Matriculation Chemistry ( Carboxylic Acid )

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CARBOXYLIC ACID AND ITS DERIVATIVES

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C OHR

O

RCOOH or RCO2H

(R ≡ alkyl, aryl or H)

Structure

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NOMENCLATURE

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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 & Common Name

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•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)

IUPAC Nomenclature

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4-bromo-3-methylpentanoic acid

5-hydroxyhexanoic acid

CH CHCH2 CH CH3COH

O CH3

5-methyl-3-hexenoic acid

CH CH2CH C OH

O

CH3

Br CH3

CH2 CH2CH2 C OH

O

CHOH

CH3

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

CH2 CH2CH2 C OH

O

COH

O

pentanedioic acid

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C C

H

CH2 H

CH2 COOH

HOOC

trans 3-hexenedioic acid

CH2 CH2CH C OH

O

CH2COH

O CH3

3-methylhexanedioic acid

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• When R is an aryl group, the parent name is benzoic acid

COOHCl

4-chlorobenzoic acid

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CH

CH3

CH3

HOOC

COOH

HOOC

COOH

1,3-benzenedicarboxylic acid

2-isopropyl-1,4-benzenedicarboxylic acid

• An aromatic dicarboxylic acid is named as 1,x-benzenedicarboxylic acid

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• A cyclic carboxylic acid is named as cycloalkanecarboxylic acid

• The C atom which is attached to —COOH is numbered as C1

COOH

cyclopentanecarboxylic acid

1

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COOH

CH3

Br

4-bromo-2-methylcyclohexanecarboxylic acid

1

COOH

cyclohexanecarboxylic acid

1

13

COOH

COOH

COOH

COOHCl

1,2-cyclohexanedicarboxylic acid

4-chloro-1,2-cyclohexanedicarboxylic acid

A cyclic dicarboxylic acid is named as 1,x-cycloalkanedicarboxylic acid

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COOH

CH3

3-methyl-2-cyclohexenecarboxylic 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.

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PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS

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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:

i. it exists as stable dimers that form hydrogen bond.

ii. molecules in dimers are arranged closely packed, therefore the hydrogen bonds are relatively strong.

iii. high energy is needed to overcome the intermolecular forces ,

boiling point

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C

O

O

R

H

C

O

O

R

H

Hydrogen bond

Hydrogen bond

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C

O

O

R

H

O

H

H

C

O

O

R

H

O H

H

Solubilitya) 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

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• 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 ↑.

R C OH

Ohydrophilic

hydrophobic

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• 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.

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Example : Descending order of solubility

CH CH2 CH2CH3

COOH COOH

CH2 CH2 CH2CH3

COOH

CH3 CH2 CH2

COOH

COOH

> >

>

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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 .

C

O

O

R

H

C

O

O

R

H

Van der Waals forces Van der Waals forces

Hydrogen bonds

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Acidity of Carboxylic Acid

• The acidity of carboxylic acid is influenced by:

i. Resonance effect

ii.Inductive effect

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Carboxylate ion :

Phenoxide ion :

R C O-

O

O-

Resonance Effect

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• 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.

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

R C OH

O

>

OH

> OH2 > R OH

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OH O-

+ H3O+

⇌phenoxide ion

(resonance structure)

+ H2O

phenol

CR

O

OH

+ H2O + H3O+

carboxylate ion (resonance structure)

CR

O

O-

CR

O-

O⇌

carboxylic acid

R—O—H + H2O ⇌ R—O– + H3O+ alkoxide ionalcohol

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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.

CEWG

O

O-

CEDG

O

O-

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i. 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.

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• 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.

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• 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

ii) The electronegativity of electron-withdrawing group in the compound

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• 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

iii) Number of electron-attracting group in the compound.

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• 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

iv) The position of electron-attracting group

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• 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.

(v ) The inductive effect of electron- releasing (electron-donating) group in the compound

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SYNTHESIS OF CARBOXYLIC ACIDS

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C OHR

H

H

C O

R

H

C O

R

OH

oxidizing agent

oxidizing agent

1o alcohol aldehyde carboxylic acid

Common oxidizing agents are :•KMnO4 / H2SO4

potassium permanganate

•K2Cr2O7 @ Na2Cr2O7 /H2SO4

potassium /sodium dichromat (VI)

1. Oxidation of primary alcohol and aldehyde

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2. Oxidation of Alkyl Benzene

Roxidizing

agent COOH

KMnO4 , H+

Δ

+ CO2 + H2O

COOH

Cl

CH

CH3

CH3

Cl

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3. Formation and Hydrolysis of nitrile

R CH2 XNaCN

R CH2 CNH2O,H+

R CH2 COOH

NaCN H2O,H+

CH2 CNCH2 Br CH2 COOH

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4. Carbonation of Grignard Reagents

R—MgX O C O

CO2

H2O, H+

R—COOH + Mg(OH)X

CH2MgBr

H2O, H+

CH2 COOH

+ Mg(OH)Br

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CHEMICAL PROPERTIES OF CARBOXYLIC ACIDS

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• Main reactions of carboxylic acid,

a. The reaction that involves the donation of H+ from –OH group.

b. The reaction that involves the substitution of OH group

c. The reaction that involves the reduction with LiAlH4 to primary alcohol

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a. 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,

CR

O

OH

+ NaOH CR

O

O-Na+

+ H2O

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+ NaOH + H2O

COOH COO– Na+

Sodium benzoate

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

R C OH

O

+ R C O

O

M + H2M

Exercise:

COH

OCl

+ K

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b. 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

R C OH

O

R C Cl

O

R C Cl

O

R C Cl

O

+ SO2 + HCl

+ POCl3 + HCl

+ H3PO3

SOCl2

PCl5

PCl3

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SOCl2

PCl5

PCl3

CH C OHCH3

CH3

O

Exercise :

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+ H—OR’C OHR

O

C OR R'

O

⇌ + H2OH+

2. EsterificationCarboxylic acids react with alcohols in the presence of mineral

acid catalyst to produce esters.

+ HOCH2CH3 ⇌H+ CH2 CCH3 OCH2

O

CH3CH2 CCH3 OH

O

+ H2O

carboxylic acid alcohol ester

propanoic acid ethanol ethyl propanoate

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3. Acid anhydride formation Acid anhydrides can be prepared from carboxylic acids by the loss of water through heating.

R C OH

O

RCOH

O

+ heat

CH3 C O C CH3

O O

R C O C R

O O

+ H2O

CH3 C OH

O

CH3 C OH

O

+ heat

+ H2O

ethanoic anhydride

acid anhydride

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4. Amides formation Reaction of carboxylic acids with an ammonia or amine give amide.

R C OH

O

NH3

RNH2

R2NH

R C NH2

O

R C NHR

O

R C NR2

O

+ H2O

+ H2O

+ H2O

1o amide

2o amide(1o amine)

(2o amine) (3o amide)

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CH C Cl

OCH3

CH3

CH3 NH2

CH3 NH

CH3

NH3

Exercise :

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c. 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 .

C OR R'

O

LiAlH4

etherCH2 OHR

1o alcohol

+ R’OH

C O CH2CH CH3CH3

O

CH3

LiAlH4

etherCH2OHCHCH3

CH3

+ HO—CH2CH3

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Methanoic acid, HCOOH as a reducing agent

• Methanoic acid molecule, has both C OHH

O

C OH

Oand

• 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.

C

O

H

carboxyliccarbonyl

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C OHH

OKMnO4 / H+

CO2 + H2O + MnO2

Ag(NH3)2+

Ag + CO2 + H2O

(Brown)

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DERIVATIVES OF CARBOXYLIC ACIDS

R C Cl

O

acid chloride

O CC RR

O O

acid anhydride

R C NH2

O

amideC OR R'

O

ester

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Reactions of carboxylic acid derivatives

i. Hydrolysis of acid chlorides

R C Cl

O

H2O R C OH

O

+ HClacid chloride carboxylic acid

ii. Hydrolysis of acid anhydrides

O CC RR

O O

H2O

carboxylic acid

2 C OHR

O

acid anhydride

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Reactions of carboxylic acid derivatives

iii. Hydrolysis of esters

H2OH+

carboxylic acid

+ ROH

H2ONaOH + ROH

alcohol

alcoholNa+C O

-R

OC ORR

O

ester

R C OH

O

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Reactions of carboxylic acid derivatives

iii. Hydrolysis of amide (acidic hydrolysis)

R C NH2

OH2OH+

R C OH

O+ NH4

+

amide Carboxylic acid

Ammonium ion

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Reactions of carboxylic acid derivatives

iii. Hydrolysis of amide (alkaline hydrolysis)

R C NH2

OH2OOH- + NH3

amideammonia

R C O-

O

Carboxylate ion

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Reactions of carboxylic acid derivatives

iii. Hydrolysis of amide (alkaline hydrolysis)

- if dilute acid is added to the carboxylate salt, the carboxylic acid are formed.

+

Carboxylic acid

R C O-

O

Carboxylate ion

H+ R C OH

O

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benzoyl chloride

H2OC Cl

O

Example :

C O CCH3 CH3

O O

ethanoic anhydride

H2O

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methyl ethanoate

H2OH+

Example :

C O CH3CH3

O

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• 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 Reactivity Of Carboxylic Acid Derivatives

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Cl– < RCOO– < RO– < HO– < NH2–

Relative basicities of the leaving group (substituent)

acidchloride

acidanhydride

ester carboxylicacid

amide

reactivity increases

R C Cl

O

C O CR

O

R

O

C ORR

O

R C NH2

O

R C OH

O

, , , ,

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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).

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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.

R C O

O

C R'

O

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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.

C O

CH3

OR

C+

O-

CH3

OR

C O-

CH3

O+

R

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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.

N

R

O H

HN

+R

O-

H

H

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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