Carboxylic Acids

Carboxylic Acids

CARBOXYLIC ACIDS CARBOXYLIC ACID are acidic compounds having one or more carboxyl group COOH, in their Molecules. The name carboxyl is derived by the combination of the term carbonyl and hydroxyl . The carboxylic acid are classified as mono , di or tri carboxylic acids according to the number of carboxyl groups present in molecule. The mono carboxylic

acids may be represented as RCOOH where R represent NOMENCLATURE 1. COMMON SYSTEM: The aliphatic mono carboxylic acids are commonly called fatty acids . branched chain acids having one branch of methyl group farthest away from the carboxyl group are named by adding prefix iso to the name of corresponding straight line acid. CH3CH(CH3)CH2COOH AROMATIC ACIDS:AS

having the carboxyl group IS KNOWN ISOVALERIC ACID directly attached to the aromatic nucleus are generally referred to as derivatives of benzoic acid example : methyl benzoic acids are generally called toluic acid. 2. IUPAC SYSTEM: the IUPAC names

of carboxyl acids are obtained in accordance with the general IUPAC rule, the suffix used for the carboxyl group being oic acid. For example: CH3CH(CH3)CH2COOH 3-Methylbutanoic acid IUPAC names of some carboxylic acids ACIDIC CHARACTER OF CARBOXYLIC Even though the carboxyl group is generally ACIDS

represented by COOH, actually it is a resonance hybrid of the following contributing structures: In the resonance hybrid, oxygen of the hydroxyl group carries some positive charge. As a result, the electron pair of the OH bond is largely displaced towards oxygen. Due to this displacement of electron pair, the hydrogen of OH can be released as H+ ion. ORBITAL STRUCTURE:

The resonance in carboxyl ion can also be understood in term of molecular orbital concept. Carboxyl ion is in state of sp hybridisation. This hybrid orbital are directed towards the corners of the triangle and lie in the same plane at an angle of 120 with each other. These three orbitals form three sigma bonds, one each with two oxygen atoms and the third with -carbon atom. GENERAL METHODS OF 1.OXIDATION OF PRIMARY ALCOHOLS: PREPARATION carboxyl acids are readily obtained by oxidation of primary alcohols. The usual

oxidising agents employed are acidified potassium dichromate, potassium permanganate or nitric acid. RCH2OH RCOOH (carboxylic acid) 2.OXIDATION OF ALKYLBENZENE: on heating alkylbenzene with oxidising agents such as acidified potassium dichromate, the alkyl side chain gets oxidised to carboxyl group. 3.HYDROLYSIS OF NITRILES: Hydrolysis of a nitrile with an aqueous acid or alkali is an important method of preparing carboxylic acids.

Acidic hydrolysis of nitriles Alkaline hydrolysis of nitriles 4. HYDROLYSIS OF ACID DERIVATIVES: 5.CARBONATION OF GRIGNARD REAGENTS: carboxylic acids may prepared by reaction of Grignard with carbon dioxide . 6. MALONIC ESTER SYNTHESIS:

diethyl malonate,CH2(COOC2H5)2, generally refer to as melonic acid forms its sodium salt on reaction with sodium ethoxide. 7.HYDROLYSIS OF TRIHALOGEN DERIVATIVES Trihalogen derivatives having all the three halogen atoms attached to the same carbon atom give carboxylic acids on hydrolysis with an alkali. 8. OXIDATION OF METHYL

KETONES: when treated with sodium hypohalite, methyl ketones are oxidised to sodium to sodium salts of Halogen + NaOH O C R CH3

solvent then H+ R O H C C OH

X X X 9. ARNDT EISTERT HOMOLOGATION: this is a convenient method of homologation of an acid i.e. conversion of a given acid into its next higher homologue. The acid firstly convert to acid chloride by treatment with SOCL2.

PHYSICAL PHYSICAL STATE: the PROPERTIES 1. lower aliphatic acids such as formic acid are color less liquids having irritating smell. Acid forms are colorless , Oily liquids having unpleasant smells.acids are 2. SOLUBILITY:

the aliphatic not soluble in water but solubility falls off rapidly with increase in molecular weight and higher acids are insoluble in water. Simplest acids i.e. benzoic acid, is only sparingly soluble in water. 3. BOILING POINT: carboxylic acid form inter molecular hydrogen bonds and exist in form of associated molecules. As carboxylic acids have high boiling points. The boiling points are higher than those of alcohols of comparable molecular weight.

cyclic dimer. 4. MELTING POINT: DONOT FOLLOW A REGULAR PATTERN WITH INC. IN MOLECULAR SIZE. IT OBSERVED FROM ACIDS CONTAINING UP TO 10 CARBON ATOMS. THE MELTING POINTS OF ACID CONTAINING EVEN NO. OF CARBON ATOMS ARE HIGHER THAN THOSE OF IMMEDIATELY LOWER AS CHEMICAL PROPERTIES CLEAVAGE OF OH BOND 1.ACIDITY: all carboxylic acids ionise in aqueous solution and exist in equilibrium with carboxyl ate anion and

the hydronium ion. RCOOH + H2O RCOO + H3O+ 2. SALT FORMATION: carboxylic acid are attacked by strongly electropositive metals, such as Na, Zn and K to form salt and liberate hydrogen. 2RCOOH+ 2Na RCOONa + H2O 3.FORMATION OF ESTER: when heated with alcohols in presence

of a strong mineral acid or a Lewis acid, carboxylic acids form esters. The reaction is reversible in nature and is known as esterification. 4. FORMATION OF ACID CHLORIDE: on treatment with phosphorus pentachloride, phosphorus trichloride or thionyl chloride, hydroxyl group of carboxylic acids is replaced by chlorine to form acid chlorides. RCOOH +PCl5 RCOCl+POCl3+HCL 3RCOOH+PCl3

3RCOCl+ H3PO3 RCOOH+SOCl2 RCOCl+SO2+SO2+HCl 5. FORMATION OF AMIDES: when treated with ammonia, acids form ammonium salts which on heating lose a molecule of water to form amides. 6.FORMATION OF ANHYDRIDES: when RCOOH+ NH3 RCOONH4 treated with acid chlorides in the RCONH2

presence of pyridine, carboxylic acids form the corresponding anhydrides. RCOOH + RCOCl+ pyridine (RCO)2O+ Pyridine HCl MECHANISM 7.REDUCTION : carboxylic acids are resistant to catalytic hydrogenation. However, they can be reduced by lithium aluminium hydride to form primary alcohols: the carbonyl group part of carboxyl group getting reduced to CH2. RCOOH

RCH2OH MECHANISM : ELIMINATION OF CARBONYL DECARBOXYLATION: GROUP SIMPLE CARBOXYLIC ACIDS GET DECARBOXYLATED TO FORM HYDROCARBONS WHEN THEIR SALTS ARE HEATED WITH SODA LIME. RCOONa+

NaOH RH+ Na2CO3 MECHANISM: 9. KOLBES ELECTROLYSIS: Alkali salts of carboxylic acids on electrolysis get decarboxylated to form hydrocarbons. DISTILLATION OF CALCIUM SALTS OF ACIDS: when

distilled in the absence of air, calcium salts of acids yield different products as given below: DECARBOXYLATION HALOGENATION ( HUNSDIECKER REACTION): THE SILVER SALT OF A CARBOXYLIC ACID REACTS WITH BROMINE OR IODINE IN DRY CARBON TETRACHLORIDE TO FORM AN ALKYL BROMIDE OR IODIDE WITH THE LOSS OF CARBON DIOXIDE. HALOGENATION OF ALKYL GROUPS: in the

presence of small amounts of red phosphorus, the - hydrogen atoms of alkyl group of aliphatic acids can be replaced by chlorine or bromine to form a halo acid. ACIDITY CONSTANT OF CARBOXYLIC ACIDS It has been already pointed out that in aqueous solution, a carboxylic acid exists in equilibrium with carboxy late anion and the hydronium ion, that is: RCOOH +H2O RCOO- + H3O+ Acc. To law of mass action, the equilibrium

constant can be expressed as : Kc = (RCOO-)(H3O+) (RCOOH)(H2O) Since water is taken in large excess and its concentration is constant Ka = (RCOO-)(H3O+) Ka is referred to as the acidity constant. Since Ka is directly proportional to H3O+ ions, it would be low in case weak highof

pKa value: theseofdays theacids acidicand strength for stronginacids . of pKa an acid is value also represented terms value which is the negative logarithm of the acidity constant. That is: pKa= - log Ka

Example pKa of acetic acid is equal to 4.75 It may be noted that a strong acid has a large pKa value but small pKa value while a weaker acid has smaller pKa value but large pKa value. Most of the simple unsubstituted carboxylic acids are of moderate strength having pKa values of about 35. EFFECT OF SUBSTITUENTS ON ACIDITIES OF CARBOXYLIC Since carboxyl group consists of carbonyl and hydroxylACIDS

groups, it is expected that acids show reactions due to both these groups. However the reactions expected of these groups are modified as a result of interaction between these groups due to close proximity. Carboxylic acids are resonance hybrid of the following structures. It is clear that the carbonyl part of the carboxyl group does not have a double bond character but a reduced double bond character. Thus it does not give the reactions of the carbonyl group. The carbonyl carbon in carboxyl group is less electrophilic than the carbonyl carbon in aldehydes and ketones due to resonance. Therefore many nucleophilic addition reactions of aldehydes and ketones do not take place with carboxylic acids. The same effect makes the hydroxyl group of carboxylic acids more

acidic than the hydroxyl group of alcohols and phenols justifying ALIPHATIC ACIDS 1. The stronger the electron withdrawing group present in an acid, the greater is the acidity. Example: flouroacetic acid is stronger than chloroacetic acid because flourine is stronger electron withdrawing than chlorine. 2. The large the no. of electron withdrawing group present in acid the greater is the effect on the acidity. 3. As a group having electron withdrawing

inductive effect moves away from the carboxylic group. Its effect on the acidity becomes less pronounced. 4. The effect of electron releasing group. It is AROMATIC BENZOIC ACID:ACIDS IT IS AN SIMPLEST ACID CONTAINING A CARBOXYL GROUP ATTACHED TO AN UNSUBSTITUTED BENZENE RING. A B

C O O H A B A B C O O H C O O H

A A B C O O H B C O O H 2. PHENYLACETIC ACID: simple

aromatic acids containing the carboxyl group in the side chain. Here resonance effect is not possible due to intervening carbon atom between the phenyl and carboxyl groups. Therefore phenyl group exerts weakly electron withdrawing inductive effect. It is a strong acid than acetic acid and higher aliphatic acids containing electron EFFECT OF SUBSTITUENTS ON ACIDIC STRENGHTS OF

AROMATIC ACIDS. THE EFFECT OF SUBSTITUENT GROUP PRESENT ON THE BENZENE RING ON THE BENZENE RINGIN THE ACIDIC STRENGTH OF AROMATIC CARBOXYLIC ACIDS, DEPENDS UPON BOTH THE INDUCTIVE AND THE RESONANCE EFFECTS OPERATING GROUP IS PRESENT IN DIFFERENT POSITIONS A) Group having electron withdrawing inductive effect as well

as electron withdrawing resonance effect such as NO2 group. Let us consider the effect of nitro group on benzene, nitro group exert an electron withdrawing inductive effect as well as electron HO HO HO withdrawing resonance effect.

NO 2 O 2 N NO 2 b) EFFECT OF HALOGENS: halogens have electron withdrawing inductive effect and electron releasing resonance

effect but the inductive effect is stronger than resonance effect. c) Group having electron withdrawing inductive effect and electron releasing resonance effect but resonance effect is stronger than inductive effect. d) Group having only electron releasing inductive effect such as CH3 and CH2CH3 groups. Such groups have

acid weakening effect when present in Para and meta position. THANK YOU

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