Alkenes

Question 1

What is an orbital overlap?

Answer 1

a singly occupied orbital on one atom overlaps with a singly occupied orbital on another atom

Question 2

What is a sigma bond?

Answer 2

2 p-orbitals each with 1 electron overlap end to end

Question 3

What is a pi bond?

Answer 3

2 p-orbitals each with 1 electron overlapping sideways

Question 4

What shape are alkenes?

Answer 4

trigonal planar (120’) around each c

Question 5

Why are alkenes rigid?

Answer 5

rotation is not possible around the double bond as it would destroy the pi bond

Question 6

Why are alkenes more reactive than alkanes?

Answer 6

pi bonds are weaker than sigma bonds

Question 7

What are alkenes?

Answer 7

unsaturated hydrocarbons that form a homolugus series

Question 8

What is the alkene general formula?

Answer 8

CnH2n

Question 9

What are stereoisomers?

Answer 9

compounds with the same formular but different arrangement in space

Question 10

What are the CIP rules for naming stereoisomers?

Answer 10

assign each of the two groups attached to 1 carbon priority - highest atomic no of atome directly bonded to carbon has priority
if they have the same atomic no. look at the next atom

Question 11

What is a Z isomer?

Answer 11

priority groups on the same side of the C=C

Question 12

What is an E isomer?

Answer 12

priority groups on the opposite sides of the C=C

Question 13

What is the Cis/trans system?

Answer 13

the C=C must be attached to two different groups and 1 of the attached groups must be hydrogen

Question 14

What is a Cis isomer?

Answer 14

two identical groups on the same side of the C=C

Question 15

What is a trans isomer?

Answer 15

two identical groups on opposite sides of the C=C

Question 16

What type of reaction is dehydration of alcohols?

Answer 16

elimination = water is removed

Question 17

Where does the double bond form after dehydration of an alcohol?

Answer 17

forms next to where the OH was removed from, there may be two possible products

Question 18

What is Markowinkoff’s rule?

Answer 18

the major product of the addition reaction will be the one formed via the more stable carbocation intermediate. The stability is in the order primary < secondary < tertiary

Question 19

What is electrophyllic addition?

Answer 19

an addition reaction where a chemical compound containing a double or triple bond has a π bond broken, with the formation of two new σ bonds.

Question 20

What do curly arrows show in a reaction mechanism?

Answer 20

represent the movements of a pair of electrons

Question 21

What is the reaction mechanism of alkene and bromine?

Answer 21

1. polarisation of bromine - the Br molecule is non-polar and it is polaried by the pi bond in a nearby alkene
   the electron pair in the pi bond is attracted to the Br+ causing the double bond to break
2. formation of the intermediate - the pi bond in the alkene breaks and the bromine acts as an electrophile accepting the electrons, the br-br bond breaks by heterolytic fission, a carbocation and bromide ion are formed
3. carbocation rapidly reacts with the bromide ion forming the product

Question 22

What is an electrophile?

Answer 22

an electron pair acceptor

Question 23

What is a carbocation?

Answer 23

an ion with a positively charged carbon atom

Question 24

What is the hydration reaction of a alkene?

Answer 24

adding steaming H2O (g)
heat and H3PO4 catalyst used
produces alkane with hydroxyl group - OH

Question 25

What is Bromination of an alkene?

Answer 25

adding liquid or aqueous bromine breaks the double bond and forms an alkane with two bromines attached
colour change - orange –> colourless if double bond present

Question 26

What happens when you add a hydride to an alkene eg HCl?

Answer 26

the chlorine atom and the H atom breaks the double bond and forms an alkane

Question 27

What is the dehydration reaction of an alcohol to form an alkene?

Answer 27

alkane with hydroxyl group heated with H2SO4 catalyst forms an alkene

Question 28

What is hydrogenation of an alkene?

Answer 28

add H2 heat and nickle catalyst to form a alkane

Question 29

What is the advantage of combusting polymers?

Answer 29

they are good fuels and burning them releases a lot of energy
also reduces landfill sites

Question 30

What is the issue with combusting polymers?

Answer 30

produces toxic waste products

Question 31

How are polymers used in the feedstock for the production of new plastics and other organic chemicals?

Answer 31

polymers can be broken down and the products reacted further to produce a range of other useful organic compounds

Question 32

What is the advantage of biodegradable and photodegradable polymers?

Answer 32

they break down in the envrionment on a relatively short timescale also reduces landfill sites

Question 33

Why do flames look different for comustion of alkanes and alkenes?

Answer 33

alkenes have more carbon to hydrogen so more soot is produced

Question 34

What are all the reactions of alkenes?

Answer 34

hydration- add steam to alkene - alcohol formed
hydrogenation - H2 and Ni = alkane formed
hydrogen halide - makes haloalkane
halogen - makes haloalkane

Question 35

What reaction forms an alkene?

Answer 35

dehydration of an alcohol with heat and acid catalyst

Question 36

What test is used to identify C=C (unsaturation)?

Answer 36

alkanes reacted with bromine
bromine water and alkene forms haloalkane
orange solution (bromine water) —> colourless soltuion
- if C=C is present

Question 37

What determines the carbocation stability?

Answer 37

the electron-donating ability of akyl groups = each alkyl group pushes and donates electrons towards the + charge of the carbocation
more alkyl groups = more the + charge is spread out so more stable

Question 38

What is a repeat unit?

Answer 38

the specific arrangement of atoms in the polymer molecules that repeats over and over again

Question 39

What are the uses of poly(ethene)?

Answer 39

supermarket bags
straws
toys
shampoo bottles

Question 40

What are biodegradable polymers?

Answer 40

made of starch/cellulose and are broken down by microorganisms

Question 41

What are photodegradable polymers?

Answer 41

oil-based polymers