ORGANIC CHEMISTRY CHM 207

CHAPTER 8:CHAPTER 8:CARBOXYL COMPOUNDS CARBOXYL COMPOUNDS

(ALIPHATIC AND (ALIPHATIC AND AROMATICS)AROMATICS)

NOR AKMALAZURA JANI

• Functional group: carboxyl group, -COOH

C

O

OH

C

O

OHR

C

O

OHAr

R = alkyl group

Ar = aryl group

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-1.

3 2 1

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

C

O

OHCHCH3

2-bromopropanoic acid

1

23C

O

OHCHCHCH3

CH3CH3Br

1234

2,3-dimethylbutanoic acid

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

-NH2 Amide

Acid anhydride

O C

O

R'

R C

O

X

R C

O

OR'

R C

O

NH2

R C

O

O C R'

O

H3C C

O

NH2

ethanamide

H3C C

O

OC2H5

ethyl ethanoate

H3C C

O

CIethanoyl chloride

H3C C

O

O C CH3

O

ethanoic anhydride

NOMENCLATURE OF AROMATIC CARBOXYLIC ACIDS

• General formula for the aromatic carboxylic acids:ArCOOH, where Ar is aryl group (aromatic group).

• Examples:

C OH

O

COOHOH

COOH

NO2benzoic acid 2-hydroxybenzoic acid 3-nitrobenzoic acid

12

3

45

6

• Carboxylic acids containing two carboxyl groups are called dicarboxyl acids.

• Their systematic names have the suffix ‘dioic’.

COOH

COOH

COOH

CH2

COOH

COOH

(CH2)4

COOHethanedioic acid(oxalic acid)

propanedioic acid(malonic acid)

hexanedioic acid(adipic acid)

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 BOILING POINTS OF ALIPHATIC CARBOXYLIC ACIDSCARBOXYLIC 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.

CH3 C

O

OH CH3 C

O

CH3CH3CH2CH2-OH

acetic acid

(RMM 60)

bp 118oC

propanone

(RMM 58)

bp 56oC

1-propanol

(RMM 60)

bp 97oC

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+

• 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.

OCR

OH

OH

H

O

H

H

H

O H

hydrogen bonds

SOLUBILITY OF ALIPHATIC CARBOXYLIC SOLUBILITY OF ALIPHATIC CARBOXYLIC ACIDSACIDS

δ-

δ+

δ- δ+δ-

δ+

• 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.

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 (H3O+).

RCOOH + H2O RCOO- + H3O+

- Carboxylic acids are stronger acids than alcohols and phenols.- Reasons:

i) the negative inductive effect of the carbonyl groupii) 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.

C Cl

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-

• 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.

CCH3

O

O

H

δ+

δ-

• The carboxylate anion is a resonance hybrid of two resonance structures.

CR

O

O

CR

O

O

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 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.

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 –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.

EFFECT OF ELECTRON-WITHDRAWING GRIUPS ON ACID STRENGTH

Formula pKa

CH3COOH 4.74

I CH2COOH 3.12

Br CH2COOH 2.90

Cl CH2COOH 2.86

F CH2COOH 2.66

O2N CH2COOH 1.67

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 (Cl3C-COOH) is more acidic than ethanoic acid and dichloroethanoic acid (Cl2CHCOOH).

Formula pKa

CH3COOH 4.74

Cl CH2COOH 2.86

CH2COOH 1.29

CH2COOH 0.65

Cl

Cl

Cl

Cl

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.

Formula pKa

CH3CH2CH COOH2.84

CH3CH CH2 COOH4.06

CH2CH2 CH2 COOH4.52

Cl

Cl

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 (CH2=CHCOOH) is a stronger acid than the corresponding saturated acid, CH3CH2COOH.

Formula pKa

4.74

4.19

4.87

4.26

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)

CH3

CH3

CH3

C O-

Increase in charge density

Formula pKa

H-COOH 3.77

CH3 COOH 4.74

CH3 CH2 COOH 4.88

4.85

5.07

CH3

ACID STRENGTH

DECREASES

CH3

CH COOH

CH3

CH3

CH3

C O-

Effect of electron-donating groups on acid strength

Trends in acidity of substituted benzoic acids

COOH

OCH3

COOHCOOH

NO24-methoxybenzoic acid pKa = 4.46

benzoic acid pKa = 4.19

4-nitrobenzoic acid pKa = 3.41

acid strength increases

* methoxy substituent (CH3O-) : electron-donating and decreases the acid strength* -NO2 : electron-withdrawing and increases the acid strength

• 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 FORMATION1) 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 + H2Opropanoic acid sodium propanoate

* carboxylate salts are soluble in 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

COOH NaOH COO-Na+ H2O

sodium benzoate

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 compoundsSolubility of carboxylate salts from base

NaOH NaHCO3

Neutral organic compounds

Insoluble Insoluble

Phenol Soluble Insoluble

Carboxylic acids Soluble Soluble

• 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:

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

• Reducing agents: LiAlH4 in dry ether• Carboxylic acids primary alcohols

REDUCTION TO ALCOHOLS

reduced

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

• 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.

R C OH

O

PCl5

R C OH

O

SOCl2

R C Cl

O

R C Cl

O

SO2

POCl3

HCl

HCl

carboxylic acids acid chlorides

carboxylic acids acid chlorides

FORMATION OF ACYL CHLORIDES / ACID CHLORIDE

Examples:

CH3 C OH

O

SOCl2

SOCl2C

O

OH

CH3 C Cl

O

C

O

Cl

SO2

SO2

HCl

HCl

ethanoic acid ethanoyl chloride

benzoic acid benzoyl chloride

• 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.

CH3 C OH

O

H OC2H5

C

O

OH H OC2H5

CH3 C OC2H5

O

C

O

OC2H5

H2O

H2O

ethanoic acid

benzoic acid

ethyl ethanoate

ethyl benzoate

FORMATION OF ESTERS

• Preparation of acid anhydrides:- reaction of sodium carboxylate with an acid chloride.

R C O- Na+

O

Cl C R'

O

CH3 C O- Na+

O

Cl C CH3

O

CH3 C O- Na+

O

Cl C

O

R C O

O

C

O

R'

CH3 C O

O

C

O

CH3

CH3 C O

O

C

O

NaCl

NaCl

NaCl

sodium carboxylate acid chloride acid anhydrides

examples

sodium ethanoate ethanoyl chloride acetic anhydride

sodium ethanoate benzoyl chloride acetic benzoic anhydride

FORMATION OF ACID ANHYDRIDES

• 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.

CH3 C OH

O

HO C CH3

OP2O5

CH3 C O

O

C

O

CH3H2O

two molecules of acetic acids acetic anhydride

• 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

FORMATION OF AMIDES

Excess RCOOH

Heat (100-200 °C)1° amide

For example:

CH3COOH + NH3 → CH3COONH4 CH3CONH2 + H2Oheat

ammonium ethanoate

ethanamide

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

R C OH

O

H N R'

H

heat (100-200 oC)H2O

1o amine 2o amideR C N

O H

R'

examples:

CH3 C OH

O

H N CH3

H

heat (100-200 oC)H2O

methylamine N-methylethanamide

CH3 C N

O H

CH3

R C OH

O

H N R'

R"

heat (100-200 oC)H2O

2o amine 3o amideR C N

O R"

R'

examples:

CH3 C OH

O

H N CH3

CH3

heat (100-200 oC)H2O

dimethylamine N,N-dimethylethanamide

CH3 C N

O CH3

CH3

• 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.

THE IMPORTANCE OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES