Lesson 6: Reactions of alkenes

Question 1

What are alkenes?

Answer 1

• Alkenes are hydrocarbons with a double carbon- carnon bond.

Question 2

What is the general formula for the homologous series of alkenes?

Answer 2

CnH2n

Question 3

Why are alkenes known as “unsaturated” hydrocarbons?

Answer 3

• Alkenes are “unsaturated” because they contain two fewer hydrogen atoms than the alkane with the same number of carbon atoms.

Question 4

What are the first 4 members of the homologous series of alkenes?

Answer 4

• Ethene
• Propene
• Butene
• Pentene

Question 5

What function group do alkenes contain?

Answer 5

C=C

Question 6

What determines the reactions of organic compounds?

Answer 6

• Generally the reactions of functional groups determine the reactions of organic compounds.

Question 7

How do alkenes react with oxygen? How do they react with this oxygen differently to other hydrocarbons?

Answer 7

• Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons.
• They tend to burn in air with smoky flames because of incomplete combustion (carbon soot produced.)

Question 8

How do alkenes react with hydrogen, water and halogens?

Answer 8

• Alkenes react with hydrogen, water and halogens by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond.

Question 9

What is a “functional group” of a molecule?

Answer 9

• A functional group (of a molecule) determines how the molecule reacts.

Question 10

What is the product when an alkene reacts with hydrogen?

Answer 10

Alkene + hydrogen —> alkane

Question 11

What are the conditions needed for an alkene to react with hydrogen to form an alkane?

Answer 11

• 150⁰C
• Nickel catalyst.

Question 12

What is the reaction of an alkene with hydrogen called?

Answer 12

• Hydrogenation.

Question 13

What is the reaction of an alkene with water called?

Answer 13

• Hydration

Question 14

What are the 4 conditions required for the hydration reaction of an alkene?

Answer 14

• Water (that is reacting with the alkene) must be in the form of steam.
• 300⁰C
• Pressure = 70 atmospheres.
• Phosphoric acid catalyst (to increase yield of alcohol, any unreacted alkene and steam are passed back through the catalyst.)

Question 15

What type of reaction is a hydration reaction?

Answer 15

• Hydration reaction is a reversible reaction.

Question 16

What is the general equation for the reaction of an alkene with water (steam)?

Answer 16

Alkene + water (steam) —-> alcohol

Question 17

What is the general formula for an alkene reacting with a halogen?

Answer 17

Alkene + halogen —> DIhalogenalkane
Ie.
Ethene + chlorine —> dichloroethane
Ethene + bromine —> dibromoethane
Ethene + iodine —> diiodethane

Question 18

What is the name of the reaction that ADDS hydrogen, halogens and water to an alkene?

Answer 18

• Addition reactions