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Chemistry > AS - Alkenes > Flashcards

AS - Alkenes Flashcards

1
Q

What are alkenes?

A

Unsaturated hydrocarbons (has one or more C=C double bonds).

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2
Q

What is the general formula for alkenes with a single double bond?

A

CnH2n

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3
Q

Why are alkenes more reactive than alkanes?

A

Because the double bond is a centre of high electron density (has high concentration of electrons).

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4
Q

What is an electrophile?

A

An electron pair acceptor.

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5
Q

Give some examples of electrophiles.

A
  1. Positively charged ions e.g - H+ and NO2+

2. Polar molecules - where the slightly positively charged atom is attracted to the high electron density double bond.

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6
Q

What are the majority of the reactions of alkenes?

A

Electrophilic addition.

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7
Q

Outline the electrophilic addition reaction between an alkene and hydrogen bromide.

A
  1. Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar with the hydrogen being slightly positively charged.
  2. The electrophile is the slightly positively charged hydrogen atom in hydrogen bromide.
  3. The H of HBr is attracted to the C=C due to the double bond’s high electron density.
  4. One of the pairs of electrons from the C=C forms a bond with the H forming a positive ion called a carbocation, whilst the electrons in the H-Br bond are drawn towards the Br.
  5. The bond in H-Br breaks and both electrons in the share pair go to the bromine atom because it’s more electronegative than hydrogen, leaving a Br- ion.
  6. The Br- ion attaches to the positively charged carbon of the carbocation forming a bond with one of its electron pairs.
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8
Q

What conditions are necessary for sulfuric acid to react with an alkene?

What type of reaction takes place?

A

Must be cold, concentrated sulfuric acid.

Electrophilic addition.

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9
Q

What is the electrophile in sulfuric acid?

A

The partially positively charged hydrogen atom H-O-SO3H.

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10
Q

Outline what happens in the electrophilic addition reaction with an alkene.

What then happens if cold water is added to the product?

A
  1. The partially positively charged hydrogen atom in the sulfuric acid is attracted to the high electron density double bond of the alkene, causing a pair of electrons to leave the alkene and form a carbocation and a negatively charged hydrogensulfate ion.
  2. The hydrogensulfate ion reacts with the carbocation forming an alkyl hydrogen sulfate.
  3. If cold water is added to the product and it is warmed, it hydrolyses to form an alcohol and reform sulfuric acid.
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11
Q

What does sulfuric acid act as in the electrophilic addition reaction with an alkene?

A

As a catalyst.

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12
Q

What is a carbocation?

A

An organic ion containing a positively charged carbon atom.

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13
Q

Outline the electrophilic addition reaction between bromine and an alkene.

A
  1. The alkene’s double bond repels the electrons in bromine, polarising the Br-Br bond.
  2. The slightly positively charged bromine atom is attracted to the double bond and forms a bond with the carbon atom on the alkene. This repels the electrons in the Br-Br bond further, until it breaks.
  3. A positively charged carbocation intermediate and a bromide ion are formed. The bromide bonds to the other carbon atom forming a dihalogenoalkane.
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14
Q

How is bromine used to test for unsaturation? Describe what happens in the test.

A

When bromine water/solution is added to an alkene (just a few drops), the solution will turn from the orange/brown colour of the bromine water to colourless as the electrophilic addition reaction occurs.

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15
Q

What is an alkyl group? Give an example.

A

Alkanes with a hydrogen atom removed, e.g methyl CH3-

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16
Q

What is an asymmetrical alkene?

A

One where the double bond is not exactly in the middle of the chain.

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17
Q

How many possible products can form in the reaction between a hydrogen halide and an unsymmetrical alkene?

A

2.

18
Q

What makes a carbocation more stable and why?

A

Having a greater number of alkyl groups. Alkyl groups have a tendency to release electrons toward the more positive charge on the carbocation, making it more stable.

19
Q

Are more or less stable carbocations more likely to form?

A

More stable carbocations.

20
Q

There are two possible products in the reaction with an unsymmetrical alkene. What does the amount of each product depend on?

A

How stable the carbocation intermediate is.

21
Q

List the least stable to the most stable types of carbocations.

A

Least: primary carbocations with one alkyl group.

Secondary carbocations with two alkyl groups.

Most: tertiary carbocations with three alkyl groups.

22
Q

What are the major and minor products of an addition reaction with an unsymmetrical alkene?

A

Major product is the one that’s formed by the most stable carbocation intermediate.

The minor product is the one formed from the least stable carbocation intermediate.

23
Q

What are addition polymers formed from?

A

Alkenes and substituted alkenes.

24
Q

What are polymers?

A

Very large molecules with long chains formed from lots of small molecules, called monomers, joined together.

25
Q

What are addition polymers made from alkenes called?

A

Polyalkenes.

26
Q

How do alkenes form polymers?

A

Their double bonds open up and join together to make long chains.

27
Q

Are polyalkene chains saturated or unsaturated?

A

Saturated.

28
Q

Where are polymers found naturally?

A

In starch, proteins, cellulose and DNA and lots more.

29
Q

How have scientists been developing polymers over the years?

A

Research into polymers has led to the knowledge and understanding of the production and properties of polymers developing over time. Scientists are developing new polymers with new properties all the time.

30
Q

What is a repeating unit?

A

A bit of a molecule that repeats over and over again. Polymers are made up of repeating units.

31
Q

How do you draw a repeating unit from an alkene monomer?

A

Draw it as you see it but replace the double bond with a single bond. Square brackets can surround the repeating unit with an n to represent the fact that this repeats n times.

32
Q

How do you draw the repeating unit from a polymer?

A

Work out which part of the chain is repeating. For an addition polymer, the repeating unit should be two carbons long.

33
Q

How are addition polymers named?

A

The word poly is written in front of the bracketed alkene monomer name. Eg poly(ethene).

34
Q

Are addition polymers reactive or in reactive? Why?

A

Unreactive - the chains are saturated and non-polar.

35
Q

Which polymers can be polar? What intermolecular forces will these polymers have?

A

Polymers that contain electronegative atoms. These have permanent dipole-dipole forces.

36
Q

What intermolecular forces do non-polar polyalkenes have?

A

van der Waals.

37
Q

What types of polyalkenes are stronger, and which are weaker? Why?

A

Long, strait chains tend to be strong and rigid because the longer the chains and the closer together they can get, the stronger the van der Waals forces are.

So short, branched chains tend to be weaker and more flexible.

38
Q

What does PVC stand for? What monomer is it formed from?

A

Poly(chloroethene), made from chloroethene.

39
Q

Describe the structure and function of PVC.

A

PVC has long, closely packed chains making it hard but brittle at room temperature. Used to make drainpipes and window frames.

40
Q

How can PVC’s properties be modified using a plasticiser?

A

Adding a plasticiser to a polymer makes it more flexible. The plasticiser molecules get between the polymer chains and push them apart. This reduces the strength of the IMF between the chains so the slide over one another making the polymer easy to bend.

Plasticised PVC is much more flexible than rigid PVC so is used to make electrical cable insulation, flooring tiles and clothing.

Chemistry (34 decks)

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