The reactivity of alkenes and benzene

Question 1

What is the type of reaction cyclohexene take part in with bromine?

Answer 1

Electrophilic addition reaction.

Question 2

Why, even though Br2 is a non-polar molecule, it still reacts with the double bond in an alkene?

Answer 2

• The pi-bond in an alkene contains localised electrons, sited above and below the two carbon atoms in the double bond. This produces a region of high electron density.
• When bromine approaches an alkene, the electrons in the pi bond repel the electrons in the Br-Br bond. This induces a dipole in the Br2 molecule, with one end of the molecule becoming slightly negative and the other end becoming slightly positive. The Br-Br molecule is now polar.
• The pi bond is now attracted to the slightly positive bromine atom, causing the double bond to break and the bromine atom joining to a carbon atom, forming a positively charged carbocation. The two Br atoms break by heterolytic fission forming a bromide ion Br-.
• Finally the Br- ion is attracted to the intermediate carbocation, forming a covalent bond.

Question 3

Draw the mechanism for the electrophilic addition for the reaction of cyclohexene with bromine.

Answer 3

Question 4

Why does bromine react much more readily with cyclohexene than it does with benzene?

Answer 4

• Benzene has delocalised pi-electrons spread over all 6 carbon atoms in the ring structure. Benzene has a lower pi electron density than alkenes.
• The Benzene ring has insufficient pi electron density above and below any two carbon atoms to cause the necessary polarisation of the bromine molecule. This makes benzene resistant to reaction with non-polar halogens.
• A halogen carrier is needed to make a more powerful electrophile for benzene, Br⁺.

Question 5

What is the general rule about the stability and reactiviness of molecules with electrons evenly distributed over the structure?

Answer 5

They tend to be more stable and less reactive than molecules with localised. Delocalised structures require more energy to react. In the case of benzene, energy is needed to disrupt the delocalised pi cloud of electrons.

Question 6

What are polychlorinated biphenyls?

Answer 6

A family of compounds made up of the elements chlorine, carbon and hydrogen. A molecule of biphenyl is composed of two benzene rings.

Question 7

What uses do PCB’s have?

Answer 7

Uses such as coolants and insulating fluids for transformers and capacitors, plasticisers in paint and cement, and in carbonless copy paper.

Question 8

Why where PCB’s banned in the late 1970s?

Answer 8

• A problem with PCBs is their fat solubility, allowing them to be stored in the body.