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

Functional group: carboxyl group, -COOH General formulae:

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

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. methanoic acid methane

Examples of carboxylic acids

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

CARBOXYLIC ACID DERIVATIVES Group replacing the –OH group of RCOOH Classes of compound General formula Example -X (halogen)Acyl halide/acid chlorides -OR’Ester -NH 2 Amide Acid anhydride

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

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

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.

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.

Reason: i)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+ δ-δ-δ+δ+

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. SOLUBILITY OF ALIPHATIC CARBOXYLIC ACIDS δ-δ- δ+δ+ δ-δ- δ+δ+ δ-δ- δ+δ+

Crystalline compound Melting points: 122 o C 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. PHYSICAL PROPERTIES OF BENZOIC ACIDS

1)The acidity of carboxylic acids compared with alcohols and phenols. -Carboxylic acids are acidic because they dissolve in water to give hydronium ions (H 3 O + ). RCOOH + H 2 O RCOO - + H 3 O + -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. ACIDITY OF CARBOXYLIC ACIDS

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. 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 δ+δ+δ-δ-

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+ δ-δ-

The carboxylate anion is a resonance hybrid of two resonance structures. Resonance effect of the carbonyl group 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 H 3 O + ions and the equilibrium RCOOH + H 2 ORCOO - + H 3 O + 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.

2) Effects of substituent groups on the acidity of carboxylic acids. i) 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 –NO 2 ) 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.

EFFECT OF ELECTRON-WITHDRAWING GRIUPS ON ACID STRENGTH FormulapK a CH 3 COOH4.74 I CH 2 COOH3.12 Br CH 2 COOH2.90 Cl CH 2 COOH2.86 F CH 2 COOH2.66 O 2 N CH 2 COOH1.67 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

ii) Number of halogen atoms and acid strength - the acid strength will increases when the number of halogen atoms increases. - trichloroethanoic acid (Cl 3 C-COOH) is more acidic than ethanoic acid and dichloroethanoic acid (Cl 2 CHCOOH). FormulapK a CH 3 COOH 4.74 Cl CH 2 COOH 2.86 CH 2 COOH 1.29 CH 2 COOH 0.65 Cl ACID STRENGTH INCREASES

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. FormulapK a CH 3 CH 2 CH COOH 2.84 CH 3 CH CH 2 COOH 4.06 CH 2 CH 2 CH 2 COOH 4.52 Cl ACID STRENGTH DECREASES

- 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 (CH 2 =CHCOOH) is a stronger acid than the corresponding saturated acid, CH 3 CH 2 COOH. FormulapK a

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. - 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) CH 3 CO-O- Increase in charge density

FormulapK a H-COOH 3.77 CH 3 COOH 4.74 CH 3 CH 2 COOH CH 3 ACID STRENGTH DECREASES CH 3 CH COOH CH 3 CO-O- Effect of electron-donating groups on acid strength

Trends in acidity of substituted benzoic 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 REACTIONS CARBOXYLIC ACIDS

SALT FORMATION 1)Neutralisation: - carboxylic acids undergo neutralisation reactions with bases to form carboxylate salts of carboxylic acids and water. - examples: CH 3 COOH (aq) + NaOH (aq) → CH 3 COONa (aq) + H 2 O (l) sodium ethanoate 2CH 3 COOH(aq) + CuO (s) → (CH 3 COO) 2 Cu(aq) + H 2 O (l) copper(II) ethanoate CH 3 CH 2 COOH + NaOH → CH 3 CH 2 COONa + H 2 O propanoic acid sodium propanoate * carboxylate salts are soluble in water

Carboxylic acids react with carbonates and hydrogen carbonates to form CO 2, water and salts of carboxylic acids. Examples: 2HCOOH (aq) + Na 2 CO 3 (aq) → 2HCOONa (aq) + CO 2 (g) + H 2 O (l) sodium methanoate CH 3 CH 2 COOH(aq) + NaHCO 3 (aq) → CH 3 CH 2 COONa (aq) + CO 2 (g)+ H 2 O(l) sodium propanoate  an aqueous solution of benzoic acid turns blue litmus paper to red.  Benzoic acids dissolves readily in alkalis to form salts (benzoates) and water.

Organic compounds Solubility of carboxylate salts from base NaOHNaHCO 3 Neutral organic compounds Insoluble PhenolSolubleInsoluble Carboxylic acidsSoluble Phenol is a weak acid compared to carboxylic acids. Phenol did not react with NaHCO 3 and only react with strong base such as NaOH to form salt. Reactions with NaHCO 3 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:

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. 2CH 3 COOH (aq) + Mg → (CH 3 COO) 2 Mg(aq) + H 2 (g) magnesium ethanoate

Reducing agents: LiAlH 4 in dry ether Carboxylic acids primary alcohols REDUCTION TO ALCOHOLS reduced

 Benzoic acid can be reduced to phenylmethanol by using LiAlH 4 in ether at low temperatures.  An alkoxide intermediate is formed first.  On adding water, hydrolysis of the intermediate yields the primary alcohols.  LiAlH 4 has no effect on the benzene ring or the double bond.  -COOH is reduced to –CH 2 OH but the C=C bonds remains unchanged. CH 3 CH 2 CH=CHCOOH CH 3 CH 2 CH=CHCH 2 OH 1) LiAlH 4 2) H 2 O

Carboxylic acids reacts with phosphorus (v) chloride or sulphur dichloride oxide (thionyl chloride) or phosphorus trichloride (PCl 3 ) at room temperature to form acyl chloride. In the case of benzoic acid, the reaction mixture is heated. FORMATION OF ACYL CHLORIDES / ACID CHLORIDE

Examples:

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. FORMATION OF ESTERS

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

Acid anhydride is also formed when a carboxylic acid is heated with phosphorus pentoxide (P 2 O 5 ) – dehydration reaction. The water is absorbed by P 2 O 5 to form H 3 PO 4.

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 - NH 4 + RCONH 2 + H 2 O FORMATION OF AMIDES Excess RCOOH Heat ( °C) 1° amide For example: CH 3 COOH + NH 3 → CH 3 COONH 4 CH 3 CONH 2 + H 2 O heat ammonium ethanoate ethanamide

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

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(CH 2 ) 4 COOH: - 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 (C 9 H 6 O 3 ) and its derivative, coumarinic acid (C 9 H 8 O 3 ): - anti-coagulants in medicine. Esters: - responsible for the smell and flavour of many fruits and flowers. Vinyl acetate: - formation of polyvinyl acetate (PVA) plastic. THE IMPORTANCE OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES