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Chemguide: Support for CIE A level Chemistry Learning outcome 10.3(a) This statement covers a number of important reactions of the halogenoalkanes. Before you go on, you should find and read the statement in your copy of the syllabus. Important Despite the fact that this is all included in one statement, there is a lot of chemistry here. Take your time over it, and don't restrict yourself to what the syllabus says - CIE certainly don't, if you look at their questions! For the purposes of this statement, ignore anything you come across in the Chemguide pages relating to mechanisms. That is covered in statement 10.3(b). Statement 10.3(a)(i) and (ii) Important background work Start by reading the page introducing halogenoalkanes. It is especially important that you understand the difference between primary, secondary and tertiary halogenoalkanes. You should also be aware of the pattern of the different carbon-halogen bond strengths. You should read the section about the physical properties of the halogenoalkanes, but there is nothing much new in this - so don't spend too much time on it. The reactions with water and hydroxide ions The reaction with water The reaction of a compound with water is described as hydrolysis. Halogenoalkanes such as bromoethane only react with water relatively slowly. The bromine atom in bromoethane is replaced by an -OH group to give an alcohol - in this case, ethanol.
This reaction has no particular importance, except that it can be used to compare the reactivities of various halogenoalkanes. You will find more about this a little further down this page when I talk about the use of silver nitrate solution in these reactions. The reaction with hydroxide ions This is a much more important reaction, because it is much faster than the reaction with water. Their are two quite different ways in which halogenoalkanes react with hydroxide ions:
You will find both of these types of reaction discussed on the page about the reactions between halogenoalkanes and hydroxide ions. There is a lot to learn here. You need to be able to write the formula for the product of any substitution or elimination reaction you are asked about. And you need to know which is the more likely type of reaction based on the kind of halogenoalkane and the conditions for the reaction. This problem mainly applies to secondary halogenoalkanes which can do either reaction. For CIE purposes, you can assume that primary halogenoalkanes mainly undergo substitution, whereas tertiary ones mainly undergo elimination. Using silver nitrate solution with these reactions Whether you use water or hydroxide ions, the halogen atom removed from the halogenoalkane ends up in solution as a halide ion - chloride, bromide or iodide. You can test for these halide ions by adding silver nitrate solution to give precipitates of the silver halide. If you are using hydroxide ions during the reaction, these must first be neutralised by adding dilute nitric acid before you add the silver nitrate solution. If you don't, you will each time get a brown precipitate of silver oxide. These reactions can be used to find out what sort of halogen was present in the halogenoalkane, and also to investigate the rates of the reactions between various halogenoalkanes and water. You will find all this discussed on the page about reactions involving halogenoalkanes and silver nitrate solution. You will find that there is no mention of this in the syllabus, but in the 7 lots of past papers (plus a specimen paper) that I had at the time of writing (December 2010), this had been asked 3 times. I suspect that CIE would justify this by saying that tests for halide ions are included under Group 7 of the Periodic Table. But few candidates would make this connection under pressure in an exam unless they had already come across it. Anyway, you now know about it! The reactions with cyanide ions You will find everything that you need to know on the page about reactions between halogenoalkanes and cyanide ions. The reactions with ammonia These reactions get seriously complicated, but fortunately you only need to know about the first stage of a series of reactions. A primary amine is formed by substituting the halogen atom by an -NH2 group. Read the beginning of the page about reactions between halogenoalkanes and ammonia, stopping when you get to the end of the bit about making primary amines. The rest of the page isn't required for CIE purposes. At the end of the section on making primary amines, you will find a green box with a link to a page about the products of the reaction. Follow that link, and look at the various ways that you might come across for writing an overall equation for the reaction. In the question papers I have available (last checked August 2013), there is no clear indication of which of these ways of writing the equation CIE are looking for. There was one question where they talked about using an excess of ammonia, so I suspect that they might be content with the second possibility for writing a single equation for the reaction. Personally, I don't believe that you can write a single meaningful equation for this reaction, and you will find wherever this reaction comes up on Chemguide that I have used the two equation version discussed at the bottom of the page. Don't worry too much about this. In principle, they should accept any accurate version. If you are asked for a single equation, use either the first or second possibility on that page. I will add to this page when it becomes clearer what CIE actually want.
© Jim Clark 2010 (modified August 2013) |
