The carboxylic group of the acids consists of a carbonyl group and a hydroxyl group. The hydroxyl group can be substituted by -Cl, -COCR, -NH2 or -OR' groups giving the compounds termed as acyl chloride (acid chloride), acid anhydrides, amides and esters correspondingly. They can be symbolized by the general formula:
Fig: Carboxylic group and acyl group
Here 'Z' stands for Cl, OCOR', NH2 or OR'
R and R' might be alkyl or aryl.
The group illustrated in the above figure, common to all the derivatives is termed as the acyl group.
General Comparative Properties of Carboxylic Acids:
Chlorides, anhydrides and esters encompass normal boiling points and are similar in value to those of aldehydes and ketones of the comparative relative molecular mass. Amides encompass a higher than expected boiling points and at normal temperatures, they exist as solid due to a fairly high degree of intermolecular hydrogen bonding. The capability of amides to form hydrogen bonds by water molecules allows them to be much more soluble than the other derivatives. Simple chlorides and anhydrides experience spontaneous hydrolysis in the water.
The derivatives of carboxylic acids experience nucleophilic substitution reactions. This is characterized through nucleophilic substitutions of the halogen, carboxylate, alkoxy or amino groups and the reaction is identical to the condensation reactions of aldehydes and ketones. All the substituent groups have lone pairs of electrons that are conjugated by the carbonyl group. As the electronegativity of the group increases, the degree of conjugation decreases and the electron availability concerning the carbonyl oxygen is reduced.
Fig: Reactivity of carboxylic group
The order of electronegativity of the substituent group is as shown below:
- Cl > OOCR' > OR' > -NH2
The overall polarity of the carbonyl group is improved by the more electronegative substituent because of the simultaneous withdrawal of electron away from the carbon atom, making it more susceptible to the nucleophilic attack. The decreasing order of reactivity of carboxylic acid itself and its derivatives are as shown:
Fig: Order of reactivity of carboxylic acid
Acyl Chlorides (Acid Chlorides):
Common Methods of Formation:
a) From acids:
Acyl chlorides can be prepared via heating carboxylic acids with phosphorus trichloride (PCl3) or phosphorus pentachloride, PCl5) or sulphur dichloride oxide (thionyl chloride), SOCl2.
RCOOH + PCl3 → RCOCl + H3PO3
RCOOH + PCl5 → RCOCl + POCl3 + HCl
RCOOH + SOCl2 → RCOCl + HCl + SO3
b) From the salt of the acids:
Acyl chloride can be made through distilling the salts of the acid by phosphorus trichloride, PCl3 or phosphorus trioxychloride, POCl3 or sulphuryl chloride, SO2Cl2.
CH3COONa + PCl3 → CH3COCl + Na3PO3
2CH3COONa + PCl5 → 2CH3COCl + NaCl + NaPO3
(CH3COO)Ca + SO2Cl2 → 2CH3COCl + CaSO4
1) The lower members are colourless, volatile liquids by irritating smell.
2) Their boiling point is much lower than those of acids from which they are obtained. This is because the absence of intermolecular hydrogen bonding.
3) They mainly fume in moist air producing vapors of the hydrogen chloride.
4) They are usually insoluble. Though they hydrolyze slowly to go to solution.
Reactions of Acyl chlorides:
The reactions of acyl chlorides are basically identical to those of carboxylic acids. The halogen readily experiences nucleophilic substitution by -OH, -OR', NH2 and so forth. The method is identical to those of condensation reactions of the aldehydes and ketones.
Benzoyl chloride is much less reactive as compare to any aliphatic acyl chloride because of the reduction in the positive nature of the carbonyl carbon caused via delocalization of electrons between it and the benzene ring (that is, resonance effect), leading to the reduction in the strength of electron deficient nucleophilic site.
Fig: Reactions of Acyl chlorides
a) Reactions with water (Hydrolysis):
Acyl chlorides hydrolyze in water to form the parent carboxylic acids and hydrogen chloride.
RCOCl + H2O → RCOOH + HCl
CH3COCl + H2O → CH3COOH + HCl
b) Ester Formation:
Acyl chlorides react by alcohols and phenols to form the repective ester and hydrogen chloride.
RCOCl + R'OH → RCOOR' + HCl
Esterification by phenol needs an alkaline medium.
Fig: Esterification with phenol
c) Amide Formation:
Acyl chlorides react by ammonia to give the respective amides and ammonium chloride.
Fig: Amide formation
d) Aldehyde Formation (Rosenmund Reduction):
Acyl chlorides are reduced to aldehydes whenever heated by palladium poisoned with barium tetraoxosulphate (VI). The barium tetraoxosulphate (VI) poison the catalytic activity of the palladium restricting the reduction to only aldehyde. Without the poisonous effect of the barium tetraoxosulphate (VI) the aldehyde may be reduced to alcohol too.
RCOCl + (H2, heat)(Poisoned Pd/BaSO4 cat) → RCHO + HCl
e) Ketone Formation (Friedel-Crafts acylation):
Acyl Chlorides reacts by aromatic hydrocarbon in the presence of an anhydrous aluminium chloride- Lewis acid, to provide a good outcome of the aromatic alkanone.
For illustration the reaction with benzene, the reaction mixture is refluxed on the water bath at 50oC.
Fig: Ketone Formation
f) Anhydride formation:
Acyl chlorides react by the sodium salts of carboxylic acids at an elevated temperature to give acid anhydrides. For illustration:
CH3COONa + CH3COCl + (distil) → (CH3CO)2O + NaCl
Acid anhydride might be regarded as being derived from acid(s) by the elimination of one molecule of water from two molecules of acids.
Fig: Acid Anhydrides
General methods of Formation of Acid Anhydrides:
a) By heating the acyl chloride with anhydrous sodium salt of the acid or the acid
CH3COCl + NaOCOCH3 → CH3COOCOCH3 + NaCl
CH3COOH + CH3COCl + (pyridine) → CH3COOCOCH3 + HCl
b) By treating surplus of anhydrous sodium salt of the acid by phosphorus oxy-chloride or thionyl chloride
3CH3COONa + POCl3 → 3CH3COCl + Na3PO4
2CH3COONa + SOCl2 → CH3COOCOCH3 + SO2 + NaCl
c) Via the dehydration of anhydrous acids in the presence of appropriate dehydrating agent such as P2O5
2CH3COOH + (heat, P2O5) → CH3COOCOCH3 + H2O
d) Anhydrides of higher acids are obtained via heating their sodium salt of acids by ethanoic anhydride.
2RCOONa + (CH3CO)2O → RCOOCOR + 2CH3COONa
Physical Properties of Acid Anhydrides:
1) They are colourless liquids or solids having a sharp irritating purgent smell.
2) They are insoluble in water.
3) Their boiling points are higher as compare to those of the acid from which they are derived as their big size and greater Van der Wall's interaction.
Chemical Properties of Acid Anhydrides:
Acid anhydrides readily experience nucleophilic substitution reaction similar to other carboxylic acid derivatives. The carbonyl carbon is the leaving group.
Acid anhydrides hydrolyse in water to provide the parent carboxylic acids.
(RCO)2O + H2O → 2RCOOH
(CH3CO)2O + H2O → 2CH3COOH
Ethanoic Ethanoic acid
b) Ester formation:
The acid anhydrides react by alcohols and phenols to provide esters and carboxylic acids. The reaction is irreversible.
(RCO)2O + R'OH → RCOOR' + RCOOH
Ester Carboxylic acid
(CH3CO)2O + CH3OH + (reflux) → CH3-COOCH3 + CH3COOH
Ethanoic acid methanol methyl ethanoate ethanoic acid
Fig: Example of Ester formation
c) Amide formation:
Amides are made up whenever acid anhydride reacts with ammonia, NH3 and primary amine, R'NH2.
(RCO)2O + NH3 → RCONH2 + RCOOH
(RCO)2O + R'NH2 → RCONHR + R'COOH
(CH3CO)2O + NH3 → CH3CONH2 + CH3COOH
Ethanamide Ethanoic acid
(CH3CO)2O + CH3CH2NH2 → CH3CONHCH2CH3 + CH3COOH
Ethanoic acid Ethylamine N-ethyl ethanamide Ethanoic acid
d) Ketone formation: Friedel - Craft acylation
Acid anhydrides react by benzene in the presence of anhydrous aluminium chloride catalyst to form aromatic ketone and carboxylic acids.
(RCO)2O + C6H6 + (AlCl3 Catalyst) → C6H5COR + RCOOH
(CH3CO)2O + C6H6 + (AlCl3 Catalyst) → C6H5COCH3 + CH3COOH
Esters encompass the general formula RCOOR'
General Methods of Preparation of esters:
a) By direct esterification:
Esters are made by refluxing the acid with alcohols in the presence of small quantity of concentrated tetraoxosulphate (vi) acid.
For illustration: CH3COOH + HOC2H5 ↔ CH3COOC2H5 + H2O
b) By the action of alcohols on acyl chloride or anhydrides:
CH3COCl + HOC2H5 → CH3COOC2H5 + HCl
(CH3CO)2O + HOC2H5 → CH3COOC2H5 + CH3COOH
c) By the action of alkyl halides on the silver salts of fatty acids
CH3COOAg + C2H5I → CH3COOC2H5 + AgI
d) From diazomethane:
For illustration: CH3COOH + CH2N2 → CH3COOC2H5 + N2
e) From Carboxylic acid and alkene:
An ester is made if an acid is treated by an alkene in the presence of boron trifluoride as the catalyst.
RCOOH + CH2 = CH2 + BF3 → RCOOC2H5
f) From ethers and carbon (II) oxide:
Ethers react by carbon (II) oxide at 125-180oC under 500 atmospheric pressure in the presence of boron trifluoride and little water to make ester.
R-O-R + CO + (Heat/BF3) → RCOOR
Physical Properties of esters:
1) They are colourless liquids having characteristic sweet odors.
2) The boiling points of esters are normal, increasing as the relative molecular masses increase. Methyl and ethyl ester though have much lower boiling points than their related parent acid despite having higher molecular masses. The boiling points of normal-chain esters are higher than those of the branched chain isomers.
3) Methyl methanoate is very soluble in water; though there is a progressive and fast decrease in solubility of the higher compounds as the relative molecular mass increase. The esters of aromatic carboxylic acids are insoluble.
Reactions of esters:
Esters experience nucleophilic substitution reactions in which the alky group -OR' is replaced by the weak nucleophiles under acid or base catalyzed conditions essential to improve the electron shortage of the carbonyl carbon atom.
Esters hydrolyzed under an acid or base catalyzed condition to make carboxylic acids and alcohols or produce the carboxylates and alcohols correspondingly. The acid-catalyzed method is the precise opposite of esterification.
i) Acid catalyzed:
RCOOR' + H2O ↔ (H+/Reflux) ↔ RCOOH + R'OH
HCOOCH2CH2CH3 + H2O ↔ (H+) ↔ HCOOH + CH3CH2CH2OH
Propyl methanoate Methanoic Propan-1-ol
ii) Alkali- catalyzed:
RCOOR' + OH- ↔ RCOO- + R'OH
CH3COOCH2CH3 + NaOH ↔ CH3COONa + CH3CH2OH
Hydrolysis in alkaline conditions is termed to as saponification as it is the kind of reaction employed in soap making process. 'Soapy detergents' are alkali metal derivatives of carboxylic acids and they hold between 10 and 18 carbon atoms. Fats and oils (that is, triesters) are hydrolyzed to produce the sodium carboxylate (that is, soap).
Fig: Example of Hydrolysis reactions
b) Amide formation:
Esters reacts by ammonia dissolved in an alcoholic medium or otherwise concentrated aqueous ammonia having dissolved ammonium salts to provide amide and alcohol.
RCOOR + NH3 + (RO- in alcohol or NH+ in concentrated Aq. solution) → RCONH2 + R'OH
c) Formation of ketones and tertiary alcohols:
Esters react by Grignard reagent by using ether as solvent, the product is a ketone that can react further by Grignard reagent to make tertiary alcohol through hydrolysis of the intermediate alkyl magnesium halide by using the aqueous ammonium chloride.
Fig: Formation of ketones and tertiary alcohols
d) Ester Exchange (Transesterification):
This is the method whereby one alcohol replaces the other in an ester. The reaction might be catalyzed through an acid or via the basic alkoxide, RO-
RCOOR' + R"OH ↔ (H+ or RO-) ↔ RCOOR" + R'OH
The reduction of esters can be taken out via reacting it by the reagent lithium tetrahydridoaluminate (LiAIH4) and then by water to produce the mixture of two alcohols.
RCOOR + 4[H] + [LiAlH4/(C2H5)O]/H2O → RCH2OH + R'OH
f) Replacement of the carboxylate group:
If dry hydrogen bromide, concentrated aqueous hydrogen iodide or concentrated tetra-oxosulphate (VI) acid.
Amides are the monoacyl derivates of ammonia. They might be categorized as primary, secondary, or tertiary amide based on the number of alkyl groups linked to the nitrogen atom.
General Methods of Preparation of Amides:
a) By heating ammonium salts of carboxylic acids:
RCOONH4 + (heat) → RCONH2 + H2O
b) Via ammonolysis of acyl chloride, anhydrides or ester by concentrated ammonia:
RCOCl + NH3 → RCONH2 + HCl
(RCO)2O + NH3 → RCONH2 + CH3COOH
RCOOR' + NH3 → RCONH2 + R'OH
c) By partial hydrolysis of alkyl cyanides through cold concentrated hydrochloric acid, polyphosphoric acid or alkaline hydrogen peroxide.
RC ≡ N + H2O → RCONH2
Physical Properties of Amides:
1) Primary amides, RCONH2 are crystalline solids due to mostly fairly strong intermolecular hydrogen bonding. Though methanamide is a liquid at room temperature (and melting point 3oC).
2) The melting points of amides are high for the size of the molecules as they can make hydrogen bonds. The hydrogen atoms in the NH2 group are adequately positive to form a hydrogen bond having a lone pair on the oxygen atom of the other molecule.
Fig: Melting point of amides
Each and every molecule consists of two slightly positive hydrogen atoms therefore there is the potential for lots of hydrogen bonds to be formed. Such hydrogen bonds require a reasonable quantity of energy to break, and therefore the melting points of the amides are pretty high.
3) The small chain amides are soluble in water as they encompass the capability to form hydrogen bond by the water molecules.
Reactions of amides:
Amides are somewhat basic in character, due to the lone pair of electrons on the nitrogen atom. Owing to resonance between the two canonical forms of amide, the lone pair of electrons is not readily available for donation as in amines forming amides much less basic.
a) Hydrolysis of amides:
The hydrolysis of amide is either acid catalyzed or alkaline catalyzed. The acid hydrolysis comprises the use of acids like dilute hydrochloric acid that acts as a catalyst for the reaction among the amide and water.
RCONH2 + (H2O, H+)/Heat → RCOOH + NH4+
CH3CONH2 + H2O + HCl → CH3COOH + NH4+Cl-
The alkaline hydrolysis comprises reaction by hydroxide ions.
RCONH2 + (OH-/Heat) → RCOO- + NH3
CH3CONH2 + NaOH → CH3COONa + NH3
The acid catalyzed hydrolysis offers carboxylic acids and ammonium salt, whereas the base catalyzed hydrolysis offers carboxylate and ammonia gas.
b) Hofmann Degradation:
The Hofmann degradation is a reaction between the amide and a mixture of bromine and sodium hydroxide solution in the presence of heat. The general effect of the reaction is a loss of the carboxyl group of the amide to obtain a primary amine the original amide.
RCONH2 + Br2 + 4NaOH → RNH2 + 2NaBr + Na2CO3 + 2H2O
CH3CONH2 + Br + 4NaOH → CH3NH2 + Na2CO3 + 2NaBr + 2H2O
The Hoffmann degradation is employed as a manner of eliminating carbon atom out of a chain.
Amides are dehydrated via heating a solid mixture of the amide and phosphorus (v) oxides, P2O5. Water is eliminated from the amide group to leave a nitrile group, -CN. The liquid nitrile is collected via simple distillation.
RCONH2 + (P2O5/heat) → RCN + H2O
CH3CONH2 + (P2O5/heat) → CH3CN + H2O
Amides can be reduced via the reagent lithium tetrahydridoaluminate (LiAlH4) to give a primary amine.
Fig: Reduction of Amides
Uses of carboxylic acids and their derivatives:
1) Benzoic acid and its sodium salt are mainly employed as food preservative.
2) Nylon 6.6 and polymers made up from the derivatives of carboxylic acids.
3) Preparation of soap.
4) Liquid esters are broadly employed as solvents for all purpose adhesives, thinners for paints and nail varnish remover.
5) As flavor enhancers in the food processing industry.
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