Alkenes 2.4

Question 1

What is the functional group in alkenes?

Answer 1

Carbon-carbon double bond, C=C

Question 2

What is the general formula for alkenes?

Answer 2

CH2n

Question 3

What type of hydrocarbons are alkenes?

Answer 3

Unsaturated hydrocarbons

Question 4

What do unsaturated hydrocarbons contain?

Answer 4

At least one C=C or C≡C

Question 5

How are alkenes named according to IUPAC rules?

Answer 5

1. Select the longest chain of C atoms containing the double bond
2. Place the ending ‘ene’ on the basic name
3. Number the chain starting from the end nearer the double bond
4. Use a number to indicate the lower number carbon of the C=C
5. Prefix with substituents; side chain positions are based on the number allocated to the first C of the C=C

Question 6

What is the ending used for the basic name of alkenes?

Answer 6

‘ene’

Question 7

What should be selected first when naming alkenes?

Answer 7

The longest chain of C atoms containing the double bond

Question 8

When numbering the carbon chain in alkenes, where should you start?

Answer 8

From the end nearer the double bond

Question 9

In the naming of alkenes, what does the number indicate?

Answer 9

The lower number carbon of the C=C

Question 10

What does the C=C bond consist of?

Answer 10

The C=C bond consists of a sigma (σ) bond and a pi (π) bond.

Question 11

How is a sigma bond formed?

Answer 11

A sigma bond is formed directly between two carbon atoms by the head-on overlap of orbitals.

Question 12

How do carbon atoms contribute to the sigma bond?

Answer 12

Each carbon atom donates one electron to the electron pair in the sigma bond.

Question 13

How is a pi bond formed?

Answer 13

A pi bond is formed above and below the plane of the carbon atoms by sideways overlap of p-orbitals.

Question 14

How do carbon atoms contribute to the pi bond?

Answer 14

Each carbon atom donates one electron to the electron pair in the pi bond.

Question 15

What is a pi bond?

Answer 15

A pi bond is a covalent bond formed by sideways overlap of p orbitals.

Question 16

What is a sigma bond?

Answer 16

A sigma bond is a covalent bond formed by the linear overlap of atomic orbitals.

Question 17

What is the shape around the double bond in alkenes?

Answer 17

There is a trigonal planar shape around the double bond, with bond angles of 120°.

Question 18

What is bond length?

Answer 18

Bond length is the distance between the nuclei of two covalently bonded atoms.

Question 19

Why are alkenes more reactive than alkanes?

Answer 19

Alkenes are more reactive than alkanes because of the C=C double bond.

Question 20

What is the reactivity of alkenes compared to alkanes?

Answer 20

Alkenes are more reactive than the corresponding alkanes due to the weaker pi bond.

Question 21

What is the significance of the C=C bond in alkenes?

Answer 21

The C=C bond is an area of high electron density, attracting electrophiles.

Question 22

What are electrophiles?

Answer 22

Electrophiles are species attracted to areas of high electron density, such as the C=C bond in alkenes.

Question 23

What type of reaction do alkenes undergo with electrophiles?

Answer 23

Alkenes react with electrophiles by adding to the double bond in an electrophilic addition reaction.

Question 24

What is the mechanism of electrophilic addition?

Answer 24

The electrophilic addition of HBr to an alkene involves a two-step reaction mechanism.

Question 25

What happens in the first step of the electrophilic addition?

Answer 25

HBr acts as an electrophile and attacks the pi bond in the alkene molecule

Question 26

What is formed when HBr attacks the double bond in ethene?

Answer 26

A covalent bond is formed with the hydrogen atom, releasing a bromide ion and creating a carbocation intermediate.

Question 27

What is a carbocation intermediate?

Answer 27

A positively charged species formed during the electrophilic addition reaction.

Question 28

What occurs in the second step of the electrophilic addition?

Answer 28

A lone pair of electrons in the bromide ion forms a covalent bond with the carbocation intermediate.

Question 29

What is the stability of a carbocation dependent on?

Answer 29

The stability of a carbocation depends on the number of alkyl groups attached to it.

Question 30

What is a primary carbocation?

Answer 30

A primary carbocation has one alkyl group attached to the carbon with a positive charge (C+). ## Footnote It is the least stable type of carbocation.

Question 31

What is a secondary carbocation?

Answer 31

A secondary carbocation has two alkyl groups attached to C+ and is more stable than a primary carbocation. ## Footnote It has greater stability due to the increased electron density from the additional alkyl group.

Question 32

What is a tertiary carbocation?

Answer 32

A tertiary carbocation has three alkyl groups attached to C+ and is more stable than both secondary and primary carbocations.

Question 33

What happens in the electrophilic addition mechanism of HBr to propene?

Answer 33

In the electrophilic addition mechanism of HBr to propene, there are two possible carbocation intermediates.

Question 34

What can alkenes be converted to in the presence of hydrogen gas and a nickel catalyst?

Answer 34

Alkenes can be converted to alkanes. ## Footnote This reaction is important in the production of margarine and other soft spreads from vegetable oils.

Question 35

What is the process of converting vegetable oils into margarine called?

Answer 35

The process is called hydrogenation or hardening.

Question 36

What is required for the hydrogenation reaction to occur?

Answer 36

Finely divided nickel, with specific temperature and pressure dependent on the alkene used.

Question 37

What temperature is required for the hydrogenation of ethene?

Answer 37

The temperature required is 180 °C.

Question 38

What are vegetable oils known for?

Answer 38

Vegetable oils are liquids containing long hydrocarbon chains, often with many double bonds, and are known as polyunsaturated.

Question 39

What does the process of hardening do to vegetable oils?

Answer 39

It partially solidifies the oil so it can act as a spread.

Question 40

What can partial hydrogenation of unsaturated fats transform?

Answer 40

It can transform some Z (cis) double bonds into E (trans) double bonds.

Question 41

What is the industrial importance of alkenes?

Answer 41

Alkenes are used to produce plastics by a process called addition polymerisation.

Question 42

What is addition polymerisation?

Answer 42

Addition polymerisation is the process where hundreds or thousands of individual alkene molecules (monomers) are joined together to form a long chain molecule known as a polymer.

Question 43

What are monomers?

Answer 43

Monomers are small molecules that join together to form a polymer.

Question 44

What is a polymer?

Answer 44

A polymer is a large molecule formed when monomers join together.

Question 45

What are some properties of polythene, polypropene, and PVC?

Answer 45

They are flexible, chemically inert, and produced relatively cheaply.

Question 46

What are some uses of polythene?

Answer 46

Plastic bags and bottles, cable insulation. ## Footnote Polythene is a type of addition polymer.

Question 47

What are some uses of polypropene?

Answer 47

Crates, toys, ropes. ## Footnote Polypropene is a type of addition polymer.

Question 48

What are some uses of PVC?

Answer 48

Guttering, window frames, waterproof clothing. ## Footnote PVC is a type of addition polymer.

Question 49

What problems can arise from the lack of reactivity of plastics?

Answer 49

The lack of reactivity can cause problems when disposing of these plastics.