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An alkene's pi electrons are exposed and loosely held, so the C=C acts as a nucleophile: in electrophilic addition it gives its pair to an electrophile, a carbocation forms, and an anion completes the addition — one reagent molecule per C=C.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will explain why an alkene reacts with electrophiles, order the steps of HBr addition, match reagents to addition products, read the bromine-water test, and count the reagent a polyene takes up.
From lesson 1 you know a C=C is one sigma bond and one pi bond, and from unit 5 that a nucleophile gives a pair of electrons to an electrophile. An alkene's pi bond is a nucleophile.
Addition turns a C=C into a C–C with one new group on each carbon. An electrophilic addition starts with an electrophile taking the pi electrons. A saturated compound has no C=C; an unsaturated one has at least one.
A pi bond's electrons sit in two lobes above and below the plane of the C=C, further from the nuclei than a sigma bond's and held less tightly. They are exposed, and an electrophile can take them.
Adding HBr to ethene:
The result, bromoethane, has lost the pi bond and gained an H and a Br. Every electrophilic addition follows this shape: H and Br from HBr, Br and Br from Br2, H and OH from acid-catalysed water.
Another way: table
Additions to ethene.
| Reagent | Adds | Product |
|---|---|---|
| HBr | H, Br | bromoethane |
| Br2 | Br, Br | 1,2-dibromoethane |
| H2O, H+ | H, OH | ethanol |
| H2, Ni | H, H | ethane |
Each C=C takes up exactly one molecule of an adding reagent. So one mole of buta-1,3-diene takes two moles of Br2 or H2, and a fat or oil can be graded by how much iodine or hydrogen it absorbs — the basis of margarine-making, which hydrogenates some of an oil's C=C bonds to make a firmer fat. Bromine water, orange, turns colourless as its bromine adds across C=C bonds, which makes a quick test for unsaturation.
An alkene pi bond cannot react because it is part of a double bond. The pi bond is the reactive part.
Addition replaces a hydrogen. It adds two groups and removes none.
Each C=C takes two Br2. It takes one.
Ethane decolourises bromine water. It has no pi bond to react.
It has two C=C bonds.
Count the double bonds.
Each takes one Br2.
One reagent per pi bond.
So one mole takes up two moles of Br2 when fully brominated.
Two C=C, two Br2.
How many C=C per molecule?
One.
How much H2 is needed?
3 mol of H2, one for each mole of C=C.
How many moles of bromine, Br2, does one mole of cyclohexene take up when every double bond reacts?
Answer:
Ethene reacts readily with HBr, but ethane does not. Why?
Put the events of HBr adding to propene in order.
Number the steps in order (write the number in the box):
Match each reagent to the product of its addition to ethene.
| bromoethane | 1,2-dibromoethane | ethanol | ethane | |
|---|---|---|---|---|
| HBr | ||||
| Br2 | ||||
| H2O with an acid catalyst | ||||
| H2 with a nickel catalyst |
Orange bromine water is shaken with four samples. Mark every sample that contains a C=C.
This task has no paper form; do it on a device.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
How many moles of hydrogen, H2, are needed to turn every C=C of $4$ mol of limonene, from orange peel, which has two C=C bonds into C–C?
The reaction needs a mol of hydrogen.
You can see the pi bond as the electron source in an addition. Tell someone why ethene decolourises bromine water and ethane does not. Next: which carbon the hydrogen goes to — regiochemistry.
8. Your turn: 3 mol of cyclohexene with hydrogen, step 3