3.4 Alkenes

Question 1

What is an alkene ?

Answer 1

An unsaturated hydrocarbon with a C=C double bond.

Question 2

What is the general formula of an alkene ?

Answer 2

CnH2n

Question 3

Why is there no rotation around a C=C bond ?

Answer 3

Due to the pi bond, there is electronegativity above and below the original bond - this prevents it from rotating.

Question 4

Are alkenes more or less reactive than alkanes ?

Answer 4

They are more reactive, due to the high electronegativity of the the double bond and the pi bond is slightly easier to break.

Question 5

What intermolecular forces of attraction do alkenes have ?

Answer 5

Only Van der Waals.

Question 6

Are alkenes soluble in water ? Why ?

Answer 6

Insoluble as they have no polar bonds.

Question 7

Name the three types of isomerism alkenes have.

Answer 7

Chain isomerism (branches)
Position isomerism (C=C on different carbon molecule)
Geometric E/Z isomerism.

Question 8

Write the formula for the complete conbustion of pent-2-ene.

Answer 8

CH3CHCHCH2CH3 + 7.5O2 -> 5CO2 + 5H2O.

Question 9

What is an electrophile ?

Answer 9

An electron deficient atom/ion which accept a pair of electrons, such as from a C=C.

Question 10

What is the most stable kind of carbocation intermediate ? Why ?

Answer 10

Alkyl groups (C+) have a positive inductive effect, so the most stable version would be a tertiary (bonded to three other carbons ).

Question 11

What conditions are needed for the electrophilic addition of H2O into an alkene ?

Answer 11

An acid catalyst, typically phosphoric acid.

Question 12

What are the products of the electrophilic addition of H2O to an alkene ?

Answer 12

An alcohol.

Question 13

Describe the mechanism for the electrophilic addition of H2O to an alkene.

Answer 13

•Arrow from C=C into a H+ ion
•This forms a carbocation (C+), draw arrow from lone pair on H2O into cation
•C-O bond forms making O+, draw arrow from an O-H bond into the O+ atom.

Question 14

Draw a mechanism for the electrophilic addition of HBr and an alkene.

Answer 14

•Arrow from C=C to H on HBR, then arrow from H-Br bond to Br
•This forms a carbocation, which the new Br- ion joins (arrow from Br- lone pair to C+).

Question 15

What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene ?

Answer 15

Standard conditions and an organic solvent.

Question 16

How does a hydrogen halide molecule with a non-polar bond react as if it is an electrophile ?

Answer 16

C=C bond with a high electron density induces a temporary dipole in the halogen -> d+ molecule attracted to double bond.

Question 17

Describe/draw the mechanism for the electrophilic addition of Br2 to an alkene.

Answer 17

•Temporary dipole on Br2, one d+ one d-, induced by high electron density of C=C
•Arrow from C=C to d+ Br, then a further arrow from Br-Br to the d- Br
•This forms a carbocation, which the Br- ion joins (arrow from lone pair of Br- to C+).

Question 18

Draw/describe the electrophilic addition reaction of sulphuric acid and an alkene.

Answer 18

•Arrow from C=C to the H of SOH, further arrow from O-H bond to O
•This forms O- which has a lone pair and a carbocation
•Draw arrow from lone pair of O- to the C+.

Question 19

What forms if you add water to the product of the electrophilic addition of sulphuric acid to an alkene ?

Answer 19

It forms an alcohol.

Question 20

What is an addition polymer ?

Answer 20

When many monomers are bonded together via the rearrangement or bonds - without losing any molecules.

Question 21

What are monomers ? What form can they take ?

Answer 21

Molecules which combine to form a polymer, an example would be an alkene as the C=C breaks to form a repeating pattern.

Question 22

How would you draw the polymerisation of ethene.

Answer 22

n CH2=CH2 -> n -[-CH-CH-]- .

Question 23

Give three uses of poly(chloroethene) / PVC.

Answer 23

•Drainpipes
•Vinyl
•Aprons.

Question 24

Give two examples of plasticisers.

Answer 24

Esters and phthalates.

Question 25

What are plasticisers ?

Answer 25

Small molecules which get between polymer chains to force them apart and allow them to slide over each other (malleability).

Question 26

How do the physical properties of PVC change due to a plasticiser ?

Answer 26

•PVC with a plasticiser is flexible
•PVC without a plasticiser is rigid.

Question 27

Why do molecules containing only C-H and C-C not decompose easily ?

Answer 27

They are non polar molecules and are unreactive.

Question 28

Why is the disposal of polymers containing only C-H and C-C a problem ?

Answer 28

As they are hard to dispose of due to their high unreactivity.

Question 29

What is mechanical recycling ?

Answer 29

Where plastics are organised into their types, melted and re-moulded.

Question 30

What is feedback recycling ?

Answer 30

Where polymers are heated to a temperature where bonds break and the original monomers form.

Question 31

What is the problem with recycling plastics ?

Answer 31

They degrade slightly each time they are melted and remoulded.