6C: Alkenes

Question 1

What is the general formula of alkenes?

Answer 1

C(n)H(2n)

Question 2

Why are alkenes unsaturated?

Answer 2

Not all bonds are with the max number of hydrogens

Double CC bond

Question 3

How do covalent bonds form?

Answer 3

Atomic orbitals from different atoms overlap
Shares electrons
Bond forms from electrostatic attraction from atoms to the bonding electrons

Question 4

What are single covalent bonds?

Answer 4

σ - sigma bonds
Overlap of two orbitals, high electron density
Strong electrostatic attraction, therefore strong bond

Question 5

What is the double bond made of in hydrocarbons?

Answer 5

π - pi bond & σ - sigma bonds
π bond forms when two p orbitals overlap sideways
Electrons spread out, weaker electrostatic attraction
Therefore weaker bond

Question 6

What is the effect on rotation by π bonds?

Answer 6

π electrons must overlap
Cannot rotate as easily
Leads to E-Z isomerism

Question 7

What kind of isomerism is E/Z isomerism?

Answer 7

Geometric - rotation is prevented about a double bond

Question 8

What is a cis isomer?

Answer 8

Same groups are on the same side of the CC double bond

Cis = Z-isomer

Question 9

What is a trans isomer?

Answer 9

Same groups are on opposite sides of the CC double bond

Trans = E-isomer

Question 10

What are the rules for E/Z isomerism?

Answer 10

The atom with the highest atomic number takes priority

If two atoms have the same atomic number and attached to same carbon, next bonded atom is taken into account

Question 11

What 4 types of addition reactions do alkenes undergo?

Answer 11

Hydrogenation
Halogenation
Hydrogen halides
Addition polymerisation

Question 12

What are the conditions for hydrogenation?

Answer 12

Nickle catalyst

150 C

Question 13

What is the process of hydrogenation?

Answer 13

Alkene reacts with hydrogen gas to form alkane

Making margarine is an example

Question 14

How does halogenation occur with alkenes?

Answer 14

Electrophilic addition

Question 15

What occurs in electrophilic addition?

Answer 15

Heterolytic fission of halide molecule, electrons go to one atom and the other bonds to C atom
Causes positively charged carbocation
Bond forms between free halide ion and carbocation
Dihalogenoalkanes formed

Question 16

How do alkenes react with bromine water?

Answer 16

Brown -> colourless if double CC bond

Forms a bromoalcohol and hydrogen bromide, e.g ethene forms 2-bromoethanol and hydrogen bromide

Question 17

What is steam hydration?

Answer 17

Alkenes hydrated by steam to form an alcohol

300 C & 60-70 atm with phosphoric (V) acid catalyst

Question 18

How do alkenes react with hydrogen halides?

Answer 18

Electrophillic addition

Forms halogenoalkane + hydrogen gas

Question 19

What is markovnikoff’s rule?

Answer 19

When a compound HX is added to an unsymmetrical alkene, the halogen binds to the carbon with the most hydrogens bonded to it

Question 20

What is a carbocation?

Answer 20

A positively charged carbon atom

Question 21

What is the stability of a carbocation determined by?

Answer 21

The more carbons it is bonded to, the more stable the carbocation is

Question 22

What is the process of the oxidation of alkenes?

Answer 22

Acidified potassium manganate (VII) added to alkene
Purple -> colourless
Produces alkene with two -OH alcohol groups (diol)

Question 23

What is the reaction between an alkene and sulphuric acid and water?

Answer 23

Forms HSO4 group on alkane

Then add water to form an alcohol group

Question 24

What is addition polymerisation?

Answer 24

Double bond in alkenes open up and join together to make polymers

Question 25

How do you convert monomers into polymers?

Answer 25

Change double CC bond to single bond

Add single bonds to each end of the carbon, which go through brackets

Question 26

How can waste plastic polymers be disposed?

Answer 26

Buried in landfill
Recycling - feedstock for cracking
Incineration- heat used for energy

Question 27

How can polymers be made to be sustainable?

Answer 27

Use reactants that are as safe and environmentally friendly as possible
Few other materials should be used
Renewable raw materials should be used
Energy use should be kept to a minimum
Limit waste products

Question 28

How are toxic gases removed when plastics are burned?

Answer 28

PVC produces HCl which is toxic

They must be neutralised in order to remove toxic gas

Question 29

How do biodegradable polymers decompose?

Answer 29

Made of renewable raw materials
Produces CO2 and if plant- based it is carbon neutral
Naturally decompose