Chemguide: Support for CIE A level Chemistry

Learning outcome 15: Halogen compounds

15.1: Halogenoalkanes

Statement 15.1.3

This statement is about various substitution reactions of halogenoalkanes. These are reactions in which the halogen atom is replaced by some other atom or group of atoms.

If you read the statement in your copy of the syllabus, you will find it also mentions the term "nucleophilic". This refers to the mechanism of the reactions, and will be dealt with in statements 15.1.5 and 15.1.6. Ignore the word for now!

Statement 15.1.3(a)

This is covered in the page Reactions between halogenoalkanes and hydroxide ions.

However, that page is complicated by the fact that there are two possible different reactions between a halogenoalkane and hydroxide ions depending on the nature of the halogenoalkane and the conditions for the reaction. One of these possibilities is a substitution reaction; the other an elimination reaction in which a halogen atom and a hydrogen atom are removed to leave a C=C double bond.

The elimination reaction is dealt with in statement 15.1.4, but I suggest you read the whole of the page now concentrating on the substitution part, and then read it again later when you come to the elimination part. By the end of this section, you will need to know all that is on that page.

Statement 15.1.3(b)

This is straightforward, and is covered on the page Reactions between halogenoalkanes and cyanide ions.

Note:  In the first version of the syllabus CIE mentioned the word "heat" in the context of this reaction. I queried this because none of the common sources at this level mention it, and I wasn't convinced by the answer I received. If it is still there in updated versions of the syllabus, and you are asked about it in an exam, you will have to tell them that heat is used. They make the rules and you have to play by them.

Statement 15.1.3(c)

This statement is really only an introduction to this reaction, and you will meet it again in the second part of the course. The reaction is actually quite complicated leading to a mixture containing various kinds of amines. For the moment, all you need to do is to read the first part of the page Reactions between halogenoalkanes and ammonia dealing with making primary amines.

Checking through quite a lot of recent versions of Paper 2, it was very difficult to find this reaction asked. The only one I found before I gave up simply asked for the reactants and conditions to convert CH3CH2Br into CH3CH2NH2. That's not very scary!

Statement 15.1.3(d)

The phrasing in this statement is a bit odd where it talks about "aqueous silver nitrate in ethanol".

The halogenoalkanes are at best only slightly soluble in water and it would be a good idea to read a bit about this on the page introducing halogenoalkanes. You will find what you want just over half-way down the page.

Silver nitrate is insoluble in organic solvents, and so these reactions are often done in a mixture with aqueous silver nitrate solution added to a solution of the halogenoalkane in ethanol. The mixture of ethanol and water produced by doing this will dissolve both the silver nitrate and the halogenoalkane. That means that the two reactants are in close contact with each other.

You will find these reactions discussed on this page.

There is overlap between this statement and statement 15.1.7. For now, you could just read the first section about testing for halogenoalkanes and leave the rest for later.

That section describes the most efficient way of testing for the halogen by first doing a substitution reaction with sodium hydroxide hydroxide to free the halogen as an ion.

It does work if you just use the silver nitrate solution on the halogenoalkane alone because the water in the solution will react with halogenoalkanes to produce some halide ions. The reactions can be very slow, though. You will see this in some detail in statement 15.1.7.

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© Jim Clark 2020