halogenalkenes

Question 1

1. Define nucleophile.

Answer 1

1. A species that donates a lone pair of electrons.

Question 2

2. Explain why nucleophiles can react with halogenalkanes.

Answer 2

2. The carbon in the functional group is δ+ as it is attached to a halogen atom that is more electronegative. The δ+ carbon will attract nucleophiles.

Question 3

3. Briefly describe homolytic fission and heterolytic fission.

Answer 3

3. Homolytic fission – breaking a covalent bond so each atom gets one unpaired electron (free radical). Heterolytic fission – breaking a covalent bond so the more electronegative atom gets a lone pair of electrons.

Question 4

4. Name and outline the mechanism for the reaction of 1-bromopropane with warm, aqueous NaOH. Write an overall equation for this reaction.

Answer 4

4. . Nucleophilic substitution

Question 5

5. Name and outline the mechanism for the reaction of 1-bromopropane with hot, ethanolic potassium cyanide. Write an overall equation for this reaction. Name the organic product that is formed.

Question 6

6. Name and outline the mechanism for the reaction of bromoethane with warm, ethanolic ammonia. Write an overall equation for this reaction. Name the organic product that is formed.

Question 7

7. When 1-chlorobutane, 1-bromobutane and 1-iodobutane react with water, 1-iodobutane reacts the fastest whilst 1-chlorobutane reacts the slowest. Explain these results.

Answer 7

7. 1-iodobutane reacts the fastest and 1-chlorobutane reacts the slowest. This is because C-I has the weakest bond enthalpy so it is easiest to break.

Question 8

8. When a halogenalkane reacts with NaOH/KOH, two different mechanisms can occur. Name these two different mechanisms and state the conditions that make it more likely to occur.

Answer 8

8. Halogenalkane + aqueous NaOH/KOH will react via nucleophilic substitution mechanism. It is more likely with primary halogenalkanes. Halogenalkanes + hot, ethanolic NaOH/KOH will react via an elimination mechanism. It is more likely with tertiary halogenalkanes.

Question 9

9. Name and outline the mechanism for the reaction of 2-chlorobutane with hot, ethanolic NaOH to form but-1-ene.

Answer 9

9. Elimination:

Question 10

10. Name and outline the mechanism for the reaction of 2-chlorobutane with hot, ethanolic NaOH to form but-2-ene.

Answer 10

10. Elimination:

Question 11

11. How are halogenalkanes classified as primary/secondary/tertiary?

Answer 11

11. Primary halogenalkanes – have one R group attached to the carbon of the functional group. Secondary halogenalkanes – have two R groups attached to the carbon of the functional group. Tertiary halogenalkanes – have three R groups attached to the carbon of the functional group.