Carboxylic Acid & Their Derivatives

CARBOXYLIC ACID & THEIR DERIVATIVES Organic compounds having –COOH group called Carboxylic group. This functional group is composed of Carbonyl O ( C ) and hydroxyl (—OH) group. O O + C Carbonyl group —OH  Hydroxyl group C OH Carboxylic group The properties of the carboxylic group are not simply the combined properties of these two groups, but it has its own distinctive properties. The acidic nature of carboxylic acids is due to the presence of replaceable H-atom in the Carboxylic group. The general formula is CnH2nO2 . Carboxylic acid derivatives are compounds with functional groups that can be converted to carboxylic acids by a simple acidic or basic hydrolysis. The most important acid derivatives are esters, amides, nitriles, acid halides and anhydrides. R–C–X acid halide O O O O R–C–O–C–R anhydride R – C – O – R' ester O R – C – NH2 amide R–C N nitrile DICARBOXYLIC ACIDS : If the compound containing two carboxyl groups, these are known as dicarboxylic acid. For example : COOH Oxalic acid Ethanedioic acid COOH COOH COOH COOH Malonic acid Propane-1, 3-dioicacid COOH COOH COOH COOH COOH COOH Glutaric acid Adipic acid Pentane-1, 5-dioicacid Hexane-1, 6-dioicacid HOOCCH2CH2 CHCOOH | Br – Bromoglutaric acid 2– Bromopentan edioic acid COOH Succinic acid Butane-1, 4-dioic acid Pimelic acid Heptane-1, 7-dioic acid CH3 | HOOCCH2CCH2COOH | CH3 , –Dimethyglutaric acid 3,3 – Dimethyl pentan edioic acid HOOCCHCH2CHCOOH | | Cl Cl , – Dichloroglutaric acid 2,4 – Dichloro–pentanedioic acid CLASSIFICATION : Monocarboxylic acid (RCOOH) : Having one carboxylic group, also called monobasic acid. General formula - CnH2nO2 ( n = 1, 2, 3, ........).Higher mono carboxylic acids are called fatty acids. Example : CH3COOH acetic acid Dicarboxylic acid : Having two carboxylic groups, also called dibasic acid. COOH Example : Oxalic acid COOH Tricarboxylic acid : Having three carboxylic groups also called tribasic acid. CH2COOH Example : HO C Citric acid COOH CH2COOH Nomenclature Acid Common name IUPAC name HCOOH Formic acid (formica-red ants) Methanoic acid CH3 COOH Acetic acid (acetum-vinegar) Ethanoic acid CH3 CH2 COOH Propionic acid (Propan-first pion-fat) Propanoic acid CH3 CH2 CH2 COOH Butyric acid (Butter-butyrums) Butanoic acid CH3 CH2 CH2 CH2 COOH Valeric acid (valerian-plant root) Pentanoic acid C5 H11 COOH Caproic acid Hexanoic acid C7 H15 COOH Caprylic acid Octanoic acid C9 H19 COOH Capric acid Decanoic acid Last three acids are found in goat fat word - (Caper-Goat). PHYSICAL PROPERTIES OF ACIDS AND ACID DERIVATIVES : (1) Boiling point : The boiling point of carboxylic acids are higher than that of alcohols, ketones or aldehydes of similar molecular weight. O CH3– C – OH acetic acid, bp 118ºC O CH3CH2CH2OH 1-propanol bp 97ºC CH3CH2CH Propionaldehyde bp 49ºC The high boiling points of carboxylic acids is the result of formation of a stable hydrogen-bonded dimer. O ----- H – O R–C C–R O – H ----- O hydrogen bonded acid dimer (2) Melting points : Melting point of carboxylic acids : There is no regular pattern in melting point of carboxylic acid (up to 10 carbon atoms) having even number of C atoms are higher than neighbouring members having odd number of CARBOXYLIC ACID & THEIR DERIVATIVES C atoms because carboxylic acid and methyl group in even members lie in opposite side of zig-zag carbon chain hence they fit better into crystal lattice resulting in higher melting points. Vice-versa is observed in case of carboxylic acid having odd no. of carbon atoms. COOH H3C Higher melting point COOH Lower melting point Amides have surprisingly high boiling points and melting points compared with other compounds of similar molecular weight. Primary and secondary amides participate in strong hydrogen bonding. (3) Solubility: Carboxylic acids form hydrogen bonds with water and the lower molecular - weight carboxylic acids (upto 4 carbon atoms) are miscible with water. Acid derivatives (esters, acid chlorides, anhydrides, nitriles and amides) are soluble in common organic solvents such as alcohols, ethers, chlorinated alkanes and aromatic hydrocarbons. Acid chlorides and anhydrides cannot be used in nucleophilic solvents such as H2O and alcohols, because they react with these solvents. GENERAL METHOD OF PREPARATION By oxidation of primary alcohol with acidic KMnO4 or acidic K2Cr2O7 : KMnO4/H + R–CH2OH + [O] K Cr O / H+ 2 2 7 RCHO + H2O [O] RCOOH By oxidation of aldehydes : Aldehydes on oxidation with usual oxidizing agent gives carboxylic acid with same number of carbon atoms as in the aldehyde. Tollen's Reagent R–CHO + [O] Tollen's Reagent C6H5CHO + [O] H R–COO– C6H5COO1 R–COOH H C6H5COOH By oxidation of alkenes : (a) RCH = CHR' (i) conc. KMnO4/OH heat (ii) H3O  RCOOH + R'COOH (i) O3 (b) RCH = CHR' (ii) H O Distill RCOOH + R'COOH 2 By Carboxylation of Grignard Reagent : O R–Br dry ether RMgBr R––C––OMgBr By hydrolysis of acyl derivatives of carboxylic acid : O R––C––Cl + H2O H R–COOH + HCl O (R – CO)2O + H2O H R – COOR' + H2O H R–COOH + R—C—OH  R–COOH + ROH O R––C––NH 2 + H2O H R–COOH + NH4 H 2O/H + O R—C—OH Cyanide hydrolysis with dilute acids : R – CN H3O  RCOOH The mechanism of hydrolysis of R–CN is as follows. By oxidation of alkyl benzene : Alkyl group having no -H atom will not be oxidized to –COOH. Any alkyl group containing at least one -H atom will be oxidized to –COOH. The product of oxidation will be benzoic acid. H2C––CH2CH3 COOH (i) KMnO4/OH (ii) H3O C(CH3)3 Neutral KMnO4 No oxidation The order of benzoic acid formation by oxidation of alkyl benzene. Methyl benzene >1° alkyl benzene >2° alkyl benzene CHEMICAL PROPERTIES OF CARBOXYLICACID 1. Acidity of carboxylic acid : Acidity is relative case with which it loses a proton leaving behind the anion. Its acid strength depends upon the difference in the stability of the acid and its anion. CARBOXYLIC ACID & THEIR DERIVATIVES both acid and its anion are stabilized by resonance, stabilization is far greater for the anion than for acid because anion gives two identical resonating structure. 2. Effect of substituents on Acidity : Any factor that stabilizes the anion more than it stabilizes the acid should increase the acidity and any factor that makes the anion less stable should decrease the acidity of the carboxylic acid. (a) An electron withdrawing substituents stabilizes the anion by dispersing the –ve charge and therefore increases the acidity of carboxylic acid. (b) Electron releasing substituents intensify the –ve charge on the anion resulting in decrease of stability of the carboxylate anion and therefore decreases the acidity of the acid. Carboxylic acids are weak acids and their carboxylate ions are strong conjugate bases. They are slightly alkaline due to the hydrolysis of carboxylate anion compared to other species. The order of acidity and basicity of corresponding conjugate bases are as follow. Acidity ––RCOOH > HOH > ROH > CH CH > NH3 > RH Basicity ––RCOO– < OH– < HC  C– < NH2– < R– O O R R O O Acidity increase Acidity decrease The effect of various number of the substituent and their distance from the carboxylic group has been illustrated with the help of following examples. (i) The effect of number of the substituent is shown by the chloro substituted acetic acids. The acid strength increases in the order given below : ClCH2––COOH < Cl2CHCOOH < Cl3CCOOH The increase in the no. of chloro substituent on -carbon atom of acetic acid make the electron withdrawing effect more pronounced and hence make carboxylate ion more stable. When electron releasing substituent is attached to the carboxylic group then acid strength decreases as the electron releasing power increases. R R CH 2 COOH > R CH R COOH > R C COOH R (ii) The effect of nature of the substituent is illustrated by the various halo acetic acids. Their acid strength follows the order : ICH2–COOH < BrCH2COOH < ClCH2COOH < F CH2COOH CH3–CH2–CH2–COOH < CH2 = CH–CH2–COOH < NC–CH2–COOH   (sp2) (sp) (iii) Effect of the position of the substituent : The effect of the substituent decreases as its distance from —COOH group increases. Cl Cl CH 3 ––CH––COOH > -chloro propanoic acid electron withdrawing effect more CH2 ––CH2––COOH -chloro propanoic acid electron withdrawing effect less Ortho Effect : The ortho substituted benzoic acid (whether the substituent is electron withdrawing or releasing) is comparatively stronger acid than the para and meta isomers. This effect is called ortho effect. It occurs due to the joint operation of steric and intra molecular H-bonding where ever it takes chance to stabilize the carboxylate anion due to nearness of the substituent. Groups like –OH, –Cl, –NO2 will cause more stabilization to anion due to direct interaction through intra molecular H-bonding. Reaction due to cleavage of ––O––H bond as acid O ––C––O––H (i) Reaction with active metals [alkali and alkaline metal] : O R–––––OH + Na (ii) O R–––––ONa + H2 ( ) Reaction with CaO : 2R –COOH + CaO  (RCOO)2Ca + H2O CARBOXYLIC ACID & THEIR DERIVATIVES (iii) Reaction with Bicarbonates and Carbonates : Carboxylic acid reacts with carbonates and bicarbonates to liberate CO2 gas NaHCO3 R–––COOH CaC O O R—C—ONa + CO2 + H2O O 3 R––C––O Ca + CO2 + H2O 2 REACTION INVOLVING CLEAVAGE OF –OH GROUP Esterification : When carboxylic acid reacts with alcohol in the presence of conc. H2SO4 to form ester, it is known as esterification R C OH + R OH Conc. H2SO4 R O C OR + H2O O MECHANISM H2SO4 H+ + HSO 4– The relative reactivity of alcohol to ester formation markedly dependent on their structure. The greater the bulk of the substituents near the –OH group, the slower the reaction would be same facts is followed by acid as well CH3OH > CH3CH2OH > (CH3)2 CHOH > (CH3)3 COH H––COOH > CH3COOH > (CH3)2CHCOOH > (CH3)3CCOOH Formation of acid chloride : R—COOH + PCl5 C6H5N R––COCl + HCl + POCl3 + SOCl2 C5H5N R––COCl + SO2 + HCl + SO2Cl2 C5H5N R––COCl + SO3 + HCl Formation of Acid Anhydride : Carboxylic acid on treatment with any dehydrating agent as P2O5 to form anhydride by elimination of water molecule. Formation of Amides : The lone pair electron on N atom undergoes resonance and hence N atom carries partial +ve charge on it. Thus extent of H-bonding increases which makes the boiling and melting points of amides higher than other acid derivatives. REACTIONS INVOLVING – COOH GROUP Schmidt Reaction : Carboxylic acid reacts with hydrazoic acid in the presence of conc. H2SO4 at 90°C forming primary amine. R—COOH + N3H Conc. H2SO4 R—NH2 + N2 + CO2 MECHANISM 2. Soda lime Decarboxylation : Carboxylic acid on heating with soda lime (NaOH and CaO) give alkane with one carbon atom less than the parent acid. R–––COOH Soda lime  R––H + CO2 Hunsdicker Reaction : R––COOH Ag2O R––COO– Ag+ Br 2 in CCl4 R––Br + AgBr + CO2 CARBOXYLIC ACID & THEIR DERIVATIVES MECHANISM Reaction proceeds through free radical mechanism in various steps. (i) Chain initiation : O O R––C––O––Br • • R––C––O + Br (ii) Chain propagation step : O • R• + CO2 R––C––O O O R––Br + R––C––O• R––C––O––Br + R• (iii) Chain termination : • • R—R R+ R O O • • R—C—O+R • • Br+ Br R—C—O—R Br2 With excess of organometallic compounds : 2 moles of organometallic compounds are needed to form ketone. Hell-Volhard-Zelinsky Reaction : This is -halogenation of a carboxylic acid. Possible Mechanism for the reaction is : Br H3C COOH H3C COBr –Br2 H3C Br COBr H2O H3C COOH The purpose of PBr3 is to convert –OH into –Br to make -hydrogen atom more acidic to be replaced by Br atom of Br2. The reaction does not stop at monosubstitution but continues till the -hydrogen are replaced. Cl Cl2 Red P H3C––COOH H2C O C OH Cl2 Red P CHCl2COOH Cl2 Red P CCl3COOH The reaction has a great synthetic importance as the halogen atom can be replaced by a number of other groups giving useful products. Heating of -keto acid : O O O R––C––CH2C––OH 100–150°C R––C––CH 3+ CO2 -keto acid There are two facts on which ease of decarboxylation depends. (i) When the carboxylate ion decarboxylate, it forms a resonance stabilized enolate anion. O O R––C––CH2C––O –CO2 O – R––C––CH2 O H R––C––CH3 O R––C CH2 Resonance stabilized enolate anion This anion is much more stable than the anion RCH 2 formed by decarboxylation of an ordinary carboxylic acid anion. (ii) When the acid itself decarboxylates it can do so through a six-membered cyclic transition state -keto acid on warming alone or in presence of a base undergoes rapid removal of CO2. H3C––C––CH2C––OH O O H3C––C––CH3 + CO 2 O CARBOXYLIC ACID & THEIR DERIVATIVES MECHANISM Here y can be substituents like OH – diacid : R – -keto acid H – aldehyde acid : X – halo acid Br O Ph Br OH KOH base Ph O O –CO2 Ph This decarboxylation proceeds through elimination. Heating of ,  and  Hydroxy Acid : (i) (ii) (iii) 8. Reaction of ,  and  halo carboxylic acid with aq. NaOH : H H3C COOH Br OH E2 H3C COO CH2 + CO2 Heating of Dicarboxylic Acids : O C CH2CH2 C O O –H2O O O H H Succinic acid O O Succinic anhydride (CH2)3 HOOC COOH O O O Glutamic anhydride Oxidation of Carboxylic acid at Alkyl Group : Acid on treatment with mild oxidising agent such as H2O2, is oxidized at the  position.   CH 3 C H 2  C H 2 COOH  [O] H2O2 H3C COOH OH 3-hydroxybutanoic acid Oxidation can also occur at  carbon atom on treatment with oxidizing agent like selenium dioxide SeO2. RCH2COOH + [O] SeO2  Abnormal Behaviour of Formic Acid : The behaviour of formic acid is different from other carboxylic acid due to presence of aldehydic group. Aldehydic Group O H––C––OH Carboxylic Group Reaction with : (a) Tollen's reagents : formic acid behaves as a reducing agent and reduces Tollen's reagent or Felhing solution. But others acid fail to do so. H–COOH + Ag2O  CO2 + H2O + 2Ag  Silver mirror H–COOH + 2CuO  CO2 + H2O + Cu2O  Red ppt. CARBOXYLIC ACID & THEIR DERIVATIVES (b) Reaction with HgCl2 : Formic acid forms white ppt. of Hg2Cl2 with HgCl2 which is converted to Grey ppt of mercury. 2HCOOH + HgCl2  CO2 + Hg2Cl2  + 2HCl white ppt. Uses of Formic Acid : (i) As an antiseptic (ii) For preservation of fruits. (iii) For leather tanning. (iv) In dying wool and cotton fabrics. (v) As a coagulating agent for rubber. (vi) For hydrogenation of oil as Ni-formate. Uses of Acetic Acid : (i) Vinegar (6 - 10% solution) used as table acid and manufacture of pickles. (ii) In the form of salts, it is used in medicine and paints. (iii) For manufacture of rubber from latex and casein from milk CH3COOH is used as coagulant. (iv) Al and Cr acetates are used as mordants. (v) In the manufacture of dyes and perfumes. (vi) As a solvent and laboratory reagent.