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  Reactions of Carboxylic Acids, Chemistry tutorial

Introduction:

Carboxylic acids comprises of a carboxyl group -COOH group (-C=O and -OH) and an alkyl group. Therefore its chemical properties are due to such two groups.

Acidic Properties of Carboxylic Acids:

The acidic properties of carboxylic acids are due to their possessing ionizable hydrogen ions (H+) that yields in the formation of carboxylate.

RCOOH + H2O ↔ RCOO- + H3O+   Ka = 5

Carboxylic acids are just partially ionized in the aqueous solution owing to the predominantly covalent nature of the molecule; they are very weak in comparison by the mineral acids. They are though 1010 to 1011 times stronger in the acidic strength than alcohols.

The comparative strengths of carboxylic acids are attributable to the stability of carboxylic anion that is a resonance hybrid of two canonical forms, A and B below.

322_stability of the carboxylic anion.jpg

Fig:  Stability of the carboxylic anion

The resonance energy help the ionization process, therefore the released of the proton. The two canonical forms might be conveniently represented through the single structure C. The structure, C describes the equivalence of both carbon-oxygen bonds and as well the equivalent distribution of the negative charge between the oxygen atoms. This effect of charge dispersal yields in the stabilization of the carboxylate ion that improves the dissociation of acids.

The effect of charge of dispersal is more important in the benzoic acid and this improves the stabilization of the carboxylate ion.

1241_stabilization of the carboxylate ion.jpg

Fig: Stabilization of the carboxylate ion

Apart from the resonance effect, the acidity of a carboxylic acid is influenced by the substituent on its hydrocarbon chain. The effect of subsituents on the acidity of carboxylic acids is based on whether the subsituents are electron-donating or electron withdrawing. Whenever an electron-donating substituent  (+I effect) is linked to a carboxylic group, it intensifies the negative charge on the carboxylate ion therefore destabilizing the anion making it complex for the carboxylic acid to form the carboxyl ate and hydrogen ions.

On contrast, whenever an electron withdrawing substituent (-I effect), is linked to a carboxylic group, it decreases the negative charge from the carboxylate ion (therefore stabilizing the anion. The higher the number of substituent with -I effect, the higher the acidity of carboxylic acid. The acidic strength reduces in the order:

1639_Order of acidic strength.jpg

Fig: Order of acidic strength

The distance between the substituent and carboxylic group as well consists of a significant effect on the acidity of carboxylic acid.

Reactions due to Carboxyl Group:

As an acid, the carboxylic acids go through ionization into proton and the acid anion.

RCOOH ↔ RCOO- + H+

a) Salt Formation:

As an acid, the carboxylic acids, irrespective of whether they are water soluble or not, discharge hydrogen in the presence of metals and carbon (iv) oxide whenever added to either sodiumtrioxocarbonate (iv) or hydrogentrioxocarbonate (iv) making the metal carboxylate in all the reactions. For illustration:

RCOOH + 2Na → 2RCOO-Na+ + H2

                         Sodium Carboxylate

2RCOOH + Mg → (RCOO-)2 Mg2+ + H2

                         Magnesium Carboxylate

RCOOH + NaOH → R COO-Na + H2O

2RCOOH + Na2CO3 → 2RCOO- Na+ + CO2 + H2O

RCOOH + NaHCO3 → RCOO- Na+ + CO2 + H2O

The carboxylic acid can be produced by treating the salt by dilute mineral acid.

RCOONa + HCl → RCOOH + NaCl

Reactions involving hydroxyl group:

The given reactions comprise the hydroxyl group.

a) Esterification:

Esters are made up if carboxylic acid reacts by an alcohol in the presence of concentrated tetraoxosulphate (vi) acid as a catalyst.

1166_Ester formation.jpg

Fig: Ester formation

The reaction is slow and reversible and the yield of ester is frequently low.

Esters are as well made up whenever carboxylic acids react by sulphur dichloride oxide followed through an alcohol.

RCOOH + (SOCl2) → RCOCl + (R'OH) → RCOOR' + HCl

The yield of ester by sulphur dichloride is good.

b) Formation of Acyl Chlorides:

Acyl chlorides can be prepared via reacting carboxylic acids with reagents such as phosphorus trichloride, PCl3, phosphorus pentachloride, PCl5 or sulphur  dichloride oxide (that is, thionyl chloride), SOCl2. For illustration:

RCOOH + PCl3 → RCOCl + H3PO3

RCOOH + PCl5 → RCOCl + POCl3 + HCl

RCOOH + SOCl2 → RCOCl + HCl + SO3

These reactions are the illustrations of substitution reaction.

Sulphur dichloride oxide is specifically convenient of all the reagents as all the products are gaseous apart from acyl chloride. As well, sulphur dichloride oxide consists of a low boiling point of 79oC making it simple for the separation of the acyl chloride from surplus sulphur dichloride oxide.

The acyl chloride can be hydrolyzed simply in the presence of water to regenerate the carboxylic acids.

c) Conversion into Acid Anhydrides:

The acid anhydrides can be formed from carboxylic acid via reacting with the dehydrating agent, phosphorus pentoxide, P2O5. In the procedure a molecule of water is eliminated. The reaction is an illustration of the condensation reaction.

1097_Conversion into Acid Anhydrides.jpg

Fig: Conversion into Acid Anhydrides

Acid anhydrides can as well be prepared through the reaction between an acyl chloride and metal carboxylate.

R COOH + (SOCl) → RCOCl

2RCOOH + 2Na → 2RCOO- Na+ + H2

R COONa + RCOCl + heat → (RCO)2O + NaCl

Reactions of Carboxyl group:

Reduction by Lithium Tetrahydrido Aluminates:

Carboxylic acids are not simply reduced and are immune to most of the common reducing agents. Though, Lithium tetrahydrido aluminate, LiAIH4 is a strong reducing agent capable of reducing the carboxylic acids to the corresponding alcohols.

RCOOH + [(1. LiAlH4/dry ether) and (2. H2O)] → RCH2OH

Reaction Due to Akyl group:

Halogenation:

Whenever the chlorine gas is bubbled via boiling ethanoic acid in the presence of either iodine or red phosphorus in the sunlight, chloroethanoic acid is formed. The chloroethanoic acid is a colourless and corrosive crystalline solid (having melting point 61oC).

I2 + 3Cl2 → 2ICl3

CH3COOH + ICl3 + (105 - 110oC) → CH2ClCOOH + ICl + HCl

                                                     Chloroethanoic acid

ICl + Cl2 → ICl3

The dichloro and trichloroethanoic acid are obtained through successive substitution of the alkyl hydrogen atoms.

CH2ClCOOH + (Cl2, I2)    →     CHCl2COOH        →        CCl3COOH

                                 Elevated  dichloroethanoic  Elevated trichloromethanoic

                                   temp             acid                temp           acid

Oxidation of Methanoic Acid:

The Oxidation of methanoic acid by using acidified potassium tetraoxomanganate (vii), KMnO4 at elevated temperature provides carbon (IV) oxide, CO2 and water.

1820_Oxidation of Methanoic Acid.jpg

Fig: Oxidation of Methanoic Acid

This is due to the reason that it consists of the - CHO group therefore it behaves as a reducing agent similar to aldehydes, unlike other carboxylic acid. This makes methanoic acid positive to Tollen's and Fehling's tests and it decolonizes the acidified potassium tetraoxomanganate (VII).

Reaction involving Salts of acids:

Conversion into Amide:

Amide can be prepared via reacting carboxylic acids with ammonia followed via dehydration of the ammonium salts formed at an elevated temperature.

1847_Conversion into Amide.jpg

Fig: Conversion into Amide

As well if sulphur dichloride oxide reacts by carboxylic acids followed by ammonia, amide is made.

RCOOH + (SOCl2) → RCOCl

2 RCOCl2 + 2NH3 → RCONH2 + NH4Cl

Decarboxylation:

Whenever anhydrous sodium salt of a fatty acid is heated by soda lime, (CaO/NaOH), alkane is formed

RCOONa + NaOH → RH + Na2CO3

For illustration: CH3COONa + NaOH → CH4 + NaOH

Formation of aldehydes and ketones:

Whenever calcium salt of a fatty acid is heated strongly, it provides ketones. Though, whenever it is heated by calcium dicarboxylic acid, it makes an aldehyde. 

For illustration:

(CH3COO)2Ca + (HCOO)2Ca → 2CH3CHO + 2CaCO3

Calcium ethanoate calcium         ethanal

                         dicarboxylic acid

Whenever calcium dicarboxylic acid is heated alone, it makes methanal

(HCOO)2Ca → HCHO + CaCO3

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