Alkenes

Question 1

what are alkenes

Answer 1

• unsaturated HC
• only C and H atoms
• 1 or more C=C bond
• = bond = more reactive

Question 2

general formula of alkenes

Answer 2

CnH2n

Question 3

physical properties of alkenes

Answer 3

• insoluble in water
• mp + bp increase with chain length - vdw forces stronger

Question 4

shape of alkenes

Answer 4

• double bonds are planar (flat)
• 120 degrees

Question 5

Why cant a double bond not rotate?

Answer 5

• pi -orbital presence
• p orbitals on each carbon atom overlap

Question 6

isomers alkenes can form and which involve the double bond?

Answer 6

• chain isomers
  2 types that involve double bond
• position isomers
• geomtrical isomers

Question 7

What are position isomers?

Answer 7

• double bond in diff positions between c atoms
• the longer the chain, the more possibilities so greater no of isomers

Question 8

geomtrical isomers

Answer 8

• E for opposite sides
• Z for same side

groups

Question 9

2 reasons why geomtric isomerism may occur?

Answer 9

• C=C bond has restricted rotation
• 2 diff groups on each carbon

Question 10

CIP
Cahn- Ingold-Prelog

Answer 10

priority is given to the atom with the largest atomic mass
* only the first atom to the c=c bond
* if 2 high priority groups same side, molcule is called Z
* opp sides = E

Question 11

BP of alkenes

Answer 11

• double bond doesnt affect BP and MP
• Van derwaals only IMF
• physical properties similar to alkanes
• MP and BP increase with C atoms
• insoluble in water

Question 12

combustion of alkenes

Answer 12

• produce co2 + h20
• burn with a sootier flame due to (large C:H ratio = more sootier)
• v reactive due to double bond

Question 13

electrophile def

Answer 13

electron pair acceptor

Question 14

What does thhe C=C bond form?

Answer 14

• electron rich area
• can be attacked by electrophiles (e liking)

Question 15

describe electrophile

Answer 15

• it is + or partially + charged

Question 16

3 regeants that act as electrophiles with alkenes

Answer 16

• HBr (HX)
• Br2 (X2)
• H2SO4 (then water)

X= halide

Question 17

mechanism of electrophilic addition reactions

Answer 17

• electrophile attracted to double bond
• they are + charged, accep pair of electrons from =
• Positve ion (carbocation) formed
• negatively charged ion forms bond with carbocation

Question 18

Describe the mechanism with HBr as a electrophile in an addition reaction

Answer 18

• EN diff between H and Br = polar bond
• H+ and Br-
• electrophile attacks double bond (H+ and double bond)
• new bond formed between 1 C atom + H
• H-Br bond breaks
• Br takes both e = Br ion
• carbocation = + carbon atom
• lone br ion attracted + bond with carbocation

Question 19

whats heterolytic bond breaking?

Answer 19

• covalent bond breaks, both e go to one atoms and none to other
• forms + and - ion

Question 20

which are more stable, secondary carbocation or primary carbocation?

Answer 20

• secondary carbocation more stable
• secondary has 2 alkyl whereas Primary only has one alkyl
• alkyl groups provide positive inductive effect (pushes e towards + charge)

Question 21

assymetrical alkenes

Answer 21

• 2 products
• one major and one minor product (due to pri/sec carbocation)
• C=C , diff in which C forms a carbocation , c1 or c2

Question 22

rank which is more stable:
tertiary carbocation
secondary carbocation
primary carbocation

Answer 22

most to least = 3,2,1

Question 23

Why is a primary carbocation less stable than a secondary carbocation?

Answer 23

• primary has a higher Ea than secondary, has more energy than secondary
• so secondary more likely to happen due to lower ea = major product

Question 24

Describe the reaction with electrophil (halogen) (Br2) with an alkene

Answer 24

• Br-Br has random e- movements = induced dipole
• electrons in = attracted to Br+, repel electrons in Br-Br bond, strengthens dipole of Br2 mol
  both makes one Br + and other -
• electrophile attracts C bond = bond formed
• Br = Br- ion with 2 electrons makes bond with carbocation

Question 25

Testing for alkene (unsaturation)

Answer 25

* bromine water (yellow) + alkene * shake * bromine water goes from yellow to colourless

Question 26

Describe the reaction with the electrophile sulfuric acid with an alkene

Answer 26

* H-O-SO3H * the H = + and O = - so polar * electrophile attacks C=C bond * H-O bond breaks heterolytically * 2 electrons on OSO3H- * attracted to carbocation * forms alkyl hydrogensulfate * u can take AH + water = alcohol + h2so4 * so h2so4 acts as a catalyst

Question 27

What is an addition polymer formed from?

Answer 27

monomers with c=c bonds (alkenes)

Question 28

general equation for addition polymerisation of alkene

Answer 28

alkene --> polyalkene unsaturated --> saturated - no = bond so unreactive

Question 29

why do polymers have a high mp?

Answer 29

* long mol * v large no of van der waal forces * more energy needd to break

Question 30

how many electrons in =

Answer 30

4

Question 31

rules for drawing a polymer

Answer 31

* single C-C bond * add lines next to C to brackets * include n

Question 32

What are plasticiers?

Answer 32

* make polymers more flexible * small mol that get between polymer chains allowing them to slide over each other * eg pvc from drainpipes to aprons

Question 33

types of polythene

Answer 33

* High density * low density

Question 34

High density polythene

Answer 34

* strong + higher MP * low temp + pressure * Ziegler - Natta catalyst * less chain branching, chains can pack together = density + mp greater

Question 35

Low density polythene

Answer 35

* flexible, can stretch * forms from higher temp + pressure * uses free radical mechanism * polymers are branched - cant pack together = lower density as chains are further apart

Question 36

uses of high density polythene

Answer 36

* milk crates * buckets * bottles

Question 37

uses of low density polythene

Answer 37

* plastic bags * sheeting * insulation for electrical cables

Question 38

biodegradability def

Answer 38

ability to be broken down by biological agents

Question 39

are polyalkenes biodegradable and why?

Answer 39

* non biodegradable * they have strong non polar c-c , c-h bonds so unreactive

Question 40

2 solutions to pollutionby plastics

Answer 40

* mechanical recycling * feedstock recycling

Question 41

mechanical recycling

Answer 41

* separate diff types of plastic * plastics washed + sorted * ground up into small pellets * pellets melted + remoulded

Question 42

Feedstock recycling

Answer 42

* plastics heated to temp - break polymer bonds = monomers * monomers used to make new plastics

Question 43

Problem with recycling

Answer 43

* plastics can be melted only a limited no of times * each heating, chains break and become shorter * degrading plastics properties

Question 44

alkene + water

Answer 44

f* double bond opens up to become single bond * one product * used to make alcohols * conditions = h20 must be steam, high temp, high pressure, catalyst = conc phosphoric acid

Question 45

alkene + hydrogen halide product

Answer 45

halogenoalkane

Question 46

reactions of alkenes

Answer 46

Electrophillic addition = electrophile + alkene