Reactions of Alkenes

Question 1

What is an Electrophilic Reaction?

Answer 1

Where an Alkenes double bond opens up and atoms are added to the carbon atoms.

Question 2

Why are carbon atoms easily attacked by Electrophiles?

Answer 2

The double bond has many electrons.

Question 3

What is an Electrophile?

Answer 3

Electrophiles are electron pair acceptors. They are attracted to areas with electrons.

Question 4

What types of molecules are Electrophiles?

Answer 4

• Positively charged ions (eg H⁺ and NO₂⁺).

- Polar Molecules.

Question 5

Which atom in a polar molecule is attracted to electrons?

Answer 5

The ℑ+ atom (less Electronegative).

Question 6

What type of reaction do Ethene and Hydrogen react in to form Ethane and what conditions do they need?

Answer 6

An addition reaction.

-150°C and a Nickel Catalyst.

Question 7

How is Ethane formed in an addition reaction? And give the equation:

Answer 7

The double bond in Ethene opens up and hydrogen is added across it.

H₂C=C₂H + H₂ → CH₃CH₃
(under 150°C and a Nickel Catalyst.)

Question 8

How is a Dihaloalkane formed?

Answer 8

It is an Electrophilitic Addition reaction between Halogens and Alkenes where Halogens are added across the double bond.

Question 9

Give the general reaction for a Dihaloalkane:

Answer 9

H₂C=CH₂ + X₂ → CH₂XCH₂X

Question 10

What is a Carbocation?

Answer 10

An organic ion containing a positively charged carbon atom.

Question 11

What is the mechanism for Electrophilic Addition Reaction?

Use Bromine as an example (although Cl₂ and I₂ react in the same way)

Answer 11

1) Double bond repels electrons in Br₂, polarising the Br-Br molecule.

2) Heterolytic (unequal) fission occurs as the closer Br molecule gives up its bonding electrons to the other Br molecule, and bonds to the C atom leaving a :Br⁻
ion.

3) A positively charged carbocation (the non bonded C atom) attracts the remaining :Br⁻ atom which bonds to it, thus forming the new molecule.

Question 12

What substance can be used to test for a carbon double bond, what are the results and how does it work?

Answer 12

Bromine water can test for Carbon double bonds.
An alkene in orange bromine water will decolourise when shaken as the Br is added across the double bond, forming a colourless Dibromoalkane.

Question 13

What process is used to make Alcohols and what conditions are needed?

Answer 13

Steam Hydration.
300°C and a 60 - 70 atm pressure
A phosphoric(v) acid (H₃PO₄) is needed as a catalyst.

Question 14

What is an example of the Steam Hydration process being used for? What type of reaction is it and what is the yield?

Answer 14

• Manufacturing ethanol from ethene.
• Reversible reaction
• Low yield (~5%) but unreacted alkene gas can be recycled giving a ~95% yield).

Question 15

How is a Haloalkane formed?

Answer 15

Formed when alkenes undergo addition reactions with Hydrogen Halides.

Question 16

Give an example reaction of a Haloalkane:

Answer 16

Ethene and Hydrogen Bromide:

H₂C=CH₂ + HBr → CH₂BrCH₃.

Question 17

If a hydrogen halide is added to an unsymmetrical molecule, what can happen and what does this depend on?

Answer 17

Two products may be formed.

The amount of each product formed depends on the stability of the carbocation formed at the middle of the reaction.

Question 18

What makes a Carbocation more stable and how?

Answer 18

More alkyl groups improve stability because they feed electrons towards the positive charge.

Question 19

Is a stable carbocation more or less likely to form?

Answer 19

MORE likely to form.

Question 20

What is a Carbocation with ONE alkyl group called and how stable is it?

Answer 20

A primary carbocation and it is the least stable.

Question 21

What is a Carbocation with TWO alkyl groups called and how stable is it?

Answer 21

A secondary carbocation and it is more stable than a primary.

Question 22

What is a Carbocation with THREE alkyl groups called and how stable is it?

Answer 22

A Tertiary carbocation and it is the most stable.

Question 23

What is a Major product formed by?

Answer 23

The more stable carbocation.

Question 24

What is a Minor product formed by?

Answer 24

The less stable carbocation.

Question 25

What is Markownikoff’s rule?

Answer 25

The major product from addition of a hydrogen halide (HX) to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached.