alkenes [O1] PAPER 2

Question 1

what is the general formula of an alkene?

Answer 1

Cn H2n

Question 2

what is an electrophile?

Answer 2

electron pair acceptor

Question 3

name and outline the mechanism to form a halogenoalkane from an alkene and Br2 / HBr

Answer 3

• electrophilic addition
• Br2 / HBr = electrophile
(mechanism is the same for Br2 - just Br instead of H)

Question 4

name and outline the mechanism for the reaction between an alkene and sulfuric acid, and state conditions required

Answer 4

• electrophilic addition
• conditions: concentrated H2SO4, cold (typically room temp.)

Question 5

how can you test for an alkene (so unsaturated compounds in general)?

Answer 5

• add bromine water
• colour change from orange to colourless indicates alkene present

[electrophilic addition reaction takes place - bromine is added to C=C]

Question 6

are addition polymers reactive?

Answer 6

no

Question 7

why are addition polymers unreactive?

Answer 7

they are saturated compounds
[i.e. only single C-C bonds]

Question 8

why is bromine, a non-polar molecule, able to react with an alkene?

Answer 8

highly electron dense C=C bond causes an induced dipole in bromine

Question 9

how do you name an addition polymer?

Answer 9

poly(name of monomer)

Question 10

typical uses of poly(chloroethene), also known as PVC

Answer 10

making drain pipes, window frames, cable insulation, floor tiles

Question 11

are alkenes polar or non-polar?

Answer 11

non-polar
(so insoluble in water and soluble in organic solvents)

Question 12

how is PVC’s properties modified using plasticisers?

Answer 12

plasticisers make PVC more flexible

Question 13

describe the intermolecular forces in an addition polymer

Answer 13

• van der Waals forces
• the longer the polymer chain, the larger the SA and the more packed together the molecules are, so stronger vdW between molecules, so polymer is stronger and more rigid

(if no polar bonds)

Question 14

why is pure PVC more rigid?

Answer 14

• chlorine is more electronegative than carbon
• dipole-dipole forces are also present, which are stronger than van der Waals

Question 15

how to draw a repeating unit and polymer from monomer? [ADDITION POLYMERISATION]

Answer 15

• blue stuff - for drawing a monomer [monomer ALWAYS has double bond]
• orange stuff - for drawing for a unit
• green + orange stuff - for drawing a polymer
• repeating unit from polymer - entire bit that is repeated in the chain given

Question 16

why is more of the major product formed than the minor product during an electrophilic addition reaction?

Answer 16

• major product formed from a more stable carbocation
• it is the most stable as it has the most alkyl groups releasing electrons towards it – POSITIVE INDUCTIVE EFFECT
• primary carbocation = least stable, tertiary carbocation = most stable

Question 17

In an electrophilic addition reaction to form an alcohol, what does the inorganic reagent (conc. H2SO4) initially act as?

Answer 17

an electrophile

Question 18

What property of poly(chloroethene) is reduced by adding plasticiser?

Answer 18

its brittleness

Question 19

Answer 19

Question 20

Suggest why a catalyst used for a process may become less efficient if the reactants contain impurities.

Answer 20

The impurities block the active site

Question 21

Ethanol can be produced by the hydration of ethene.
The operating conditions for the process are a temperature of 300°C and a pressure of 7 MPa.
Under these conditions, the conversion of ethene into ethanol is 5%.
Deduce how an overall yield of 95% is achieved in this process without changing the operating conditions.

Answer 21

• recycle the unreacted ethene and steam
• pass the gases over a catalyst many times