Ketones

Introduction to Ketones

Importance of the Carbonyl Group

• The position of the carbonyl group is crucial in distinguishing ketones from aldehydes.
• It significantly affects their chemical reactions, behaviours, and applications.

Structural Formulae

Structural Formulae serve as the Blueprints for understanding molecule behaviour and are vital to organic chemistry.

• General Structure of Ketones:
  • Contains a central carbonyl group, located between two R groups (alkyl or aryl).
  • Impact of R groups:
    • Variations in the R groups alter ketone properties, affecting physical characteristics and application.
    • Example: Propanone (acetone) is a prevalent ketone where R groups influence its role as a solvent.

Nomenclature of Ketones

• IUPAC Naming Steps:
  1. Identify the longest carbon chain that includes the carbonyl group.
  2. Determine and name the attached alkyl groups.
  3. Number the carbon chain to give the carbonyl group the lowest possible number.
  4. Use the suffix '-one' to denote a ketone.
  • Mnemonic: "Locate, Label, Number, Name" aids memorisation.
• Interactive Example: What would you name a three-carbon ketone with a methyl and an ethyl group? (Answer: 2-Pentanone)

Example: Butanone

Steps	Example: Butanone
1. Longest chain	Four carbons = Butane
2. Alkyl group	Methyl group at C-2
3. Numbering	Carbonyl at C-2
4. Final Name	2-Butanone

Comparison Between Ketones and Aldehydes

Structural Comparison

• Ketones:
  • Carbonyl group positioned internally.
  • Attached to two alkyl or aryl groups.
  • Example: Acetone - a widely used solvent in nail polish remover due to its volatility.
• Aldehydes:
  • Carbonyl group located terminally.
  • Generally attached to one alkyl group and a hydrogen atom.
  • Example: Formaldehyde - used for preserving biological specimens due to its preservative qualities.

Reactivity and Oxidation Comparison

• Nucleophilic Attack:
  • Both ketones and aldehydes participate in nucleophilic addition reactions.
  • Aldehydes exhibit greater reactivity due to reduced steric hindrance.
  • The internal vs terminal position of the carbonyl greatly impacts reactivity.
• Oxidation Behaviour:
  • Aldehydes: Easily oxidised to carboxylic acids and react positively with Tollens' reagent.
  • Ketones: Resistant to oxidation, providing stability, and show no reaction with Tollens' reagent.

Physical Properties of Ketones

• Physical Properties: Characteristics observed without altering a compound's composition.
  • Significance: Facilitate predictions of ketone behaviour and usage in various applications.

Key Physical Properties

Boiling Point

• Influence of Carbonyl Group:

  • Results in moderate boiling points, typically higher than aldehydes.

  

Solubility in Water

• Polarity Contribution: Enhances solubility in polar solvents like water.

  

Chemical Properties of Ketones

Reactivity with Nucleophiles

• Carbonyl Group Polarisation: Facilitates reactions with nucleophiles, leading to the formation of a tetrahedral intermediate.

$\text{C}=\text{O} + \text{Nucleophile} \rightarrow \text{Alcohol}$

Resistance to Oxidation

• Stability: The presence of the carbonyl group contributes to inherent stability compared to aldehydes.

Introduction to Reaction Types

• Nucleophilic Addition: The primary reaction type involving ketones.
  • Driven by the polarisation of the carbonyl group.
  • Essential for the synthesis of various organic compounds.

Mechanism of Nucleophilic Addition

• Step-by-Step Mechanism:
  • Step 1: The nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate.
  • Step 2: Protonation restores the carbonyl group.
• Example of Important Reagents:
  • Grignard Reagents transform ketones into alcohols.
  • Hydride Ions convert ketones into secondary alcohols.

Reduction of Ketones

• Reduction Process:
  • Ketones are converted into secondary alcohols using reducing agents like $\text{NaBH}_4$ (sodium borohydride).

$\text{R}_2\text{C}=\text{O} + \text{NaBH}_4 + \text{H}_2\text{O} \rightarrow \text{R}_2\text{CHOH} + \text{NaBO}_2$

Laboratory Techniques for Identifying Ketones

• Conducting the DNPH Test
  1. Preparation of DNPH Reagent
  2. Mixing with Ketone Samples
  3. Observation: Look for a yellow/orange precipitate.

Applications of Ketones

• Industrial Applications:

  • Pharmaceuticals and Solvents:
    • Propranolol, a ketone-based drug, is used in managing heart conditions.
    • Acetone is used in nail polish removers and coatings.

• Biological and Metabolic Functions

  • Ketogenesis: Involves the formation of ketone bodies in the liver, providing energy during periods of low carbohydrate availability.

• Risks and Environmental Impact:
  • Volatility and Flammability: Proper storage is essential.