Ketones 

Ketones are a class of organic compounds characterized by a carbonyl group (C=O) bonded to two alkyl or aryl groups. They have the general formula R-CO-R', where "R" and "R'" can be the same or different carbon-containing groups. Ketones are polar molecules due to the carbonyl group, which makes them soluble in polar solvents and gives them relatively high boiling points compared to nonpolar compounds.

1.0General Structure of Ketone

• Ketones have a carbonyl group (C=O) bonded to two alkyl or aryl groups.
• General Formula: Represented as R-CO-R', where R and R' are alkyl or aryl groups.
• Bonding: The carbonyl carbon is sp² hybridized.
• Bond Angles: The structure around the carbonyl carbon has bond angles of about 120°, giving it a trigonal planar shape.

2.0Physical Properties of Ketones

3.0Classification and Nomenclature of Ketones

Classification of ketones

1. Based on the type of R Groups:

• Symmetrical Ketones: Both R groups attached to the carbonyl are the same (e.g., acetone, CH₃COCH₃).
• Asymmetrical Ketones: The R groups attached to the carbonyl are different (e.g., methyl ethyl ketone, CH₃COC₂H₅).

2. Based on the Type of Carbon Chain:

• Aliphatic Ketones: R groups are alkyl groups without aromatic rings (e.g., propanone).
• Aromatic Ketones: At least one R group is an aromatic ring (e.g., acetophenone, C₆H₅COCH₃).

IUPAC Nomenclature

• Ketones are named by identifying the longest chain that includes the carbonyl carbon.
• The suffix "-one" is used, and the position of the carbonyl group is indicated by a number if needed.
• Example: Butan-2-one for CH₃COCH₂CH₃.

Common Nomenclature:

• Common names are derived by naming the alkyl groups attached to the carbonyl and adding the word “ketone.”
• Example: Methyl ethyl ketone for CH₃COC₂H₅.
• Aromatic Ketones: Aromatic ketones are often named based on the aromatic group and the suffix “-phenone.”
• Example: Benzophenone for C₆H₅COC₆H₅.

4.0Examples of IUPAC Names and Common Names

5.0Preparation of Ketones

1. Oxidation of Secondary Alcohols: Ketones can be prepared by the oxidation of secondary alcohols using oxidizing agents like potassium dichromate (K₂Cr₂O₇) or PCC (Pyridinium chlorochromate). 

     $RCH(OH)R\prime \to RCOR\prime +H2O$

• Friedel-Crafts Acylation: Aromatic ketones can be synthesized by reacting an aromatic compound with an acyl chloride in the presence of AlCl₃. 

   $C_6{H}_6+\text{RCOCl}\stackrel{{\text{AlCl}}_3}{\to }{C}_6{H}_5\text{COR}+\text{HCl}$

• Hydration of Alkynes: Alkynes can be converted into ketones by hydration using acid and mercuric sulfate as a catalyst. 

$\text{RC}\equiv \text{CR’}+{\text{H}}_2\text{O}\stackrel{{\text{HgSO}}_4,{\text{H}}_2{\text{SO}}_4}{\to }\text{RCOR’}$

6.0Chemical Properties of Ketones

Ketones are known for their reactivity, primarily due to the polar nature of the carbonyl group (C=O). Here are some significant chemical properties:

1. Nucleophilic Addition Reactions:

Mechanism:

• The carbonyl carbon is electrophilic, making it susceptible to attack by nucleophiles.

Examples:

• Addition of Hydrogen Cyanide (HCN): Forms cyanohydrins, where CN⁻ attacks the carbonyl carbon.

R₂​C=O + HCN → R₂​C(OH)CN

• Addition of Alcohols: Forms hemiacetals and acetals in the presence of an acid catalyst. 

R₂C=O + R′OH → R₂C(OR′)(OH)

2. Reduction Reactions:

• Catalytic Hydrogenation: Ketones can be reduced to secondary alcohols using hydrogen gas (H₂) with a catalyst like Pt, Pd, or Ni. 

     R₂C=O + H₂ → R₂CH−OH

• Reduction with Metal Hydrides: Sodium borohydride (NaBH₄) or lithium aluminum hydride (LiAlH₄) can also reduce ketones to secondary alcohols. 

     R₂C=O + 2[H] → R₂CH−OH

3. Oxidation:

• Unlike aldehydes, ketones are relatively resistant to oxidation. However, strong oxidizing agents, such as potassium permanganate (KMnO₄), can cleave the carbon-carbon bond next to the carbonyl, producing carboxylic acids.

     RCOCR′ + [O] → RCOOH + R′COOH

7.0Tests for Ketones

Here are two major tests used for Ketones are:

1.  2,4-Dinitrophenylhydrazine (DNPH) Test: Ketones react with DNPH to form yellow or orange precipitates, indicating the presence of a carbonyl group.
2. Iodoform Test: Ketones with a methyl group adjacent to the carbonyl (methyl ketones) react with iodine and sodium hydroxide to give a yellow precipitate of iodoform (CHI₃).