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A-Level Chemistry Kieren > 9 - Alkenes > Flashcards

9 - Alkenes Flashcards

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1
Q

What is an alkene?

A

An unsaturated hydrocarbon

it has a double bond

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2
Q

What is general formula of an alkene?

A

C~n H~2n

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3
Q

What is the functional group of alkenes?

A

C=C

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4
Q

What type of reactions do alkenes take part in?

A

Addition reactions

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5
Q

How can you test to see if a substance is an alkene?

A

You react the substance in bromine water

If it is an alkene, the bromine water will turn from brown to colourless

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6
Q

What is the general formula of cycloalkenes?

A

C~n H~(2n-2)

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7
Q

What is the general formula of arenes?

A

C~n H~n

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8
Q

What is an example of an arene?

A

Benzene

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9
Q

How do you need to draw alkenes as displayed formulas?

A

All bond angles have to be at their maximum

No matter what it ends up looking like

A double bond has the same repulsion as a single bond

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10
Q

What does the number in the middle of an alkene mean?

A

The carbon which the double bond is on

E.g. but-2-ene:

Double bond is between carbon 2 and carbon 3

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11
Q

What does diene mean?

A

When there are 2 double bonds

There will be 2 numbers before

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12
Q

What is a double bond made from?

A

The first C-C bond is a sigma-bond

The second part is a pi-bond

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13
Q

What are valence electrons?

A

Outer electrons used in bonding

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14
Q

In what 3 ways are sigma-bonds formed?

A

When 2 S-orbitals overlap

When an S and a P overlap

When 2 P-orbitals overlap

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15
Q

Generally, when do sigma-bonds form?

A

When there is a direct overlap of atomic orbitals

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16
Q

How is the pi-bond of a double bond formed?

A

The P-orbitals around the carbons which have a sigma-bond overlap

This overlap is the pi-bond

in short: The pi-bond is caused by sideways overlap of P-orbitals

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17
Q

What effect does the pi-bond have on alkenes?

A

The pi-bond prevents rotation of atoms around the double bonded carbons

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18
Q

What are stereoisomers?

A

Molecules with the same structural formula but different arrangements of atoms in space

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19
Q

What are the 2 types of stereoisomers?

A

E-isomers (trans)

Z-isomers (cis)

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20
Q

What are E-isomers and Z-isomers?

A

E-isomers:
When the 2 largest groups which are bonded to a central carbon are on the opposite sides of the molecule

Z-isomers:
When the largest groups are on the same side

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21
Q

How do you show if an alkene etc is an E or Z isomer in the name?

A

Just put E- or Z- before the name

Like E-but-2-ene

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22
Q

What 2 things have to be true for a substance to have stereoisomers?

A

Double bond (so that there is no rotation)

Each carbon in the double bond is bonded to 2 different atoms or groups

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23
Q

What are Cis and Trans isomers?

A

A Cis isomer is the Z isomer with a hydrogen on each double bonded carbon

A Trans isomer is the E isomer with a hydrogen in each double bonded carbon

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24
Q

What do you do when classifying stereoisomers in very complex organics?

A

use the C-I-P rules:

If the 2 highest priority are on the same side, it’s Z and the opposite for E

The priority of an atom is higher if it’s atomic mass is greater

You have to compare the different parts of the structural formulas until one has a higher priority part than another

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25
Q

How reactive are alkenes compared to alkanes and why?

A

alkenes are more reactive

The pi-bond can break and more sigma-bonds can form

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26
Q

How do pi and sigma bonds compare in terms of strength?

A

Sigma-bonds are stronger (higher bond enthalpy) than pi-bonds

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27
Q

How do you test for alkenes and cycloalkenes?

A

Add bromine water

If it is an alkene or cycloalkene

The bromine water will turn from orange to colourless

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28
Q

What is the reaction for bromine water added to alkenes?

A

Example:

Ethene + Bromine –> 1,2-dibromoethane

29
Q

What type of reaction is the one between ethene and bromine and why?

A

An addition reaction

Because the bromine is added across the double bond with one bromine bonding to each carbon

30
Q

Which bond breaks in the reaction of ethene and chlorine?

A

The pi-bond between the double bonded carbon

31
Q

What is the reaction between cyclohexene and fluorine?

A

They make 1,2-difluorocyclohexane

32
Q

Define electrophile

A

Electron pair acceptor

33
Q

What are electrophiles attracted to and why?

A

The double bond

There is a high electron density there

34
Q

What is the full name of reactions between alkenes and halogens etc?

A

Electrophilic addition reactions

35
Q

What are the electrophiles in the electrophilic addition reactions?

A

The halogen or hydrogen which adds across the double bond

36
Q

What is the name of the reaction where hydrogen is added?

A

Hydrogenation

37
Q

What are the conditions for hydrogenation?

A

Nickel catalyst at 423K

38
Q

How is margarine made?

A

Hydrogenation of plant oils

39
Q

What is the addition of halogens called?

A

Halogenation

40
Q

What is the charge of electrophiles and why?

A

Positive

Because they are attracted to electron pairs

41
Q

What is Markownikoff’s rule?

A

The isomer with the tertiary carbocation is favoured over the isomer with the secondary carbocation

Secondary > primary as well

42
Q

How can you tell which carbocation the isomer had?

A

You look at the branches off the main chain

43
Q

How do you denote a carbocation?

A

The number then a circle as a superscript

Secondary carbocation is 2^o

44
Q

Explain why hexane is saturated

A

There is only C-C bonds

No C double bond C

(You have to say C-C instead of single bonds)

45
Q

Define polymer

A

A long molecular chain built up from monomer units

46
Q

Define monomer

A

A small molecule that combines with many other monomers to form a polymer

47
Q

What are the 2 requirements for a monomer?

A

Based on alkenes

Unsaturated

48
Q

What is the name of the reaction used to make polymers?

A

Addition polymerisation

49
Q

How does addition polymerisation work?

A

Double bonds are opened up

A long chain of carbons forms

50
Q

How do you draw the addition polymerisation of poly(propene)?

A

Do the whole thing but only draw the double bond part as displayed and draw the CH~3 coming off a bond

Do the same for the polymer side of the equation

51
Q

What is the repeat unit?

A

The thing before the n on the polymer side

52
Q

What is the advantage and disadvantage of the lack of reactivity of polymers?

A

They are suitable for storing food

They often take a very long time to biodegrade

53
Q

What are the 2 advantages of recycling polymers?

A

It reduces the amount of polymers going to landfill

There is only a finite amount of polymers that can be made and recycling reuses the same polymers over and over again

54
Q

Describe the polymer recycling process

A

The polymers are first sorted by type

Mixed polymers can’t be recycled

The polymers are then chopped into flakes

They are then washed and dried

Then they are melted and reformed into small pellets

55
Q

What are the 2 disadvantages of recycling PVC?

A

Hazardous due to:

High chlorine content

Range of additives present in the polymers

56
Q

What are the 2 disadvantages of putting PVC into landfill?

A

When it is burnt:

The corrosive gas hydrogen chloride is released

Along with other pollutants like toxic dioxins

57
Q

Describe the old and new way to recycle PVC

A

old and new way to recycle PVC
We used to grind it and reuse it

We now use solvents to dissolve the PVC and high grade PVC is recovered by precipitation from the solvent

The solvent can now be reused

58
Q

What do we do with some polymers instead of recycling and why?

A

We use them as fuels by burning them to generate heat

Because they have a high stored energy value

59
Q

What is feedstock recycling?

A

The chemical and thermal processes which can reclaim monomers, gases or oil from waste polymers

60
Q

What is a big advantage of feedstock recycling?

A

It can handle unsorted or unwashed polymers

61
Q

What are bioplastics?

A

Plastics produced from starch, cellulose, plant oils and proteins

62
Q

What are the 2 advantages of bioplastics?

A

Protect our environment

Help conserve our finite oil reserves

63
Q

What are biodegradable polymers made from normally?

A

Starch or cellulose

Or a normal polymer contains additives so it can be broken down

64
Q

How are biodegradable polymers broken down?

A

Broken down by microorganisms into water, carbon dioxide and biological compounds

65
Q

What are compostable polymers and what are they usually based on?

A

Degrade and leave no visible or toxic residues

Based on poly(lactic acid)

66
Q

How can supermarket bags be changed so that they are compostable?

A

Made from plant starch

Used as bin liners so that the food and the bag can be composted together

67
Q

What are compostable plates, cups etc made from?

A

Sugar cane fibre to replace polystyrene

68
Q

Sugar cane fibre to replace polystyrene

A

Oil based polymers which contains bonds which are weakened by light to start the degradation

Or light absorbing additives can be used

69
Q

What are the reagents and conditions for electrophilic addition of alkenes?

A

Steam

Acid catalyst

A-Level Chemistry Kieren (25 decks)

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