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Chemistry > Chapter 13 - Alkenes > Flashcards

Chapter 13 - Alkenes Flashcards

1
Q

What kind of hydrocarbons are alkenes ?

A

Unsaturated

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2
Q

What is the general formula of alkenes ?

A

CnH2n

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3
Q

How is a double bond formed ?

A
  • Three sigma bonds are formed to other atoms from the carbon atom
  • One electron is left in a p-orbital
  • A pi bond forms from the sideways overlap of two p orbitals, one from each carbon atom of the double bond
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4
Q

Where is the electron density focused ?

A
  • The pi electron density is concentrated above and below the line joining the nuclei of the two bonding atoms
  • The double bond has a high electron density
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5
Q

What prohibits rotation in alkenes?

A

The pi bond as it locks the two carbon atoms in position and prevents them from rotating

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6
Q

What is the shape around the carbon atoms in the double bond ?

A
  • Trigonal planar

- 120 degree bond angle

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7
Q

What is stereoisomerism ?

A

The same structural formula but a different arrangement of the atoms in space

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8
Q

What are the two types of stereoisomerism ?

A
  • E/Z isomerism

- Cis/trans isomerism

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9
Q

When is cis-trans isomerism applicable ?

A
  • When molecules have a C=C bond

- Each carbon in the double bond is attached to two different groups.

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10
Q

What is the cis isomer ?

A

Hydrogen atoms on each carbon on the same side.

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11
Q

What is the trans isomer ?

A

Hydrogen atoms diagonally opposite each other.

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12
Q

What are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?

A

Higher atomic number is higher priority.

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13
Q

E isomer

A

Groups of higher priority on different sides of the double bond, then E isomer.

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14
Q

Z isomer

A

Groups of higher priority on the same side of the double bond, then Z isomer.

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15
Q

How can you remember whether it is a Z or E isomer ?

A
  • For a Z isomer you can remember it as, Zee Zame Zide, the groups of higher priority are on the same side
  • If they arent then it is an E isomer
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16
Q

Why are alkenes more reactive than alkanes ?

A
  • They are more reactive due to the fact that they have a pi bond
  • The electrons in the pi bond are more exposed, as they are on the outside
  • The fact that the electrons are exposed leads to their higher reactivity
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17
Q

How are alkenes tested for ?

A

When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.

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18
Q

Describe the hydrogenation of alkenes

A

An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.

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19
Q

What does hydrogenation form ?

A

An alkane

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20
Q

Describe the halogenation of alkenes

A

Addition reaction between an alkene and either chlorine or bromine at room temperature to form a halogenoalkane.

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21
Q

What does the halogenation of alkenes form ?

A

Halogenoalkane

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22
Q

Describe the hydration of alkenes

A

An alkene is reacted with steam in the presence of a phosphoric acid catalyst

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23
Q

What does the hydration of alkenes form ?

A

An alcohol

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24
Q

How many products can be formed in the hydration of alkenes ?

A

Two possible products can be formed

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25
Q

How is the hydration of alkenes used in industry ?

A

This addition reaction is used widely in industry to produce ethanol from ethene

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26
Q

Describe the hydrohalogenation of alkenes

A

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes

27
Q

What does the hydrohalogenation of alkenes form ?

A

Haloalkanes

28
Q

How many products can be formed in the hydrohalogenation of alkenes ?

A

Two products can be formed

29
Q

How do you test for unsaturation ?

A

The reaction of alkenes with bromine water can be used to identify if there is a C=C bond present and the organic compound is unsaturated

30
Q

What would the observation be when adding bromine water to an alkene ?

A

If it is an alkene, the orange colour of the bromine water would disappear, indicating the presence of a C=C bond

31
Q

What does the high electron density of the pi bond attract ?

A

Electrophiles

32
Q

What are electrophiles ?

A

An electrophile is an atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair

33
Q

What are electrophiles said to be ?

A

Electron pair acceptors

34
Q

What is the mechanism for addition reactions ?

A

Electrophilic addition

35
Q

Outline an electrophilic addition reaction between an alkene and hydrogen bromide

A
  • H-Br is a polar molecule
  • Electron pair in pi bond attracted to H
  • Double bond breaks
  • Bond forms between H (in HBr) and C atom
  • H-Br bond breaks by heterolytic fission
  • Bromide ion and carbocation formed
  • Br- reacts with carbocation to form the addition product
36
Q

What is a carbocation ?

A

A compound that contains a positively charged carbon ion

37
Q

How does electrophilic addition take place with molecules that are non polar ?

A

When the molecule approaches the pi bond, the electrons in the bond are repelled and this induces polarity

38
Q

Outline an electrophilic addition reaction between an alkene and bromine

A
  • Br-Br is now an induced polar molecule
  • Electron pair in pi bond attracted to Br
  • Double bond breaks
  • Bond forms between B (in Br2) and C atom
  • Br-Br bond breaks by heterolytic fission
  • Bromide ion and carbocation formed
  • Br- reacts with carbocation to form the addition product
39
Q

What is Markownikoff’s rule ?

A
  • In electrophilic addition reactions, when the carbocation is formed it can be primary, secondary or tertiary
  • The major product is the one in which the carbocation is attached to the carbon atom with the most alkyl groups
40
Q

How are carbocations classified ?

A

They are classified by the number of alkyl groups attached to the positively charged carbon

41
Q

Which carbocation is the most stable ?

A

The tertiary carbocation is the most stable so is most likely to be the product

42
Q

Why is the tertiary carbocation the most stable ?

A
  • The connecting alkyl groups donate and push electrons towards the positive charge of the carbocation.
  • Therefore more alkyl groups means a more stable ion
43
Q

What is the name of the reaction for a polymers formation from alkenes ?

A

Addition polymerisation

44
Q

How is the monomer unit displayed ?

A

Displayed formula with an ‘n’ before it to signify many units

45
Q

How is the repeat unit of the polymer displayed ?

A
  • Displayed formula
  • In square brackets
  • An n placed after to show a large number of repeats
46
Q

What are the conditions for a polymerisation reaction ?

A

High temperature and pressure

47
Q

What are the uses of polyethene ?

A

Supermarket bags, shampoo bottles, childrens toys

48
Q

What are the uses of polychloroethene aka polyvinylchloride ?

A

Pipes, films and sheeting, insulation, bottles and flooring

49
Q

What are the uses of polypropene ?

A

Children toys, packing crates, guttering, uPVC windows, rope fibre

50
Q

What are the uses of polyphenylethene aka polysterene ?

A

Packaging material, food trays and cups

51
Q

What are the uses of polytetrafluoroethene aka PTFE or Teflon ?

A

Coating for non stick pans, permeable membrane for clothing and shoes, cable insulation.

52
Q

What are the uses of phenylethene aka styrene ?

A

Used in packing material and in food trays and cups due to its thermal properties

53
Q

What is the downside of current disposal of waste polymers ?

A

Waste polymers are very unreactive so they are non biodegradable

54
Q

How are polymers recycled ?

A

They are chopped into flakes, washed, dried, melted and then cut into pellets for use

55
Q

What are the benefits of recycling polymers ?

A

It reduces environmental impact by conserving fossil fuels

56
Q

What are the weaknesses of recycling polymers ?

A
  • Polymers have to be sorted by type

- Process is undermined if polymers are mixed as this renders the product unusable

57
Q

What is the method of recycling PVC ?

A
  • Dissolved by solvents
  • High grade PVC is recovered by precipitation
  • Solvent can be used again
58
Q

What are the dangers of PVC disposal ?

A

High chlorine content makes it hazardous

59
Q

How can polymers be used as fuel ?

A

Can be incinerated to form heat

60
Q

What is feedstock recycling ?

A

Chemical and thermal processes that can reclaim monomers, gases and oils from waste polymers

61
Q

What is a major advantage of feedstock recycling ?

A

Able to handle unsorted and unwashed polymers

62
Q

What are biodegradable polymers ?

A
  • Produced from renewable sources and can be broken down by microorganisms
  • Made from starch or cellulose
63
Q

What are photodegradable polymers ?

A

Polymers that contain bonds that are weakened by absorbing light to start degradation, or light absorbing additives are used

Chemistry (29 decks)

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