Chapter 13- Alkenes

Question 1

what are alkenes

Answer 1

unsaturated hydrocarbons where the molecule contains at least one carbon to carbon double bond in their structure

Question 2

alkenes general formula

Answer 2

CnH2n but note cyclic alkenes and alkenes with more than one double bond do not follow this general formula

Question 3

How are pi bonds made

Answer 3

, the sideways overlap of the 2 p-orbitals result in the formation of a pi bond.

Question 4

why is rotation possible around every atom in alkanes and not alkenes?

Answer 4

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

Question 5

what is the shape of an alkene

Answer 5

trigonal planar, 3 regions repel each other as far aprt as possible so bond angle is 120.

Question 6

give one example of alkenes in the natural world

Answer 6

Alkene Limonene C10H16 is found naturally in the rinds of citrus fruits and is responsible for the smell and flavour of oranges and lemons

Question 7

define stereoisomers

Answer 7

compounds that have the same structural formula but different arrangements of the atoms in space
2 types; E/Z and cis/trans

Question 8

give a reason for how stereoisomerism is possible in alkenes.

Answer 8

The restricted rotation of groups attached to each c atom around the double bond by position of pi- bonds electron density below the plane of the sigma bond allows for this

Question 9

what are the two conditions necessary for a molecule to have E/Z isomerism?

Answer 9

• a c to c double b

- different groups attached t oeach carbon atom of the double bond.

Question 10

how is E/Z isomerism different to cis/trans isomerism ?

Answer 10

Cid/trans isomerism requires one of the attached groups on each c atom of the double bond to be a h atom

Question 11

Using the cahn-ingold-prelog nomenclature, how are groups prioritised?

Answer 11

if the groups of higehr priority are on the same size then= z isomer
if grops of higher P are diagonally placed across double b then = E isomer
The higher the atomic numebr the higehr the priority

Question 12

Difference between E/Z isomerism and cis/trans isomerism

Answer 12

E/Z - isomers with different arrangments in space and locked rotation around double bond
cis/trans - a type of E/Z isomerism that arises when there are 2 H’s and Two non-hydrogen functional groups attached to each carbon.

Question 13

why are alkenes unsaturated hydrocarbons?

Answer 13

contains 1 or more carbon to carbon double bonds

Question 14

electrophile

Answer 14

electron pair acceptor

Question 15

addition reaction

Answer 15

2 or more molecules bonded to make one product

Question 16

what is hydrogenation of an alkene and what are the conditions?

Answer 16

• hydrogen added across double bond
• over a nickel catalyst at 423K in a c
• ADDITION REACTION where alkene becomes saturated = alkane

Question 17

give an example of an everyday use of alkanes and the effects of this on health?

Answer 17

vegetable and animal oil goes under hydrogenation t o form margarine - forming trans double bonds - trans- fats
raises cholesterol levels

Question 18

hydration and the conditions

Answer 18

• water molecule in form of steam added across the C-C double bond - producing alcohol
• ## high pressure and a phosphoric/sulfuric acid catalyst

Question 19

halogenation and conditions

Answer 19

• halogen molecule added across C-C double bond
• room temp .
• alkene becomes saturated and a dihaloalkane is formed .

Question 20

describe the test for saturation?

Answer 20

mix alkene with BR2 water = bromine molecules add across double bond = change of colour from orange brwn to colourless

Question 21

Through what mechanism does halogenation take place?

Answer 21

electrophilic addition

Question 22

reaction of alkene with hydrogen halides = what and under what conditions?

Answer 22

• hydrogen halide is added across the C-C double bond .
• room temperature
• if alkene is a gas (ethene) then two gases are mixed and reaction takes place
• if alkene is a liquid then the hydrogen halide is bubbled through it.
• forms dihaloalkanes

Question 23

what is Markownikoff’s rule?

Answer 23

states that when a hydrogen halide reacts with an unsymmetrical alkene , the H of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms.

Question 24

polymers

Answer 24

repeating units of monomers

Question 25

conditions required to form addition polymers ?

Answer 25

high temperature

high pressure

Question 26

pro’s and cons of recycling polymers ;

Answer 26

• conserving finite fossil fuels can be conserved and the amount that goes into landfill is reduced
  • Polymers can be reused by a manufacturer to make another product
• con; time and money intensive

Question 27

pro’s and con’s of stability of polymers (lack of reactivity )

Answer 27

• pro ; makes it useful for storing food and chemicals safely
• con ; challenge of disposal , alkene based polymers are non-biodegradable so polymer waste can kill marine life

Question 28

pro’s and cons of burning polymers?

Answer 28

• can be used as a fuel due to high stored energy value releasing carbon dioxide
• CO2 gas contributes to global warming
• HCL may be produced because addition polymers usually consist of hydrogen and cl attached to C

Question 29

feedstock recycling ?

Answer 29

the monomers of a waste polymer are collected, and can then be reused to make other polymers or new organic molecules.

Question 30

what are the two types of biodegradable polymers?

Answer 30

biodegradable polymers

photodegradable polymers

Question 31

biodegradable polymers pros?

Answer 31

Polymers that are affected by the action of
microorganisms and environmental conditions,
leading to decomposition
- used as bioplastics for supermarket bags and bin liners

Question 32

photodegradable polymers pros?

Answer 32

• used when plant-based materials are not available. Polymers that chemically break down when supplied with sunlight.

Question 33

define hydrocarbon

Answer 33

hydrocarbons that have double or triple covalent bonds between the adjacent carbon atoms.

Question 34

How does boiling point vary amongst alkenes?

Answer 34

• BP increases with molecular mass (increased induced dipole-dipole interactions)
• ## greater degree branching lowers the BP- less surface area of contact = weaker London forces. (induced D-D interactions)

Question 35

solubility of alkenes?

Answer 35

• alkenes are non-polar molecules so tehy are insoluble in water but soluble in organic solvents

Question 36

why are alkenes non-polar molecules ?

Answer 36

• made of carbons and Hydrogens and C-H bonds so little diff. in EN = non- polar.

Question 37

what is the shape around the double bond of an alkene?

Answer 37

planar