MSc Reactive Intermediates: Radicals Formation and Stability

These flashcards cover key terms and concepts related to the formation, stability, and reactions of radicals as discussed in the lecture notes.

Radicals

Molecular entities with unpaired electrons, characterized by S = ½.

Homolysis

The process by which radicals are formed by breaking weak bonds, usually involving the movement of single electrons.

Bond dissociation energy (BDE)

The measure of stability of carbon-based radicals, indicating the energy required to break a bond homolytically.

Capto-stabilisation

A form of stabilization that lowers the energy of the singly-occupied molecular orbital (SOMO) by π-acceptor groups.

Dative-stabilisation

Stabilisation that raises the SOMO energy but stabilises the lone pair through π-donor groups.

Radical chain reaction

A reaction that proceeds via initiation, propagation, and termination steps, often involving radicals forming and reacting to create products.

Markovnikov selectivity

The preference for the electrophilic addition of HX to an alkene to occur where the hydrogen atom attaches to the carbon with more hydrogen atoms.

Radical halogenation

The process of substituting hydrogen atoms in hydrocarbons by halogen atoms (like Cl or Br) through radical reactions.

Allylic bromination

The radical bromination of alkenes to create allylic bromides, often using N-bromosuccinimide (NBS).

Dehalogenation

Reactions that involve breaking weak carbon-halogen bonds and forming stronger bonds, often using bu3SnH (tributyltin hydride).

Radicals

Molecular entities with unpaired electrons, characterized by S = ½.

Homolysis

The process by which radicals are formed by breaking weak bonds, usually involving the movement of single electrons.

Bond dissociation energy (BDE)

The measure of stability of carbon-based radicals, indicating the energy required to break a bond homolytically.

Capto-stabilisation

A form of stabilization that lowers the energy of the singly-occupied molecular orbital (SOMO) by π-acceptor groups.

Dative-stabilisation

Stabilisation that raises the SOMO energy but stabilises the lone pair through π-donor groups.

Radical chain reaction

A reaction that proceeds via initiation, propagation, and termination steps, often involving radicals forming and reacting to create products.

Markovnikov selectivity

The preference for the electrophilic addition of HX to an alkene to occur where the hydrogen atom attaches to the carbon with more hydrogen atoms.

Radical halogenation

The process of substituting hydrogen atoms in hydrocarbons by halogen atoms (like Cl or Br) through radical reactions.

Allylic bromination

The radical bromination of alkenes to create allylic bromides, often using N-bromosuccinimide (NBS).

Dehalogenation

Reactions that involve breaking weak carbon-halogen bonds and forming stronger bonds, often using bu3SnH (tributyltin hydride).

Anti-Markovnikov Addition

The addition of HBr to alkenes in the presence of peroxides, where the bromine adds to the less substituted carbon, typically via a radical mechanism.

Relative Stability of Alkyl Radicals

The order of stability for carbon-based radicals: tertiary (3^\circ) > secondary (2^\circ) > primary (1^\circ) > methyl, due to hyperconjugation.

Radicals

Molecular entities with unpaired electrons, characterized by S = ½.

Homolysis

The process by which radicals are formed by breaking weak bonds, usually involving the movement of single electrons.

Bond dissociation energy (BDE)

The measure of stability of carbon-based radicals, indicating the energy required to break a bond homolytically.

Capto-stabilisation

A form of stabilization that lowers the energy of the singly-occupied molecular orbital (SOMO) by π-acceptor groups.

Dative-stabilisation

Stabilisation that raises the SOMO energy but stabilises the lone pair through π-donor groups.

Radical chain reaction

A reaction that proceeds via initiation, propagation, and termination steps, often involving radicals forming and reacting to create products.

Markovnikov selectivity

The preference for the electrophilic addition of HX to an alkene to occur where the hydrogen atom attaches to the carbon with more hydrogen atoms.

Radical halogenation

The process of substituting hydrogen atoms in hydrocarbons by halogen atoms (like Cl or Br) through radical reactions.

Allylic bromination

The radical bromination of alkenes to create allylic bromides, often using N-bromosuccinimide (NBS).

Dehalogenation

Reactions that involve breaking weak carbon-halogen bonds and forming stronger bonds, often using bu3SnH (tributyltin hydride).

Anti-Markovnikov Addition

The addition of HBr to alkenes in the presence of peroxides, where the bromine adds to the less substituted carbon, typically via a radical mechanism.

Relative Stability of Alkyl Radicals

The order of stability for carbon-based radicals: tertiary (3^\circ) > secondary (2^\circ) > primary (1^\circ) > methyl, due to hyperconjugation.

Initiation step (Radical Chain Reaction)

The first step in a radical chain reaction where radicals are created from non-radical precursors, often by homolysis of a weak bond through heat or UV light.

Propagation step (Radical Chain Reaction)

The middle step in a radical chain reaction where product radicals are formed, but new radicals are also generated, continuing the chain with no net change in the number of radicals.

Termination step (Radical Chain Reaction)

The final step in a radical chain reaction where two radicals combine to form a stable, non-radical product, thereby ending the chain.

Hyperconjugation (in radicals)

The stabilizing interaction in radicals where electrons from adjacent C-H or C-C sigma bonds delocalize into the singly-occupied molecular orbital (SOMO), contributing to the stability of alkyl radicals (e.g., tertiary > secondary > primary).

Role of N-bromosuccinimide (NBS)

In allylic bromination, NBS serves as a source of low concentration of bromine atoms, ensuring that the radical chain reaction primarily targets the weaker allylic C-H bonds rather than adding across the alkene double bond.

Tributyltin hydride (bu3SnH)

A commonly used reagent in radical dehalogenation reactions, where it acts as a hydrogen-atom donor, reducing alkyl halides to alkanes via a radical mechanism.