Chapter 4

CARBOXYLIC ACID Norfazrin Mohd Hanif

Faculty of Applied Science

UiTM Negeri Sembilan

CHAPTER OUTLINE

General formula : RCOOH OR ArCOOH Nomenclature –IUPAC and common Physical properties of aldehyde : Boiling points and solubility Acidity Preparation

Oxidation of Primary Alcohols, Aldehydes, Alkenes And Alkyl Benzene

Carboxylation of Grignard reagent Hydrolysis of nitrile

Reactions Salt formation Reduction to alcohols Formation of Acyl Chlorides Formation of Esters Formation of Acid Anhydrides Formation of Amides

The functional group of a carboxylic acid is a carboxyl group, which can be represented in any one of three ways:

, R-COOH, R-CO2H R COH

O

Carboxylic acids named by replacing –e of the corresponding alkane name with –oic acid

–CO2H carbon atom is numbered C1

Compound containing multiple –COOH group, add di- or tri- to the suffix –oic acid

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)

Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids.

Aromatic acids are named as benzoic acids.

C OH

O

COOH

OH

COOH

NO2benzoic acid 2-hydroxybenzoic acid 3-nitrobenzoic acid

12

3

45

6

All common names of acid end in –ic acid. Positions of substituents on the chain are labeled

with Greek letters.

CH3CH2CHC

Cl

OH

O

-chlorobutyric acid

CH3CH2CH2CHCH2COOH

Ph

-phenylcaproic acid

Boiling point (BP)

Carboxylic Acids have higher BP than other organic molecules with comparable MW.

Higher boiling points than similar alcohols, due to dimer formation.

Acetic acid, b.p. 118C

In the presence of water, hydrogen bond are formed between water molecules & individual molecules of acid.

Carboxylic acid with up to 4 Carbon completely miscible in water at room temperature.

Water solubility decreases with the length of the carbon chain.

δ+ δ-…………

H O

H Hydrogen bond with water.

R COH

O

More soluble in water than comparable alcohols, ethers, aldehydes, and ketones

CH3COOH

CH3CH2CH2OH

CH3 CH2CHO

CH3 (CH2 ) 2COOH

CH3 (CH2 )3CH2OH

CH3 (CH2 ) 3CHO

acetic acid

1-propanol

propanal

60.5

60.1

58.1

118

97

48

16388.1butanoic acid

1-pentanol 88.1 137

103pentanal 86.1

Structure NameMolecular

Weight

Boiling Point

(°C)

Solubility

(g/100 mL H2O)

infinite

infinite

16

infinite

2.3

slight

Carboxylic acids are acidic because of the hydrogen in the -COOH group.

Recall that acids are compounds which yield H+ ions in solution.

Carboxylic acids in solution dissociate into the following ions:

Carboxylate ion

Carboxylic acids – stronger acids than ROH The pKa of ethanol is ~16, compared to ~5 for acetic

acid

Alkoxide ion (R-O-): negative charge is localized on O Carboxylate ion (COO-): negative charge is delocalized

over two equivalent O atoms: Resonance stabilization

Oxidation of Alkyl benzene Alkene Alcohol Aldehyde

Carboxylation of Grignard reagent

Hydrolysis of nitrile

KMnO4 ,

H+

R COOH

o General Formula:

o Example:

+KMnO

4 ,H

3O+

R C

OH

O

CH CHR R' R C

OH

O

o General Formula:

o Example:

o General Formula:

KMnO4 ,

H3O+

R C

OH

O

CH2OHR

o Example:

o General Formula:

o Example:

KMnO4 ,

H3O+

R C

OH

O

CR

H

O

R = alkyl, allyl, benzyl, aryl CO2 = dry ice or bubbling gaseous CO2

• nucleophilic addition - alkyl or aryl magnesium halide is added to a double bond C=O of CO2 to form carboxylate ion. • when treated with acid, carboxylic acid is formed

H3O+R MgX O C O R C

O

O- MgX+ R C

O

OH

o General Formula:

RMgX CO2 RCOOHH3O+

o Example:

CH3CH2MgCl i) CO2

ii) H3O+CH3CH2COOH

MgCl COOH

i) CO2

ii) H3O+

Alkyl halides react with NaCN to form nitriles which in turn undergo hydrolysis in acidic solution to produce carboxylic acid.

R X R CN R C

O

OH + NH4+NaCN H3O+

Formation of nitriles involves nucleophilic substitution.

Primary alkyl halides (Cl, Br, I) – SN2 substitution

Aromatic nitriles cannot be prepared through this method but still can hydrolysed to give aromatic carboxylic acids.

The nitrile is reflux with a dilute acid such as H2SO4 or HCl to form carboxylic acid.

BrNaCN

CNH

+

H2O

COOH

o General Formula:

o Example:

Salt formation

- neutralisation

- reactions with electropositive metals

Reduction to alcohols

Formation of Acyl Chlorides

Formation of Esters

Formation of Acid Anhydrides

Formation of Amides

1) Neutralisation: - carboxylic acids undergo neutralisation reactions with strong

bases or some weak base such as NaOH, NaHCO3, Na2CO3, NH3 and amines to form carboxylate salts of carboxylic acids and water.

RCO2H + NaOH → RCO2

-Na+ + H2O RCO2H + NaHCO3 → RCO2

-Na+ + H2O + CO2

2RCO2H + Na2CO3 → 2RCO2-Na+ + H2O + CO2

RCO2H + NH3 → RCO2-NH4

+

* carboxylate salts are soluble in water

Examples: CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)

sodium ethanoate

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.

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

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, zinc and iron.

2CH3COOH (aq) + Mg → (CH3COO)2Mg(aq) + H2

(g) magnesium ethanoate

RCO2H + Na RCO2-Na+ + H2(g)

1) Reaction with active metals

RCO2H + NaOH RCO2-Na+ + H2O

2) Reaction with base

3) Reaction with carbonate

RCO2H + NaHCO3 RCO2-Na+ + CO2 + H2O

o General reaction

o Example

LiAlH4 ,

H3O+

CH3 CH2 OHCCH3 OH

O

LiAlH4 ,

H3O+

R CH2 OHCR OH

O

Carboxylic acids reacts with phosphorus (v) chloride (PCl5) 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

carboxylic acids

PCl5 R C Cl

O

POCl3 HCl

acid chlorides

R C OH

O

SOCl2 R C Cl

O

SO2 HCl

carboxylic acids acid chlorides

R C OH

O

carboxylic acids

PCl3 R C Cl

O

H3PO3

acid chlorides

3 3

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.

Known as Fischer esterification.

Since the reaction is reversible, the mixture must be heated or reflux.

CH3 C OH

O

C

O

OH

ethanoic acid

benzoic acid

H OC2H5 CH3 C OC2H5

O

H2O

ethyl ethanoate

H OC2H5 C

O

OC2H5H2O

ethyl benzoate

H+

H+

heat or reflux

heat or reflux

H+ = H2SO4

Preparation of acid anhydrides:

- reaction of carboxylic acid with an acid chloride in the presence of pyridine.

- pyridine, C5H5N (base) is added to neutralize the HCl formed in the reaction to prevent unnessary side reaction (if any).

R C OH

O

Cl C R'

O

CH3 C OH

O

Cl C CH3

O

CH3 C OH

O

Cl C

O

R C O

O

C

O

R'

CH3 C O

O

C

O

CH3

CH3 C O

O

C

O

HCl

acid chloride acid anhydrides

examples

ethanoyl chloride acetic anhydride

benzoyl chloride acetic benzoic anhydride

pyridine

HCl

HCl

pyridine

pyridine

carboxylic acid

acetic acid

acetic acid

Symmetrical anhydride is formed when two carboxylic acid is heated with phosphorus pentoxide (P2O5) (dehydration reaction) or heating with ZnO.

CH3 C OH

O

HO C CH3

O

CH3 C O

O

C

O

CH3H2O

two molecules of acetic acids acetic anhydride

P2O5

or ZnO/heat

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 (poor method).

RCOO-NH4

+ RCONH2 + H2O

1° amide

For example:

CH3COOH + NH3 → CH3COONH4 CH3CONH2 + H2O heat

ammonium ethanoate

ethanamide

Excess RCOOH

Heat (100-200 °C)

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)

H2O1

o amine 2

o amide

R 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)

H2O2

o amine 3

o amide

R 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

Other method: reaction of acid chloride with ammonia or amines

R C Cl

O

NH4+ Cl-R C NH2

O

examples:

CH3 C Cl

O

CH3 C NH2

O

ammonium chloride

2NH3

2NH3 NH4+ Cl-

amide

2CH3NH2 CH3NH3+ Cl-

R C Cl

O

RNH3+ Cl-R C N

O

ammonium chloride

2RNH2

secondary amide

H

R

R C Cl

O

R2NH2+ Cl-R C N

O

ammonium chloride

2R2NH

tertiary amide

R

R

ammonia

amine

secondary amine

CH3 C Cl

O

CH3 C N

O CH3

H

Thank you…