properties of alkenes

Question 1

how many sigma bonds are in a carbon atom in a double bond

Answer 1

3 sigma bonds - one to the other carbon atom in the double bond and 2 to another carbon or hydrogen, leaving one electron

Question 2

how is a pi bond formed in a double bond

Answer 2

remaining electron in carbon atom is in p orbital so double bond forms from the sideways overlap of 2 p orbitals, one from each carbon atom in the double bond

Question 3

where is the pi electron density concentrated

Answer 3

pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms

Question 4

why cant the carbon atoms rotate around the double bond

Answer 4

the pi bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond.

Question 5

what is the shape around each of the carbon atoms in a double bond

Answer 5

trigonal planar because there are 3 regions of electron density around each of the carbon atoms. These 3 regions repel each other as much as possible forming a bond angle of 120 degrees. All of the atoms are in the same plane

Question 6

what are stereoisomers

Answer 6

same structural formula but different arrangement of atoms in space

Question 7

what is the difference between optical isomerism and E/Z isomerism?

Answer 7

optical isomerism also occurs in alkanes with no functional groups whereas E/Z isomerism only occurs in Carbon-carbon double bonds

Question 8

what conditions are needed for E/Z isomerism?

Answer 8

Carbon-carbon double bond
each carbon atom in double bond must have different functional groups

Question 9

what conditions are needed for cis-trans isomer

Answer 9

carbon-carbon double bond
each carbon atom in double bond must have a hydrogen atom

Question 10

what is a cis isomer?

Answer 10

hydrogen atoms of both carbon atoms in same plane.

Question 11

Is a cis isomer , a Z or E isomer

Answer 11

Z

Question 12

what is trans isomer

Answer 12

hydrogen atoms are opposite each other

Question 13

is a trans isomer, Z or E?

Answer 13

E

Question 14

why does pi bond break but not sigma bonds when alkenes react?

Answer 14

pi bonds are weaker than sigma bonds so are broken more readily

Question 15

what is the hydrogenation of alkene?

Answer 15

alkene is mixed with hydrogen over a nickel catalyst at 423 k in an addition reaction to form an alkane

Question 16

what are the conditions for hydrogenation of alkene?

Answer 16

nicket catalyst at 424K

Question 17

what type of reaction is for hydrogenation of alkene

Answer 17

addition

Question 18

what is the halogenation of alkene?

Answer 18

alkenes undergo addition reaction with chlorine or bromine at room temperature

Question 19

what is the test for a double in alkenes?

Answer 19

add drops of bromine to alkene. If bromine orange colour fades away, that means it has reacted with double bond indicating a positive result

Question 20

what is the reaction of alkenes with hydrogen halides?

Answer 20

alkene reacts with gaseous hydrogen halides in an addition reaction at room temperature to form haloalkanes.

Question 21

how does the reaction of alkene with hydrogen halide occur if alkene is a gas?

Answer 21

both gases are mixed together.

Question 22

how does the reaction of alkene with hydrogen halide occur if alkene is a liquid?

Answer 22

the hydrogen halide is bubbled through it

Question 23

what is the hydration reaction of alkenes?

Answer 23

when alkenes are reacted with steam , H20 , in the presence of a phosphoric acid catalyst, H3PO4, it forms an alcohol

Question 24

what catalyst is needed for hydration of alkenes?

Answer 24

phosphoric acid, H3PO4

Question 25

why does electrophilic addition happen?

Answer 25

double bond in alkene has high electron density because of pi bonds which attract electrophiles

Question 25

what is an electrophile

Answer 25

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

Question 26

what charge is an electrophile

Answer 26

positive

Question 27

what is a carbocation

Answer 27

a carbocation contains a positively charged carbon atom

Question 27

what is the full mechanism for electrophilic addition in addition of but-2-ene with hydrogen bromide

Answer 27

bromide is slightly negative and hydrogen is slightly positive. The electron pair in pi bond is attracted to partially positive hydrogen atom causing the double bond to break and forming a bond between carbon atom and the hydrogen atom. H-Br bond breaks by heterolytic fission with electron pair going to bromine atom. Carbocation in hydrocarbon which reacts with negative bromine atom.

Question 28

which carbocations are most stable

Answer 28

tertiary carbocation

Question 29

which carbocations are least stable?

Answer 29

primary carbocation

Question 30

what is addition polymerisation?

Answer 30

unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains

Question 31

what conditions are needed for addition polymerisation?

Answer 31

high temperature and high pressure using catalysts

Question 32

what are poly(chloroethene) used for

Answer 32

used for bottle, pipes, insulation cable due to being flexible and rigid

Question 33

what is poly(propene) used for

Answer 33

children toys, packing crates, fibre for ropes , guttering

Question 34

what is poly(phenylethene) used for

Answer 34

packaging material ,food trays and cups due to thermal insulating properties

Question 35

what is poly(tetrafluorethene) used for

Answer 35

coating for non-stick pans, permeable membrane for clothing an shoes , and cable insulation

Question 36

what are the benefits of polymers?

Answer 36

readily available, cheap, more convenient , lack of reactivity makes it suitable for storing food

Question 37

what are the disadvantages of polymers?

Answer 37

they are non-biodegradable so has negative environmental effects

Question 38

what are the ways to prevent environmental damage from polymers?

Answer 38

1) recycling polymers - chop polymers into flakes washed , dired and melted. 2) PVC recycling - recycling pvc is hazardous due to high chlorine content so releases HCL when burnt. Instead, dissolve sorted polymers using solvent and recover it by precipitation from solvent/ 3) feedstock recycling - Reclaim monomers, gases and oils from polymers which can be used as raw materials for new polymers. Polymers can be mixed and unsorted.

Question 39

what is PVC recyling?

Answer 39

PVC recycling - recycling pvc is hazardous due to high chlorine content so releases HCL when burnt. Instead, sort the polymers so they arent mixed and dissolve polymer using solvent and recover it by precipitation from solvent/

Question 40

what is feedstock recycling

Answer 40

feedstock recycling - Reclaim monomers, gases and oils from polymers which can be used as raw materials for new polymers. Polymers can be mixed and unsorted.

Question 41

what is an advantage of feedstock recycling

Answer 41

polymers can be unsorted and mixed