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OCR A level Chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

Are alkenes saturated or unsaturated

A

Unsaturated because they contain at least one carbon - carbon double bond

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2
Q

What is the general formula for an alkene

A

CnH2n

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3
Q

Do alkenes form a homologous series

A

Yes

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4
Q

For a carbon of the double bond, how many sigma bonds are formed and between which atoms are they formed

A

3 sigma bonds, 1 between the carbon in which it’s double bonded too and 2 to the other atoms e.g. hydrogen or carbon

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5
Q

How is a Pi bond formed

A

Formed by the sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond

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6
Q

Where is the Pi- electron density concentrated in a double bond

A

Above and below the line joining the nuclei of the bonding atoms

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7
Q

What bond (sigma or Pi) locks the carbon atoms in an alkene in place, preventing them from rotating around the double bond

A

Pi bond

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8
Q

How does the geometry differ in alkanes and alkenes

A

Rotation is possible in alkanes since there is no Pi bond whereas rotation cannot happen in alkenes due to the presence of a Pi bond.

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9
Q

What is the bond angle and shape around the carbon atoms in the double bond in an alkene

A

Trigonal planar (120)
- There are 3 regions of electron density around each carbon atom

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10
Q

What is a stereoisomer

A

A compound with the same structural formula but a different arrangement of atoms in space

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11
Q

What type of isomerism occurs in compounds with C=C double bond

A

E/Z isomers

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12
Q

Why does stereoisomers arise around a double bond

A

Because rotation around the double bond is restricted so the groups attached to each carbon is fixed

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13
Q

Why are the groups attached to each double bonded carbon restricted

A

Due to the position of the Pi bond above and below the C=C double bond

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14
Q

What 2 conditions are required for a molecule to have E/Z isomerism

A
  • A C=C double bond
  • Different groups attached to each carbon atom of the double bond
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15
Q

Why can But-1-ene not form E/Z isomers whereas But-2-ene can

A

Because one of the carbons in the double bond in But-1-ene is attached to 2 hydrogens meaning that there isn’t a different group attached to each carbon

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16
Q

What is cis-trans isomerism

A

The name commonly used to describe a special case of E/Z isomerism.

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17
Q

What extra rule differs E/Z isomerism and cis-trans isomerism

A

In cis-trans isomerism one of the attached groups on each carbon atom of the double bond must be hydrogen

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18
Q

Is But-2-ene an example of a molecule with E/Z isomers or cis-trans isomers

A

Both, because there is hydrogen attached to each carbon involved in the double bond

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19
Q

In a cis isomer, where are the hydrogens

A

On the same side of the molecule

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19
Q

In a trans isomer, where are the hydrogens

A

Diagonally opposite each other in the molecules

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20
Q

In an E isomer, are the same atoms/groups opposite each other in the molecule or on the same side of the molecule

A

Opposite

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21
Q

In a Z isomer, are the same atoms/ groups opposite each other in the molecule or on the same side of the molecule

A

The same side

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22
Q

In cis-trans isomerism, is the cis isomer the E or the Z

A

Z isomer

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23
Q

In cis-trans isomerism, is the trans isomer the E or the Z

A

E isomer

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24
Q

When naming isomers, how do you assigns priority

A

Upon their atomic number

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25
Q

If the groups of higher priority are on the same side of the double bond, what isomer is the compound

A

Z isomer

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26
Q

If groups of higher priority are diagonally placed across the double bond, what isomer is the compound

A

E isomer

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27
Q

If the 2 atoms attached to a carbon atom in the double bond are the same, what do you do when trying to identity the isomerism

A

Find the first point of difference

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28
Q

Why are alkenes much more reactive than alkanes

A

Because of the presence of the Pi bond

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29
Q

Why do Pi bonds readily break

A

Because they’re on the outside of the double bond, so they are more exposed than the electrons in the sigma bond

30
Q

What are the 4 addition reactions of alkenes

A
  • Hydrogen in the presence of a nickel catalyst
  • Halogens
  • Hydrogen halides
  • Steam in the presence of an acid catalyst
31
Q

What bond breaks when alkenes undergo addition reactions

A

The Pi bond

32
Q

What are the 2 conditions required for the hyrogenation of alkenes

A
  • Heat
  • Nickel catalyst
33
Q

What is the conditions required for the halogenation of alkenes

A

Room temperature

34
Q

What element is used when testing for unsaturation

A

Bromine

35
Q

What happens to the bromine water when it is added to an organic compound

A

The orange colour disappears and becomes colourless

36
Q

If bromine is added to any C=C bond what will happen

A

It will be decolourised

37
Q

What is the name of product when an alkene reacts with a hydrogen halide

A

Haloalkane

38
Q

How many products are there when an unsymmetrical alkene reacts with an unsymmetrical molecule e.g. HCl

A

2 possible products

39
Q

When alkenes react with steam what is the name of the group formed

A

Alcohols

40
Q

What catalyst is required when alkenes react with steam

A

Phosphoric acid (H3PO4)

41
Q

How many products can be produced when alkenes react with steam

A

2 products

42
Q

What is an electrophile

A

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

43
Q

What is the name of the mechanism, where alkenes take part in addition reactions to form saturated compounds

A

Electrophilic addition

44
Q

Does the double bond in alkenes, with the presence of a Pi bond, represent a region of high or low electron density

A

High electron density

45
Q

In an electrophilic addition reaction, what does the high electron density of the Pi bond attract

A

It attracts electrophiles

46
Q

Is an electrophile, usually an positive or negative ion or a molecule with a slightly positive or negative charge

A

Positive ion or a molecule with a slightly positive charge

47
Q

What are the 6 steps in an electrophilic addition reaction of But-2-ene and HBr

A
  1. Br is more electronegative than hydrogen, so HBr is polar and contains dipoles
  2. The electron pair in the Pi bond is attracted to the slightly positive hydrogen atom, breaking the double bond
  3. A bond is now formed between the H and the C which had the initial double bond
  4. H-Br bond breaks by heterolytic fission, the electron pair goes to the bromine
  5. Leaving a Br- ion and a carbocation
  6. Br- ion reacts with the carbocation
48
Q

What is a carbocation

A

A positively charged carbon atom

49
Q

When a non-polar molecule wants to react in a elecrophilic addition reaction how are the dipoles decided

A

When one of the atoms in the non-polar molecule approaches the Pi bond, the electrons in the non-polar molecule bond are repelled, so polarity has been induced

50
Q

What are the 3 types of carbocations

A
  • Primary
  • Secondary
  • Tertiary
51
Q

How many carbons is a primary carbocation bonded to

A

1 carbon

52
Q

How many carbons is a secondary carbocation bonded to

A

2 carbons

53
Q

How many carbons is a teritary carbocation bonded to

A

3 carbons

54
Q

What is the order of stability of carbocations

A

Primary < Secondary < Tertiary

55
Q

When drawing the electrophilic addition reaction mechanism what do the curly arrow represent

A

The bonds breaking

56
Q

What is Markownikoff’s rule

A

The major product of the addition reaction will be the one formed via the more stable carbocation intermediate and the stability is in the order primary<secondary<tertiary

57
Q

What is a polymer

A

Extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers

58
Q

What type of polymerisation do unsaturated alkenes undergo to produce long, saturated chains containing no double bonds

A

Addition polymerisation

59
Q

What 3 conditions are required in industrial polymerisation

A
  • High pressure
  • High temperature
  • Using a catalyst
60
Q

Do addition polymers have a high or low molecular mass

A

High molecular mass

61
Q

When writing out an addition polymerisation equation, what types of brackets is the repeat unit always written in

A

Square brackets

62
Q

What is the common prefix of addition polymers

A

Poly(alkene)

63
Q

What is Poly(chloroethene) more commonly known as

A

PVC

64
Q

Are the major of alkene-based products biodegradable

A

No, so they tend to grow in landfills

65
Q

Why is recycling polymers good

A

It conserves finite fossil fuels and decreases the amount of waste going into landfill

66
Q

What is the disposal of PVC so toxic

A

Due to the high chlorine content

67
Q

What is released when PVC is burnt

A

HCl, which is a corrosive gas, and other pollutants like toxic dioxins

68
Q

How do we use waste polymers as fuel

A

Waste polymers can be incinerated to produce heat, which generates steam to drive a turbine producing electricity

69
Q

What is feedstock recycling

A

Describes the chemical and thermal processes that can reclaim monomers, gases, or oil from waste polymers

70
Q

The products from feedstock recycling resemble products similar to what

A

Products produced from crude oil refineries

71
Q

What is an advantage of feedstock recycling

A

It is able to handle unsorted and unwashed polymers

72
Q

What are biodegradable polymers usually made from

A
  • Starch
  • Cellulose
  • Additives that alter the structure
73
Q

What does photodegradable mean

A

Can be broken down by light-driven chemical processes

OCR A level Chemistry (17 decks)

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