Chemistry Question #2003

hart, a 40 year old female from the Internet asks on March 23, 2004,

I understand that alkenes are more reactive than alkanes. I recently read however, in a new high school text, that ethane undergoes combustion faster than ethene. Is this an exception to the generalization above or merely a typo in the text. Also, what is the mechanism of the combustion reaction? Is the oxygen molecule split to form free radicals (as occurs in addition reactions) and attach to the ethane or ethene or is there another mechanism?

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The answer

Reg Mitchell answered on March 23, 2004

Alkenes are more reactive generally because most of the reactions in the lab would be addition reactions to the double bond of an alkene, so bromination, hydrogenatation etc all are fast with alkenes and do not go with alkanes.

Substitution reactions however, which are much more difficult to do, are faster with ethane than with ethylene because the C-H bond in ethylene is stronger than ethane. Oxidation probably breaks the C-H bond and so that is why you might have read that combustion is faster.

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