Alkenes

Question 1

What is the general formula of an alkene?

Answer 1

CnH2n

Question 2

Why are alkenes described as ‘unsaturated’?

Answer 2

Alkenes contains at least 1 double bond.

Question 3

What bonds is a double covalent bond made of ?

Answer 3

A sigma and pi bond.

Question 4

Why are sigma bonds the strongest type of covalent bond?

Answer 4

Sigma bonds are when 2 S orbitals overlap and aligns horizontally. There is a strong electrostatic attraction between nuclei and shared electrons, due to a high electron density. This strong electrostatic attractions means more energy is needed to break the bond, giving a high bond enthalpy.

Question 5

What makes a single covalent bond?

Answer 5

A sigma bond.

Question 6

Why are pi bonds weaker than sigma bonds?

Answer 6

Electron density is spread out above and below the nuclei. Electrostatic attraction is lower, and this also gives a lower bond enthalpy.

Question 7

Why are alkanes less reactive than alkenes?

Answer 7

Alkanes have sigma bonds, which have a high bond enthalpy, meaning more energy is needed to break this bond, hence is less reactive. Alkanes are also non-polar.

Question 8

What is an electrophile?

Answer 8

An electron pair acceptor.

Question 9

What is halogenation?

Answer 9

When a halogen molecule reacts with an alkene, to form a dihaloalkane.

Question 10

How can haloalkanes be formed?

Answer 10

Reacting alkenes with hydrogen halides.

Question 11

The formation of haloalkanes and dihaloalkanes occur by what mechanism?

Answer 11

Electrophilic addition mechanism.

Question 12

In the formation of a dihaloalkane, how is a dipole initially caused in the halogen molecule? What type of bond fission is this?

Answer 12

The double bond in the alkene is electron rich as it shares 4 electrons. When the halogen molecule approaches the double bond, the double bond pushes the electrons within the halogen molecule (as electrons repel), inducing a dipole. Both electrons goes to one bonding atom in the halogen molecule, which is heterolytic bond fission. The bonding atom that has lost its shared electron is now delta positive and now acts as an electrophile.

Question 13

How may we test for an alkene?

Answer 13

Testing an alkene with bromine water is also a halogenation reaction. If the bromine water turns from orange to colourless, there is an alkene present.

Question 14

When a hydrogen halide reacts with an alkene, there is no need to induce a dipole on the hydrogen halide. Why is this?

Answer 14

The hydrogen halide bond is already polar, with the halogen being more electronegative.

Question 15

When forming a haloalkane, why may there be a mixture of products?

Answer 15

The positions the hydrogen and halogen reacts with on the alkene can alternate.

Question 16

In the formation of haloalkanes, there will be major and minor products from the mixture. How do we determine which is the major product?

Answer 16

The major product is the product which has more carbons bonded to the carbocation. If there is more carbons bonded to the carbocation (primary, secondary, tertiary), it is more stable.

Question 17

What are the reagents (and their states) in a hydrogenation reaction? What are the products?

Answer 17

In a hydrogenation reaction, an alkene reacts with hydrogen gas. This forms an alkane.

Question 18

State the conditions needed for a hydrogenation reaction.

Answer 18

150 degrees celcius and a nickel catalyst.

Question 19

What covalent bond breaks in a hydrogenation reaction?

Answer 19

A pi bond.

Question 20

What is the product of a hydration reaction?

Answer 20

An alcohol.

Question 21

What conditions are needed for a hydration reaction?

Answer 21

Phosphoric acid catalyst.

Question 22

Why may there be a mixture of products formed?

Answer 22

The positions the -OH and -H reacts on the alkene can alternate.

Question 23

What is a polymer?

Answer 23

A long chain made of monomers.

Question 24

State the conditions needed for a polymerisation reaction to occur?

Answer 24

High temperature, high pressure and a catalyst.

Question 25

Why may storing polymers in landfill sites cause pollution?

Answer 25

Alkenes are non-biodegradable.

Question 26

What are the advantages and disadvantages of burning polymers?

Answer 26

Burns them instead of a non-renewable source like fossil fuels. But burning polymers releases HCl gas which is toxic.

Question 27

What are ways to dispose of polymers?

Answer 27

Recycling, feedstock recycling, burning, landfills.