CARBOXYLIC ACID AND ITS DERIVATIVES

Structure RCOOH or RCO2H (R ≡ alkyl, aryl or H)

NOMENCLATURE

IUPAC Nomenclature & Common Name HCOOH Methanoic acid Formic acid CH3COOH Ethanoic acid Acetic acid CH3CH2COOH Propanoic acid Propionic acid CH3CH2CH2COOH Butanoic acid Butyric acid CH3CH2CH2CH2COOH Pentanoic acid Valeric acid

IUPAC Nomenclature The longest chain must contain the carboxyl group. The carboxyl group is at the terminal, therefore the carbon of the carboxyl group is not numbered. One COOH – carboxyl group is at one end Two COOH – carboxyl groups are at both ends Name the compound as alkane, drop ‘e’ in alkane and add ‘oic acid’ (eg: methanoic acid)

4-bromo-3-methylpentanoic acid 2 4-bromo-3-methylpentanoic acid C H 2 O 3 5-hydroxyhexanoic acid 5-methyl-3-hexenoic acid

Two COOH groups, the compound will be named as alkanedioic acid’ (Example: ethanedioic acid, propanedioic acid and etc) pentanedioic acid

3-methylhexanedioic acid trans 3-hexenedioic acid

When R is an aryl group, the parent name is benzoic acid 4-chlorobenzoic acid

An aromatic dicarboxylic acid is named as 1,x-benzenedicarboxylic acid 2-isopropyl-1,4-benzenedicarboxylic acid

A cyclic carboxylic acid is named as cycloalkanecarboxylic acid The C atom which is attached to —COOH is numbered as C1 1 cyclopentanecarboxylic acid

cyclohexanecarboxylic acid 1 4-bromo-2-methylcyclohexanecarboxylic acid 1

A cyclic dicarboxylic acid is named as 1,x-cycloalkanedicarboxylic acid 1,2-cyclohexanedicarboxylic acid 4-chloro-1,2-cyclohexanedicarboxylic acid

When a compound contains a carboxyl group and other functional group(s), the priority is given to the carboxylic acid as the parent name. 3-methyl-2-cyclohexenecarboxylic acid

PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS

Boiling Point The boiling point of carboxylic acid is higher than an alcohol, a ketone or an aldehyde (with Mr that almost the same) because: it exists as stable dimers that form hydrogen bond. molecules in dimers are arranged closely packed, therefore the hydrogen bonds are relatively strong. high energy is needed to overcome the intermolecular forces ,  boiling point 

Hydrogen bond Hydrogen bond

Solubility a) Solubility in water Carboxylic acids are soluble in water due to the formation of hydrogen bond between the water molecules and carboxylic acid molecules. Hydrogen Bonds

The solubility of carboxylic acid in water is almost the same as alcohol. Aliphatic carboxylic acids with C > 5 are insoluble in water. Size of R ↑, hydrophobic area ↑. hydrophilic hydrophobic

Aromatic carboxylic acids are slightly soluble in water due to the huge aromatic ring. Dicarboxylic acids are relatively more soluble since more hydrogen bonds are formed.

Example : Descending order of solubility > > >

b. Solubility in non polar solvent Carboxylic acids are soluble in non polar solvent such as benzene due to the Van der Waals forces between the benzene and alkyl group of carboxylic acids . Van der Waals forces Van der Waals forces Hydrogen bonds

Acidity of Carboxylic Acid The acidity of carboxylic acid is influenced by: Resonance effect Inductive effect

Resonance Effect Carboxylate ion : Phenoxide ion :

Carboxylic acids are more acidic due to the resonance stabilisation of the carboxylate ion. The electrons in carboxylate ion are delocalised between two oxygen atoms, whereas in phenoxide ion, the electrons are delocalized in the benzene ring. The C=O group in carboxylic acid is a electron-withdrawing group which reduce the electron density of –OH, therefore the –OH bond becomes weaker. Thus H+ is easily donated and carboxylic acid is more acidic than phenol.

Carboxylic acid is relatively a weak acid, however it is stronger than phenol & alcohol

(resonance structure) (resonance structure) + H2O + H3O+ carboxylate ion (resonance structure) ⇌ carboxylic acid + H3O+ ⇌ phenoxide ion (resonance structure) + H2O phenol R—O—H + H2O ⇌ R—O– + H3O+ alkoxide ion alcohol

Inductive Effect An electron withdrawing group (deactivating group) that attached to a carboxylate ion will delocalise the negative charge, thereby stabilises the carboxylate ion and increases acidity. An electron donating group, (activating group) will destabilise the carboxylate ion and decreases acidity.

The inductive effect electron-withdrawing group in the compound electron-withdrawing groups (e.g –NO2 ,-F,-Cl,-Br, -I ) reduce the electron density of –O H. Thus the O-H bond becomes weaker and H+ can be easily released. The compound is said to be more acidic  Electron- withdrawing group increases the acidity.

Example: CH3CHCl-COOH and CH3CH2COOH Cl is an electron-withdrawing groups, therefore reduce the electron density of –OH. Thus the O-H bond becomes weaker and H+ can be easily released. Acidity : CH3CHCl-COOH > CH3CH2COOH Electron-withdrawing groups increase the acidity.

ii) The electronegativity of electron-withdrawing group in the compound Example: CH3CHF-COOH and CH3CHCl-COOH Both F and Cl are electron-attracting group. The electronegativity of F > Cl The electron density of –OH in CH3CHF-COOH is less, thus the –OH bond is weaker than in CH3CHCl-COOH. Therefore, H+ is easily donated. Acidity : CH3CHF-COOH > CH3CHCl-COOH

iii) Number of electron-attracting group in the compound. Example: CH3C(Cl)2-COOH and CH3CHCl-COOH CH3C(Cl)2-COOH contains 2 Cl atoms that make the bond of –OH weaker than CH3CHCl-COOH (with only one Cl atom). Thus, H+ is easily donated. Acidity : CH3C(Cl)2-COOH > CH3CHCl-COOH

iv) The position of electron-attracting group Example: CH3CH2CH(Cl)COOH and CH2(Cl)CH2CH2COOH The distance between Cl atom and carboxyl group in CH3CH2CHCl-COOH is nearer compare to in CH2ClCH2CH2-COOH. The –OH bond in CH3CH2CH(Cl)COOH is weaker than in CH2ClCH2CH2-COOH, so H+ is easily donated. Acidity : CH3CH2CH(Cl)COOH > CH2(Cl)CH2CH2COOH

(v ) The inductive effect of electron- releasing (electron-donating) group in the compound Example: CH3COOH and CH3CH2COOH -R is an electron –releasing group. The size of –R group in CH3CH2COOH is larger than in CH3COOH, so CH3CH2- is a stronger releasing group than CH3-. The electron density of –OH in CH3CH2COOH increases and H+ is difficult to be donated.  Electron-releasing groups reduce the acidity of a carboxylic acid.

SYNTHESIS OF CARBOXYLIC ACIDS

1. Oxidation of primary alcohol and aldehyde oxidizing agent oxidizing agent 1o alcohol aldehyde carboxylic acid Common oxidizing agents are : KMnO4 / H2SO4 potassium permanganate K2Cr2O7 @ Na2Cr2O7 /H2SO4 potassium /sodium dichromat (VI)

2. Oxidation of Alkyl Benzene oxidizing agent KMnO4 , H+ Δ + CO2 + H2O

3. Formation and Hydrolysis of nitrile NaCN H2O,H+ H2O,H+ NaCN

4. Carbonation of Grignard Reagents R—MgX R—COOH + Mg(OH)X H2O, H+ CO2 H2O, H+ + Mg(OH)Br

CHEMICAL PROPERTIES OF CARBOXYLIC ACIDS

Main reactions of carboxylic acid, The reaction that involves the donation of H+ from –OH group. The reaction that involves the substitution of OH group The reaction that involves the reduction with LiAlH4 to primary alcohol

The reaction that involves the donation of H+ from –OH group 1. Neutralisation Carboxylic acids are acidic, it can react with base such as NaOH (aq) to give metal carboxylate salts, + NaOH + H2O Na+

– Na+ + NaOH + H2O Sodium benzoate

2. Reaction with electropositive metals such as Na, K, Ca, Mg and Fe. Exercise:

The reaction that involves the substitution of –OH group (to form its derivatives) 1. Acid chloride formation Acid chloride can be prepared from the reaction of carboxylic acids with thionyl chloride, SOCl2 ; phosphorous pentachloride, PCl5 ; phosphorous trichloride, PCl3 SOCl2 + SO2 + HCl PCl5 + POCl3 + HCl PCl3 + H3PO3

Exercise : SOCl2 PCl5 PCl3

2. Esterification Carboxylic acids react with alcohols in the presence of mineral acid catalyst to produce esters. H+ ⇌ + H—OR’ + H2O carboxylic acid alcohol ester H+ + HOCH2CH3 ⇌ propanoic acid ethanol ethyl propanoate + H2O

3. Acid anhydride formation Acid anhydrides can be prepared from carboxylic acids by the loss of water through heating. + heat + H2O acid anhydride heat + ethanoic anhydride + H2O

Reaction of carboxylic acids with an ammonia or amine give amide. 4. Amides formation Reaction of carboxylic acids with an ammonia or amine give amide. NH3 + H2O 1o amide RNH2 + H2O (1o amine) 2o amide R2NH + H2O (2o amine) (3o amide)

Exercise : NH3

The reaction that involves the reduction with LiAlH4 to primary alcohol Carboxylic acid are reduced to primary alcohols by reaction with lithium aluminium hydride, LiAlH4 . LiAlH4 ether + R’OH 1o alcohol LiAlH4 ether + HO—CH2CH3

Methanoic acid, HCOOH as a reducing agent Methanoic acid molecule, has both and carbonyl carboxylic It shows the properties of both carboxylic acid and aldehyde. It also shows reducing properties in reactions with acidified KMnO4 or K2Cr2O7 and Tollens’ reagent.

KMnO4 / H+ CO2 + H2O + MnO2 (Brown) Ag(NH3)2+ Ag + CO2 + H2O

DERIVATIVES OF CARBOXYLIC ACIDS

Reactions of carboxylic acid derivatives i. Hydrolysis of acid chlorides H2O + HCl acid chloride carboxylic acid ii. Hydrolysis of acid anhydrides H2O 2 acid anhydride carboxylic acid

Reactions of carboxylic acid derivatives iii. Hydrolysis of esters H2O H+ carboxylic acid + ROH NaOH alcohol Na+ ester

Example : H2O benzoyl chloride H2O ethanoic anhydride

Example : H2O H+ methyl ethanoate

Relative Reactivity Of Carboxylic Acid Derivatives The reactivity of a carboxylic acid derivative depends on the basicity of the substituent (leaving group) that attached to the acyl group The less basic the substituent, the more reactive the carboxylic acid derivative.

Relative basicities of the leaving group (substituent) Cl– < RCOO– < RO– < HO– < NH2– , acid chloride acid anhydride ester carboxylic acid amide reactivity increases

ACYL CHLORIDE Acyl chloride is the most reactive because of its electropositive carbonyl group is attach to the electronegative Cl atom (which is a releasing group).

ANHYDRIDE ACID Anhydride acid is more reactive than ester and amide because the carboxyl group of anhydride is attached to the carbonyl carbon. This makes the carbonyl carbon becomes more electropositive and can be easily attack by nucleophile.

ESTER Ester is less reactive towards nucleophile because the delocalization of electron makes the positive charge of carbon can be shifted to oxygen. That makes the carbonyl carbon less electropositive.

AMIDE Amide is the least reactive because, NH2 group is an electron-donating group that makes the carbonyl less electropositive. The resonance structure of amide shows that the carbonyl carbon is not electropositive.

The Uses of Carboxylic Acid Carboxylic acid / derivatives Uses Polyamide (Nylon) carpet, apparel Ester Artificial flavors Acetic acid Vinegar Ethanoic anhydride Drug aspirin Salicylic acid analgesic