VIEWS: 206 PAGES: 39 POSTED ON: 5/8/2011
Carboxylic Acid A. Preparation 1. Oxidation of primanry alcohol H O O MnO4 / H+ MnO4 / H+ CH3 C OH C C CH3 H CH3 OH H A. Preparation 2. From acid nitrile H+ R C N + H2O RCOOH A. Preparation 3. For aromatic acid CH3 MnO4 / H+ COOH CH2CH3 COOH MnO4 / H+ B. Physical properties 1. The boiling point and melting point are higher than corresponding ester due to the existence of H-bond between the molecules of acid. Lower members of aliphatic acids are liquid with pungent smell. 2. Lower members of aliphatic acids are soluble in water due to the formation of extensive H-bond with water molecules. B. Physical properties 3. Molecular mass of acids is found to be doubled when dissolved in benzene due to the formation of dimer via H-bond formation. O H O R C C R O H O C. Chemical Properties 1. Acid character RCOOH + H2O RCOO + H3O+ [RCOO ][ H 3O ] Ka = [RCOOH ] C. Chemical Properties 1. Acid character (cont’d) Acid Ka at 25oC / mol dm-3 Methanoic acid (HCOOH) 1.7 x 10-4 Ethanoic acid (CH3COOH) 1.7 x 10-5 Propanoic acid (CH3CH2COOH) 1.3 x 10-5 Benzenecarboxylic acid (Ph-COOH) 6.5 x 10-5 Phenol (Ph-OH) 1.3 x 10-10 Methanol (CH3OH) 1.0 x 10-16 C. Chemical Properties 1. Acid character (cont’d) Carboxylic acid is more acidic than alcohol and phenol due to the comparatively more stable carboxylate anion than alkoxide and phenoxide ion. The equilibrium position thus lie closer to the right hand side. O O R C R C O O C. Chemical Properties 1. Acid character (cont’d) The resulting resonance hybrid of the carboxylate anion is more stable than that of phenoxide ion due to the equal contribution of the two canonical forms. On passing up the homologous series, the members become less acidic since alkyl group is electron donating which makes the negative charge on the resulting carboxylate anion to be more intensified and thus less stable. C. Chemical Properties Acid Ka / 25oC H 1.4 x 10-3 Cl C COOH H H 5.1 x 10-2 Cl C COOH Cl Cl 2.2 x 10-1 Cl C COOH Cl C. Chemical Properties 1. Acid character (cont’d) Chlorine replacing hydrogen in alkyl group of the acid causes the acid to be more acidic since the chlorine has an negative inductive effect. This causes the negative charge in the carboxylate anion to be spread through the ion more effectively, i.e. more stable. C. Chemical Properties 2. Esterification O conc. H2SO4 O R C + HOR' R C + H2O OH reflux OR' Forward reaction is an acid-catalyzed esterification while the backward reaction is acid-catalyzed hydrolysis of ester. O + O H R C + H2O R C + HOR' OR' OH C. Chemical Properties 2. Esterification (cont’d) Saponification: In alkaline medium (hydrolysis of ester). (Not a equilibrium reaction) O O OH R C + H2O R C + HOR' OR' O C. Chemical Properties 3. Reduction H O 1. LiAlH4 / dry ether R C R C OH 2. water OH H LiAlH4 is a very strong reduction agent (especially in organic compound). N.B. NaBH4 cannot reduce carboxylic acid. C. Chemical Properties 4. Replacement of –OH by -X O O PCl5 R C OH R C Cl + POCl3 + HCl O O PCl3 R C OH R C Cl + H3PO3 O O SOCl2 R C OH R C Cl + SO2 + HCl C. Chemical Properties 5. Formation of amide O O NH3 R C OH R C O NH4+ (RCOONRH3) (RNH2) O + H2O R C NH2 O ( R C NRH ) C. Chemical Properties 6. Decarboxylation 1. NaOH RCOOH RH + CO2 2. NaOH / C. Chemical Properties 7. Chlorination red P CH3COOH + Cl2 CH2COOH + HCl o 100 C Cl C. Chemical Properties – Special reaction for methanoic acid 1. Oxidation O + [O] H2O + CO2 H C OH O + Ag2O + NH3 Ag + (NH4)2CO3 + H2O H C OH Tollen's reagent C. Chemical Properties – Special reaction for methanoic acid 2. Dehydration O conc. H2SO4 H2O + CO H C OH How can you test CO? 1. turn citrated blood cherry red. 2. burn with blue flame the gas turn lime water milky. C. Chemical Properties – Special reaction for methanoic acid 3. Decomposition by heating O H2 + CO2 H C OH (absent of air) C. Chemical Properties – Special reaction for methanoic acid 4. With neutral FeCl3 O H C OH + FeCl3 Fe(HCOO)3 red solution O CH3 C OH + FeCl3 (CH3COO)3Fe reddish brown ppt. Ethanedioic acid (Oxalic acid) A. Preparation 1. Oxidation of ethene O O MnO4 / OH excess [O] CH2=CH2 CH2-CH2 HO C C OH OH OH Ethanedioic acid (Oxalic acid) A. Preparation 2. Ozonolysis of ethyne O3 / CCl4 O O HC CH HO C C OH Zn dust / H+ Ethanedioic acid (Oxalic acid) B. Physical Properties 1. It is a colourless solid. 2. It is quite soluble in water. Ethanedioic acid (Oxalic acid) C. Chemistry Properties 1. As an acid 2. Formation of acid chloride 3. Formation of ester Ethanedioic acid (Oxalic acid) c. Chemistry Properties 4. With oxidising agent MnO4 / H+ (COOH)2 2 CO2 + H2O 60 oC - 70 oC It is used as a primary standard in volumetric analysis. Ethanedioic acid (Oxalic acid) c. Chemistry Properties 5. With conc. sulphuric acid conc. H2SO4 (COOH)2 CO + CO2 + H2O Butenedioic acid H H H COOH C C C C HOOC COOH HOOC H maleic acid fumaric acid Butenedioic acid A. Difference in physical porperties Maleic acid (cis-) Fumaric acid (trans-) Melting point 132oC 302oC Density 1.50 g / dm3 1.64 g / dm3 Solubility Soluble Sparingly soluble Acidity: Ka1 1 x 10-3 1 x 10-12 Ka2 6 x 10-7 4 x 10-5 Butenedioic acid A. Difference in physical porperties (cont’d) OH2 C4H4O4 C3H3O2COO + H3O+ [C3H3O2COO ][H3O ] K a1 [C4 H 4O4 ] OH2 C3H3O2COO OOCC2H2COO + H3O+ [ OOCC2 H 2COO ][H3O ] Ka2 [C3H3O2COO ] Butenedioic acid B. Reasons for the differences 1. Melting point Melting involves the breaking of intermolecular bond. The cis-isomer has intramolecular H-bond since the acid group are close together and this reduce the extent of formation of intermolecular H-bond. Butenedioic acid B. Reasons for the differences 1. Melting point (cont’d) H H C C HO C C O O HO Trans-isomer can form intermolecular H-bond only since the 2 acid groups are too far apart. Also, packing of the molecules in trans-isomer is better as it has a more symmetrical structure. Butenedioic acid B. Reasons for the differences 2. Density The intermolecular force in cis-isomer is weaker and so molecules are packed less closely together, leading to lower density than the trans-isomer. Butenedioic acid B. Reasons for the differences 3. Solubility Since intermolecular force in cis-isomer is weaker, it is more easily broken on adding to water and thus more soluble. In cis-isomer, there is a net dipole moment, it is thus more soluble in a polar solvent. Butenedioic acid B. Reasons for the differences 4. Acidity Ka1 for trans-isomer is smaller than that of cis-isomer. In cis-isomer, after the first proton is released, the anion exists in resonance which causes it to be more stable than that of trans- isomer. And thus the equilibrium position shift to the right hand side. Butenedioic acid B. Reasons for the differences 4. Acidity (cont’d) H H H H C C C C O C C O O C C O O H O O H O But on releasing the second proton, such extra stability may be lost and therefore Ka2 is smaller for cis-isomer. Butenedioic acid C. Difference in chemical properties H H o H H heated to 100 C C C C C O C C O C C O O OH H O O The ease of formation of acid anhydride indicates that maleic acid is the cis-isomer. Since the C=C bond cannot be rotated, the two acid groups in trans-isomer are not close enough to lose a molecule of water in the same way.
"Carboxylic Acid (PowerPoint)"