S.y. carboxylic acid

Acid chemistry

1. Dr. B. R. Thorat Government of Maharashtra Ismail Yusuf Arts, Science and Commerce College, Mumbai Carboxylic acid and Salfonic acid

2. Nomenclature Structure and physical properties Acidity of carboxylic acids Effects of substituents on acid strength of aliphatic and aromatic carboxylic acids. Preparation of carboxylic acids: Oxidation of alcohols and alkyl benzene Preparation of carboxylic acids: Carbonation of Grignard and hydrolysis of nitriles Acidity Salt formation Decarboxylation Reduction of carboxylic acids with LiAlH 4 Reduction of carboxylic acids with diborane Hell- Volhard - Zelinsky reaction Conversion of carboxylic acid to acid chlorides, esters, amides and acid anhydrides and their relative reactivity Mechanism of nucleophilic acyl substitution Acid-catalysed nucleophilic acyl substitution Interconversion of acid derivatives by nucleophilic acyl substitution Mechanism of Claisen condensation Dieckmann condensation Content Preparations Reactions

3. Nomenclature A carboxylic acid - Contains a carboxyl group, which is a carbonyl group (C=O) attached to a hydroxyl group (—OH) . The carboxyl group carbon is numbered as 1. Some naturally occurring carboxylic acid: General Molecular formula of Saturated aliphatic monocarboxylic acids

4. Some important examples of carboxylic acid Some important Derivatives of Carboxylic Acid

5. Common Names H CO 2 H formic acid L. formica ant, bees, etc CH 3 CO 2 H acetic acid L. acetum vinegar CH 3 CH 2 CO 2 H propionic acid G. “pro-first, pion - fat” CH 3 CH 2 CH 2 CO 2 H butyric acid L. butyrum butter CH 3 CH 2 CH 2 CH 2 CO 2 H valeric acid L. valerans CH 3 (CH 2 ) 4 CO 2 H caproic acid L. caper goat milk CH 3 (CH 2 ) 6 CO 2 H caprylic acid -- CH 3 (CH 2 ) 8 CO 2 H capric acid -- CH 3 (CH 2 ) 10 CO 2 H lauric acid oil of lauryl C—C—C—C—C=O used in common names δ γ β α 5 4 3 2 1 Carboxylic acids containing six or fewer carbons are frequently called by their common names. The position of a substituent is designated by a lowercase Greek letter , and the carbonyl carbon is not given a designation.

6. Special names

7. IUPAC Nomenclature The IUPAC names of carboxylic acids - Find the longest carbon chain that contains the – COOH group . Number the longest chain. Carbon number 1 is the carboxyl carbon . Write the name of parent hydrocarbon and replace the - e in the hydrocarbon name with - oic acid . Name and number other substituents. The ring is numbered to give the lowest possible numbers for any substituents . Aromatic acid names are derived from the parent compound, benzoic acid, naphthaloic acid . The prefixes ortho , meta , and para may be used to show the position of one other substituent.

8. Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids . Double bonds in the main chain are signaled by the ending - enoic acid , and their position is designated by a numerical prefix . Double-bond stereochemistry is specified by using either the cis–trans or the E – Z notation

9. For these compounds, both ends of a chain will end with a –COOH group. The parent chain is the one that involves both –COOH groups. Compounds with two carboxyl groups are distinguished by the suffix - dioic acid or - dicarboxylic acid as appropriate. The final –e in the base name of the alkane is retained . Dicarboxylic acid

10. Salts of carboxylic acids Writing the name of the cation followed by the name of the acid with the – ic acid ending replaced by an –ate ending.

11. Acyl halides By using the acid name and replacing “ ic acid” with “ yl chloride” (or “ yl bromide”). For acids ending with “carboxylic acid,” “ carboxylic acid” is replaced with “carbonyl chloride ” (or “bromide”)

12. Acid anhydride If the two carboxylic acid molecules forming the acid anhydride are the same, the anhydride is a symmetrical anhydride . Symmetrical anhydrides are named by using the acid name and replacing “acid” with “anhydride. ” If the two carboxylic acid molecules are different, the anhydride is a mixed anhydride . Mixed anhydrides are named by stating the names of both acids in alphabetical order , followed by “anhydride.”

13. Ester The name of the group (R’) attached to the carboxyl oxygen is stated first, followed by the name of the acid , with “ ic acid” replaced by “ate.”

14. Lactones In systematic nomenclature, they are named as “ 2-oxacycloalkanones .” Their common names are derived from the common name of the carboxylic acid , which designates the length of the carbon chain, and a Greek letter to indicate the carbon to which the carboxyl oxygen is attached. Thus, four- membered ring lactones are β-lactones (the carboxyl oxygen is on the β-carbon), five- membered ring lactones are γ-lactones and six- membered ring lactones are δ-lactones. The “ ic acid ” replaced with “ lactone .”

15. Amide Amides are named by using the acid name, replacing “ oic acid ” or “ ic acid ” with “ amide .” For acids ending with “ carboxylic acid ,” “ ylic acid ” is replaced with “ amide .” If a substituent is bonded to the nitrogen, the name of the substituent is stated first (if there is more than one substituent bonded to the nitrogen, they are stated alphabetically), followed by the name of the amide. The name of each substituent is preceded by a capital N to indicate that the substituent is bonded to nitrogen.

16. Lactams They are named as “ 2-azacycloalkanones ” in systematic nomenclature (“ aza ” is used to designate the nitrogen atom). In their common names, the length of the carbon chain is indicated by the common name of the carboxylic acid , and a Greek letter indicates the carbon to which the nitrogen is attached. The “ ic acid ” replaced with “ lactam .”

17. Carboxylic acids with other functional groups Carboxylic acids are given the highest nomenclature priority by the IUPAC system . This means that the carboxyl group is given the lowest possible location number. If molecules containing carboxylic acid and alcohol functional groups the OH is named as a hydroxyl substituent. If molecules containing a carboxylic acid and aldehydes and/or ketones functional groups the carbonyl is named as a " Oxo " substituent. If molecules containing a carboxylic acid an amine functional group the amine is named as an "amino" substituent.

18. Give the IUPAC names of following compounds Write the structure of following compounds Methyl salicylate 2. Succinic acdid 3. Phthalic acid 4. p- Toluic acid Benzoic anhydride 6. Methyl salicylate 7. Propionamide 8. Malonic acid 9. 2-methyl butanoic acid

19. Structure The C-O single bond of a carboxylic acid is shorter than the C- O single bond of an alcohol . In the alcohol, the carbon is sp 3 hybridized, whereas in the carboxylic acid the carbon is sp 2 hybridized. As a result, the higher percent s -character in the sp 2 hybrid orbital shortens the C-O bond in the carboxylic acid. Partial double bond character due to resonance effect . The carbonyl carbon in carboxylic acids and derivatives is sp 2 hybridized. The three atoms attached to the carbonyl carbon via sigma bond and are in the same plane, and their bond angles are each approximately 120°.

20. Physical properties At ordinary temperature, aliphatic carboxylic acids (upto nine carbon) are colorless liquids at room temperature with unpleasant smell . The higher acids are wax like solids and odorless due to their low volatility.. The melting points and boiling points of carboxylic acids are higher than those of hydrocarbons and oxygen-containing organic compounds because of strong intermolecular attractive forces. The carboxylic acid molecules are held together by strong H-bonding.

21. Simple aliphatic carboxylic acids, lower carboxylic acids having higher melting than higher member because of increasing alkyl porting, decrease the strength of hydrogen bonding.

22. Among the derivatives, amides have the highest boiling points , because they have strong dipole–dipole interactions between the resonance structures of the molecules having charge separation. Amide > Carboxylic acid > Nitrile >> Ester ~ Acyl chloride ~ Aldehyde ~ Ketone

23. Solubility Simple aliphatic carboxylic acids having upto four carbon atoms miscible in water due intermolecular hydrogen bonding with water. The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are insoluble in water due to the increased hydrophobic interaction of hydrocarbon part of acid with water . Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform etc.

24. Solubility – Some examples of carboxylic acids

25. Acidity of carboxylic acids Carboxylic acids are the most acidic class of hydrocarbons containing C,H, and O. The p K a value of the acid-base pair in the equilibrium reaction was used to explain the strength of acid. Lesser the value of p K a < 1 value, stronger the acid; moderately strong acids p K a value ranges from 1 to 5 ; for weak acids p K a values are from 5 to 15 ; for extremely weak acids p K a > 15 .

26. Carboxylic acids are weaker than mineral acids but stronger than alcohols and phenols. Structure K a pK a HCl 10 7 -7 0.23 0.64 3.3 x 10 -2 1.48 1.4 x 10 -3 2.85 1.77 x 10 -4 3.75 6.46 x 10 -5 4.19 5.6 x 10 -5 4.25 1.76 x 10 -5 4.75 10 -16 16 Ionization energy = 91 kJ/mol Ionization energy = 27 kJ/mol

27. Effects of substituent's on acid strength of aliphatic and aromatic carboxylic acids Inductive effect of substituent Resonance effect of substituent

28. Inductive effect of substituent Resonance effect of substituent The carbonyl group of acetate ion is electron-withdrawing, and by attracting electrons away from the negatively charged oxygen, acetate anion is stabilized. Electron delocalization, expressed by resonance between the following Lewis structures, causes the negative charge in acetate to be shared equally by both oxygens .

29. Effects of substituent's on acid strength of aliphatic and aromatic carboxylic acids Inductive effect of substituent The electron donating groups such as alkyl group has slightly decreases the acidic character of carboxylic acid. An electronegative substituent, particularly if it is attached to the  -carbon, increases the acidity of a carboxylic acid. All the monohaloacetic acids are about 100 times more acidic than acetic acid. The  electrons in the carbon–chlorine bond of chloroacetate ion are drawn toward chlorine, leaving the  -carbon atom with a slight positive charge.

30. Effect of the halogen on the strength of acetic acid is shown below. Fluoroacetic acid is more acidic than iodoacetic acid. Inductive effects fall off rapidly as the number of  bonds between the carboxyl group and the substituent increases. Structure K a pK a 0.23 0.64 3.3 x 10 -2 1.48 1.4 x 10 -3 2.85 1.76 x 10 -5 4.75

31. Effects of substituent's on acid strength of aliphatic and aromatic carboxylic acids Resonance effect of substituent Substituents on a benzene ring either donate or withdraw electron density, depending on the balance of their inductive and resonance effects. An electron-donor group (+R/+I; +R is more dominant than –I) destabilizes a conjugate base by donating electron density onto a negatively charged carboxylate anion.

32. An electron-withdrawing group (-R/-I; -R is more dominant than +I) stabilizes a conjugate base by removing electron density from the negatively charged carboxylate anion.

33. Match each of the following p K a values (3.2, 4.9, and 0.2) to the appropriate carboxylic acid: (a) CH 3 CH 2 COOH; (b) CF 3 COOH; (c) ICH 2 COOH. Explain why HCOOH (formic acid) has a lower p K a than acetic acid (3.8 versus 4.8). Which would be you expect to be the strongest acid – benzoic acid or p- nitrobenzoic acid. Rank the compounds in each group in order of increasing acidity. a. CH 3 COOH, HSCH 2 COOH, HOCH 2 COOH b. ICH 2 COOH, I 2 CHCOOH, ICH 2 CH 2 COOH.

34. Rank the compounds in each group in order of increasing acidity and explain.

35. Preparation of carboxylic acids Oxidation of Alkyl benzenes alkyl group at benzylic position (activated by benzene) is oidized to –COOH by using oxidizing agent such as chromic acid (H + , Na 2 Cr 2 O 7 ) , HNO 3 , alkaline or aqueous KMnO 4 , heating in air in presence of catalyst such as V 2 O 5 . Regardless of the length of the alkyl substituent, it will be oxidized to a COOH group, provided that hydrogen is bonded to the benzylic carbon i.e. CH or CH 2 or CH 3 group attach to aromatic ring . tert-benzylic carbon is not susceptible to the oxidation under these conditions.

36. Oxidation of Alkyl benzenes

37. Oxidation of alcohols Strong oxidizing agents - Chromic acid, CrO 3 or Na 2 Cr 2 O 7 , HNO 3 , alkaline or aqueous KMnO 4 , etc oxidizes alcohol to acid. Mild oxidizing agents such as MnO 2 oxidize alcohol to aldehyde or ketone. Carbonyl compound formed is an aldehyde, a ketone, or a carboxylic acid depends on the alcohol and on the oxidizing agent.

38. Oxidation of Alkyl benzenes - V 2 O 5 catalyst Preparation of phthalic acid Oxidation by using KMnO 4

39. Oxidation of alcohols Chromic acid (H 2 CrO 4 ) oxidation – primary and secondary alcohol sodium dichromate in presence of sulfuric acid (chromic acid) The carbon bearing the OH group in a tertiary alcohol is not bonded to hydrogen, so the OH group cannot be oxidized to a carbonyl group chromate ester

40. Acidic or basic aqueous potassium permanganate - Oxidation Aldehyde is oxidized to carboxylic acid with Tollens reagent (AgNO 3 in NH 4 OH).

41. Hydrolysis of nitriles or cyanides Nitriles are slowly hydrolysis than amides. It slowly hydrolyzed to carboxylic acids when heated with water and an acid. Protonation make the carbon of the cyano group more electrophilic

42. Hydrolysis of nitriles or cyanides The nitrile is important intermediate to convert alkyl halide into carboxylic acid which has one more carbon than the alkyl halide. Synthesis of propanoic acid from ethyl bromide Synthesis of benzoic acid from chlorobenzene

43. Carbonation of Grignard Grignard reagents treated with carbon dioxide ; addition reaction takes place to yield magnesium salts of carboxylic acids which on acidification converts these magnesium salts to the desired carboxylic acids.

44. Reactions of carboxylic acids The polar C-O and O-H bonds, nonbonded electron pairs on oxygen, and the π bond give a carboxylic acid many reactive sites. Carboxylic acids are strong organic acids so that they are showing acid–base reactions, any nucleophile that is also a strong base will react with a carboxylic acid by removing a proton first, before any nucleophilic substitution reaction can take place.

45. Carboxylic acids can be converted to a variety of other acyl derivatives using special reagents, with acid catalysis, or sometimes, by using rather forcing reaction conditions. These nucleophilic substitution reactions take place in two steps - formation of a tetrahedral intermediate and collapse of the tetrahedral intermediate. The weaker the base attached to the acyl group , the easier it is for both steps of the reaction to take place i.e. higher the rate of reaction. Reactions of carboxylic acids

46. Salt formation Reaction of polar O– H bond carboxylic acid Neutralization of carboxylic acids

47. Decarboxylation Carboylic acid or Carboxylate ions or carboxylate salt do not undergoes decarboxylation (loss of CO 2 ) on heating, because of least stabilization of intermediate carbanion or shifting of H from O to C. If, the –COO - or –COOH group is bonded to a carbon that is adjacent to a carbonyl carbon, CN, C=N, NO 2 , etc the CO 2 group can be removed because the electrons left behind on carbon (carbanion) can be delocalized onto the carbonyl oxygen forming enolate ion or enol like intermediates. The loss of carbon dioxide is called decarboxylation . Carboylic acid or Carboxylate ions or carboxylate heated with soda lime goes decarboxylation at higher temperature.

48. Decarboxylation Examples: Under acidic conditions:

49. Reduction of carboxylic acids with LiAlH 4 Sodium borohydride (NaBH 4 ) is not a sufficiently strong hydride donor to react with the less reactive (compared with aldehydes and ketones) esters, carboxylic acids, and amides, so esters, carboxylic acids, and amides must be reduced with lithium aluminum hydride (LiAlH 4 ) a more reactive hydride donor. primary alcohol The chemoselectivity of these two most commonly used reducing agents is listed -

50. electrophile

51. Reduction of carboxylic acids with Borane The best reducing reagent for reduction of carboxylic acid to alcohol is borane, BH 3 . Solvents used are - ether (Et 2 O), THF, or dimethyl sulfide (DMS, Me 2 S). Esters are usually less electrophilic because of conjugation between the carbonyl group and the lone pair of the sp 3 hybridized oxygen atom - but, in triacylborates, the oxygen next to the boron has to share its lone pair between the carbonyl group and the boron’s empty p orbital, so they are considerably more reactive than normal esters, or the lithium carboxylates formed from carboxylic acids and LiAlH 4 .

52. Borane is a highly chemoselective reagent for the reduction of carboxylic acids in the presence of other reducible functional groups such as esters, and even ketones. borane’s reactivity ( Lewis acid ) is dominated by its desire to accept an electron pair into its empty p orbital. In the context of carbonyl group reductions, this means that it reduces electron-rich carbonyl groups fastest - acyl chlorides and reduces esters only slowly, but it will reduce amides.

53. Hell- Volhard - Zelinsky reaction (C  -bromination) A carboxylic acid is treated with PBr 3 and Br 2 then the  -carbon can be brominated . (Red phosphorus can be used in place of PBr 3 , since P and excess Br 2 react to form PBr 3 ). This halogenation reaction is called the Hell– Volhard – Zelinski reaction or, more simply, the HVZ reaction . example

54. Formation of carbonyl compounds (by using calcium carboxylate ) When carboxylic acids are heated with MnO , carbonyl compounds are formed When calcium salt of fatty acids are dry distilled, carbonyl compounds are formed

55. Hunsdiecker reaction When silver salt of carboxylic acids are treated with halogen such as bromine in suitable solvent forming alkyl halide (alkyl bromide) with loss of one carbon as carbon dioxide How will you prepared – a. Butanoic acid by oxidation of alcohol. b. Benzoic acid from aromatic alcohol. c. p- tuloic acid by using Grignard reagent. d. Phenyl acetic acid by hydrolysis of suitable nitrile .

56. Complete the following reactions – Acetic acid + Ca(OH) 2 Butanoic acid + Br2 (in red P) Benzoic acid + BH3 Butyl bromide + Mg followed by carbonylation How benzoic acid prepared by using – Aromatic alcohol A nitrile Grignard Reagent Alkyl benzene

57. Conversion of carboxylic acid to their derivatives Synthesis of Acid halides A carboxylic acid can be converted into an acyl chloride by heating it either with SOCl 2 , PCl 3 , PBr 3 , POCl 3 , PCl 5 , COCl 2 , C 2 O 2 Cl 2 , etc in presence of base such as Et 3 N, pyridine, etc . All these reagents convert the OH group of a carboxylic acid into a better leaving group than the halide ion.

58. Conversion of carboxylic acid to their derivatives Synthesis of anhydride Dicarboxylic acids readily lose water when heated or by treating dehydrating agent such as P 2 O 5 form a cyclic anhydride with a five- or a six- membered ring. Anhydride was synthesized by treating acid with dehydrating agent or acid chloride with carboxylic acid or its salt . Cyclic anhydrides are more easily prepared if the dicarboxylic acid or ester is heated in the presence of acetyl chloride or acetic anhydride.

59. Conversion of carboxylic acid to their derivatives Synthesis of ester Carboxylic acids have approximately the same reactivity as esters because the HO - of a carboxylic acid and RO - as leaving group of an ester has approximately same basicity . Carboxylic acids react reversibly with alcohols to form esters therefore reaction should be catalyzed. Ester could be prepared by treating a carboxylic acid with excess alcohol in the presence of an acid catalyst, so the reaction is called a Fischer esterification .

60. Conversion of carboxylic acid to their derivatives Synthesis of Amide Carboxylic acids do not undergo nucleophilic acyl substitution reactions with amines. Because a carboxylic acid has a lower pK a than a protonated amine, the carboxylic acid immediately donates a proton to the amine when the two compounds are mixed forming salt. But at higher temperature, ammonium salts undergoes dehydration forming amide.

61. Synthesis of phthalimide from phthalic acid A carboxylic acid and an amine readily react to form an amide in the presence of an dehydrating agent such as, dicyclohexylcarbodiimide (DCC) .

62. Exercise

63. Mechanism of nucleophilic acyl substitution Attack of the nucleophile takes place on carbonyl carbon atom forming tetrahedral intermediate . The carboxylic acid their derivatives bearing group (OH, OR, Cl , OCOR, NR 2 ) which acts as leaving group therefore the intermediate further undergoes elimination reaction forming substitution product. Step I : Nucleophile attacks the carbonyl carbon of a carboxylic acid derivative forming intermediate is called a tetrahedral intermediate because the trigonal sp 2 -carbon in the reactant has become a tetrahedral sp 3 -carbon in the intermediate – Addition reaction . Step II : The tetrahedral intermediate is unstable because Y and Z are both electronegative atoms. A lone pair on the oxygen reforms the π-bond, and either Y - (k 2 ) or Z - (k -1 ) is expelled with its bonding electrons – elimination reaction .

64. A carboxylic acid derivative will undergo a nucleophilic acyl substitution reaction – if newly added nucleophile should stronger base than the group depart .

65. Acid-catalysed nucleophilic acyl substitution The nonbonded electron pairs on oxygen create electron-rich sites that can be protonated by strong acids (H – A). Example

66. The equilibrium is pushed towards the ester side by using an excess of alcohol or carboxylic acid (usually the reactions are done in a solution of the alcohol or the carboxylic acid).

67. Inter-conversion of acid derivatives by nucleophilic acyl substitution Acid chlorides example

68. Acid anhydrides Inter-conversion of acid derivatives by nucleophilic acyl substitution

69. Esters Inter-conversion of acid derivatives by nucleophilic acyl substitution transesterification reaction Both hydrolysis and alcoholysis of an ester can be catalyzed by acids or base (HO - or RO - ). A reaction with an amine that converts one compound into two compounds is called aminolysis .

70. Amides Inter-conversion of acid derivatives by nucleophilic acyl substitution Amides do, however, react with water and alcohols if the reaction mixture is heated in the presence of an acid. Amide with an NH 2 group can be dehydrated to a nitrile .

71. Which of the following reactions will not give the carbonyl product shown?

72. Mechanism of Claisen condensation When two molecules of an ester in presence of base undergo a condensation reaction forming β-keto ester , the reaction is called a Claisen condensation . Mechanism Enolate ion strong base Removing a proton from keto ester prevents the reverse reaction from occurring, because the negatively charged alkoxide ion will not react with the negatively charged β-keto ester anion. prevents the reverse reaction

73. In the Claisen condensation, the negatively charged oxygen reforms the carbon–oxygen π-bond while in Aldol addition, the negatively charged oxygen obtains a proton from the solvent.

74. A mixed Claisen condensation is a condensation reaction between two different esters. One ester does not have  -hydrogen otherwise it gives mixture of products – two self condensed and two cross condensed. Example:

75. Dieckmann condensation Diester molecule undergoes intramolecular condensation reaction in presence of base forming five- or six- membered ring cycloketone containing ester group at C 2 -position. An intramolecular Claisen condensation is called a Dieckmann condensation . A six- membered ring β-keto ester is formed from a Dieckmann condensation of a 1,7-diester.

76. Salfonic Acids Nomenclature: Name of hydrocarbon followed by sulfonic acid Acidity of arene sulfonic acid A sulfonic acid is a strong acid ( pKa ~ -1) because its conjugate base is particularly stable due to delocalization of its negative charge over three oxygen atoms.

77. Acidity Explain - Sulfonic acids are more acidic than carboxylic acids. Resonance stabilization Solvation of conjugated base.

78. Mechanism

79. An advantage of the sulfonation reaction is its reversibility . Simply heating benzenesulfonic acid with an aqueous acid removes the sulfonic acid group.

80. Sulfonation of toluene Direct sulfonation of toluene with concentrated sulfuric acid gives a mixture of ortho and para sulfonic acids from which about 40% of toluene para sulfonic acid can be isolated as the sodium salt. SO 3 as the also used as electrophile and draw the intermediate with the charge at the ipso carbon to show the stabilization from the methyl group.

81. Sulfonation of naphthalene Sulfonation of naphthalene does not always lead to substitution at the 1-position; may occur at 2-position. If the reaction is carried out under conditions which cause it to be ir reversible (80 C) , substitution occurs at the 1-position (90% yield; kinetically control ). the reaction is carried out under conditions which cause it to be readily reversible (160 C) , substitution occurs predominantly at the 2-position (90% yield, thermodynamically control).

82. Sulfonation of naphthalene The 1-substituted product is easier to form because the carbocation leading to its formation is more stable. The 2-substituted product is more stable because there is more room for the bulky sulfonic acid group at the 2-position. A sulfonic acid group on C 1 comes within the van der Waals radius of the hydrogen at C 8 .

83. Substitution Reactions:

84. Orientation of monosubstituted benzene Ortho/Para-Directing Groups Meta-Directing Groups Groups shows electron donating effect as +I and +R effect. +R effect is strong than +I. Groups shows electron withdrawing effect as -I and -R effect.

85. Acidity of Arene Sulfonic acids Sulfonic acid donates proton in its aqueous solution. The acidic proton of the sulfonic acid reacts with base forming salt and water The negative charged sulfonate stabilized by three resonance structures (three oxygen atoms) The negative charged carboxylate stabilized by two resonance structures (two oxygen atoms)

86. Sulfonic Acids Reagents: NaOH , NaHCO 3 Heat to 100-175 C NH 3 ROH -SO 3 H is solubilising Agent The sulfonic acid group is more polar than any other group. It increases the solubility of drugs, dyes, detergent in water. -SO 3 H is blockinig Agent -It is easily added by sulfonation and removed by desulfonation on heating. It is very useful to block the reacting position. E.g. Chlorination of toluene

87. IPSO substitution In addition to o-, m- and p-attack on substituted benzene C 6 H 5 X, the electrophilic substitution at the carbon atom bearing substituent X is occurred. The C 6 H 5 X undergoes electrophilic substitution reactions with electrophile E + forming an intermediate reversible as shown below which would undergoes the displacement of X + . Such overall reaction is called as ipso substitution Ipso-substitution usually occurs with reversible electrophilic substitution reaction such as sulfonation .