Ch 10- Radical Reactions. Radical Reactions All the reactions we have considered so far have been...
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Transcript of Ch 10- Radical Reactions. Radical Reactions All the reactions we have considered so far have been...
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.