Alkenes

Question 1

How can alkenes be prepared?

Answer 1

Dehydration of alcohols when treated with sulfuric acid.
Base induced elimination of hydrogen halides from monohaloalkanes

Question 2

Type of reactions alkenes can undergo?

Answer 2

Electrophillic addition reactions:
Hydrogenation to form alkanes - platinum catalysed.
Halogenation to form dihaloalkanes.
Hydrohalogenation to form monohaloalkanes.
Acid catalysed hydration to form alcohols

Question 3

Markovnikovs rule

Answer 3

When H-X (hydrogen halide) is added to an asymmetric alkene. The major product formed is the one where the H attaches to the double bonded carbon already attached to the most hydrogen atoms.
Can also apply for H-OH, water.

Question 4

When doesn’t Markovnikov’s rule apply?

Answer 4

When alkene is symmetrical.

Question 5

Halogenation precursors

Answer 5

Diatomic halogen molecule and alkene

Question 6

Halogenation intermediate

Answer 6

Cyclic ion intermediate

Question 7

Halogenation Step 1

Answer 7

Example for Bromine:
Br2 becomes polarised when one side approaches double bond as it pushes electrons away to further Br atom. Closer S+ atom attacks and cyclic ion intermediate formed alongside Br- ion.

Question 8

Halogenation Step 1 nucleophile and electrophile

Answer 8

Nucleophile is the alkene and electrophile is the bromine molecule.

Question 9

Halogenation Step 2

Answer 9

Br- ion attacks from opposite side and makes covalent bond with other carbons on double bond, breaking the carbons intermediate bond with the other bromine.

Question 10

Halogenation Step 2 Nucleophile and electrophile

Answer 10

Nucleophile is the Br- ion.
Electrophile is the cyclic ion intermediate.

Question 11

In Halogenation why does Bromine ion attack from opposite side?

Answer 11

The Br atom already bonded is so large that it prevents access from that side.

Question 12

Hydrohalogenation precursor

Answer 12

Alkene and hydrohalide.

Question 13

What is intermediate in hydrohalogenation

Answer 13

Carbocation

Question 14

Step 1 for hydrohalogenation

Answer 14

Polar hydrohalide will have slightly positive hydrogen. This will act as an electrophile, attacking the electron rich double bond. Bond forms between carbon and hydrogen. Hydrohalide bind will split leaving a negative halogen ion. And carbocation will be formed.

Question 15

Step 2 for hydrohalogenation

Answer 15

Halogen ion is nucleophile and attacks the carbocation forming a covalent bond between.

Question 16

Does it matter where halide ion attacks from in hydrohalogenation

Answer 16

No

Question 17

Which is more stable C-C+-C or C-C-C+ and why.

Answer 17

C-C+-C is more stable due to positive inductive effect since the positive carbon is bonded to two alkyl as opposed to one so it is more stable.

Question 18

Acid catalysed hydration precursors

Answer 18

Precursors - water, alkene and hydronium (catalyst)

Question 19

Step 1 for acid catalysed hydration

Answer 19

Hydronium electrophile attacks the electron rich double bond on the alkene.
Bond between hydrogen and oxygen brakes and bond forms between carbon and hydrogen.
Carbocation intermediate and water is left behind.

Question 20

Step 2 for acid catalysed hydration

Answer 20

Carbocation undergoes rapid nucleophillic attack from the water molecule to give leave a protonated alcohol molecule.

Question 21

Step 3 for acid catalysed hydration

Answer 21

The protonated alcohol is very acidic so is fine to give away hydrogen to water, to form alcohol and hydronium