Ch 10- Radical Reactions

Radical Reactions

• All the reactions we have considered so far have been ionic reactions.

• Ionic reactions are ones where covalent bonds break heterolytically.

• Another type of reaction, called radical reactions, have mechanisms where bonds break homolytically.

• This is called Homolysis

Radical Reactions

• These reactions produce intermediates with unpaired electrons called radicals (aka free radicals)

• Example

• Note: we are using single headed arrows to show the movement of single electrons!

Radical Reactions

• To produce radicals, energy must be supplied• This is usually done by heating or irradiating

with light• For example, the oxygen-oxygen bond in

peroxides is broken easily with heat to produce two alkoxyl radicals

• ex

Radical reactions

• Halogen molecules (X2) also contain relatively weak bonds which undergo homolysis

• example

Reactions of Radicals

• Almost all small radicals are short-lived, very reactive species.

• They react to pair their unpaired electron• If the other molecule is a radical, it produces a

normal molecule• Ex• If the other molecule is not a radical, then a radical

is produced• Ex.

Reactions of Radicals

• The last example is another form of Hydrogen Abstraction.

• The radical can also add to another molecule• Ex

• Radicals are classified as primary, secondary, and tertiary according to the Carbon that has the unpaired electron

Radical stability

• General stability of radicals:

• Note: This is the same stability series as we saw for carbocations, and it is for the same reasons!

• The carbon of the alkyl radical is sp2 hybridized with the unpaired electron located in the pure p orbital

Reactions of alkanes with Halogens

• Alkanes react with the first three members of the halogen family through radical reactions

• Order of reactivity:

• This reaction is a substitution reaction called halogenation

• General reaction:• Example:

Multiple Substitution reactions vs Selectivity

• One characteristic of alkane halogenation is that multiple substitutions almost always take place

• Monosubstitution can be maximized by using a large excess of alkane.

• Chlorinations of most alkanes give a mixture of monosubstituted products because the chlorine radical is not very selective.

• The ratio of products is governed by statistics, that is more opportunity

Mechanism

• Because radicals are so reactive, we can’t show the mechanism in a linear fashion

• Instead, we have to list possible steps, and group the steps into three groups:– Initiation steps– Propagation steps– Termination steps

Mechanism of the Chlorination of Methane

Higher Alkanes are shown the same way

Multiple products

• Chlorination of most alkanes that contain more than 2 carbons gives a mixture of monochlorinated products

• Examples:

Multiple products

• The ratio of products that we obtain from chlorination reactions of higher alkanes are not identical with what we would expect if all hydrogen atoms were equally reactive.

• There is a correlation between reactivity of the different hydrogens and the type of hydrogen being replaced

• The tertiary hydrogens are most reactive, followed by secondary, and then primary hydrogens are the least reactive

Statistics vs Reactivity

• Consider the following reaction:

• Stats:

• Experimentally, we see we actually get twice as much secondary substitution, so we can conclude that secondary hydrogens are twice as reactive as primary hydrogens

Statistics vs Reactivity

• We can do the same thing to get information on tertiary hydrogens with the following:

• Experimentally, we see that we get almost 4 times the amount of tertiary substitution, so tertiary hydrogens are 4 times as reactive as primary hydrogens!

Estimate the product percentages

• Estimate the percentage of each product formed when 2-methylbutane undergoes chlorination.

Selectivity of Bromine

• Bromine is less reactive towards alkanes and as a result, much more selective in the site of reaction than chlorine.

• Bromine shows a much greater ability to discriminate among the different types of hydrogen atoms

• Bromine will go almost exclusively to the site of the most reactive hydrogen

• Examples:

Summary

• For chlorinations, all products should be shown

• For brominations, only the product with substitution at most reactive hydrogen needs to be shown

• NBS-

Reactions that generate tetrahedral stereogenic carbons

• When achiral molecules react to produce a compound with a single, tetrahedral, stereogenic carbon, the product will be obtained as a racemic form.

• This will always be true in the absence of any chiral influence on the reaction such as an enzyme, or the use of a chiral solvent

Reactions that generate tetrahedral stereogenic carbons

• This is true for the same reasons that Sn1 reactions yielded racemic mixtures

• Both proceed through an achiral immediate

• example:

Reactions that generate tetrahedral stereogenic carbons

• If a molecule already has a chiral center and creates another during a radical reaction, the products will be diastereomers

• They will not be produced equally!• The two faces are different because of the original

chiral center

Reactions that generate tetrahedral stereogenic carbons

• The radical reacts with chlorine to a greater extent at one face than the other although we can not easily predict which.

• Because the two products are diastereomers, they should have different physical properties and can be easily separated.

Anti-Markovnikov addition of HBr to Alkenes

• Markovnikov created his rule in 1870.• However, reactions would sometimes give

Markovnikov products and sometimes give the Anti-Markovnikov product.

• Scientists at the University of Chicago were able to discover what was going on

Anti-Markovnikov addition of HBr to Alkenes

• They found that when the reaction was done in the presence of peroxides, the Anti-Markovnikov was formed.

• The reason people were getting different products was that peroxides were formed by the action of atmospheric oxygen

• HF, HCl, and HI DO NOT give anti-markovnikov products even with peroxides present

Mechanism for Anti-product

• Reaction still proceeds through the most stable radical!

Summary of HBr addition to alkenes

Polymerizations

• These radical reactions are also very useful in some polymerizations called chain-growth polymerizations or Addition polymerizations

• Examples include the synthesis of polyethylene.

Mechanism with special steps

• Combination• Disproportionation• Backbiting

• Special catalyst, such as the Ziegler-Natta catalyst were developed specifically to prevent backbiting.

Polymer Properties

• The less branching a polyethylene polymer has, the tighter the chains can pack together, thus the higher the density, higher the melting point, and the stronger it is.

• Other examples of Addition polymers:

Extra Reading

• Read the Special Topics I passed out and be able to write a brief essay describing polymerizations including such terms as:

• Chain-growth/addition polymers, cationic polymerizations, anionic polymerizations, copolymer, atactic, syndiotactic, and isotactic and give examples of each.