9 - Alkenes

Question 1

What is an alkene?

Answer 1

An unsaturated hydrocarbon

it has a double bond

Question 2

What is general formula of an alkene?

Answer 2

C~n H~2n

Question 3

What is the functional group of alkenes?

Answer 3

C=C

Question 4

What type of reactions do alkenes take part in?

Answer 4

Addition reactions

Question 5

How can you test to see if a substance is an alkene?

Answer 5

You react the substance in bromine water

If it is an alkene, the bromine water will turn from brown to colourless

Question 6

What is the general formula of cycloalkenes?

Answer 6

C~n H~(2n-2)

Question 7

What is the general formula of arenes?

Answer 7

C~n H~n

Question 8

What is an example of an arene?

Answer 8

Benzene

Question 9

How do you need to draw alkenes as displayed formulas?

Answer 9

All bond angles have to be at their maximum

No matter what it ends up looking like

A double bond has the same repulsion as a single bond

Question 10

What does the number in the middle of an alkene mean?

Answer 10

The carbon which the double bond is on

E.g. but-2-ene:

Double bond is between carbon 2 and carbon 3

Question 11

What does diene mean?

Answer 11

When there are 2 double bonds

There will be 2 numbers before

Question 12

What is a double bond made from?

Answer 12

The first C-C bond is a sigma-bond

The second part is a pi-bond

Question 13

What are valence electrons?

Answer 13

Outer electrons used in bonding

Question 14

In what 3 ways are sigma-bonds formed?

Answer 14

When 2 S-orbitals overlap

When an S and a P overlap

When 2 P-orbitals overlap

Question 15

Generally, when do sigma-bonds form?

Answer 15

When there is a direct overlap of atomic orbitals

Question 16

How is the pi-bond of a double bond formed?

Answer 16

The P-orbitals around the carbons which have a sigma-bond overlap

This overlap is the pi-bond

in short: The pi-bond is caused by sideways overlap of P-orbitals

Question 17

What effect does the pi-bond have on alkenes?

Answer 17

The pi-bond prevents rotation of atoms around the double bonded carbons

Question 18

What are stereoisomers?

Answer 18

Molecules with the same structural formula but different arrangements of atoms in space

Question 19

What are the 2 types of stereoisomers?

Answer 19

E-isomers (trans)

Z-isomers (cis)

Question 20

What are E-isomers and Z-isomers?

Answer 20

E-isomers:
When the 2 largest groups which are bonded to a central carbon are on the opposite sides of the molecule

Z-isomers:
When the largest groups are on the same side

Question 21

How do you show if an alkene etc is an E or Z isomer in the name?

Answer 21

Just put E- or Z- before the name

Like E-but-2-ene

Question 22

What 2 things have to be true for a substance to have stereoisomers?

Answer 22

Double bond (so that there is no rotation)

Each carbon in the double bond is bonded to 2 different atoms or groups

Question 23

What are Cis and Trans isomers?

Answer 23

A Cis isomer is the Z isomer with a hydrogen on each double bonded carbon

A Trans isomer is the E isomer with a hydrogen in each double bonded carbon

Question 24

What do you do when classifying stereoisomers in very complex organics?

Answer 24

use the C-I-P rules:

If the 2 highest priority are on the same side, it’s Z and the opposite for E

The priority of an atom is higher if it’s atomic mass is greater

You have to compare the different parts of the structural formulas until one has a higher priority part than another

Question 25

How reactive are alkenes compared to alkanes and why?

Answer 25

alkenes are more reactive The pi-bond can break and more sigma-bonds can form

Question 26

How do pi and sigma bonds compare in terms of strength?

Answer 26

Sigma-bonds are stronger (higher bond enthalpy) than pi-bonds

Question 27

How do you test for alkenes and cycloalkenes?

Answer 27

Add bromine water If it is an alkene or cycloalkene The bromine water will turn from orange to colourless

Question 28

What is the reaction for bromine water added to alkenes?

Answer 28

Example: Ethene + Bromine --> 1,2-dibromoethane

Question 29

What type of reaction is the one between ethene and bromine and why?

Answer 29

An addition reaction Because the bromine is added across the double bond with one bromine bonding to each carbon

Question 30

Which bond breaks in the reaction of ethene and chlorine?

Answer 30

The pi-bond between the double bonded carbon

Question 31

What is the reaction between cyclohexene and fluorine?

Answer 31

They make 1,2-difluorocyclohexane

Question 32

Define electrophile

Answer 32

Electron pair acceptor

Question 33

What are electrophiles attracted to and why?

Answer 33

The double bond There is a high electron density there

Question 34

What is the full name of reactions between alkenes and halogens etc?

Answer 34

Electrophilic addition reactions

Question 35

What are the electrophiles in the electrophilic addition reactions?

Answer 35

The halogen or hydrogen which adds across the double bond

Question 36

What is the name of the reaction where hydrogen is added?

Answer 36

Hydrogenation

Question 37

What are the conditions for hydrogenation?

Answer 37

Nickel catalyst at 423K

Question 38

How is margarine made?

Answer 38

Hydrogenation of plant oils

Question 39

What is the addition of halogens called?

Answer 39

Halogenation

Question 40

What is the charge of electrophiles and why?

Answer 40

Positive Because they are attracted to electron pairs

Question 41

What is Markownikoff's rule?

Answer 41

The isomer with the tertiary carbocation is favoured over the isomer with the secondary carbocation Secondary > primary as well

Question 42

How can you tell which carbocation the isomer had?

Answer 42

You look at the branches off the main chain

Question 43

How do you denote a carbocation?

Answer 43

The number then a circle as a superscript Secondary carbocation is 2^o

Question 44

Explain why hexane is saturated

Answer 44

There is only C-C bonds No C double bond C (You have to say C-C instead of single bonds)

Question 45

Define polymer

Answer 45

A long molecular chain built up from monomer units

Question 46

Define monomer

Answer 46

A small molecule that combines with many other monomers to form a polymer

Question 47

What are the 2 requirements for a monomer?

Answer 47

Based on alkenes Unsaturated

Question 48

What is the name of the reaction used to make polymers?

Answer 48

Addition polymerisation

Question 49

How does addition polymerisation work?

Answer 49

Double bonds are opened up A long chain of carbons forms

Question 50

How do you draw the addition polymerisation of poly(propene)?

Answer 50

Do the whole thing but only draw the double bond part as displayed and draw the CH~3 coming off a bond Do the same for the polymer side of the equation

Question 51

What is the repeat unit?

Answer 51

The thing before the n on the polymer side

Question 52

What is the advantage and disadvantage of the lack of reactivity of polymers?

Answer 52

They are suitable for storing food They often take a very long time to biodegrade

Question 53

What are the 2 advantages of recycling polymers?

Answer 53

It reduces the amount of polymers going to landfill There is only a finite amount of polymers that can be made and recycling reuses the same polymers over and over again

Question 54

Describe the polymer recycling process

Answer 54

The polymers are first sorted by type Mixed polymers can't be recycled The polymers are then chopped into flakes They are then washed and dried Then they are melted and reformed into small pellets

Question 55

What are the 2 disadvantages of recycling PVC?

Answer 55

Hazardous due to: High chlorine content Range of additives present in the polymers

Question 56

What are the 2 disadvantages of putting PVC into landfill?

Answer 56

When it is burnt: The corrosive gas hydrogen chloride is released Along with other pollutants like toxic dioxins

Question 57

Describe the old and new way to recycle PVC

Answer 57

old and new way to recycle PVC We used to grind it and reuse it We now use solvents to dissolve the PVC and high grade PVC is recovered by precipitation from the solvent The solvent can now be reused

Question 58

What do we do with some polymers instead of recycling and why?

Answer 58

We use them as fuels by burning them to generate heat Because they have a high stored energy value

Question 59

What is feedstock recycling?

Answer 59

The chemical and thermal processes which can reclaim monomers, gases or oil from waste polymers

Question 60

What is a big advantage of feedstock recycling?

Answer 60

It can handle unsorted or unwashed polymers

Question 61

What are bioplastics?

Answer 61

Plastics produced from starch, cellulose, plant oils and proteins

Question 62

What are the 2 advantages of bioplastics?

Answer 62

Protect our environment Help conserve our finite oil reserves

Question 63

What are biodegradable polymers made from normally?

Answer 63

Starch or cellulose Or a normal polymer contains additives so it can be broken down

Question 64

How are biodegradable polymers broken down?

Answer 64

Broken down by microorganisms into water, carbon dioxide and biological compounds

Question 65

What are compostable polymers and what are they usually based on?

Answer 65

Degrade and leave no visible or toxic residues Based on poly(lactic acid)

Question 66

How can supermarket bags be changed so that they are compostable?

Answer 66

Made from plant starch Used as bin liners so that the food and the bag can be composted together

Question 67

What are compostable plates, cups etc made from?

Answer 67

Sugar cane fibre to replace polystyrene

Question 68

Sugar cane fibre to replace polystyrene

Answer 68

Oil based polymers which contains bonds which are weakened by light to start the degradation Or light absorbing additives can be used

Question 69

What are the reagents and conditions for electrophilic addition of alkenes?

Answer 69

Steam Acid catalyst