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AS chemistry UNIT 2 module 1 > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What are alkenes?

A

Alkenes are unsaturated hydrocarbons with at least one C=C double bond.

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2
Q

What three things can alkenes do that alkanes can’t?

A
  • Aliphatic alkenes with one double bond have the general formula : CnH2n
  • Alkenes are more reactive than alkanes, and typically take part in addition reactions.
  • Alkenes can form E/Z isomers, including cis-trans isomers/
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3
Q

How is ethene made?

A

It is obtained from catalytic cracking of crude oil fractions.

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4
Q

What is the main use of ethene?

A

It is used to make polymers for the plastics industry.

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5
Q

What is the C=C bond in an alkene?

A

The C=C double bond is the functional group in an alkene and is responsible for its reactions.

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6
Q

Define pi-bond

A

A pi-bond is a reactive part of a double bond formed above and below the plane of the bonded atoms by sideways overlap of p-orbitals.

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7
Q

What two parts is a C=C bond made from?

A

The C=C bond is made up from two parts: a sigma bond and a pi bond.

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8
Q

Which of the sigma and pi-bonds in a double bond is present in alkanes?

A

The sigma bond.

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9
Q

Describe the sigma bond in a C=C.

A

A sigma bond is formed directly between two carbon atoms by the overlap of orbitals. Each carbon atom contributes one electron to the electron pair in the sigma bond.

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10
Q

Describe the pi-bond in a C=C.

A

A pie-bond is formed above and below the plane of the carbon atoms by sideways overlap of p-orbitals. Each carbon contributes one electron from a p-orbital to the electron pair in the pie-bond.

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11
Q

Why can’t the C=C double bond rotate?

A

The pie-bond fixes the carbon atoms in position, at either end of the double bond. This prevents any rotation of the bond.

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12
Q

In alkenes, each carbon involved in a double bond uses how many electrons to form pie-bonds and how many electrons to form sigma bonds?

A
  • Uses three of its electrons in the formation of three sigma bonds.
  • Uses one of its electrons in the formation of a pie bond.
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13
Q

What shape do alkene molecules have and whats their bond angle? Also explain why.

A
  • There are three regions of electron density that surround each carbon atom in the double bond.
  • The electrons pairs repel each other to give a trigonal planar shape, with bond angles of approximately 120.
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14
Q

What shape does ethene have?

A

Ethene is a planar molecule with all its atoms in the same plane.

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15
Q

What are cyclic alkenes?

A

Cyclic alkenes have closed rings of carbon atoms containing one or more double bonds.

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16
Q

What is most common cyclic alkene?

A

Cyclohexene

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17
Q

Why are alkenes more reactive than alkanes?

A

Alkenes are more reactive than alkanes due to their double bond.

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18
Q

What do bond enthalpies measure?

A

Bond enthalpies measure the strengths of covalent bonds.

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19
Q

Give the average bond enthalpy of C-C.

A

+347 KJmol^-1

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20
Q

Give the average bond enthalpy of a C=C double.

A

+612 KJmol^-1

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21
Q

What do the bond enthalpies of the C-C and C=C bond show?

A
  • A double bond is stronger than a single bond;

- A pie-bond is weaker than a sigma bond.

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22
Q

When an alkene reacts, what happens to the bonds?

A
  • The pie-bond breaks

- The sigma bond remains intact.

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23
Q

What generally happens when a alkene takes part in an addition reaction?

A
  • A small molecule is added across the double bond causing the pie bond to be broken.
  • Two reactant molecules react together to form one product molecule.
  • An unsaturated alkene reacts and forms a saturated product.
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24
Q

Show the general equation for an alkene reacting with a small molecule in an addition reaction.

A

> C=C< + a—b ——-> b>C—-C<a

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25
Q

What are the conditions needed for an addition that forms an alkene into a alkane?

A

A mixture of hydrogen gas and a gaseous alkene is passed over a catalyst of nickel at a temperature of 150* C

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26
Q

What is the name of the reaction and what happens in the reaction between an alkene and hydrogen gas in the right conditions?

A
  • The hydrogen adds across the double bond and alkane is formed.
  • The reaction is sometimes known as hydrogenation and is an example of reduction.
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27
Q

Draw the reaction for an alkene reacting with hydrogen gas.

A

Look at the bottom of page 128 for answer.

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28
Q

What conditions are needed for an alkene to react with halogens?

A

Alkenes react rapidly with halogens such as chlorine, bromine and iodine at room temperature.

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29
Q

What is the reaction name and what happens bond wise when a halogen reacts with an alkene?

A
  • The halogen adds across the double bond to give a di-substituted halogenalkane.
  • The reaction is sometimes known as halogenation.
30
Q

Draw the reaction of an alkene such as ethene with Cl2.

A

Go to page 129 to find the answer.

31
Q

What is the test for unsaturation and shows the presence of a C=C double bond?

A

-When bromine is added to a sample containing an alkene, the colour changes from orange to colourless. The colour change indicates that the bromine has reacted with the double bond. This reaction is used as a test for unsaturation and shows the presence of a double C=C bond.

32
Q

What are the conditions needed for a hydrogen halide to react with an alkene?

A

Hydrogen halides are gases at room temperature and are bubbled into liquid alkanes.

33
Q

Name some hydrogen halides and give the name of what the addition reaction between an alkene and a hydrogen halide produces?

A
  • Hydrogen halides include HCl, HBr, HI.

- Hydrogen halides are gases at room temperature and are bubbled into liquid alkanes.

34
Q

What reaction is reaction required to make an alcohol, what are the reactants and what are the conditions required? What is the name of the process?

A
  • Steam and the gaseous alkene are heated to a high temperature and pressure in the presence of a phosphoric acid catalyst.
  • The process is sometimes called the hydration of an alkene.
35
Q

Draw the reaction of ethene with steam.

A

Go to page 130 for the answer.

36
Q

Why are electrophiles attracted to the double bond in alkenes?

A

The double bond in alkenes represents a region of high electron density. This density is due to the pie- electrons in the double bond. Electrophiles are attracted to these pie- electrons.

37
Q

Define electrophilic addition.

A

Electrophilic addition is a type of addition reaction in which an electrophile is attracted to an electronrich centre or atom, where it accepts a pair of electrons to form a new covalent bond.

38
Q

Define carbocation.

A

Carbocation is an organic ion in which a carbon atom has a positive charge.

39
Q

Explain the curly arrow symbol.

A

A curly arrow is a symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of a covalent bond.

40
Q

Explain the addition reaction of hydrogen bromide with ethene.

Use diagrams with curly arrows to also explain what happens. The diagrams can be found on page 131.

A

1) Hydrogen bromide is a polar molecule. Bromine is more electronegative than hydrogen. This causes a dipole.
2) The electron pair in the pie-bond is attracted to the slightly positive hydrogen atom causing the double bond to break.
3) A new bond forms between one of the carbon atoms and the hydrogen atom.
4) The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine.
5) A bromine ion, Br- and a carbocation is formed.
6) The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product.
7) The product of this reaction is a halogenoalkane.

41
Q

What do curly arrows represent?

A

The movement of a pair of electrons.

42
Q

Explain the mechanism for the reaction of ethene with bromine.

Use diagrams with curly arrows to explain what happens also. The diagrams can be found on page 132.

A

When bromine approaches an alkene, the electrons in the pie bond repel the electrons in the Br-Br bond. This induces a dipole in the Br2 molecule.

1) The electron pair in the pie bond is attracted to the slightly positive bromine atom, causing the double bond to break.
2) A new bond forms between one of the carbon atoms and the bromine atom.
3) The Br-Br bond breaks by heterolytic fission, with the electron pair going to the bromine.
4) A bromide ion Br- and a carbocation are formed.
5) The positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product.

43
Q

What are terpenes?

A

Terpenes are alkenes built from numbers of isoprene (C5H8) molecules. The isoprene molecules can either be linked together head to tail forming chains or can e formed into rings.

44
Q

Propene reacts with Cl2 in what conditions to form what?

A

Propene reacts with Cl2 at room temperature to form 1,2-dichloroporpane.

45
Q

Propene reacts with HBr in what conditions to form what?

A

Propene reacts with HBr(g) to form either 1-bromopropane or 2-bromopropane.

46
Q

Propene reacts with H2O in what conditions to form what?

A

Propene reacts with H2O(g) with H3P04 catalyst at high temperatures and pressures to form Propan-2-ol.

47
Q

Propene reacts with H2 in what conditions to form what?

A

Propene reacts with H2 and Ni catalyst at a 150* C to form propane.

48
Q

Define polymer

A

A polymer is a long molecular chain built up from monomer units.

49
Q

Define monomer

A

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

50
Q

Define addition polymerisation.

A

Addition polymerisation is the process in which unsaturated alkene molecules(monomers) add on to a growing polymer chain one at a time, to form a very long saturated molecular chain.

51
Q

What is margarine?

A

-Margarine is made from animal or vegetable fats, mixed with skimmed milk and slat.
-Margarine made from vegetable oil is becoming popular on health grounds,as people turn away from animal fats in their diet.
-

52
Q

What are vegetable oils?

A

Vegetable oils are liquids containing long hydrocarbon chains, often with many double bonds, and are polyunsaturated. The oil must be hardened, so that it can spread on bread without soaking into it.

53
Q

Explain the process hydrogenation which hardens oils such as vegetable oils.

A
  • Hydrogenation adds hydrogen molecules across double bonds in an addition reaction.
  • Hydrogenation alters the individual molecule in the oil in such a way that the oil partially solidifies and hardens.
  • By adding hydrogen molecules across different numbers of the many double bonds, we can make margarines with different amounts of hardness. This affects the spread-ability of margarine.
54
Q

What is the disadvantage of hardening margarine through hydrogenation?

A

Partial hydrogenation of unsaturated fats can transform some cis double bonds into trans double bonds as a by-product. These trans fats are now thought to be bad for health.

55
Q

Are monomers unsaturated or saturated?

A

Monomers are unsaturated and have a double bond.

56
Q

Are addition polymers unsaturated or saturated?

A

They are saturated.

57
Q

Define addition polymer.

A

An addition polymer is a very long molecular chain, formed by repeated addition reactions of many unsaturated alkene molecules(monomers).

58
Q

Ethene monomers for what polymer?

A

They form a polymer called poly(ethene)

59
Q

How many different types of monomers are addition polymers made from?

A

Addition polymers are made from one type of monomer only.

60
Q

Describe radical polymerisation.

A

Radical polymerisation requires temperatures of 200*C and very high temperatures.

This reaction can lead to branching of the polymer chain and the production of polymer mixtures. Typically poly(styrene) and poly(ethene) are made in this way.

61
Q

Describe the Ziegler-Natta process.

A

The Ziegler-Natta process involvs the use of catalysts such as TiCl3 and Al(C2H5)2Cl at 60*C.

In this process, the alkene is passed over the catalyst. The conversion is low and any unreacted alkene is recycled and passes over the catalyst repeatedly.

62
Q

What is the most common method for the manufacture of non-branched poly(ethene)?

A

The Ziegler-Natta process.

63
Q

Define repeat unit

A

A repeat unit is a specific arrangement of atoms that occurs in the structure over and over again. Repeat units are included in brackets, outside of which is the symbol,n.

64
Q

Draw the general equation for addition polymerisation.

A

Answer in page 136.

65
Q

Why are repeat units drawn instead of the full polymer?

A

Polymers have been formed from thousands of monomer units, so its impossible to draw the full structural formula of any particular polymer. So we represent polymers in repeat units.

66
Q

Draw ethene going to its repeat unit and name it.

A

Answer found in figure 3 at page 136.

67
Q

Draw chloroethene going to its repeat unit and name it.

A

Answer can be found on figure 5 on page 136.

68
Q

Draw propene going to its repeat unit, and name it.

A

Answer can be found on page 136 on figure 4.

69
Q

Draw Phenylethene going to its repeat unit, and name it.

A

Answer can be found on figure 6 on page 136.

70
Q

Draw tetrafluoroethene going to its repeat unit, and name it.

A

Answer can be found on figure 8 on page 137.

AS chemistry UNIT 2 module 1 (13 decks)

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