4.1.3 alkenes

Question 1

what are alkenes

Answer 1

unsaturated hydrocarbons

Question 2

what is the general formula of alkenes

Answer 2

cnH2n

Question 3

what is the molecule shape around the alkene carbons and what is the bond angle

Answer 3

trigonal planar
120

Question 4

what is a sigma bond

Answer 4

a bond formed when two s orbitals overlap. the

Question 5

what is a pi bond

Answer 5

a pi bond is formed by the sideways overlap of two p orbitals

Question 6

why is a sigma bond stronger than a pi bond

Answer 6

high electron density between nuclei means there is a strong electrostatic attraction between nuclei and shared pair. this gives sigma bonds high enthalpy. in a pi bond the electron density is spread out so it is weaker

Question 7

why are alkenes more reactive than alkanes

Answer 7

the pi bond from the c=c stick out slightly above and below the bond and attacked by electrophiles

Question 8

what are stereoisomers

Answer 8

they have the same structural formula but a different arrangement of atoms in space

Question 9

what are E/Z isomers and what causes them

Answer 9

stereoisomers occur because the double bond cannot rotate.
E isomers have highest priority groups on opposite sides
Z isomers have the highest priority groups on the same side

Question 10

what groups are given priority in E/Z isomerism

Answer 10

highest atomic number has priority

Question 11

what are cis/trans isomers

Answer 11

stereoisomers when the carbon atoms have at least one group in common
cis - same groups are on the same side
trans - same groups are on opposite sides

Question 12

what is electrophilic addition

Answer 12

the double bond opens up and atoms are added to the carbon atoms

Question 13

what are the reagents/ condition to produce alkanes from alkenes

Answer 13

hydrogen (H2)
nickel catalyst and 150 degrees

Question 14

what are the reagents/ condition to produce dihaloalkanes from alkenes

Answer 14

halogens (X2)
room temperature

Question 15

how can we test for the carbon-carbon double bond

Answer 15

shaking with bromine water causes a colour change from orange to colourless

Question 16

what are the reagents/ condition to produce haloalkanes from alkenes

Answer 16

hydrogen halides (H-X)
room temperature

Question 17

how doe we determine the major product

Answer 17

the most stable carbocation forms most of the time which forms the major product

Question 18

what is markownikoff’s rule

Answer 18

the major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached

Question 19

what are the reagents/ condition to produce alcohols from alkenes

Answer 19

steam
300 degrees
60-70 atm
H3PO4 catalyst

Question 20

how do alkenes form polymers

Answer 20

the double bond opens up and bond to multiple monomers. this is called addition polymerisation

Question 21

why are polymers useful

Answer 21

they are unreactive and cheap to produce

Question 22

what is a limitation of polymers

Answer 22

they are hard to dispose of

Question 23

what is the advantages and disadvantages of land fill

Answer 23

it can pollute the land and takes up lots of space
it wastes the polymers
it is cheap and easy and quick

Question 24

what is the advantages and disadvantages of reusing polymers

Answer 24

we have to sort them which takes time
it and their properties are reduced for each reuse

Question 25

what is the advantages and disadvantages of burning polymers

Answer 25

it releases emissions and sometimes toxic gases
it can be used as an energy source

Question 26

what is a biodegradable polymer

Answer 26

a polymer that degrades pretty quickly in certain conditions

Question 27

what are biodegradable polymers made of

Answer 27

renewable raw materials such as starch

Question 28

what is a photodegradable polymer

Answer 28

polymers that decompose when exposed to sunlight