cl_{2} \rightarrow P \rightarrow ClCH_{2}COOH + HCl$

$ClCH_{2}COOH + Cl_{2} \rightarrow P \rightarrow Cl_{2}CHCOOH + HCl$

$Cl_{2}CHCOOH + Cl_{2} \rightarrow P \rightarrow Cl_{3}C.COOH + HCl$

In case of propionic acid CH3CH2COOH the reaction with bromine in presence of ‘P’ gives alpha (α) bromo propionic acid and further reaction gives α, α di bromo-propionic acid.

$CH_{3}CH_{2}COOH + Br_{2} \rightarrow P \rightarrow CH_{3}CH-BrCOOH + Br_{2} \rightarrow P \rightarrow CH_{3}C-Br_{2}COOH$

These alpha ($\alpha$) halogenation reactions are better known as Hell Volhard Zellnsky (HVZ) reaction.

Reactions due to the presence of carboxylic group:
The carboxylic acids quite like the inorganic acids react with the aqueous hydroxides, the carbonates and bicarbonates to form alt and water. In case of carbonates and bicarbonates the reaction ends up in formation of carbon dioxide as well.

$CH_{3}COOH + NaOH \rightarrow CH_{3}COONa + H_{2}O$

$2 CH_{3}COOH + Na_{2}CO_{3} \rightarrow 2 CH_{3}COONa + H_{2}O + CO_{2}$

$CH_{3}COOH + NaHCO_{3} \rightarrow CH_{3}COONa + H_{2}O + CO_{2}$

Any metal lower than hydrogen in electrochemical series are not able to displace the hydrogen from carboxylic acids and hence no reaction.
$2 CH_{3}COOH + Zn \rightarrow (CH_{3}COO)_{2} Zn + H_{2}$

Reactions involving carboxylic acid group as a whole:
Decarboxylation: the monocarboxylic acids do not undergo decarboxylation reaction or loss of CO2 if heated alone, but if the soda salts of these acids are heated with soda lime (NaOH + CaO) then they tend to lose CO2 and form hydrocarbon.

$CH_{3}COONa + (NaOH + CaO) \rightarrow CH_{4} + Na_{2}CO_{3} (CaO)$

The acetoacetic acid or $\beta$-keto acid are found to be thermally unstable and very quickly lose CO$_2$ when heated.

$CH_{3}C$ = $OCH_{2}COOH \rightarrow \Delta \rightarrow CH_{3}-CO- CH_{3} + CO_{2}$

Formation of aldehydes and ketones

The calcium salts of these carboxylic acids when heated to very high temperature the formation of aldehyde and ketones are observed.

$(HCOO)_{2} Ca \rightarrow \Delta \rightarrow HCHO + CaCO_{3}$ (aldehyde)
$(RCOO)_{2} Ca \rightarrow \Delta \rightarrow RCOR + CaCO_{3}$ (ketone)

Reactions due to the presence of –OH group:
The hydroxyl group of any carboxylic acid can be replaced by any alkyl group, the halogen group or amino group resulting in the formation of esters.

$RCOOH + R’OH \rightarrow H+ \rightarrow RCOOR’ + H_{2}O$

The formation of acyl halides takes place in presence of either SOCl2 or PCl5 to give acid chloride.

Reactions due to the presence of carbonyl group:
The common reactions of carbonyl group (CO) do not occur in carboxylic acids due to the resonance effect in carboxylate anion. But the carboxyl group is reduced to –CH{2- by hydrogen under pressure either in presence of Ni or LiAlH4.

$RCOOH + 3H_{2} \rightarrow Ni / LiAlH_{4} \rightarrow R-CH_{3} + 2H_{2}O$

Carboxylic Acid Functional Group Isomer

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The functional group isomers are basically a class of structural isomers that have the same molecular formula with different structural formula and also with a different functional group.

For example the molecular formula of $C_{4}H_{8}O_{2}$ has a completely different structure. While one is ethyl ethanoate or an ester and the other is Butanoic acid or a carboxylic acid. 
Ethyl Ethanoate Butanoic Acid

Carboxylic Acid Functional Group Structure

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The carbonyl group C=O is common to many of the group of molecules. This carbonyl group are present in a large majority of organic compounds and in practically all biologically active molecules.

These compounds behave in similar manner in many aspects but differ depending upon the identity of the atoms bonded to carbonyl group carbon.

The aldehydes have at least one hydrogen bonded to the C=O while ketones have two carbon bonded to C=O.

The carboxylic acids have one –OH group bonded to the C=O and esters have an ether like oxygen bonded to the C=O. similarly the thioesters have a sulphide like sulphur bonded to the C=O and amides have an amine like nitrogen bonded to the C=O.

The carbonyl carbon atom shows a partial positive charge (+ $\delta $) and the oxygen bears a partial negative charge (- $\delta $).

Typical carbonyl compound
Acetone   Carboxylic Acid    Ester    Ketone   Aldehyde
           Acetone                  Carboxylic Acid                     Ester                         Ketone                Aldehyde

Test for Carboxylic Acid Functional Group

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The carboxylic acid functional group can be identified by three different tests, and they are litmus test, sodium bicarbonate test and ester test.

Litmus test:

The carboxylic acid functional group shows a blue to red litmus change when tested for litmus. The hydroxyl group in –COOH is far more acidic than in alcohol and hence it turns from blue to red.

$RCOOH + H_{2}O \rightarrow RCOO- + H_{3}O+$

Sodium bicarbonate test:

The carboxylic acid functional group reacts with NaHCO3 to give off CO2 and sodium salt. The brisk effervescence produced turns lime water milky.

$RCOOH + NaHCO_{3} \rightarrow RCOONa + CO_{2} + H_{2}O$ 

Esterification:

The carboxylic acid functional group reacts with an alcohol to produce fruity smelling ester and water. The reaction takes place in presence of conc. $H_{2}SO_{4}$ and 170 C temperature.

$RCOOH + R’OH \rightarrow RCOOR (fruity smelling) + H_{2}O$
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