Chapter 13

Question 1

What is an alkene?

Answer 1

An unsaturated hydrocarbon consisting of at least one C=C bond

Question 2

What two types of bonds are in a C=C bond?

Answer 2

Pi and sigma bonds

Question 3

What is a pi bond?

Answer 3

A pi bond is a type of covalent bond formed from the sideways overlap of adjacent p orbitals, above and below the bonding C atoms

Question 4

What is a special feature of a pi bond?

Answer 4

It has restricted rotation i.e. it does not allow bonds around the C atom to rotate

Question 5

What is a stereoisomer?

Answer 5

Compounds with the same structural formula but with a different arrangement in space

Question 6

What are the conditions needed for a molecule to have E/Z isomerism?

Answer 6

A C=C bond
Different groups attached to each carbon atom of the double bond

Question 7

How does stereoisomerism arise?

Answer 7

Stereoisomerism around double bonds arises because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.

Question 8

What is cis-trans isomerism?

Answer 8

A special case of E/Z isomerism in which there is one Hydrogen atom on each side of the carbon-carbon double bond.

Question 9

What are the conditions needed for a molecule to have cis/trans isomerism?

Answer 9

A C=C bond
Different groups attached to each carbon atom of the double bond
One of the attached groups on each carbon atom of the double bond must be hydrogen

Question 10

Where there is a hydrogen on each of the double bonded carbon atoms in cis-trans isomerism:

Answer 10

The cis isomer is the Z isomer
The trans isomer is the E isomer

Question 11

What are the Cahn-Ingold-Prelog rules for naming isomers?

Answer 11

If the groups of higher priority are on the same side of the double bond, the compound is a Z isomer

If the groups of higher priority are diagonally placed across the double bond, the compound is an E isomer

Question 12

How to assign priority to molecules attached to the C=C bond when deducing the type of isomer (E/Z isomer)?

Answer 12

Examine the atoms attached directly to the carbon atoms of the double bond and see which atom has the higher atomic number. Higher atomic number = higher priority.

If the two atoms attached to a carbon atom in the double bond are the same, then you will need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority.

Question 13

Why are alkenes so reactive?

Answer 13

Alkenes are much more reactive than alkanes because of the presence of the pi bond. Being on the outside of the double bond, the pi electrons are more exposed than the electrons in the sigma bond, therefore a pi bond readily breaks and alkenes undergo addition reactions relatively easily.

The pi bond also readily breaks due to its lower bond enthalpy compared to a sigma bond.

Question 14

What are the different types of addition reactions that alkenes undergo?

Answer 14

With hydrogen in the presence of a nickel catalyst
With halogens
With hydrogen halides
With steam in the presence of an acid catalyst
Electrophilic addition reactions

Question 15

What is the hydrogenation of alkenes?

Answer 15

When an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction takes place to form an alkane.

Question 16

What are the conditions needed for hydrogenation of alkenes?

Answer 16

Suitable catalyst (most commonly nickel)
Temperature of 423K

Question 17

What is the halogenation of alkenes?

Answer 17

When alkenes undergo a rapid addition reaction with a halogen at room temperature to form a dihaloalkane.

Question 18

What is the colour change observed during a positive test for unsaturation?

Answer 18

From orange to colourless

Question 19

How do you test for unsaturation?

Answer 19

The halogenation of alkenes can be used to identify if there is a C=C bond present and the organic compound is unsaturated.
When bromine water (orange solution) is added to a sample of an alkene, bromine is added across the double bond. The orange colour disappears, indicating the presence of a C=C bond.

Question 20

What is the addition reaction of alkenes with hydrogen halides?

Answer 20

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. if the alkene is a gas, then the reaction takes place when the two gases are mixed. If the alkene is a liquid, then the hydrogen halide is bubbled through it.

Question 21

What is the hydration of alkenes?

Answer 21

When alkenes react with steam in the presence of a phosphoric acid catalyst, H₃PO₄, alcohols are formed

Question 22

What is an electrophile?

Answer 22

An electron pair acceptor

Question 23

Why do alkenes undergo electrophilic addition?

Answer 23

The double bond in an alkene represents a region of high electron density because of the presence of the electrons in the pi bond
The high electron density of these electrons attracts electrophiles (an electron pair acceptor)

Question 24

Describe the electrophilic addition reaction between but-2-ene and hydrogen bromide:

Answer 24

1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar. Hydrogen has a positive dipole and bromine has a negative dipole.
2. The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break
3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond.
4. The H-Br bond breaks my heterolytic fission, with the electron pair going to the bromine atom
5. A bromide ion Br^- and a carbocation are formed.
6. In the final step, the Br^- ion reacts with the carbocation to form the addition product

Question 25

What is Markownikoff's rule?

Answer 25

When a hydrogen halide reacts with an unsymmetrical alkene, the halide of the hydrogen halide attaches itself to the carbon atom of the alkene with the least number of hydrogen atoms and the greatest number of carbon atoms

Question 26

What is carbocation stability?

Answer 26

Carbocation stability is linked to the electron donating ability of alkyl groups. Each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The more alkyl groups attached to the positively-charged carbon atom, the more the charge is spread out, making the ion more stable.

Question 27

What is the explanation used when explaining why a major product has formed?

Answer 27

Because the major product has formed via the most stable carbocation

Question 28

How many alkyl groups does a primary carbocation contain?

Answer 28

1

Question 29

How many alkyl groups does a secondary carbocation contain?

Answer 29

2

Question 30

How many alkyl groups does a tertiary carbocation contain?

Answer 30

3

Question 31

Which carbocation is the most stable?

Answer 31

Tertiary

Question 32

Which carbocation is the least stable?

Answer 32

Primary

Question 33

What is addition polymerisation?

Answer 33

When many unsaturated alkene molecules join together to form long saturated chains containing no double bonds

Question 34

What is a repeat unit?

Answer 34

A specific arrangement of atoms in a polymer molecule that repeats over and over again

Question 35

How to name a polymer?

Answer 35

poly(name of monomer) e.g.: propene --> poly(propene)

Question 36

What are the benefits of processing waste polymers by combustion?

Answer 36

It can produce heat, as well as steam which can be used to generate electricity

Question 37

What are the benefits of processing waste polymers by using them as organic feedstock?

Answer 37

It can reclaim monomers, gases or oil from waste polymers. A major advantage is that it can handle unsorted and unwashed polymers

Question 38

What are the benefits of using biodegradable polymers?

Answer 38

Broken down by microorganisms into water, CO₂ and other biological compounds Leaves no visible or toxic residues

Question 39

What are the benefits of using photodegradable polymers?

Answer 39

Gets rid of waste quickly