addition reactions of alkenes

Question 1

addition reaction

Answer 1

a reaction where two molecules react together to produce one
e.g C2H4 + H2 = C2H6

Question 2

reaction of alkenes with hydrogen

Answer 2

change in functional group: alkene –> alkane
reagent: hydrogen
conditions: nickel catalyst
type of reaction: addition/reduction

Question 3

electrophile

Answer 3

an electron pair acceptor

Question 4

electrophillic addition: reactions of alkenes

Answer 4

a pi bond is weaker than a sigma bond so less energy needed to break the pi bond. the pi bonds in alkenes are areas with high electron density. this is more accessible to electrophillic attack by electrophiles. alkenes undergo addition reactions.

Question 5

reaction of alkenes with a dihalogenoalkane

Answer 5

change in functional group: alkene –> dihalogenoalkane
reagent: halogen (bromine)
conditions: room temperature
mechanism: electrophillic addition
type of reagent: electrophile, halogen δ+ (Br δ+)
type of bond fission: heterolytic

Question 6

what happens as a Br2 molecule approaches an alkene?

Answer 6

the pi bond electrons repel the elctron pair in the Br-Br bond. this induces a dipole. Br2 becomes polar and electrophillic (Br δ+).

Question 7

carbocation

Answer 7

positive charge on a carbon atom

Question 8

reaction of alkenes with a halogenoalkane

Answer 8

change in functional group: alkene –> halogenoalkane
reagent: halogen halide (HCl or HBr)
conditions: room temperature
mechanism: electrophillic addition
type of reagent: electrophile, H δ+
type of bond fission: heterolytic

Question 9

why is HBr a polar molecule?

Answer 9

Br is more electronegative than H. H δ+ is attracted to the electron-rich pi bond.

Question 10

what if the alkene is unsymmetrical?

Answer 10

addition of hydrogen bromide can lead to two isomeric products
e.g. but-1-ene will form a mixture of 1-bromobutane and 2-bromobutane

Question 11

in electrophillic addition to alkenes where is the major product formed?

Answer 11

via the most stable carbocation intermediate

Question 12

what is the order of stability for the carbocations?

Answer 12

tertiary < secondary < primary

Question 13

what makes a carbocation stable?

Answer 13

the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stablises it

Question 14

diol mechanism

Answer 14

reagent: KMnO4 in an acidified solution
conditions: room temperature
type of reaction: oxidation
observation: purple colour of MnO4- ion will decolourise to colourless

Question 15

reaction of bromine water with alkenes

Answer 15

reagent: bromine dissolved in water (BrOH)
conditions: room temperature
type of reaction: addition
observation: orange colour of bromine water will decolourise to colourless

Question 16

hydration of alkenes to form alcohols

Answer 16

CH2CH2 (g) + H2O (l) = CH3CH2OH (l)

Question 17

essentional condition for hydration of alkenes to form alcohols

Answer 17

high temperature - 300 - 600 degrees Celsius
high pressure - 70 atm
catalyst of concentrated H3PO4

Question 18

hydration reaction

Answer 18

a reaction where water is added to a molecule

Question 19

polyalkenes

Answer 19

unreactive due to strong C-C and C-H bonds

Question 20

polyethene use

Answer 20

used to make plastic bags, buckets, bottles.
it is a flexible, easily moulded, waterproof, chemical proof and low density plastic

Question 21

polypropene use

Answer 21

stiffer polymer, used in utensils and containers and fibres in rope and carpets

Question 22

methods of disposal of waste polymers

Answer 22

incineration
recycling
feedstock for cracking

Question 23

incineration

Answer 23

rubbish is burnt and energy produced is used to generate electricity.
some toxins can be released on incineration, e.g. combustion of halogenated plastics can lead to formation of toxic, acidic waste products such as HCl.
modern incinerators burn more efficiently and most toxins and pollutants can be removed.
greenhouse gases still emitted
volume of rubbish greatly reduced

Question 24

recycling

Answer 24

saves raw materials - nearly all polymers are formed from compounds sourced/produced from crude oil. saves precious resources.
polymers need collecting/sorting - expensive process in terms of energy and manpower. polymers can only be recycled into the same type - so careful separation needs to be done.
thermoplastic polymers can be melted down and reshaped

Question 25

feedstock for cracking

Answer 25

polymers can be cracked into small molecules which can be used to make other chemicals and new polymers - saves raw materials

Question 26

biodegradeable polymers

Answer 26

can be made from substances like maize and starch