Radicals and Free Radical Reactions

Radicals and Free Radical Reactions

• A radical is an atom or species with an unpaired number of electrons, which can have an odd number of electrons.
• Radicals are formed through two types of bond cleavages: homolytic and heterolytic.

Homolytic Bond Cleavage

• A bond breaks equally, resulting in two radicals with one unpaired electron each.
• Example: Bromine (Br2) breaks down into two bromine radicals (Br·) under heat or UV light.

Heterolytic Bond Cleavage

• A bond breaks unequally, resulting in two ions with opposite charges.
• Example: Carbon-bromine bond breaks, with bromine pulling the electrons towards itself, forming a carbocation and a bromide ion.
• Carbon-hydrogen bond breaks, with carbon pulling the electrons towards itself, forming a carbanion and a hydrogen ion.

Free Radical Reactions

• Initiation: A neutral molecule turns into two radicals.
• Propagation: One radical reacts with a molecule to form another radical.
• Termination: Two radicals react to form a molecule.

Chlorination of Methane

• Methane (CH4) reacts with chlorine gas (Cl2) to form methyl chloride (CH3Cl) and hydrochloric acid (HCl).
• Mechanism:
  • Initiation: Cl2 breaks down into two chlorine radicals (Cl·) under heat or UV light.
  • Propagation: Cl· reacts with methane to form a methyl radical (CH3·) and HCl.
  • Propagation: Cl· reacts with CH3· to form CH3Cl and regenerate Cl·.

Reactivity of Halogens

• Chlorine (Cl2) is highly reactive and non-selective, substituting hydrogen atoms randomly.
• Bromine (Br2) is less reactive and selective, preferring to substitute secondary hydrogens.
• Iodine (I2) is not reactive enough to replace hydrogen atoms.
• Fluorine (F2) is too reactive and dangerous to use.

Selectivity of Halogens

• The reactivity of halogens decreases from top to bottom of the periodic table (F > Cl > Br > I).
• As reactivity decreases, selectivity increases.
• Chlorine is less selective due to small differences in activation energy for abstracting primary, secondary, and tertiary hydrogens.
• Bromine is more selective due to large differences in activation energy for abstracting different hydrogens.

Relative Reactivity Rates

• Chlorine: 5 (tertiary) > 3.8 (secondary) > 1 (primary)
• Bromine: 1600 (tertiary) > 82 (secondary) > 1 (primary)

Radicals and Free Radical Reactions

• Radicals are atoms or species with an unpaired number of electrons, which can have an odd number of electrons.
• Radicals are formed through homolytic and heterolytic bond cleavages.

Homolytic Bond Cleavage

• A bond breaks equally, resulting in two radicals with one unpaired electron each.
• Example: Bromine (Br2) breaks down into two bromine radicals (Br·) under heat or UV light.

Heterolytic Bond Cleavage

• A bond breaks unequally, resulting in two ions with opposite charges.
• Example: Carbon-bromine bond breaks, with bromine pulling the electrons towards itself, forming a carbocation and a bromide ion.
• Example: Carbon-hydrogen bond breaks, with carbon pulling the electrons towards itself, forming a carbanion and a hydrogen ion.

Free Radical Reactions

• Initiation: A neutral molecule turns into two radicals.
• Propagation: One radical reacts with a molecule to form another radical.
• Termination: Two radicals react to form a molecule.

Chlorination of Methane

• Methane (CH4) reacts with chlorine gas (Cl2) to form methyl chloride (CH3Cl) and hydrochloric acid (HCl).
• Mechanism involves initiation, propagation, and termination steps:
  • Initiation: Cl2 breaks down into two chlorine radicals (Cl·) under heat or UV light.
  • Propagation: Cl· reacts with methane to form a methyl radical (CH3·) and HCl.
  • Propagation: Cl· reacts with CH3· to form CH3Cl and regenerate Cl·.

Reactivity of Halogens

• Chlorine (Cl2) is highly reactive and non-selective, substituting hydrogen atoms randomly.
• Bromine (Br2) is less reactive and selective, preferring to substitute secondary hydrogens.
• Iodine (I2) is not reactive enough to replace hydrogen atoms.
• Fluorine (F2) is too reactive and dangerous to use.

Selectivity of Halogens

• The reactivity of halogens decreases from top to bottom of the periodic table (F > Cl > Br > I).
• As reactivity decreases, selectivity increases.
• Chlorine is less selective due to small differences in activation energy for abstracting primary, secondary, and tertiary hydrogens.
• Bromine is more selective due to large differences in activation energy for abstracting different hydrogens.

Relative Reactivity Rates

• Chlorine: 5 (tertiary) > 3.8 (secondary) > 1 (primary)
• Bromine: 1600 (tertiary) > 82 (secondary) > 1 (primary)

Radicals and Free Radical Reactions

• Radicals are atoms or species with an unpaired number of electrons, which can have an odd number of electrons.
• Radicals are formed through homolytic and heterolytic bond cleavages.

Homolytic Bond Cleavage

• A bond breaks equally, resulting in two radicals with one unpaired electron each.
• Example: Bromine (Br2) breaks down into two bromine radicals (Br·) under heat or UV light.

Heterolytic Bond Cleavage

• A bond breaks unequally, resulting in two ions with opposite charges.
• Example: Carbon-bromine bond breaks, with bromine pulling the electrons towards itself, forming a carbocation and a bromide ion.
• Example: Carbon-hydrogen bond breaks, with carbon pulling the electrons towards itself, forming a carbanion and a hydrogen ion.

Free Radical Reactions

• Initiation: A neutral molecule turns into two radicals.
• Propagation: One radical reacts with a molecule to form another radical.
• Termination: Two radicals react to form a molecule.

Chlorination of Methane

• Methane (CH4) reacts with chlorine gas (Cl2) to form methyl chloride (CH3Cl) and hydrochloric acid (HCl).
• Mechanism involves initiation, propagation, and termination steps:
  • Initiation: Cl2 breaks down into two chlorine radicals (Cl·) under heat or UV light.
  • Propagation: Cl· reacts with methane to form a methyl radical (CH3·) and HCl.
  • Propagation: Cl· reacts with CH3· to form CH3Cl and regenerate Cl·.

Reactivity of Halogens

• Chlorine (Cl2) is highly reactive and non-selective, substituting hydrogen atoms randomly.
• Bromine (Br2) is less reactive and selective, preferring to substitute secondary hydrogens.
• Iodine (I2) is not reactive enough to replace hydrogen atoms.
• Fluorine (F2) is too reactive and dangerous to use.

Selectivity of Halogens

• The reactivity of halogens decreases from top to bottom of the periodic table (F > Cl > Br > I).
• As reactivity decreases, selectivity increases.
• Chlorine is less selective due to small differences in activation energy for abstracting primary, secondary, and tertiary hydrogens.
• Bromine is more selective due to large differences in activation energy for abstracting different hydrogens.

Relative Reactivity Rates

• Chlorine: 5 (tertiary) > 3.8 (secondary) > 1 (primary)
• Bromine: 1600 (tertiary) > 82 (secondary) > 1 (primary)