Aldehydes, Ketones and Carboxylic Acids

Carbonyl Compounds

Carbonyl Compounds

• Aldehydes (R-CHO)

• Ketones (R-CO-R’)

• Carboxylic Acids (R-COOH)

Carboxylic Acid Derivatives

• Esters (R-COOR)

• Acyl Chloride (R-COCl)

• Amide (R-CONH₂)

Common and IUPAC Names of Some Aldehydes and Ketones

Preparation of Aldehyde

1. Rosenmund reduction

2. Stephen reaction

   From hydrocarbons:

3. Etard Reaction

4. By side chain chlorination followed by hydrolysis

5. Gatterman Koch Reaction

6. Ozonolysis

7. Hydration of Alkynes

8. Catalytic Dehydration

1. Rosenmund Reduction (from acyl chloride)

R-COCL → [H₂, Pd-BaSO₄] R-CHO

C₇H₅ClO (benzoyl chlrodie) → [H₂, Pd-BaSO₄] C₆H₅CHO (benzaldehyde)

CH₃COCl → [H₂, Pd-BaSO₄] CH₃CHO

The role BaSO₄ is to prevent further reduction of aldehydes to 1ᵒ alcohol (since COOH and COOH derivatives (acyl chlorides) on reduction, convert to 1ᵒ alcohol)

2. Stephen Reaction

RCN + SnCl₂ + HCl → RCH=NH →[H₂O]RCHO

CH₃-CH₂-CN→ [SnCl₂+HCl] CH₃-CH₂-CH=NH→ [H₃O⁺] CH₃-CH₂-CHO

CH₂=CH-CN → [DIBAL-H] CH₂=CH-CH=NH → CH₂=CH-CHO (not stephen’s reaction)

CH₂=CH-CN → [SnCl₂+HCl] CH₃-CH₂-CHO

3) Etard Reaction

4) By Side Chain Chlorination Followed By Hydrolysis

5) Gatterman Koch Reaction

6) Ozonolysis

see cw

7) Hydration of Alkynes

R-C≡C-R→[H₂O,H⁺] R-C(OH)=CH-R→ [Proton shift'/Tautomerism] R-C(O)-CH₂-R

CH≡CH→[H₂O,H⁺] CH(OH)=CH₂ → [H⁺ Shift] CH₃CHO

8)Catalytic Dehydration

R-CH₂-OH → [Cu, 573K] R-CHO

R-CH(OH)-R’ → [Cu, 573K] R-C(O)-R’

Preparation of Ketones

1. From Grignard Reagents

2. From benzene (Friedel Craft’s Acylation)

3. From Acyl Chlorides

1. From Grignard Reagents

2. From Benzene (Friedel Craft’s Acylation)

anhy.AlCl₃

3. From Acyl Chlorides

Physical Properties

• Methanal is a gas and ethanal is a volatile liquid while other aldehydes are mostly liquids and other soft solids

• Comparatively, aldehydes have higher boiling points due to dipole-dipole interactions but have lesser boiling points than alcohols due to the absence of inter-molecular hydrogen bonding

• The lower members of aldehydes and ketones are completely miscible in water as they can form hydrogen bond

• with water and solubility decreases with an increase in size

• The lower members have a sharp pungent odor but as the size increases, they become more and more fragrant. Hence, they are naturally used as flavouring agents and perfume blends.

Chemical Properties

1. Nucleophilic Addition Reaction:

   1. with HCN

   2. with NAHSO₃

   3. with Alcohol

   4. Alcohol + Ketones

   5. with Diols

   6. with Grignard Reagents

   7. with Ammonia and its derivatives

1. Nucleophilic Addition Reaction

always remove hydrogen and consider the rest as nucleophile

1. with HCN

   Q; Cyanohydrin of Acetaldehyde

(R)C(R’)=O + HCN → (CN)(R)C(R’)(OH)

2. with NAHSO₃

can directly give the final product, no need to write the middle product

3. with Alcohol

   Q: Acetal and semiacetal of acetaldehyde and ethanol

same can be done for ketone

4. Alcohol + Ketones

see note

5. with Diols

6. with Grignard Reagents

7. with Ammonia and its derivatives

Aldol Condensation

Cannizzaro reaction

Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

Electrophilic Substitution Reaction

carbonyl group acts as a meta-directing group.

Order of Reactivity of Aldehydes and Ketones towards Nucleophilic Addition

• Aldehydes are more reactive than ketones due to lesser +I effect and lesser steric hindrance of aldehydes than ketones.

• Lighter aldehydes are more reactive than heavier aldehydes due to lesser +I effect and lesser steric hindrance.

• Lighter ketones are more reactive than heavier ketones.

• Benzaldehydes are less reactive than aldehydes due to resonance in the aromatic ring, the electrophilicity of carbonyl carbon is decreased.

• Benzaldehydes with electron-withdrawing groups (e.w.g) are more reactive than benzaldehydes with electron-donating groups (e.d.g).

Tests for Aldehydes and Ketones

1. 2,4-Dinitrophenyl Test

2. Tollen’s Test

3. Fehling’s Test

4. Iodoform Test

1) 2,4-Dinitrophenyl Test

• Test for carbonyl carbon

• Orange precipitate is formed

2) Tollen’s Test

• Test for aliphatic/aromatic aldehydes

• Tollen’s Reagent: Ammoniacal silver nitrate solution Ag(NH₃)₂OH

• Oxidation takes place

• Bright silver mirror is formed

  R-CHO+Ag⁺ → R-COO⁻ + Ag↓

3) Fehling’s Test

• Test for Aliphatic Aldehyde

• Does not work for aromatic

• Fehling’s Reagent A: aq.CuSO₄

• Fehling’s Reagent B: Na/K tartarate

• Oxidation takes place

4) Iodoform Test

• Test for methyl carbonyl (compounds with CH₃-C(O)- group/acetyl group)

• Reagent: NaOI or NaOH + I₂

R-C(O)-CH₃ + NaOI → R-COONa + CHI₃↓

Uses of Aldehydes and Ketones

• Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite, urea-formaldehyde glues and other polymeric products.

• Benzaldehyde is used in perfumery and in dye industries.

Carboxylic Acids

-C(O)-O - carboxyl

Some higher members of aliphatic carboxylic acids (C₁₂ – C₁₈) known as fatty acids, occur in natural fats as esters of glycerol.

Names and Structures of Some Carboxylic Acids

Structure of Carboxyl Group

The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown in image

Preparation of Carboxylic Acids

1. From alcohols (by oxidation)

2. From Alkyl Benzene

3. From Grignard Reagent

4. From Acid Anhydrides

5. From Esters

Preparation of Carboxylic Acids

1) From Alcohols (by oxidation)

• Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO₄) in neutral, acidic or alkaline media or by potassium dichromate (K₂Cr₂O₇ ) and chromium trioxide (CrO₃) in acidic media (Jones reagent).

Preparation of Carboxylic Acids

2) From Alkyl Benzene

• The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain

• Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected.

Preparation of Carboxylic Acids

3) From Grignard Reagent

Carbon dioxide is in the form of solid dry ice

Preparation of Carboxylic Acids

4) From Acid Anhydrides

CH₃COOCOCH₃ → [H₃O⁺] 2CH₃COOH

(CH₃CO)₂O

Preparation of Carboxylic Acids

5) From Esters

Physical Properties

• Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours

• Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding

• Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. The solubility decreases with increasing number of carbon atoms.

• Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water.

Chemical Properties

1) Acidic Character

2) Esterification

3) Dehydration

4) Chlorination
5) Addition of Ammonia

6) Hell-Volhard-Zelinsky reaction (Halogenation)

7) Electrophilic Subsitution

8)Reduction

9) Decarboxylation

Chemical Properties

1) Acidic Character (Reactions with metals and alkalies)

2R-COOH + 2Na → 2R-COONa⁺ + H₂

R-COOH + NaOH → R-COONa⁺ + H₂O

Bicarbonate Test (Test for carboxylic acids):

R-COOH + NaHCO₃ → R-COONa⁺ + H₂O + CO₂↑ (brisk effervescence)

Carboxylic acids dissociate in water to give resonance-stabilised carboxylate anions and hydronium ions.

Order of Acidic Character

HCOOH > CH₃COOH

CH₃COOH < CH₂(Cl)COOH < (Cl)CH₂(Cl)COOH

Acidic character ∝ [H⁺] ∝ 1/pH ∝ Ka ∝ i/pKa

The electron-withdrawing effect of the following groups in increasing acidity order is

Ph < I < Br < Cl < F < CN < NO2 < CH₂X < CHX₂ < CX₃

X - any e.w.g

Chemical Properties

2) Esterification

R-COOH = R’-OH → RCOOR’ + H₂O

CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O

Chemical Properties

3) Dehydration

2R-COOH → [P₂O₅] R-C(O)-O-C(O)-R + H₂O

2CH₃COOH → [P₂O₅] CH₃-C(O)-O-C(O)-CH₃ + H₂O

R-COOH+ R’COOH → [P₂O₅] R-C(O)-O-C(O)-R’ + H₂O

CH₃-COOH + C₆H₅COOH → [P₂O₅] C₆H₅-C(O)-O-C(O)-CH₃ + H₂O

Chemical Properties

4) Chlorination

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

3RCOOH + PCl₃ → 3RCOCl + H₃PO₃

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Chemical Properties

5) Addition of Ammonia

Chemical Properties

6) Hell-Volhard-Zelinsky reaction (Halogenation)

CH₂(Cl)-COOH +HCl

Chemical Properties

7) Electrophilic Substitution

They do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).

Chemical Properties

8) Reduction

Chemical Properties

9) Decarboxylation

NaOH + CaO = Soda Lime