Ch 10- Radical Reactions

1. Ch 10- Radical Reactions

2. 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

3. 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!

4. 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

5. Radical reactions • Halogen molecules (X2) also contain relatively weak bonds which undergo homolysis • example

6. 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.

7. 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

8. 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

9. 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:

10. 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

11. 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

12. Mechanism of the Chlorination of Methane

13. Higher Alkanes are shown the same way

14. Multiple products • Chlorination of most alkanes that contain more than 2 carbons gives a mixture of monochlorinated products • Examples:

15. 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

16. 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

17. 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!

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

19. 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:

20. Summary • For chlorinations, all products should be shown • For brominations, only the product with substitution at most reactive hydrogen needs to be shown • NBS-

21. Reactions that generate tetrahedral stereogeniccarbons • 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

22. 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:

23. 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

24. 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.

25. 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

26. 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

27. Mechanism for Anti-product • Reaction still proceeds through the most stable radical!

28. Summary of HBr addition to alkenes

29. Polymerizations • These radical reactions are also very useful in some polymerizations called chain-growth polymerizations or Addition polymerizations • Examples include the synthesis of polyethylene.

30. Mechanism with special steps • Combination • Disproportionation • Backbiting • Special catalyst, such as the Ziegler-Natta catalyst were developed specifically to prevent backbiting.

31. 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:

32. 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.