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 (X 2 ) 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 sp 2 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 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.