Chapter 10 - Organic reactions

Question 1

What are alkenes?

Answer 1

A homologous group of unsaturated hydrocarbons consisting of covalent bonds and one double carbon-carbon bond.

Question 2

What is the functional group for alkenes?

Answer 2

C=C

Question 3

What is the general formula for alkenes?

Answer 3

CnH2n

Question 4

Why are alkenes unsaturated?

Answer 4

Because they have a double carbon to carbon bond.

This covalent bond can break in order for the carbon atoms to bond an atom from another molecule.

Question 5

State the 4 alkenes you are required to know for GCSE chemistry.

Answer 5

• Ethene (C2H4)
• Propene (C3H6)
• Butene (C4H8)
• Pentene (C5H10)

Question 6

State the 3 reactions of alkenes you should know for GCSE chemistry

Answer 6

• Hydration (reaction with water)
• Hydrogenation (reaction with hydrogen)
• Halogenation (reaction with halogens)

Question 7

How do alkenes react with oxygen during combustion.

Answer 7

• When alkenes burn/react with oxygen, they produce smokier flames than alkanes.
• They are very reactive due to their double bond, so they need a rapid supply of oxygen. If this isn’t enough, incomplete combustion will occur.
• INCOMPLETE COMBUSTION OF ALKENES:

Alkene + Oxygen -> Carbon monoxide + Carbon

• COMPLETE COMBUSTION OF ALKENES:

Alkene + Oxygen -> Carbon dioxide + water

Question 8

Why are alkenes so reactive?

Answer 8

Because of their double bond/unsaturation.

Question 9

What are the products when alkenes react with halogens (halogenation)?

Answer 9

Alkene + Halogen -> Di - halogen prefix ending with o - alkane version of alkene.

e.g Ethene + Bromine -> Dibromoethane

Question 10

What are the conditions needed for alkenes to react with halogens?

Answer 10

No conditions

Question 11

What is an addition reaction?

Answer 11

an organic reaction were a hydrocarbon chemically bonds with another molecule to produce one single product.

Question 12

What are the products when alkenes react with hydrogen (hydrogenation)?

Answer 12

Alkene + Hydrogen -> Alkane

Question 13

What are the conditions needed for hydrogenation?

Answer 13

1) High Temperature: 150°C

2) Catalyst: Nickel

Question 14

What are the products when alkenes react with water (hydration)?

Answer 14

Alkene + Water (vapour) ⇌ Alcohol

Question 15

What are the conditions needed for hydration?

Answer 15

1) High Temperature: 300°C

2) High Pressure: 70 atm (atmospheres)

3) Catalyst: Phosphoric acid

Question 16

True or False, Hydration of alkenes is a reversible reaction?

Answer 16

True

Question 17

How can we increase the yield of the alcohol produced from hydration of alkenes?

Answer 17

1 - By passing the by-products (e.g unreacted alkene and water) back through the catalyst since the reaction is reversible.

2 - Cooling the mixture of alcohol, unreacted ethene and water then fractional distillation

STEP 1) Cool mixture to get ride of alkene. Alkene has a higher BP so remains as a gas, but alcohol and water condense.

STEP 2) Fractional distillation: heat alcohol and water mixture, alcohol has a higher BP so vaporises rises and then condenses in beaker.

Water w lower BP remains in distillation flask.

Question 18

What is the functional group for alcohols?

Answer 18

OH, they are also known as the hydroxyl group.