Aldehydes and Ketones

Aldehydes and ketones are organic compounds containing the carbonyl group (C=O). In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, whereas in ketones, it is bonded to two carbon atoms.

1. Structure of Carbonyl Group

The carbonyl carbon is sp² hybridized. The molecule is planar and the bond angle around the carbonyl carbon is approximately 120 degrees.

The C=O bond consists of one sigma bond and one pi bond.
Oxygen is more electronegative than carbon.
The bond is polar: carbon is partially positive and oxygen is partially negative.

Due to this polarity, aldehydes and ketones readily undergo nucleophilic addition reactions.

2. Nomenclature

Aldehydes (IUPAC)

Select the longest chain containing -CHO group.
Replace the ending -e of alkane with -al.
The aldehyde carbon is always carbon number 1.

Examples:

HCHO - Methanal
CH₃CHO - Ethanal

Ketones (IUPAC)

Select the longest chain containing the carbonyl group.
Replace -e with -one.
Give the carbonyl carbon the lowest possible number.

Examples:

CH₃COCH₃ - Propanone
CH₃CH₂COCH₃ - Butan-2-one

3. Methods of Preparation

Preparation of Aldehydes

Controlled oxidation of primary alcohols
Ozonolysis of alkenes
Reduction of acyl chlorides
Hydrolysis of gem-dihalides

Preparation of Ketones

Oxidation of secondary alcohols
Friedel Crafts acylation
Hydration of alkynes
Ozonolysis of alkenes

4. Physical Properties

Lower members are colorless liquids.
Have higher boiling points than hydrocarbons.
Have lower boiling points than alcohols.
Lower aldehydes and ketones are soluble in water.

5. Chemical Reactions

A. Nucleophilic Addition Reactions

Nucleophilic Addition Reaction

Aldehydes and ketones undergo nucleophilic addition due to the polar nature of the carbonyl group. The nucleophile attacks the positively polarized carbon atom of the C=O group.

Equation:
CH₃CHO + HCN → CH₃CH(OH)CN

Reactant	Product
Aldehyde / Ketone	Cyanohydrin

Oxidation of Aldehydes

Aldehydes are easily oxidized to carboxylic acids because the carbonyl carbon is attached to a hydrogen atom. Ketones do not undergo oxidation under mild conditions.

Equation:
R-CHO + [O] → R-COOH

Compound	Observation
Aldehyde	Oxidized easily
Ketone	No reaction

Reduction of Aldehydes and Ketones

Aldehydes and ketones are reduced to alcohols in presence of reducing agents. Aldehydes form primary alcohols, while ketones form secondary alcohols.

Equations:
R-CHO → R-CH₂OH
R-CO-R → R-CHOH-R

Reactant	Product
Aldehyde	Primary alcohol
Ketone	Secondary alcohol

Aldol Condensation

Aldehydes and ketones containing alpha hydrogen atoms undergo aldol condensation in the presence of dilute base to form beta-hydroxy compounds.

Equation:
2 CH₃CHO → CH₃CH(OH)CH₂CHO

Requirement	Condition
Alpha hydrogen	Must be present

Cannizzaro Reaction

Aldehydes without alpha hydrogen undergo Cannizzaro reaction in presence of strong base. One molecule is oxidized while the other is reduced.

Equation:
2 HCHO + NaOH → HCOONa + CH₃OH

Condition	Requirement
Alpha hydrogen	Absent

F. Haloform Reaction

Methyl ketones react with halogen in alkaline medium to give haloform.

CH₃COCH₃ + I₂ + NaOH -> CHI₃ + CH₃COONa

CHI₃ is a yellow precipitate.

6. Distinction Tests

Tollens Test

Aldehydes produce silver mirror with Tollens reagent.

Fehling Test

Aliphatic aldehydes give brick red precipitate.

Iodoform Test

Methyl ketones give yellow precipitate of iodoform.

7. Uses of Aldehydes and Ketones

Formaldehyde is used as preservative.
Acetone is used as solvent.
Used in plastics, dyes, perfumes, and medicines.

Numericals & HOTS

Q1. Reactivity Comparison

Arrange the following compounds in increasing order of their reactivity towards nucleophilic addition reactions:
Ethanal, Propanal, Propanone, Butanone.

Solution:
Reactivity depends on two factors:
1. Steric Hindrance: Larger alkyl groups hinder the approach of the nucleophile.
2. Electronic Effect (+I): Alkyl groups release electrons, reducing the positive charge on the carbonyl carbon.

Aldehydes are more reactive than ketones. Within aldehydes, smaller alkyl groups are better.
Order: Butanone < Propanone < Propanal < Ethanal

Q2. Identification of Compound A

An organic compound A (C₅H₁₀O) forms a phenylhydrazone and gives a negative Tollens' test. It also gives a positive Iodoform test and on reduction yields n-pentane. Identify A.

Solution:
1. Forms Phenylhydrazone: Indicates it is a carbonyl compound (Aldehyde or Ketone).
2. Negative Tollens' Test: Confirms it is a Ketone.
3. Positive Iodoform Test: Indicates it is a Methyl Ketone (contains CH₃CO- group).
4. Reduction to n-pentane: Indicates a straight chain of 5 carbons.

Possible Methyl Ketone with 5 carbons: Pentan-2-one.
Ans: Pentan-2-one (CH₃COCH₂CH₂CH₃)

Q3. Crossed Aldol Products

How many total structural aldol condensation products are possible when a mixture of Ethanal and Propanal is treated with dilute NaOH?

Solution:
There are 4 products in total:
1. Self Aldol of Ethanal: But-2-enal.
2. Self Aldol of Propanal: 2-Methylpent-2-enal.
3. Crossed (Ethanal nucleophile + Propanal electrophile): Pent-2-enal.
4. Crossed (Propanal nucleophile + Ethanal electrophile): 2-Methylbut-2-enal.
Ans: 4 Products

Q4. pH Effect on Nucleophilic Addition

Why is the addition of HCN to benzaldehyde catalyzed by a base?

Solution:
HCN is a weak acid and does not ionize easily to produce the nucleophile CN^-. The base (OH^-) reacts with HCN to generate the strong nucleophile CN^- ion, which then attacks the carbonyl carbon efficiently.
Reaction: HCN + OH^- → H₂O + CN^-

Q5. Exceptional Carbohydrate

Fructose is a keto-hexose (ketone), yet it reduces Tollens' reagent. Why?

Solution:
Under the basic conditions of Tollens' reagent (ammoniacal silver nitrate), Fructose undergoes Lobry de Bruyn-van Ekenstein transformation (tautomerization). This converts the ketose (fructose) into aldoses (glucose and mannose), which then react with Tollens' reagent.
Ans: Due to base-catalyzed tautomerization to Aldose.

Q6. Distinction Logic

Out of Pentan-2-one and Pentan-3-one, which one will give a yellow precipitate with I₂ and NaOH?

Solution:
The Haloform/Iodoform test is given by compounds containing the CH₃CO- (Methyl Ketone) group.
• Pentan-2-one: CH₃-CO-CH₂CH₂CH₃ (Has the group).
• Pentan-3-one: CH₃CH₂-CO-CH₂CH₃ (Does NOT have the group).

Ans: Pentan-2-one.

Q7. Acidity of Alpha-Hydrogens

Why are the α-hydrogens of aldehydes and ketones acidic in nature?

Solution:
1. Strong Electron Withdrawing Effect: The carbonyl group (>C=O) withdraws electrons from the α-carbon, weakening the C-H bond.
2. Resonance Stabilization: Once the proton is lost, the resulting conjugate base (enolate ion) is stabilized by resonance with the carbonyl oxygen.

Q8. Hydride Transfer

In the Cannizzaro reaction of formaldehyde, what is the rate-determining step?

Solution:
The rate-determining step in the Cannizzaro reaction is the transfer of hydride ion (H^-) from the anion formed by the attack of OH^- on one carbonyl molecule to the second carbonyl molecule.

Q9. Imine Formation

Predict the product when Benzaldehyde reacts with Aniline.

Solution:
Aldehydes react with primary amines to form Schiff's bases (Azomethines).
C₆H₅CHO + H₂N-C₆H₅ → C₆H₅CH=N-C₆H₅ + H₂O.
Ans: Benzalaniline (Schiff's base).

Q10. Boiling Point Trends

Arrange in decreasing order of Boiling Point:
CH₃CH₂CH₂OH, CH₃CH₂CHO, CH₃CH₂CH₂CH₃.

Solution:
1. Propan-1-ol: Highest due to strong intermolecular hydrogen bonding.
2. Propanal: Intermediate due to dipole-dipole interactions.
3. n-Butane: Lowest due to weak van der Waals forces.

Order: Alcohol > Aldehyde > Hydrocarbon

Numericals & HOTS

Q1. Reactivity Comparison

Arrange the following compounds in increasing order of their reactivity towards nucleophilic addition reactions:
Ethanal, Propanal, Propanone, Butanone.

Q2. Identification of Compound A

An organic compound A (C₅H₁₀O) forms a phenylhydrazone and gives a negative Tollens' test. It also gives a positive Iodoform test and on reduction yields n-pentane. Identify A.

Q3. Crossed Aldol Products

How many total structural aldol condensation products are possible when a mixture of Ethanal and Propanal is treated with dilute NaOH?

Q4. pH Effect on Nucleophilic Addition

Why is the addition of HCN to benzaldehyde catalyzed by a base?

Q5. Exceptional Carbohydrate

Fructose is a keto-hexose (ketone), yet it reduces Tollens' reagent. Why?

Q6. Distinction Logic

Out of Pentan-2-one and Pentan-3-one, which one will give a yellow precipitate with I₂ and NaOH?

Q7. Acidity of Alpha-Hydrogens

Why are the α-hydrogens of aldehydes and ketones acidic in nature?

Q8. Hydride Transfer

In the Cannizzaro reaction of formaldehyde, what is the rate-determining step?

Q9. Imine Formation

Predict the product when Benzaldehyde reacts with Aniline.

Solution:
Aldehydes react with primary amines to form Schiff's bases (Azomethines).
C₆H₅CHO + H₂N-C₆H₅ → C₆H₅CH=N-C₆H₅ + H₂O.
Ans: Benzalaniline (Schiff's base).

Q10. Boiling Point Trends

Arrange in decreasing order of Boiling Point:
CH₃CH₂CH₂OH, CH₃CH₂CHO, CH₃CH₂CH₂CH₃.

Important Reactions

1. Alcohol Preparation Trends

Hydroboration-Oxidation:

Alkene + (BH₃)₂ / H₂O₂, OH^- → 1° Alcohol

(Anti-Markovnikov Hydration; No rearrangement)

Grignard + Carbonyl:

HCHO + RMgX → 1° Alcohol
RCHO + RMgX → 2° Alcohol
R₂CO + RMgX → 3° Alcohol

2. Phenol Name Reactions

Reimer-Tiemann Reaction:

Phenol + CHCl₃ + NaOH → Salicylaldehyde

[Image of Reimer-Tiemann reaction mechanism]

Kolbe’s Reaction:

Phenoxide + CO₂ &xrightarrow{H^+} Salicylic Acid

3. Ether Cleavage with HI

Case 1: 1° or 2° alkyl groups (S_N2):

R-O-R' + HI → Smaller R-I + Larger R'-OH

Case 2: One 3° alkyl group (S_N1):

(CH₃)₃C-O-CH₃ + HI → (CH₃)₃C-I + CH₃OH

20 Golden Facts (NEET)

1. Boiling Point: Alcohols and phenols have much higher boiling points than ethers or hydrocarbons of comparable mass due to Intermolecular Hydrogen Bonding.
2. Solubility: Lower alcohols are miscible with water in all proportions due to H-bonding, but solubility decreases as the size of the alkyl (hydrophobic) group increases.
3. Acidity Order: Water > 1° Alcohol > 2° Alcohol > 3° Alcohol. (Alcohols are even weaker acids than water, except Methanol).
4. Phenol Acidity: Phenol is more acidic than alcohols because the phenoxide ion is resonance-stabilized, whereas the alkoxide ion is not.
5. Picric Acid: 2,4,6-Trinitrophenol is called Picric Acid. It is very acidic due to the presence of three electron-withdrawing nitro (-NO₂) groups.
6. Oxidation: 1° alcohols → Aldehydes → Acids; 2° alcohols → Ketones; 3° alcohols are resistant to oxidation but undergo dehydration under drastic conditions.
7. PCC Reagent: Pyridinium Chlorochromate (PCC) is the best reagent to oxidize 1° alcohols to Aldehydes without further oxidation to carboxylic acids.
8. Victor Meyer Test: Red colour = 1° alcohol; Blue colour = 2° alcohol; Colourless = 3° alcohol. (RBC rule).
9. Lucas Reagent: A mixture of conc. HCl and ZnCl₂. It distinguishes alcohols based on the rate of formation of alkyl chlorides (turbidity).
10. Dehydration of Ethanol: At 443 K (conc. H₂SO₄) → Ethene. At 413 K → Ethoxyethane (Ether).
11. Electrophilic Substitution: The -OH group in phenol is ortho and para directing and highly activating due to resonance.
12. Bromine Water/Phenol: Reaction of phenol with bromine water gives a white precipitate of 2,4,6-tribromophenol.
13. Aspirin Synthesis: Acetylation of salicylic acid with acetic anhydride in the presence of acid gives Acetylsalicylic acid (Aspirin).
14. Williamson Synthesis: It involves an S_N2 attack of an alkoxide ion on a primary alkyl halide. 3° halides give alkenes.
15. Ethers as Lewis Bases: Ethers can donate lone pairs of electrons to Lewis acids like BF₃ to form coordination complexes called etherates.
16. Peroxide Formation: Ethers on long exposure to air and light form highly explosive peroxides.
17. Denatured Alcohol: Ethyl alcohol made unfit for drinking by adding Methanol, Pyridine, or Copper Sulphate.
18. Dow's Process: Industrial preparation of phenol from Chlorobenzene using NaOH at high temperature (623 K) and pressure (300 atm).
19. Power Alcohol: A mixture of 20% Ethanol and 80% Petrol used as fuel in internal combustion engines.
20. Catalyst in Williamson: If we want to prepare tert-butyl methyl ether, we must use Sodium tert-butoxide and Methyl bromide, NOT Sodium methoxide and tert-butyl bromide.

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