ORGANIC CHEMISTRY CHM 207

1. ORGANIC CHEMISTRY CHM 207 CHAPTER 8: CARBOXYL COMPOUNDS (ALIPHATIC AND AROMATICS) NOR AKMALAZURA JANI

2. Functional group: carboxyl group, -COOH • General formulae:

3. NOMENCLATURE OF CARBOXYLIC ACIDS • The carbonyl group (-C=O) is always at the beginning of a carbon chain. • The carbonyl carbon atom is always designated as C-1. 2 1 3

4. The IUPAC name of a carboxylic acid is derived from the name of the alkane corresponding to the longest carbon chain that contains the carboxyl group. The parent name is formed by changing the –e ending of the alkane to –oic acid. methane methanoic acid

5. Examples of carboxylic acids

6. acetic acid formic acid common name common name • Organic acids are usually known by common names. • These names usually refer to a natural source of the acid. ethanoic acid methanoic acid IUPAC name IUPAC name

8. CARBOXYLIC ACID DERIVATIVES

9. NOMENCLATURE OF AROMATIC CARBOXYLIC ACIDS • General formula for the aromatic carboxylic acids: ArCOOH, where Ar is aryl group (aromatic group). • Examples:

10. NOMENCLATURE OF ALIPHATIC DICARBOXYLIC ACIDS • Carboxylic acids containing two carboxyl groups are called dicarboxyl acids. • Their systematic names have the suffix ‘dioic’.

11. PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS • Methanoic acid has a pungent odour. • Ethanoic acid and propanoic acids have strong vinegar smell. • The higher members of the homologous series (acids with four to eight carbon atoms) have a very strong unplesent odour of rancid butter. • Butanoic acid is present in human sweat and in butter.

12. BOILING POINTS OF ALIPHATIC CARBOXYLIC ACIDS • Aliphatic acids with one to 10 carbon atoms are liquids. • The boiling points increase with increasing relative molecular mass. • Carboxylic acids have higher melting and boiling points than alkanes of similar relative molecular mass. - reason:carboxylic acid can form hydrogen bonds with one another in the solid and liquid states. • Boiling points of carboxylic acid is higher than alcohols, aldehyde or ketone with similar relative molecular mass.

13. Reason: • Carboxylic acids form stronger hydrogen bonds than that alcohols. The carbonyl, C=O group in a carboxylic acid is an electron withdrawing group. This causes the –OH group in a carboxylic acid to be more polarised than that the –OH group in an alcohol. • ii) Formation of dimers between two molecules of carboxylic acids to form a single molecule via hydrogen bonding. δ- δ+ δ- δ+

14. SOLUBILITY OF ALIPHATIC CARBOXYLIC ACIDS • Carboxylic acids of fairly low relative molecular mass (one to four carbon atoms) – completely miscible in water. - reason: the –COOH group is able to form hydrogen bonds with water molecules. • The solubility of carboxylic acids in water decreases as the relative molecular mass increases. For examples, propanoic acid is very soluble in water, butanoic acid and pentanoic acid are soluble in water, but hexanoic acid is only slightly soluble in water. • The solubility of carboxylic acids in non-polar solvents such as hexane increases as the carbon chain gets longer. δ+ δ- δ+ δ- δ- δ+

15. PHYSICAL PROPERTIES OF BENZOIC ACIDS • Crystalline compound • Melting points: 122 oC • Slightly soluble in water at room temperature but dissolve readily in hot water. • Soluble in benzene and other organic solvents. • In organic solvent, it exists as a dimer through hydrogen bonding.

16. ACIDITY OF CARBOXYLIC ACIDS • The acidity of carboxylic acids compared with alcohols and phenols. • Carboxylic acids are acidic because they dissolve in water to give hydronium ions (H3O+). RCOOH + H2O RCOO-+ H3O+ • Carboxylic acids are stronger acids than alcohols and phenols. • Reasons: i) the negative inductive effect of the carbonyl group ii) the resonance effect of the carbonyl group.

17. Strong negative inductive effect of the carbonyl oxygen • Inductive effect: the shift in electron density from one atom to another to form a polar bond. • Indicated by an arrow showing the direction of the shift of the electronic charge. δ+ δ- • The arrow in the representation of the inductive effect shows that a) the carbon atom repels electrons b) the chlorine atom attracts electrons because of its higher electronegativity

18. The oxygen atom in –C=O group is electronegative and acts as a powerful electron-withdrawing atom. • The withdrawal of electrons away from the carboxyl hydrogen atom weakens the O-H bond. The carbonyl group can lose a proton readily. This means that a carboxylic acid is much stronger acid than an alcohol. δ- δ+

19. Resonance effect of the carbonyl group • The carboxylate anion is a resonance hybrid of two resonance structures. • In the carboxylate anion, the negative charge is delocalised over two carbon-oxygen bonds. • The delocalisation or resonance stabilises the carboxylate anion. • The carboxylate anion has less tendency to accept H3O+ ions and the equilibrium RCOOH + H2O RCOO- + H3O+ tends to the right. • Delocalisation of electrons in the carboxylate anion promotes the release of a proton and makes the carboxylic acid a stronger acid than alcohols. • The carboxylate anion is delocalised to a far greater extent than the corresponding phenoxide ion. Carboxylic acid is a stronger acid than phenol.

20. 2) Effects of substituent groups on the acidity of carboxylic acids. electron-withdrawing groups increase acidity - any factor that stabilises the carboxylate anion relative to undissociated carboxylic acid will shift the equilibrium to the right and result in increased acidity. - any factor that destabilises the carboxylate anion relative to the undissociated acid will result in decreased acidity. - for example, an electron-withdrawing atom (such as halogen atom) or an electron-withdrawing group (such as –NO2) in the carboxylic acid molecule will withdraw electron density from carboxylate anion and delocalise the negative charge. - the carboxylate anion is stabilised and acidity increases.

21. EFFECT OF ELECTRON-WITHDRAWING GRIUPS ON ACID STRENGTH ACID STRENGTH INCREASES • Fluorine is more electronegative than chlorine and therefore has a stronger electron-withdrawing effect. • Fluoroethanoic acid is a stronger acid than chloroethanoic acid

22. ii) Number of halogen atoms and acid strength - the acid strength will increases when the number of halogen atoms increases. - trichloroethanoic acid (Cl3C-COOH) is more acidic than ethanoic acid and dichloroethanoic acid (Cl2CHCOOH). ACID STRENGTH INCREASES Cl Cl Cl Cl Cl

23. iii) Effect of position of halogen atom on acid strength - The magnitude of the inductive effect is dependent on its distance from the carboxyl group. - substituents on the α-carbon (the carbon atom next to the –COOH group) are the most effective in increasing acid strength. - the effect of a chlorine substituent decreases rapidly as the substituent moves further from the carboxyl group. - the inductive effect is negligible after the second carbon. Cl ACID STRENGTH DECREASES Cl Cl

24. - the aromatic nucleus (benzene ring) and multiple bonds are electron-withdrawing groups and possess negative inductive effects. - benzoic acid is a stronger acid than ethanoic acid and the unsaturated acid (CH2=CHCOOH) is a stronger acid than the corresponding saturated acid, CH3CH2COOH. 4.74 4.19 4.87 4.26

25. CH3 CH3 C O- CH3 iv) electron-donating groups decrease acidity - an electron-donating group destabilises the carboxylate anion by increasing the charge density of the oxygen atom in the C-O bond. Increase in charge density - The increase in charge density strengthens the –OH bond. - This makes proton loss more difficult. - Thus, the presence of electron-donating groups decreases the strength of an acid. - Example of electron-donating groups: alkyl and ethoxy (-OR)

26. CH3 CH3 C O- CH3 Effect of electron-donating groups on acid strength ACID STRENGTH DECREASES CH3 CH COOH CH3

27. Trends in acidity of substituted benzoic acids

28. REACTIONS CARBOXYLIC ACIDS • Salt formation - neutralisation - reactions with electropositive metals • Reduction to alcohols • Formation of Acyl Chlorides • Formation of Esters • Formation of Acid Anhydrides • Formation of Amides

29. SALT FORMATION 1) Neutralisation: - carboxylic acids undergo neutralisation reactions with bases to form carboxylate salts of carboxylic acids and water. - examples: CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l) sodium ethanoate 2CH3COOH(aq) + CuO (s) → (CH3COO)2Cu(aq) + H2O (l) copper(II) ethanoate CH3CH2COOH + NaOH → CH3CH2COONa + H2O propanoic acid sodium propanoate * carboxylate salts are soluble in water

30. an aqueous solution of benzoic acid turns blue litmus paper to red. • Benzoic acids dissolves readily in alkalis to form salts (benzoates) and water. • Carboxylic acids react with carbonates and hydrogen carbonates to form CO2, water and salts of carboxylic acids. • Examples: 2HCOOH (aq) + Na2CO3 (aq) → 2HCOONa (aq) + CO2 (g) + H2O (l) sodium methanoate CH3CH2COOH(aq) + NaHCO3(aq) → CH3CH2COONa (aq) + CO2(g)+ H2O(l) sodium propanoate

31. Phenol is a weak acid compared to carboxylic acids. Phenol did not react with NaHCO3 and only react with strong base such as NaOH to form salt. Reactions with NaHCO3 can be used to distinguish carboxylic acid with phenol and other organic compounds. Comparison of the solubility of organic compounds are listed in table below:

32. 2) Reaction with electropositive metals - reactive metals (i.e. metals that are very electropositive) react with carboxylic acids to form hydrogen gas and salts of carboxylic acids. - examples of metals: calcium, magnesium and iron. 2CH3COOH (aq) + Mg → (CH3COO)2Mg(aq) + H2 (g) magnesium ethanoate

33. REDUCTION TO ALCOHOLS • Reducing agents: LiAlH4 in dry ether • Carboxylic acids primary alcohols reduced

34. Benzoic acid can be reduced to phenylmethanol by using LiAlH4 in ether at low temperatures. • An alkoxide intermediate is formed first. • On adding water, hydrolysis of the intermediate yields the primary alcohols. • LiAlH4 has no effect on the benzene ring or the double bond. • -COOH is reduced to –CH2OH but the C=C bonds remains unchanged. CH3CH2CH=CHCOOH CH3CH2CH=CHCH2OH 1) LiAlH4 2) H2O

35. FORMATION OF ACYL CHLORIDES / ACID CHLORIDE • Carboxylic acids reacts with phosphorus (v) chloride or sulphur dichloride oxide (thionyl chloride) or phosphorus trichloride (PCl3) at room temperature to form acyl chloride. • In the case of benzoic acid, the reaction mixture is heated.

36. Examples:

37. FORMATION OF ESTERS • When a carboxylic acid is heated with an alcohol in the presence of a little concentrated sulphuric acid, an ester is formed. • Simple esters have fragrant odours. They are used as flavouring agents in the food industy.

38. FORMATION OF ACID ANHYDRIDES • Preparation of acid anhydrides: - reaction of sodium carboxylate with an acid chloride.

39. Acid anhydride is also formed when a carboxylic acid is heated with phosphorus pentoxide (P2O5) – dehydration reaction. • The water is absorbed by P2O5 to form H3PO4.

40. FORMATION OF AMIDES • Amides can be synthesised directly from carboxylic acids, but the yield is poor. • A better method of synthesising amides is by using acid chlorides. • When ammonium carboxylates are heated in the presence of the free acid, dehydration occurs to form the primary amide. • Ammonium carboxylates are obtained by the reaction of carboxylic acids with ammonia. RCOO-NH4+ RCONH2 + H2O Excess RCOOH Heat (100-200 °C) 1° amide • For example: CH3COOH + NH3→ CH3COONH4 CH3CONH2 + H2O heat ammonium ethanoate ethanamide

41. Secondary and tertiary amides can be synthesised by using primary amines and secondary amines respectively.

42. THE IMPORTANCE OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES • Methanoic acid and ethanoic acid: coagulate rubber latex. • Ethanoic acid: - used in the food industry as vinegar. - making cellulose ethanoates for producing artificial fibres. • Hexanedioic acid, HOOC(CH2)4COOH: - manufacture of nylon 6,6 • Benzoic acid and sodium benzoate: - as preservatives in foodstuff. • 2-hydroxybenzoic acid: - making aspirin • 1,4-benzenedicarboxylic acid: - making PET plastic • Coumarin (C9H6O3) and its derivative, coumarinic acid (C9H8O3): - anti-coagulants in medicine. • Esters: - responsible for the smell and flavour of many fruits and flowers. • Vinyl acetate: - formation of polyvinyl acetate (PVA) plastic.