14.2 - Reaction of alkanes

Question 1

Addition of hydrogen halides

Answer 1

Question 2

Bromine water reacting with alkenes:

Answer 2

• The alkene reacts with bromine via electrophilic addition.
• This reaction removes the orange colour as the bromine is consumed.
• The product is a colourless dibromoalkane solution.

Question 3

Addition of concentrated sulphuric acid

Answer 3

Question 4

What do alkyl groups stabilise:

Answer 4

stabilise the positive charge through inductive effects, with more groups providing greater stability

Question 5

What does water and heat do to the alkyl hydrogensulphate

Answer 5

hydrolyses the alkyl hydrogen sulfate back to alcohols and sulphuric acid

Question 6

Mechanism for addition of halogens to alkenes

Answer 6

Step 1 -The high electron density of the C=C double bond repels electrons in the bromine molecule, polarising the Br-Br bond.

Step 2 -This leads to heterolytic fission of the Br-Br bond, with one bromine atom taking the bonding pair of electrons, creating a positively charged bromine which bonds to one of the carbon atoms.

Step 3 -A positively charged carbocation intermediate forms. The negatively charged bromide ion then bonds to the other carbon atom.

Step 4 - The final product, 1,2-dibromoethane, has a bromine atom bonded to each of the original alkene carbon atoms.

Question 7

Carbocation stability structures (3)

Answer 7

• Primary carbocations- These have one alkyl group attached to the positively charged carbon amd are the least stable.
• ** Secondary carbocations**- These have two alkyl groups at the carbocation centre and offer moderate stability.
• Tertiary carbocations - These have three alkyl groups and are the most stable. - most common product as it’s the most stable one

Question 8

Why does electrophilic addition occur?

Answer 8

This happens because the C=C double bond is rich in electrons, making it a target for electrophiles

Question 9

Electrophilic addition:

Answer 9

The π bond of the C=C double bond breaks.
Atoms or groups add across the carbon atoms.

Question 10

Hydration to form alcohols (use of catalyst, temperature and pressure):

Answer 10

phosphoric acid catalyst
300 degrees
60-70 atm pressure

Question 11

Electrophilic addition:

Answer 11

The π bond of the C=C double bond breaks.
Atoms or groups add across the carbon atoms.