Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.2: Alkenes

Statement 14.2.1

This statement looks at three ways that alkenes can be made, in the lab or industrially. Find and read the statement in your copy of the syllabus.

Of the three parts of this statement, only one relates to material that you are likely to have met already. In the other cases, I suggest you just make a note of the reaction, including conditions, and an equation for now. Leave looking at it in any detail until it comes up later in the syllabus.

Statement 14.2.1(a)

Halogenoalkanes are compounds which have a halogen atom substituted into an alkane chain. If the compound is heated with sodium hydroxide in solution in ethanol (the ethanolic solution mentioned in the syllabus), the halogen atom is removed together with a hydrogen from a next-door carbon atom.

You will find this discussed at length on the page about Reactions between halogenoalkanes and hydroxide ions. For now just look at the section titled "Elimination reactions" and read and make notes on the few paragraphs down to the green box.

"Heating under reflux" mentioned on the page means heating with a condenser placed vertically in the flask. This stops the loss of any liquids before they have reacted - they condense and run back down into the reaction mixture. Any gas, of course, passes out through the top of the condenser and can be collected.

Statement 14.2.1(b)

This is discussed in detail on the page about Dehydration of alcohols. For now, just look at the beginning of the page showing the formation of ethene in two different ways. You will come back to this again when you look at the chemistry of alcohols in Section 16.

Statement 14.2.1(c)

This has already been covered in statement 14.1.4

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© Jim Clark 2020