8303120-040-Reactions-Mechanisms - Chem Factsheet September...

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2. Homolytic Free Radical Addition. Example: Polymerisation of ethene. The initiator is a free radical, R (which could be a peroxide radical from the oxygen catalyst). Stage 1 - Initiation. The new radical formed can then react with further ethene molecules to lengthen the polymer chain. Stage 2 - Propagation. Termination can occur when two radical chains meet, or when a radical chain attacks an established chain. Stage 3 - Termination. The end product, polyethene, consists of long chain molecules (of varying length) which are branched. Any such reaction caused by a free radical, where “replacement” free radicals are produced by the reaction, hence the reaction process continues unaided. Reaction Mechanisms C hem F actsheet September 2002 Number 40 1 Exam Hint: - Reaction mechanisms are commonly examined. As well as learning the standard examples given here, you need to practise applying the mechanisms to other similar reactions. 1. Homolytic Free Radical Substitution These reactions involve free radicals , very reactive species which have a single, unpaired electron. Example: Alkanes with chlorine. The reaction between methane and chlorine occurs in 3 stages. Stage 1 - Initiation The Cl-Cl bond is broken homolytically to form two chlorine free radicals. Energy is required to break the bond, hence the reaction only begins in the presence of UV light. Stage 2 - Propagation Cl Cl uv light Cl . Cl . + HCl + Cl . H H H H C H H H C . H 3 C Cl Cl H 3 C Cl + Cl . Cl . . Cl Cl Cl Cl . CH 3 . Cl 3 uv light Free radicals are used up and not regenerated, causing the reaction to eventually stop. The overall reaction equation is as follows: CH 4 + Cl 2 CH 3 Cl + HCl But be aware that “by-products” are also formed in such reactions, e.g. the formation of ethane (when chloromethane is the target molecule). CH 3 + CH 3 C 2 H 6 Most free radical reactions are extremely unstable and explosive. This reaction between methane and chlorine is controlled using subdued UV light. To succeed in this topic you need to: Have a good knowledge of the AS and A2 level organic chemistry reactions covered so far; Be confident in using organic terminology, nomenclature and structural formulae. After working through this Factsheet you will: Be able to represent movement of electron using the “curly arrow” convention; Have been shown the key reaction mechanisms. All chemical reactions involve the movement of electrons, and reaction mechanisms allow us to display these movements and understand more about how a reaction occurs.
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303120-040-Reactions-Mechanisms - Chem Factsheet September...

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