Module 4.1.3 - Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons (have at least one multiple bond between carbon atoms)

Question 2

Why are alkenes unsaturated?

Answer 2

In alkanes, there is at least one double bond

Question 3

What is the general formula for alkenes?

Answer 3

CnH2n

Question 4

What bond is in a C=C bond?

Answer 4

Made up of 1 sigma bond and 1 pi bond (which restricts its rotation)

Question 5

How is a sigma bond formed in an alkene?

Answer 5

Directly between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms

Question 6

How is a pi bond formed in an alkene?

Answer 6

The double, sideways overlap of p-orbitals ABOVE AND BELOW the bonding C atoms

Question 7

What shape and bond angle is around the carbons in C=C?

Answer 7

Trigonal planar, 120 degrees
3 bonding regions of electrons which repel each other equally to get as far apart as possibly

Question 8

How do you name alkenes?

Answer 8

• Find the longest C chain that includes the alkene group & name the stem
• Number parent chain from end to lowest number possible for alkene group
• If more than 1 alkene group, use prefix (di-,tri-..) and add an ‘a’ at the end of stem of alkene
• Name and number any alkene groups, with multiple list in alphabetical order by name
• Cyclic alkenes have the prefix ‘-cyclo’

Question 9

What is stereoisomerism?

Answer 9

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

Question 10

What does E/Z stand for?

Answer 10

Entgegen/Zusammen

Question 11

What is the criteria for E/Z stereoisomerism? (2)

Answer 11

• Must have a carbon-carbon double bond, so the compound cannot rotate
• EACH carbon atom of the C=C must have two different groups attached to it

Question 12

How do you draw E/Z stereoisomerism?

Answer 12

Draw the molecule with its trigonal planar shape and attached groups are displayed in structural formula, has to condense only to the C=C

Question 13

How do you find priority in alkene stereoisomers?

Answer 13

Compare the atoms that are attached on either side of the C=C and look at their atomic number (top and bottom)
Higher atomic number = Higher priority

Question 14

How do you identify a Z-stereoisomer?

Answer 14

The highest priority groups are on the same side of the double bond (either both highest priority groups are top, top OR bottom, bottom)

Question 15

How do you identify an E-stereoisomer?

Answer 15

Highest priority groups are on opposite sides of the C=C (one highest priority group on bottom, one on top)

Question 16

What is the criteria for cis-trans stereoisomerism? (3)

Answer 16

• must have a carbon-carbon double bond because it cannot rotate
• Each carbon on the C=C must have two different groups attached
• Two groups on the C=C must be identical

Question 17

How do you identify a cis-stereoisomer?

Answer 17

The two same groups from each C atom are adjacent to the C=C

Question 18

How do you identify a trans-stereoisomer?

Answer 18

Two same groups from each C atom are opposite the C=C bond (across from each other)

Question 19

Why are alkenes more reactive than alkanes?

Answer 19

Alkenes have C=C bond, which is their functional group

Question 20

What type of reactions do alkenes undergo?

Answer 20

Addition reactions - A group is added across the C=C double bond

Question 21

Why does the pi bond break when an alkene reacts?

Answer 21

It has relatively low bond enthalpy

Question 22

Why is the atom economy 100% for alkenes?

Answer 22

Since addition, only one product is formed so no waste product

Question 23

What are the different reactions for alkenes? (5)

Answer 23

1. Bromination
2. Chlorination
3. Hydrogenation
4. Hydration
5. Hydrogen halide

Question 24

What is the test for identifying alkenes?

Answer 24

Bromine water (bromination) as it goes from orange to colourless

Question 25

What conditions and reagents are needed for hydrogenation?

Answer 25

Conditions: Nickel catalyst and 150 degrees C reagents: alkene + H2 Forms an **alkane**

Question 26

What conditions and reagents are needed for bromination/chlorination?

Answer 26

Conditions: None Reagents: Br2/Cl2 + alkene Forms a **haloalkane**

Question 27

What conditions and reagents are needed for halogen halide?

Answer 27

Conditions: None Reagents: alkene + HX (where X is a halogen) Forms a **haloalkane**

Question 28

What conditions and reagents are needed for hydration?

Answer 28

Conditions: Conc. H3PO4 catalyst, high temperature & pressure, **H2O has to be as a gas** Reagents: H2O (g) + alkene Forms an **alcohol**

Question 29

What is electrophilic addition?

Answer 29

The mechanism that alkene reactions undergo, as C=C attracts electrophiles due to its high electron density

Question 30

What is an **electrophile?**

Answer 30

Electron pair **acceptor**

Question 31

How is the electrophilic addition mechanism shown?

Answer 31

Using curly arrow model **(shows movement of electron pair)**

Question 32

What must you show in the electrophilic addition mechanism?

Answer 32

The dipoles on the polar molecules and curly arrows showing where electrons are moving

Question 33

When is a mixture of two organic products formed from alkenes? (2)

Answer 33

- When either hydrogen halide/steam reacts - Where alkene is unsymmetrical around the C=C double bond

Question 34

What are the two products formed from unsymmetrical alkenes catagorised as?

Answer 34

Major / Minor product

Question 35

What is **Markownikoff’s rule**?

Answer 35

The **major** product is the tertiary/secondary carbocation which is more stable than the primary carbocation (minor product), as the tertiary carbocation is the most stable and primary is the least stable

Question 36

What is the carbocation intermediate stage?

Answer 36

After the pi bond is broken, the carbocation is where one C atom has a positive charge and attracts the negative ion

Question 37

How many alkyls groups do each primary,secondary and tertiary carbocation have around it?

Answer 37

P - 1 S - 2 T - 3

Question 38

Which product is more likely to form?

Answer 38

Major product

Question 39

What are **addition polymers?**

Answer 39

Long, saturated molecules formed from alkene monomers

Question 40

What conditions and reagents are needed for addition polymerisation?

Answer 40

Conditions: high temperature and pressure + catalyst Reagents: One type of alkene monomers

Question 41

What are the problems with disposing addition polymers? (2)

Answer 41

- Non-biodegradable - Burning/Incineration of toxic chloro-gases

Question 42

In what ways are waste addition polymers processed which are more sustainable? (4)

Answer 42

- Combustion for energy production - Removal of toxic waste products (like HCl) - Use as an organic feedstock - Recycles

Question 43

How can polymers be produced to minimise environmental damage? (2)

Answer 43

- develop biodegradable polymers - develop photodegradable polymers