Haloalkanes and Haloarenes

  • Reaction of Haloarenes with Metals

Introduction to Haloalkanes

  • Haloalkanes are organic compounds containing halogen atoms (fluorine, chlorine, bromine, or iodine) attached to an alkane chain.
  • General formula: R-X (R is alkyl group and X is a halogen atom)
  • Also known as alkyl halides or alkane halides.

Properties of Haloalkanes

  • Physical state: Liquid or solid at room temperature.
  • Boiling point: Higher than that of corresponding alkanes.
  • Melting point: Higher than that of corresponding alkane.
  • Solubility: Limited solubility in water.
  • Density: Usually higher than that of water.

Classification of Haloalkanes

  1. Primary (1°): Carbon with the halogen is attached to only one other carbon atom.
  1. Secondary (2°): Carbon with the halogen is attached to two other carbon atoms.
  1. Tertiary (3°): Carbon with the halogen is attached to three other carbon atoms.

Nomenclature of Haloalkanes

  • Named as alkyl halides, with the halogen name as a suffix.
  • The carbon chain is numbered to have the halogen atom as the lowest possible number.
  • Common naming examples:
    • Chloroethane: CH3CH2Cl
    • 1-Bromobutane: CH3CH2CH2CH2Br

Reaction of Haloalkanes with Metals

  • Haloalkanes can react with metals to produce organometallic compounds.
  • The reaction is a nucleophilic substitution where a metal takes the place of the halogen atom.
  • Example: CH3Br + 2 K → CH3K + KBr

Mechanism of Reaction with Metals

  1. Formation of carbocation intermediate: Halogen atom leaves and forms a carbocation.
  1. Nucleophilic attack: Metal acts as a nucleophile and attacks the carbocation, leading to an organometallic compound.
  1. Ionic bond formation: The metal replaces the halogen, creating a new ionic bond.

Example 1: Reaction with Sodium

  • Haloalkanes react with sodium to form alkyl sodium compounds.
  • Example: CH3Br + 2 Na → CH3Na + NaBr

Example 2: Reaction with Magnesium

  • Haloalkanes react with magnesium to form Grignard reagents.
  • Example: CH3Br + Mg → CH3MgBr

Reaction with Magnesium (cont.)

  • Grignard reagents are highly reactive and can be used in various organic synthesis reactions.
  • They can react with a wide range of electrophiles such as carbonyl compounds (aldehydes, ketones, esters) to form new carbon-carbon bonds.

Haloalkanes and Haloarenes - Reaction of Haloarenes with Metals

  • Haloarenes are aromatic compounds containing a halogen atom (fluorine, chlorine, bromine, or iodine) attached to an aromatic ring.
  • The reaction of haloarenes with metals is quite different from that of haloalkanes.

Reaction of Haloarenes with Sodium

  • When a haloarene is treated with sodium metal in the presence of dry ether, the halogen atom is replaced by a sodium atom.
  • Example: C6H5Br + 2 Na → C6H5Na + NaBr

Reaction of Haloarenes with Magnesium

  • Haloarenes can also react with magnesium to form arylmagnesium halides, also known as Grignard reagents.
  • Example: C6H5Br + Mg → C6H5MgBr

Mechanism of Reaction with Metals

  • The reaction of haloarenes with metals involves the formation of a radical intermediate.
  • The halogen atom leaves, forming a radical, which then reacts with the metal to form an organometallic compound.

Uses of Organometallic Compounds

  • Organometallic compounds formed from the reaction between haloalkanes or haloarenes and metals have several uses in organic synthesis.
  • They can be used as nucleophiles in various reactions.
  • They can react with carbonyl compounds to form new carbon-carbon bonds.

Example: Reaction of Haloalkane with Grignard Reagent

  • Haloalkanes can react with Grignard reagents to form alcohols by nucleophilic substitution.
  • Example: CH3Br + C6H5MgBr → CH3C6H5 + MgBr2

Example: Reaction of Haloarene with Grignard Reagent

  • Haloarenes can react with Grignard reagents to form new carbon-carbon bonds.
  • Example: C6H5Br + C6H5MgBr → C6H5C6H5 + MgBr2

Limitations of the Reaction

  • The reaction of haloalkanes or haloarenes with metals may not work for certain substrates that are unstable or highly reactive.
  • Some haloarenes may require the use of special conditions or catalysts for the reaction to proceed.

Safety Considerations

  • When working with haloalkanes or haloarenes and metals, it is important to follow proper safety precautions.
  • Use appropriate protective equipment, such as gloves and safety goggles.
  • Work in a well-ventilated area or under a fume hood to minimize exposure to fumes.

Summary

  • Haloalkanes and haloarenes can undergo reactions with metals to form organometallic compounds.
  • Haloalkanes react with metals to form alkyl metal compounds, while haloarenes form aryl metal compounds.
  • The reaction mechanism involves the formation of a carbocation or radical intermediate.
  • Organometallic compounds have various applications in organic synthesis.

Reaction of Haloarenes with Metals

  • Haloarenes are aromatic compounds containing a halogen atom attached to an aromatic ring.
  • The reaction of haloarenes with metals is different from that of haloalkanes.
  • Example: C6H5Br (Bromobenzene)

Reaction of Haloarenes with Sodium

  • When a haloarene is treated with sodium metal in the presence of dry ether, the halogen atom is replaced by a sodium atom.
  • Example: C6H5Br + 2 Na → C6H5Na + NaBr

Reaction of Haloarenes with Magnesium

  • Haloarenes can also react with magnesium to form arylmagnesium halides, known as Grignard reagents.
  • Example: C6H5Br + Mg → C6H5MgBr

Mechanism of Reaction with Metals

  • The reaction of haloarenes with metals involves the formation of a radical intermediate.
  • The halogen atom leaves, forming a radical, which then reacts with the metal to form an organometallic compound.

Uses of Organometallic Compounds

  • Organometallic compounds formed from the reaction between haloarenes and metals have several uses in organic synthesis.
  • They can be used as nucleophiles in various reactions.
  • They can react with carbonyl compounds to form new carbon-carbon bonds.

Example: Reaction of Haloarene with Grignard Reagent

  • Haloarenes can react with Grignard reagents to form new carbon-carbon bonds.
  • Example: C6H5Br + C6H5MgBr → C6H5C6H5 + MgBr2

Example: Reaction of Haloarene with a Carbonyl Compound

  • Grignard reagents can react with carbonyl compounds such as aldehydes and ketones.
  • Example: C6H5MgBr + CH3COCH3 → C6H5CH3 + MgBr(OCCH3)

Limitations of the Reaction

  • The reaction of haloarenes with metals may not work for certain substrates that are unstable or highly reactive.
  • Some haloarenes may require the use of special conditions or catalysts for the reaction to proceed.

Safety Considerations

  • When working with haloarenes and metals, it is important to follow proper safety precautions.
  • Use appropriate protective equipment, such as gloves and safety goggles.
  • Work in a well-ventilated area or under a fume hood to minimize exposure to fumes.

Summary

  • The reaction of haloarenes with metals forms organometallic compounds.
  • Sodium and magnesium are commonly used metals in these reactions.
  • Organometallic compounds have various applications in organic synthesis, including carbon-carbon bond formation and nucleophilic reactions.