Alkenes - Reactions of Alkenes

Question 1

What type of reaction is typical of the alkenes?

Answer 1

Electrophilic addition

Question 2

Describe the hydrogenation of alkenes.

Answer 2

Addition of H across the C=C double bond in the presence of heat (423K) & a nickel/platinum catalyst to form an alkane.
e.g. C2H4 + H2 -> C2H6

Question 3

Describe the halogenation of alkenes.

Answer 3

Addition of halogens (Cl & Br) across the C=C double bond at room temperature to form haloalkanes.
e.g. C2H4 + Br2 -> C2H4Br2

Question 4

Describe the hydration of alkenes.

Answer 4

Addition of steam across the C=C double bond in the presence of a phosphoric acid catalyst to form an alcohol.
e.g. C2H4 + H20 -> C2H5OH

Question 5

State the reagent & explain the positive result in the test for the alkene C=C double bond.

Answer 5

• Bromine water turns form orange to colourless in the presence of an alkene.
• Colour change occurs because orange bromide reacts with the alkene, adding across the C=C double bond to form a colourless bromoalkane.

Question 6

What type of bond fission (homolytic or heterolytic) occurs in the reaction of alkenes with hydrogen halides?

Answer 6

Heterolytic fission

Question 7

What causes the Br-Br bond to break in the reaction mechanism of Br2 with an alkene?

Answer 7

The high electron density of the pi-electrons with polarises the Br-Br bond causing heterolytic fission.

Question 8

How does an unsymmetrical molecule like HBr act as an electrophile?

Answer 8

The H-Br bond is polar, so the partially positive H atom acts as an electrophile because it is attracted to the electon rich double bond.

Question 9

What is Markownikoff’s rule?

Answer 9

• The H of the hydrogen halide adds to the C atom of the unsymmetrical alkene with the greater number of H atoms already attached.
  (It determines the major product in the reaction).

Question 10

How does carbocation stability determine the major product when hydrogen halides are added to unsymmetrical alkenes?

Answer 10

• The major product is on which is formed via the most stable carbocation intermediate.
  (The more substituted a carbocation is, the greater the stablility).

Question 11

What is the definition of an electrophile?

Answer 11

• An electron-pair acceptor.
  (An electron-deficient species that accepts a pair of electrons to form a new covalent bond).

Question 12

What is a carbocation?

Answer 12

• An ion that contains a positively charged C atom.

Question 13

State the order of stability or primary, secondary & tertiary carbocations.

Answer 13

The stability of carbocations increases in the order: primary < secondary < tertiary.