organic chemistry - alkenes

Question 1

What is the general formula for alkenes?

Answer 1

CₙH₂ₙ

Question 2

What is meant by ‘unsaturated’?

Answer 2

A molecule that contains a carbon-carbon double bond (C=C)

Question 3

What types of bond make up a double bond? And describe each of them.

Answer 3

It is made of a σ-bond and a π-bond.

The sigma bond is between the two carbons.

Adjacent p-orbitals above and below the bonding C atoms overlap, creating a pi bond.

(pi bond surrounds sigma bond)

Question 4

What is a sigma bond?

Answer 4

The result of the overlap of two orbitals, one from each atom. It has two electrons that are shared between the bonding atoms. They are single covalent bonds.

Question 5

What is the bond angle around the double bond?

Answer 5

3 bonding regions, so 120° with trigonal planar shape

Question 6

What is positional (or structural) isomerism?

Answer 6

Different isomers formed when the C=C appears between different carbon atoms.

(number added before -ene to indicate first carbon involved)

Question 7

What are stereoisomers?

Answer 7

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

Question 8

Why do you get stereoisomers?

Answer 8

Because the double bond locks the carbon atoms in place and prevents free rotation.

(their geometry is different to alkanes, where rotation is possible around every atom)

Question 9

What is E/Z isomerism?

Answer 9

A type of stereoisomerism:

• E is for when the same group is on opposite sides
• Z is when the same groups are on the same side.

Question 10

What are conditions for stereoisomerism?

Answer 10

molecules must:

• have a C=C double bond
• have different groups attached to each carbon in the double bond.

Question 11

In E/Z isomerism, what happens when there are more than 2 different groups?

Answer 11

Each atom (directly) attached to the carbon in the C=C is given a priority based on its atomic number. If first atom is the same, look for the first point of difference.

(Higher priority adjacent -> Z isomer, diagonal -> E isomer)

(Cahn-Ingold Prelog Rules)

Question 12

What is cis/trans isomerism?

Answer 12

A subset of E/Z isomerism. At least two of the groups attached to the carbon need to be the same. (usually hydrogen)

Question 13

What is an addition reaction?

Answer 13

A reaction where 2 molecules react together to produce one

Question 14

What is hydrogenation of alkenes?

Answer 14

Alkenes react with hydrogen to produce their equivalent alkane

Question 15

What catalyst is needed for hydrogenation of of alkenes?

Answer 15

nickel catalyst (and temp. 150°C)

Question 16

What is hydration of alkenes?

Answer 16

Alkenes are converted to alcohols by reacting with steam

Question 17

What catalyst is needed for hydration of alkenes?

Answer 17

An acid catalyst eg. phosphoric acid

reaction is reversible

Question 18

What is halogenation of alkenes?

Answer 18

Alkenes react with halogen molecules to form dihaloalkanes (no catalyst needed)

Question 19

What is the test for alkenes?

Answer 19

Bromine water goes from orange to colourless

Question 20

What is the reaction of alkenes with hydrogen halides?

Answer 20

Forms haloalkanes (room temp, gaseous)

Question 21

What is electrophilic addition?

Answer 21

The mechanism alkenes use for addition reactions

Question 22

What is an electrophile?

Answer 22

An atom or group of atoms that accepts an electron pair

and is attracted to an electron-rich centre) (usually positive ion or atom with δ+ charge

Question 23

Describe step 1 of electrophilic addition with hydrogen halides.

Answer 23

The hydrogen halide has a permanent dipole (because the halogen is more electronegative than H): H is δ+ and halogen is δ-

Draw curly arrow from hydrogen halide bond to halogen - shows electrons moving to δ- atom.

Draw another curly arrow from double bond to δ+ Hydrogen. Double bond breaks because electrons are more attracted to δ+ atom and a bond is formed between C and H.

Question 24

Describe step 2 of electrophilic addition with hydrogen halides

Answer 24

Indicate positive charge on the other carbon atom that was in the double bond. It is now a carbocation.

Draw halide ion with lone pair, and draw curly arrow from lone pair to carbocation. They react to form the addition product.

Question 25

How does electrophilic addition work with halogen molecules?

Answer 25

The molecule needs to be polarised as halogen molecules aren’t polar. An induced dipole is formed. The electrons in the pi bond repel the electrons in the halogen molecule - so the closer atom becomes δ+ and the further one becomes δ- Then electrophilic addition can occur.

Question 26

Multiple products can be made when hydrogen halides react with unsymmetrical alkenes. How do we know which is the major product/minor product?

Answer 26

The hydrogen attaches to the carbon with larger no. of hydrogens and smaller no. of carbons. (Markownikoff’s Rule)

Question 27

Describe different carbocations

Answer 27

primary carbocation (1°) has 1 carbon chain attached secondary carbocation (2°) has 2 carbon chains attached tertiary carbocation (3°) has 3 carbon chains attached primary is least stable, then secondary, and tertiary is most stable.

Question 28

What is an alkyl group?

Answer 28

carbon and hydrogen chain

Question 29

Why do carbocations get more stable as they are attached to more alkyl groups?

Answer 29

Each alkyl group pushes electrons towards the C⁺ charge of carbocation. The positive charge of the carbocation is spread over the alkyl groups. More alkyl groups mean the positive charge is spread out more so the carbocation is more stable.

Question 30

What is a monomer?

Answer 30

Small molecule that combines to make polymers

Question 31

What is a polymer?

Answer 31

Long molecular chain made from repeating units

Question 32

What is a repeating unit?

Answer 32

An arrangement of atoms that occurs in a structure repeatedly.

Question 33

General equation for addition polymerisation?

Answer 33

W X [ W X ] \ / [ | | ] n C=C -> —[—C—C—]— / \ [ | | ] Y Z [ Y Z ] n

Question 34

What is poly(ethene) also known as and what is it used for?

Answer 34

Polythene, used for plastic bags and bottles

Question 35

What is the equation for the formation of poly(ethene)?

Answer 35

H H [ H H ] \ / [ | | ] n C=C -> —[—C—C—]— / \ [ | | ] H H [ H H ] n

Question 36

What is poly(propene) also known as and what are its uses?

Answer 36

Polypropene, used for plastic bowls, ropes and crates.

Question 37

What is the equation for the formation of poly(propene)?

Answer 37

H CH₃ [ H CH₃ ] \ / [ | | ] n C=C -> —[—C—C—]— / \ [ | | ] H H [ H H ] n

Question 38

What is poly(chloroethene) also known as and what are its uses?

Answer 38

PVC, used for electrical cable insulation

Question 39

What is the equation for the formation of poly(chloroethene)?

Answer 39

H Cl [ H Cl ] \ / [ | | ] n C=C -> —[—C—C—]— / \ [ | | ] H H [ H H ] n

Question 40

What is poly(phenylethene) also known as and what are its uses?

Answer 40

Polystyrene, used in packaging

Question 41

What is the equation for the formation of poly(phenylethene)?

Answer 41

H H [ H H ] \ / [ | | ] n C=C -> —[—C—C—]— / \ [ | | ] H C₆H₆ [ H C₆H₆ ] n (C₆H₆ is benzene)

Question 42

How are polymers disposed of?

Answer 42

- Landfill - Combustion (incineration) - Reuse - Sorted and recycled - Use as organic feedstock

Question 43

Why might the landfill be used?

Answer 43

To dispose of plastics that are: - Too difficult to recycle - Too difficult to separate from other materials - Don’t have enough plastic to extract to make it economically viable

Question 44

What are issues with disposing of plastics in the landfill?

Answer 44

- Not sustainable as large amounts of land are needed - Becoming increasingly expensive to use land for waste disposal - We need to reduce our reliance on landfill

Question 45

What ways are scientists trying to reduce environmental impacts of disposing of plastics?

Answer 45

- develop photodegradable polymers - develop biodegradable polymers - develop techniques for cracking polymers - develop ways of making polymers from plant-based substances - design processes with high atom economy - develop ways of sorting AND recycling

Question 46

How can waste polymers be processed usefully?

Answer 46

- Combustion for energy regeneration - Sorted and recycled - Sort, melt and remould - Cracking - Organic feedstock

Question 47

What is the issue when disposing of halogenated plastics and how is this combated?

Answer 47

- Produces toxic HCl gas when burnt (as well as other pollutants like dioxins) - Flue gas scrubbers used to neutralise acidic gases