POC-1ST

UNIT-4

CARBONYL COMPOUNDS

ALDEHYDES AND KETONES

SYLLABUS + IMPORTANT QUESTIONS

• Nucleophilic Addition, Electromeric effects, Aldol Condensation, Crossed Aldol Condensation, Cannizaro reactions, Crossed cannizaro reactions, Benzoin condensation, Perkin condensation, Qualitative tests.
• Structure and uses of formaldehyde, Paraldehyde, Acetone, chloral hydrate, Hexamine, Benzaldehyde, Vanillin, Cinnamaldehyde.

INTRODUCTION

• Carbonyl compound are those organic compounds which contain carbonyl group ($C=O$), which is carbon-oxygen double bond.
• Several compounds are formed from carbonyl group.

Properties

• These are highly reactive compounds.
• They can perform Nucleophilic addition, Electrophilic substitution etc.
• Most carbonyl compounds are polar and soluble in water & organic solvents.
• The carbonyl group has a $sp^2$ hybridised carbon and oxygen.

ALDEHYDES

• These are those carbonyl compounds which contain carbonyl group ($C=O$) at the end of carbon chain.

KETONES

• These are those carbonyl compounds which contain carbonyl group ($C=O$) within the carbon chain.

CHEMICAL REACTIONS OF CARBONYL COMPOUNDS (AFK)

1. Nucleophilic Addition Reactions
2. Electromeric effect
3. Aldol Condensation
4. Cross Aldol Condensation
5. Cannizaro reactions
6. Cross-cannizaro reactions
7. Benzoin Condensation
8. Perkin Condensation

NUCLEOPHILIC ADDITION REACTIONS

• These are those reactions, in which a nucleophile (a negatively charged or electron rich species) attacks the electrophilic carbonyl carbon of an aldehyde or ketone, resulting in the addition of the nucleophile to the carbonyl group.

• Both Aldehyde and ketone give nucleophilic addition reactions.
• In normal, one substance act as reagent and other reactant.

For Aldehydes:

• In this, Aldehyde act as reactant and HCN act as reagent.
• Now, HCN form nucleophile ($CN^-$) and release $H^+$ ion.
• Then this nucleophile ($Nu^-$) attacks on the electrophile of the aldehyde.
• finally, Nu attacked on Carbon (electrophile) atom and $H^+$ ion attached with O and form cyanohydrins.

Mechanism:

1. Step 1: protonation of reagent / formation of nucleophile ($Nu^-$)

2. Step 2: Attack of nucleophile on reactant.

3. Step 3: Addition of electrophile ($H^+$)

For Ketones:

• In this, positively polarised carbon atom in $C=O$ act as electrophile and reagent act as nucleophile ($Nu^-$).

• firstly, Nu attacks on electrophile and form intermediate which then react with $H^+$ ion and form final product.

• Eg: Addition of HCN mechanism is the same as aldehyde.

ELECTROMERIC EFFECTS

• It is a temporary and reversible shift of electron in a multiple bond ($\pi$ bond) toward one of the bonded atom, formed polarised bond.

• Electromeric addition reaction: Shifting of $\pi$ electron and attachment of electrophile ($E^+$) at same side.

• Nucleophilic addition reaction: Sifting of $\pi$ electron and attachment of nucleophile ($Nu^-$) are at different side.

• It is of two types:

  • +E Effects (Positive Electromeric Effects): Electron-withdrawing group (EWG) pull electrons away from the multiple bond, increasing the positivity of adjacent atom.

• -E Effects (Negative Electromeric Effects): Electron-donating groups (EDG) push electrons towards the multiple bond, increasing the negativity to adjacent atom.

ALDOL CONDENSATION

• It is a chemical reactions that combines two carbonyl compounds (aldehydes or ketones) to form a new carbon-carbon bond.

Conditions for Aldol Condensation:

• All aldehydes and ketones, if they have $\alpha$-hydrogen.
• dil. NaOH (for aldehydes), $Ba(OH)_2$ $\rightarrow$ for ketones.

For Aldehydes:

• In this reaction two molecules of aldehyde are condensed together to form Aldol, in the presence of dil. NaOH.

Mechanism:

1. Step 1: formation of nucleophile (from reagent).

2. Step 2: Attack of Nucleophile on reactant

Note : Presence of $\alpha$-hydrogen of aldehyde or ketone is necessary for aldol condensation.

• $\alpha$-hydrogen is necessary to form Nucleophile ($Nu^-$).

• $\alpha$-hydrogen

For Ketones:

• Some mechanism as aldehyde.

CROSSED ALDOL CONDENSATION

• It involves reaction between two different carbonyl compound (aldehydes or ketones) react to form a new carbon-carbon bond.

Conditions:

• One of the reactants does not have $\alpha$-hydrogen.

Mechanism:

1. Step 1: formation of Nucleophile (from reagent).
2. Step 2: Attack of nucleophile on reactant.
3. Step 3: Addition of electrophile ($H_2O$).
4. Step 4: removal of $H_2O$ through $\alpha, \beta$ dehydration.

CANNIZZARO REACTIONS

• It is a reaction which involves reaction between two non $\alpha$-hydrogen containing aldehydes.
• They form an alcohol and a carboxylic acid salt in the presence of conc. NaOH (strong base).

Conditions:

• only for aldehydes ($\alpha$-hydrogen absent eg. formaldehyde, Benzaldehyde).
• Involve redox mechanism.

Example: formaldehyde + formaldehyde $\xrightarrow{conc. NaOH}$ Methanol (Alcohol) + Sodium formate (Carboxylic acid salt)

Mechanism:

1. Step 1: Nucleophilic addition of $OH^-$ ion to the 1st aldehyde.

2. Step 2: Shifting of hydride ion ($H^-$) from one molecules to other.

3. Step 3: formation of alcohol and carboxylic acid salt.

Crossed-Cannizzaro Reactions

• Crossed cannizzaro reactions is a variation of cannizzaro reaction in which two different aldehydes react to form an alcohol and a carboxylic acid salt.

Mechanism:

• firstly, ($OH^-$) hydroxide ion attached on the carbon atom and form alkoxide ion.
• shifting of hydride ion from one molecules of aldehyde to another one.
• then transfer of proton from one molecules to another.
• After addition of $Na^+$ ion, it form Sodium formate (carboxylic acid salt) and Alcohol (Benzyl alcohol).

BENZOIN CONDENSATION

• It is a chemical reaction between two molecules of an aromatic aldehyde (typically Benzaldehyde) to form a Benzoin compound.
• It occurs in lack of $\alpha$-hydrogen compounds as aromatic condensation occurs in the presence of potassium cyanide (KCN).

Mechanism:

1. Step 1: Cyanide ion react with Benzaldehyde to form Cyanohydrine.

2. Step 2: Condensation b/w Cyanohydrin and 2nd molecules of Benzaldehyde.

3. Step 3: Rearrangement and Removal of Cyanide ion (forms Benzoin).

PERKIN CONDENSATION

• It is a reaction which occurs between an aldehyde and acid anhydride to form an $\alpha, \beta$-unsaturated carboxylic acid, in the presence of carboxylic acid salt.

Mechanism:

QUALITATIVE TESTS

1. Sodium Bisulphite test: when Aldehyde and ketones are react with sodium bisulphite it forms a white precipitate (crystalline compounds).
2. 2,4-Dinitrophenylhydrazine (DNPH) test: forms a yellow to orange precipitate with aldehydes and ketones.
3. Schiff's test: forms a pink to red color with aldehydes.
4. Fehling's test: forms a brick-red precipitate with aldehydes.
5. ferric hydroxide test: forms a brown color with ketones.
6. Tollen's test: forms a silver mirror with aldehydes.

STRUCTURE AND USES

I. formaldehyde

• It is colorless, strong-smelling gas.

• Uses:
  • It is used to produce adhesives, plastic & resins.
  • It is used as a disinfectant and preservatives in pharmaceuticals, cosmetics and food industries.
  • Also used in cosmetics, soaps, shampoos, lotions etc.

II. Vanillin

• It is a white or very slightly yellow colored needle-like compound.

• Uses:
  • It is used as a flavouring agent in foods, beverage and pharmaceuticals.
  • It is also used in perfumes, fragrances.
  • Also used in food additives and cosmetics.

III. Benzaldehyde

• It is a colorless to yellow liquid having a bitter almond odour. It is slightly water soluble but completely insoluble in ethanol and diethyl ether.

• Uses:
  • It is used in perfumery and dye.
  • It is used as a solvent for oils, resins, etc.
  • Also used as a flavoring agent in cinnamic acid, benzoic acid, and some pharmaceuticals.
  • also used as photographic chemicals, baking chemicals, soap.

IV. Acetone

• Uses:
  • It is widely used as a solvent in various industries, such as cosmetic, pharmaceutical.
  • used for manufacturing plastics, artificial fibers.
  • making paper coating, adhesives & heat-seal coating.

V. Paraldehyde

• Uses:
  • In ancient time, it was used as sedative and hypnotics medication, but it discontinued due to its toxicity & flammability.
  • It has also been used as an anticonvulsant for treating seizures.

VI. Chloral Hydrate ($C_2H_3Cl_3O_2$)

• Uses:
  • It is a sedative used in the short-term treatment of insomnia.
  • It relieves anxiety and induce sleep before surgery.
  • also used in post-surgery as pain killer.

VII. Hexamine ($C_6H_{12}N_4$)

• Methenamine

• Uses:
  • It is an anti-infective agent used for treating urinary tract infections.
  • It also has antibiotic activity.
  • also used as a fuel source in camping stoves, emergency rations and military applications.

VIII. Cinnamaldehyde ($C_9H_8O$)

• Uses:
  • It have antifungal and antibacterial property.
  • It enhance oral health as it fight against tooth decay and bad breath.
  • Also used as flavouring agent in liquid refreshment, ice-creams, chewing etc.
  • also used in perfumes.