Chapter 10: Radical Reactions

Front 1

Homolysis

Back 1

- The cleavage of a covalent bond so that each fragment departs with one of the electrons of the covalent bond that joined them.

- generates radicals

-

Energy in the form of heat or light must be supplied to cause homolysis of covalent bonds

Front 2

Peroxides

Back 2

- A compound with an oxygen-oxygen single bond.

- undergo homolysis readily when heated, because the oxygen.oxygen bond is weak.

- produces 2 alkoxyl radicals

Front 3

Halogen Molecules

Back 3

- contain a relatively weak bond

- halogens undergo homolysis readily when heated or when irradiated with light of a wavelength that can be absorbed by the halogen molecule

Front 4

Reactions of Radicals

Back 4

- Almost all small radicals are short-lived, highly reactive species

- When radicals collide with other molecules, they tend to react in a way that leads to pairing of their unpaired electron

Front 5

Hydrogen Abstraction

Back 5

- To abstract an atom means to remove an atom by homolytic bond cleavage as the atom forms a bond with another radical

- gives the halogen atom an electron (from the hydrogen atom) to pair with its unpaired electron.

- the other product of this abstraction is another radical intermediate, in this case, an alkyl radical, R⋅, which goes on to react further,

Front 6

Radical Addition to a Pi Bond

Back 6

Front 7

Homolytic Bond Dissociation Energies (DH)

Back 7

- The energies required to break covalent bonds homolytically

- The order of stability of alkyl radicals is the same as for carbocations

Front 8

Radical Holgenation

Back 8

- Alkanes react with molecular halogens to produce alkyl halides

- the mechanism involves species with unpaired electrons called radicals.

- This reaction is not a nucleophilic substitution reaction

- A halogen atom replaces one or more of the hydrogen atoms of the alkane, and the corresponding hydrogen halide is formed as a by-product

- Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive due to unfavorable reaction energetics

Front 9

Multiple Halogen Substitution

Back 9

- multiple substitutions almost always occur unless we use an excess of the alkane

- ix an equimolar ratio of methane and chlorine (both substances are gases at room temperature) and then either heat the mixture or irradiate it with light

- At the outset, the only compounds that are present in the mixture are chlorine and methane, and the only reaction that can take place is one that produces chloromethane and hydrogen chloride

- As the reaction progresses, however, the concentration of chloromethane in the mixture increases, and a second substitution reaction begins to occur. Chloromethane reacts with chlorine to produce dichloromethane

- The dichloromethane produced can then react to form trichloromethane, and trichloromethane, as it accumulates in the mixture, can react with chlorine to produce tetrachloromethane.

- Each time a substitution of ─Cl for ─H takes place, a molecule of H─Cl is produced.

Front 10

Chloronation

Back 10

- A reaction in which one or more chlorine atoms are introduced into a molecule

- Chlorination of most higher alkanes gives a mixture of isomeric monochlorinated products as well as more highly halogenated compounds.

- Chlorine is relatively unselective; it does not discriminate greatly among the different types of hydrogen atoms (primary, secondary, and tertiary) in an alkane

- An exception is the halogenation of an alkane (or cycloalkane) whose hydrogen atoms are all equivalent (i.e., homotopic).

Front 11

Radical Chlorination of Methane

Back 11

- A relatively small number of light photons permits the formation of relatively large amounts of chlorinated product

- react in the dark if the gaseous mixture is heated to temperatures greater than 100 °C

- chain reaction making light reactions efficient

Front 12

Chain Reaction

Back 12

- sequential, stepwise mechanism, in which each step generates the reactive intermediate that causes the next cycle of the reaction to occur

- Step 1 is called the chain-initiating step. In the chain-initiating step radicals are created.

-Steps 2 and 3 are called chain-propagating steps. In chain-propagating steps one radical generates another

Front 13

Radical Halogenation of Ethane

Back 13

- chloronation is not selective

Front 14

Selectivity of Bromine

Back 14

- Bromine is less reactive than chlorine toward alkanes in general but bromine is more selective in the site of attack

- Bromination is selective for substitution where the most stable radical intermediate can be formed.

Front 15

Selectivity of Flourine

Back 15

- being much more reactive than chlorine, is even less selective than chlorine

- Because the energy of activation for the abstraction of any type of hydrogen by a fluorine atom is low, there is very little difference in the rate at which a 1°, 2°, or 3° hydrogen reacts with fluorine

- Reactions of alkanes with fluorine give (almost) the distribution of products that we would expect if all of the hydrogens of the alkane were equally reactive.

Front 16

Geometry of Alkyl Radicals

Back 16

- geometric structure of most alkyl radicals is trigonal planar at the carbon having the unpaired electron

- This structure can be accommodated by an sp2-hybridized central carbon

- the p orbital contains the unpaired electron

Front 17

Chloronation of Pentane

Back 17

-

When achiral molecules react to produce a compound with a single tetrahedral chirality center, the product will be a racemic form

Front 18

Stereochemistry of Chlorination at C2 of Pentane

Back 18

- the C2 hydrogens of pentane are enantiotopic because enantiomers are formed by reaction at each C2 hydrogen.

Front 19

Stereochemistry of Chlorination at C3 of (s)-2-Chloropentane

Back 19

- products of the reactions are diastereomers

- The two diastereomers are not produced in equal amounts because the intermediate radical itself is chiral, reactions at the two faces are not equally likely.

- because the two compounds are diastereomers, they have different physical properties

Front 20

Allylic Group

Back 20

- An atom or group that is bonded to an sp3-hybridized carbon adjacent to an alkene double bond

- Allylic hydrogens are especially reactive in radical substitution reactions.

Front 21

Allylic Substitution

Back 21

- Allylic hydrogens are especially reactive in radical substitution reactions

- When propene reacts with bromine or chlorine at high temperatures or under radical conditions where the concentration of the halogen is small

Front 22

Allylic Chlorination (High Temperature)

Back 22

- Propene undergoes allylic chlorination when propene and chlorine react in the gas phase at .

- the chain-initiating step, the chlorine molecule dissociates into chlorine atoms

- In the first chain-propagating step the chlorine atom abstracts one of the allylic hydrogen atoms. The radical that is produced in this step is called an allylic radical.

- In the second chain-propagating step the allyl radical reacts with a molecule of chlorine

- The chain reaction continues until the usual chain-terminating steps consume the radicals

Front 23

Allylic Bromination with N-Bromosuccinimide

Back 23

- Propene undergoes allylic bromination when it is treated with N-bromosuccinimide (NBS) in the presence of peroxides or light

- The reaction is initiated by the formation of a small amount of Br radical

- The main propagation steps for this reaction are the same as for allylic chlorination

- N-Bromosuccinimide is a solid that provides a constant but very low concentration of bromine in the reaction mixture.

Front 24

Relative Stability of Radicals

Back 24

- The reason that allylic radicals are more stable than alkyl radicals is due to electron delocalization

Front 25

Benzylic Group

Back 25

- An atom or group bonded to an sp3-hybridized carbon adjacent to a benzene ring

- The group is said to be bonded at the benzylic position

- Benzylic hydrogens are even more reactive than allylic hydrogens in radical substitution reactions due to the additional delocalization that is possible for a benzylic radical intermediate

Front 26

Anti-Markovnikov Addition of HBr

Back 26

Front 27

Radical Polymerization of Ethene

Back 27

Front 28

Ziegler-Natta catalysts

Back 28

- organometallic complexes of transition metals

- this process no radicals are produced, no back biting occurs, and, consequently, there is no chain branching

- The polyethylene that is produced is of higher density, has a higher melting point, and has greater strength

Front 29

acid-catalyzed polymerizations

Back 29

- growing chains in acid-catalyzed polymerizations are cations rather than radicals

- The catalysts used for cationic polymerizations are usually Lewis acids that contain a small amount of water

Front 30

Polymerization of Alkenes with Electron-Withdrawing Groups

Back 30

- polymerize in the presence of strong bases

- Acrylonitrile, for example, polymerizes when it is treated with sodium amide (NaNH2) in liquid ammonia

- The growing chains in this polymerization are anions

Front 31

Autooxidation

Back 31

- The reaction of an organic compound with oxygen to form a hydroperoxide

Front 32

Combustion of Alkanes

Back 32

- When alkanes react with oxygen (e.g., in oil and gas furnaces and in internal combustion engines) a complex series of reactions takes place, ultimately converting the alkane to carbon dioxide and water.

- the important reactions occur by radical chain mechanisms with chain-initiating and chain-propagating steps

- One product of the second chain-propagating step is R─OOH, called an alkyl hydroperoxide. The oxygen-oxygen bond of an alkyl hydroperoxide is quite weak, and it can break and produce radicals that can initiate other chains: