4.1.3 - alkenes

Question 1

what is the general formula of alkenes?

Answer 1

CnH2n

Question 2

what is the functional group in an alkene?

Answer 2

carbon-carbon double bond

Question 3

what is a sigma bond?

Answer 3

end on end overlap of orbitals directly between atoms

Question 4

what is a pi bond?

Answer 4

sideways overlap of p-orbitals to give a pi bond above and below a sigma bond

Question 5

how many sigma and pi bonds does a double bond contain?

Answer 5

1 pi, 1 sigmah

Question 6

how many sigma and pi bonds does a triple bond contain?

Answer 6

2 pi, 1 sigma

Question 7

state and explain the shape around the carbon in a carbon-carbon double bond.

Answer 7

120 - trigonal planar. all electron pairs repel. lone pairs repel more than bonding pairs. around the c atom, there are 3 regions of electron density and no lone pairs.

Question 8

why cant C-C double bonds rotate?

Answer 8

p-orbitals must stay in the same position to overlap and form a pi bond

Question 9

what is stereoisomerism?

Answer 9

stereoisomers have the same structural formulas but different arrangements of atoms in space.

Question 10

why does stereoisomerism occur?

Answer 10

C-C double bond restricts rotation

Question 11

what is “Z-isomerism”?

Answer 11

when both priority groups are on the “same side”

Question 12

what is “E-isomerism”?

Answer 12

when the priority groups are on different “sides”

Question 13

what are the Cahn-ingold-prelog rules?

Answer 13

help to assign E or Z isomerism to molecules with different atoms on either side of the bond.
assign a priority group to each side of the double bond (the atom with the highest atomic number on each side is the priority group)

Question 14

what is cis/trans isomerism?

Answer 14

a special case of E/Z isomerism in which two of the substituent groups either side of the double bond are the same e.g. pent-2-ene, where there’s a hydrogen on both sides of the double bond

Question 15

why are alkenes more reactive than alkanes?

Answer 15

pi bonds are weaker then sigma bonds. pi bonds can break and electrons can be donated to form another bond.

Question 16

what type of reactions are alkenes involved in and what is the % atom economy of these reactions

Answer 16

addition reactions
100% atom economy, only 1 product is formed

Question 17

What are the reagents and reaction conditions for the synthesising a dihaloalkane from an alkene (halogenation)?

Answer 17

X2 (e.g. Br2)
20 degrees celsius

Question 18

What are the reagents and reaction conditions for synthesising a haloalkane from an alkene (halogenation)?

Answer 18

HX (eg HBr)
20 degrees celsius

Question 19

that is the test for alkenes and what is the positive result? why does this happen?

Answer 19

add bromine water to the solution and shake.
if a double bond is present, halogenation will occur and the bromine will be used up.
bromine is orange, so when the bromine is used up, there is a colour change from orange to colourless.

Question 20

What are the reagents and reaction conditions for synthesising an alkane from an alkene (hydrogenation)?

Answer 20

Ni catalyst
150 degrees celsius
H2

Question 21

What are the reagents and reaction conditions for synthesising an alcohol from an alkene (hydration)?

Answer 21

H3PO4 catalyst
60-70 atm
300 degrees celsius
water (steam)

Question 22

what is a nucleophile?

Answer 22

electron pair donor

Question 23

what is an electrophile?

Answer 23

electron pair acceptor

Question 24

what do curly arrows represent in a mechanism?

Answer 24

movement of a pair of electrons

Question 25

describe in words the 4 steps in electrophilic addition where the electrophile is X-Y

Answer 25

1. 2 electrons from the c-c double bond attack the partially positive X creating a new bond 2. X-Y bond breaks by heterolytic fission 3. a positively charged carbocation intermediate is formed 4. Y- acts as a nucleophile and attacks the positively charged carbocation intermediate donating its lone pair of electrons, forming a new bond.

Question 26

how do unsymmetrical alkenes form a mixture of products when reacting with a hydrogen halide of water?

Answer 26

they form a mixture of carbocation intermediates, based on which carbon reacts with the hydrogen and which with the X- or OH-

Question 27

what is Markownikoff's rule?

Answer 27

the major product of the hydration/halogenation of an unsymmetrical alkene is the one that forms the most stable carbocation intermediate primary carbocation intermediates are the least stable (only bonded to 1 other alkyl group) and tertiary carbocation intermediates are the most stable (bonded to 3 other alkyl groups)

Question 28

what is a monomer?

Answer 28

single repeating unit (usually an alkene in addition polymers)

Question 29

what is a polymer?

Answer 29

long chain of repeating units

Question 30

explain the reactivity of polymers

Answer 30

polymers are saturated compounds so are chemically inert and are non-biodegradable

Question 31

give a pro and con of polymers.

Answer 31

polymers are chemically inert and so are non-biodegradable. pro - this means they're long lasting and reusable con - it means they're difficult to dispose of

Question 32

give 3 methods of disposing of waste polymers

Answer 32

burying plastic waste recycling plastics burning waste plastic

Question 33

when can waste plastic be buries (put in landfill)

Answer 33

when too difficult to separate from other waste when not in sufficient quantities to make separation financially worthwhile when too difficult to technically recycle

Question 34

explain the recycling of plastice

Answer 34

we can sort plastics into different types 1. eg polypropene can be melted and remoulded 2. some can be cracked into monomers, which can be used as organic feedstock to make more plastics/chemicals overall process involved sorting and processing

Question 35

explain the combustion as a method of disposal of waste plastic.

Answer 35

waste plastics ban be burned, and the heat is used to generate electricity toxic gases can be released i.e. PVC (polyvinyl chloride) releases HCl gas passing through scrubbers containing NaCHO3 neutralises and removes HCl

Question 36

what are the benefits of biodegradable/photodegradable polymers

Answer 36

renewable material will not run out carbon neutral for plant-based polymers plant-based polymers save energy