13. Alkenes

Question 1

What is a pi bond?

Answer 1

the sideways overlap of the 2 p orbitals creating electron density above and below the plane of atoms

Question 2

Can C=C double bonds rotate?

Answer 2

no <3

Question 3

What is the double bond in alkenes?

Answer 3

an area of localised high electron density which attracts electrophiles

Question 4

What is the relative strength of pi to sigma bonds

Answer 4

pi is weaker so is relatively easy to break

Question 5

What is the bond angle around a C=C bond?

Answer 5

120 degrees because there are 3 bonded regions around each atom and they repel as far apart as possible
all atoms are in the same plane

Question 6

Define a stereoisomer

Answer 6

isomers have the same structural formula, but the atoms occupy a different position in space

Question 7

Why does E/Z isomerism occur?

Answer 7

due to the restricted rotation of C=C bonds

Question 8

What is the difference between E/Z and cis/trans isomerism?

Answer 8

E/Z just must contain a C=C bond where each carbon is bonded to 2 different groups
cis/trans requires one of those groups to be a the same on each carbon

Question 9

How does cis/trans relate to E/Z ?

Answer 9

cis=Z : same side of C=C

trans=E : opposite side of C=C

Question 10

Define an electrophile

Answer 10

an electron pair acceptor

: usually possesses a d+ dipole or positive charge somewhere in its structure

Question 11

Why does a pi bond attract an electrophile?

Answer 11

a pi bond is an area of high electron density and an electrophile wants to “accept” those electrons

Question 12

What is a C=C bond made of?

Answer 12

a sigma bond and a pi bond, with the electron density above and below the plane of the sigma bond

Question 13

What are the conditions for the hydrogenation of alkenes?

Answer 13

alkene + hydrogen + nickel/platinum catalyst @ 423K

Question 14

How do you test for unsaturation?

Answer 14

bromine water

Question 15

addition reactions of alkenes with halides

Answer 15

learn

Question 16

What are the conditions for reaction of alkene hydration to alcohols?

Answer 16

alkene + steam + phosphoric acid catalyst

Question 17

What breaks in electrophilic addition?

Answer 17

the pi bond

Question 18

How does a dipole become induced in Br2 in electrophilic addition?

Answer 18

as the Br2 approaches the area of high electron density around the C=C bond, the electrons of the Br2 molecule move as far away as possible, creating a d +ve charge at the end of the molecule closer to the alkene

Question 19

Where should the + go on a forming carbocation to get the major product?

Answer 19

on the carbon bonded to the most other carbons

Question 20

Which is the most stable carbocation?

Answer 20

tertiary

Question 21

Why is a tertiary carbocation always the major product when possible?

Answer 21

it is the most stable so is formed more readily

Question 22

What are the conditions for the dehydration of alcohols to alkenes?

Answer 22

alcohol + conc. sulphuric/phosphoric acid + reflux

Question 23

What are the problems with polymers?

Answer 23

unreactive and non-biodegradable (carbon chain is non-polar and covalent bonds are strong)
adds to landfill problem on disposal

Question 24

How can we dispose polymers safely/less dangerously? (4 ways)

Answer 24

recycling : but is high cost
burn waste : saves landfill and produces energy
purification: toxic fumes can be removed from burnt chlorinated polymers
feedstock : reuse polymers as useful organic compounds

Question 25

How can polymers be used for feedstock?

Answer 25

new tech converts waste to hydrocarbons

hydrocarbons turned back into polymers

Question 26

Draw the mechanism for the reaction of H-Cl with ethene.

Answer 26

go google it or smth idk

Question 27

What is the name of the mechanism used to form haloalkanes from alkenes?

Answer 27

electrophilic addition

Question 28

What are the conditions for the electrophilic addition of HBr or Br2 to ethene?

Answer 28

room temp.

no catalyst or UV light needed

Question 29

What acts as the electrophile in electrophilic addition of a diatomic molecule? How does it do this?

Answer 29

one of the atoms (closest to the alkene C=C)

dipole is induced into diatomic molecule due to high e- density of the C=C bond

Question 30

What is the atom economy of an electrophilic addition mechanism?

Answer 30

100%

only 1 product made

Question 31

Which is the major product?

Answer 31

the more substituted one e.g. tertiary over secondary over primary

Question 32

What are the uses of polystyrene and polypropene?

Answer 32

ps: packaging, cups
pp: ropes & carpets

Question 33

What are the uses of PVC and polyethene?

Answer 33

PVC: window frames, toys, umbrellas
pe: bags & bottles

Question 34

Name some useful alkene polymers.

Answer 34

PVC, polyethene, polypropene, polystyrene

Question 35

What are biodegradable polymers broken down into?

Answer 35

water, CO2 and biological compounds

Question 36

What are biopolymers made of?

Answer 36

starch, cellulose & additives that allow it to be broken down 
lactic acid (compostable)

Question 37

What are photodegradable polymers?

Answer 37

made of oil-based plastics with bonds that can be weakened by absorbing light to begin degradation

Question 38

What does Markownikoff’s rule state?

Answer 38

H of H-halide goes to C bonded to more H atoms

halide goes to C bonded to more C atoms

Question 39

Why can some alkenes have cis and trans isomerism?

Answer 39

they contain a C=C double bond which restricts rotation and each C in the C=C is bonded to 2 different atoms or groups