-CHO AND - CO -

Definition Of Aldehyde

“Organic compounds that contain monovalent functional group –CHO are known as aldehydes.”

General Formula Of Aldehyde

 They are denoted by RCHO, where R may be any alkyl group. The general formula for aldehyde is:

C_nH_2n – CHO

Where n may be any natural number.

Structure Of Aldehyde

The structural formula of aldehyde shows that it contains a carbonyl group.

Examples Of Aldehyde

·        CH₃CHO   (Acetaldehyde)

·        HCHO                   (Formaldehyde)

Definition Of Ketone

“Organic compounds that contain divalent functional group – CO – are called Ketones.”

General Formula Of Ketone

They are denoted by R may be any alkyl group. The general formula for Ketone is:

C_nH_2n+1 – CO – C_nH_2n+1

Where n may be any natural number.

Structure of Ketone

The structural formula of ketone shows that it contains a carbonyl group.

Example Of Ketone

·        CH₃COCH₃           (Acetone)

Preparation of Aldehyde & Ketone

Aldehyde and Ketone can be prepared by the following methods:

1. From Dehydrogenation of Alcohol

Removal of hydrogen from a compound is called dehydrogenation. In presence of catalyst, Cu – Ni couple when alcohol is heated at 180°C then dehydrohalogenation takes place. As a result, aldehyde and ketone are formed.

H-CH₂OH        ®       H-CH=O         +          H₂

                                                                        Form Aldehyde

CH₃-CH₂OH    ®       H-CH=O         +          H₂

                                                                         Acetaldehyde

(CH₃)₂-CH-OH            ®       (CH₃)₂-C=O    +          H₂

                                                                              Acetone

2. From Oxidation of Alcohol

In presence of oxidizing agents, K₂Cr₂O₇ or concentrated H₂SO₄, alcohols are oxidized to form aldehyde or ketone.

H-CH₂OH        + [O]   ®       H-CH=O         +          H₂O

                                                                        Form Aldehyde

CH₃-CH₂OH + [O]      ®       H-CH=O         +          H₂O

                                                                         Acetaldehyde

(CH₃)₂-CH-OH + [O]  ®       (CH₃)₂-C=O    +          H₂O

                                                                              Acetone

3. From Dry Distillation

By the dry distillation of calsium formate (Calsium salt of formic acid), form aldehyde is obtained.

Ca(COOH)₂    ®       HCHO +          CaCO₃

By the dry distillation of calsium salt of formic acid and calsium salt of carboxylic acid, aldehydes are obtained.

Ca(CH₃OOH)₂            +          Ca(COOH)₂     ®       CH₃CHO         +          CaCO₃

By the dry distillation of calsium salt of carboxylic acid, Ketone is formed.

Ca(CH₃OOH)₂            ®       CH₃CHO         +          CaCO₃

1. From Ethyne

This method is used for the preparation of acetaldehyde. In presence of catalyst H₂SO₄ and HgSO₄ when ethyne reacts with water than unstable, intermediate vinyle alcohol is formed which on rearrangement of atoms convert into acetaldehyde.

H-CºC-H        +          H₂0      ®       H₂C-HCOH     ®       H₃C-COH

Preparation of Acetone By Pyrolysis Of Acetic Acid

In presence of catalyst MnO₂, when the acetic acid is heated at about 500°C then acetone is formed.

CH₃COOH                  ®       CH₃-CHO        +          CO₂     +          H₂0

Chemical Properties Of Aldehyde & Ketone

Some important chemical reactions of aldehyde and ketone are given below:

1. Addition Reaction due to Carbonyl Group

The carbonyl group of aldehyde and ketone is polarized by due to the high electronegativity of oxygen atom. The shared pair of electron (p electrons) is shifted towards oxygen atom as a result the carbonyl carbon becomes partial positively charged and oxygen becomes partial negatively charged. Due to the presence of double bond, carbonyl group shows addition reaction. When aldehyde and ketone reacts with electorphillic reagent then addition reaction takes place as a result product is formed.

=C⁺=O^-            +          A⁺B^-    ®       =CB-OA

                                  Carbonyl      Electrophilic     Addition

                                                 Group              Reagent           Product

Some important addition reactions of aldehyde and ketone are given below:

a)Reaction with Hydrogen Cyanide

When aldehyde or ketone reacts with hydrogen cyanide then cynohydrin is formed.

H₂C=O            +          HCN    ®       H₂C-CN-OH

                          Form Aldehyde                              CynoHydrin

CH₃HC=O       +          HCN    ®       CH₃HC-CN-OH

                           Acetaldehyde

(CH₃)₂C=O      +          HCN    ®       (CH₃)₂C-CN-OH

                                Acetone

b)Reaction with Ammonia

When aldehyde and ketone reacts with ammonia then hydroxyl amine is formed.

H₂C=O            +          HNH₂   ®       H₂C- NH₂-OH

                          Form Aldehyde                          Hydroxile Amine

CH₃HC=O       +          HNH₂   ®       CH₃HC- NH₂-OH

                           Acetaldehyde

(CH₃)₂C=O      +          HNH₂   ®       (CH₃)₂C- NH₂-OH

                                Acetone

c)Reaction with Alcohol

When aldehyde or ketone reacts with alcohol (ethyl alcohol) the hemi acetal is formed.

H₂C=O            +          C₂H₅OH          ®       H₂C- C₂H₅O-OH

                          Form Aldehyde

CH₃HC=O       +          C₂H₅OH          ®       CH₃HC- C₂H₅O-OH

                           Acetaldehyde

(CH₃)₂C=O      +          C₂H₅OH          ®       (CH₃)₂C- C₂H₅O-OH

                                Acetone

2. Substitution Reaction due to Carbonyl Group

When aldehyde and ketone reacts with the compound containing H₂, then oxygen of carbonyl group combines with Hydrogen of the attacking molecule to form water and substituted product.

Some important substitution reactions of aldehyde and ketone are given below:

a)Reaction with Hydroxyl Amine

When aldehyde and ketone reacts with hydroxyl amine then oxine is formed.

H₂C=O            +          H₂NOH            ®       H₂C=NOH       +          H₂O

                          Form Aldehyde

CH₃HC=O       +          H₂NOH            ®       CH₃HC=NOH +          H₂O

                           Acetaldehyde

(CH₃)₂C=O      +          H₂NOH            ®       (CH₃)₂C=NOH            +          H₂O

                                Acetone

b)Reaction with Hydrazene (NH₂ – NH₂)

When aldehyde and ketone reacts with Hydrazene then hydrazone is formed.

H₂C=O            +          H₂N-NH₂         ®       H₂C=N-NH₂    +          H₂O

                          Form Aldehyde

CH₃HC=O       +          H₂N-NH₂         ®       CH₃HC=N-NH₂           +          H₂O

                           Acetaldehyde

(CH₃)₂C=O      +          H₂N-NH₂         ®       (CH₃)₂C=N-NH₂          +          H₂O

                                Acetone

c)Reaction with Phenyl Hydrazene

When aldehyde and ketone reacts with Phenyl hydrazene then Phenyl Hydrazene is formed.

H₂C=O            +          H₂N-NHC₆H₅   ®       H₂C= N-NHC₆H₅         +          H₂O

                          Form Aldehyde

CH₃HC=O       +          H₂N-NHC₆H₅   ®       CH₃HC= N-NHC₆H₅   +          H₂O

                           Acetaldehyde

(CH₃)₂C=O      +          H₂N-NHC₆H₅   ®       (CH₃)₂C= N-NHC₆H₅  +          H₂O

                               Acetone

3. Oxidation of Aldehyde & Ketone

In presence of oxidizing agent such as a mixture of K₂Cr₂O₇ and H₂SO₄, when aldehyde is oxidized then acids are formed.

H₂C=O            +          [O]       ®       HCOOH

                          Form Aldehyde

CH₃HC=O       +          [O]       ®       CH₃COOH

                           Acetaldehyde

(CH₃)₂C=O      +          [O]       ®       CH₃COOH + CO₂ + H₂0

                                       Acetone

4. Reaction with Fehling’s Reagent

The alkaline solution of copper hydroxide and sodium hydroxide is called Fehling’s solution. When aldehydes are warmed with Fehling’s reagent then red precipitates of cuprous oxide are formed. In this reaction Cu⁺² is reduced to Cu⁺¹. Due to the presence of terminal hydrogen i.e. the hydrogen bonded with carbonyl carbon atom. Aldehyde act as reducing agent.

CH₃HC=O + Cu(OH)₂ + NaOH ® CH₃COONa + Cu₂O + 3H₂O

Ketones do not react with Fehling’s reagent because in ketone hydrogen is not directly bonded with carbonyl carbon atom. Therefore, this test is use to distinguish between aldehyde and ketone in laboratory.

5. Reaction with Tollen’s Reagent

The ammonical silver nitrate solution is called Tollen’s reagent. When aldehyde reacts with Tollen’s reagent then metallic silver is produced which is deposited on a fine smooth surface forming silver mirror. In aldehyde the carbonyl carbon is bonded with hydrogen atom, due to presence of terminal hydrogen aldehyde acts as reducing agent and it reduces Ag⁺¹ to Ag⁰ which is deposited on smooth surface forming silver mirror.

CH₃HC=O + [Ag(NH₃)₂]OH