Chemguide: Support for CIE A level Chemistry

These statements deal with the lack of reactivity of the halogenoalkanes used in a variety of products.

Before you go on, you should find and read the statements in your copy of the syllabus.

I am unsure exactly what CIE want for this despite what the syllabus says, and the limited amount of support material available. Very little has been asked about it in exam papers so far. I will modify this page if or when it becomes more obvious what they are looking for.

What follows explores around the topic enough, I hope, for you to be able to answer any questions which arise.

The relative strengths of the carbon-halogen bonds

You should have already read the necessary bits about this. Have another quick look at:

• "the importance of bond strengths", about 3/4 of the way down the page introducing halogenoalkanes;

• "comparing halogenoalkane reactivities", about half-way down the page about reactions involving halogenoalkanes and silver nitrate solution. Just read the "Background" and "Comparing the reaction rates as you change the halogen" sections.

  The syllabus talks about hydrolysis, and the silver nitrate reaction involves hydrolysis by water in the solution, followed by reaction of the silver ions with liberated halide ions.

All you need to realise from this is that the reactivity increases as the halogen gets bigger. That means that halogenoalkanes containing chlorine and, more particularly, fluorine are relatively unreactive. In fact, the energy needed to break the carbon-fluorine bond is so high that fluoroalkanes are virtually unreactive. A fluoroalkane is a halogenoalkane containing fluorine.

Relating uses to their relative chemical inertness

For some uses and comments about some commonly used halogenoalkanes containing fluorine and chlorine, read the page uses of halogenoalkanes.

Concentrate on the CFCs (what the syllabus describes as fluorohalogenoalkanes) and fluoroalkanes (HFCs).

If you think about the uses (for example as refrigerants, or in aerosols, or in making foamed plastics), it is important that the compounds should be as chemically unreactive as possible.

You will have read that CFCs have (or, increasingly, once had) a lot of different uses partly because of their low reactivity. Carbon-chlorine bonds in the molecules are, however, broken by UV light in the high atmosphere and have caused problems with the ozone layer.

They are being replaced by HFCs which are simple fluoroalkanes. The fluorine-carbon bond isn't easily broken in the atmosphere. (More about that in statement 10.3(e).)

The relative lack of reactivity of compounds like chlorobenzene

Important! This won't be examined until the end of a two year course. If you don't already know about the structure of benzene, there is no point in reading about this for now.

The lack of reactivity of compounds like chlorobenzene is covered on the page about reactions of aryl halides. It is important that you follow the link in the section about the strength of the carbon-halogen bond to the introductory page about aryl halides. You can ignore the stuff about physical properties on that second page.

For exam purposes, CIE will ask you about the lack of reactivity of chlorobenzene in terms of the extra strength of the carbon-chlorine bonds. Don't use the additional explanation about the repulsion between the incoming nucleophile and the ring electrons.

If you are asked why chlorobenzene is unreactive towards nucleophiles, say "Delocalisation of a lone pair on a chlorine atom over the benzene ring strengthens the carbon-chlorine bond."

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