Reactions of alkenes

Question 1

Why are alkenes more reactive than alkanes?

Answer 1

Alkenes are more reactive than alkanes due to the presence of the π-bond

The C=C double bond consists of a σ-bond and a π-bond, where the π-electrons are more exposed.

Question 2

What is the bond enthalpy of a C=C double bond?

Answer 2

612 kJ mol⁻¹

The C=C double bond is made up of a σ-bond and a π-bond.

Question 3

What is the bond enthalpy of a C–C single bond in ethene?

Answer 3

347 kJ mol⁻¹

This value helps to understand the strength differences between single and double bonds.

Question 4

What is the bond enthalpy of the π-bond in an alkene?

Answer 4

265 kJ mol⁻¹

It is calculated as 612 kJ mol⁻¹ (C=C double bond) - 347 kJ mol⁻¹ (C–C single bond).

Question 5

What type of reactions do alkenes primarily undergo?

Answer 5

Addition reactions

Alkenes add small molecules across the double bond, breaking the π-bond.

Question 6

What is hydrogenation of alkenes?

Answer 6

An addition reaction where hydrogen is added across a double bond to form an alkane

This occurs in the presence of a nickel catalyst.

Question 7

What is the product of hydrogenating propene?

Answer 7

Propane

The reaction involves adding hydrogen across the C=C bond.

Question 8

What is the general outcome of halogenation of alkenes?

Answer 8

Formation of haloalkanes

Alkenes react rapidly with halogens like chlorine or bromine.

Question 9

How can the presence of a C=C bond be tested?

Answer 9

By adding bromine water, which decolorizes if a C=C bond is present

The orange color disappears indicating unsaturation.

Question 10

What happens when alkenes react with hydrogen halides?

Answer 10

They form haloalkanes

The reaction can occur with gaseous or liquid hydrogen halides.

Question 11

What are the possible products when propene reacts with hydrogen chloride?

Answer 11

1-chloropropane and 2-chloropropane

The reaction can yield two different products due to the unsymmetrical nature of propene and HCl.

Question 12

What is the result of the hydration reaction of alkenes?

Answer 12

Formation of alcohols

This occurs when alkenes react with steam in the presence of a phosphoric acid catalyst.

Question 13

What is typically produced when ethene reacts with steam?

Answer 13

Ethanol

This industrial process is used widely for ethanol production.