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| Qu1: | |
| (a)
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Chlorine atoms can be introduced fairly simply by treating benzene with chlorine in the presence of a Lewis acid catalyst like iron (III) chloride. |
| (b)
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The most common method for introducing fluorine is via the Schiemann reaction of the diazonium salt using HBF4. Diazonium salts are obtained from anilines. |
| (c)
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The most common method for introducing iodine is again via the diazonium salt , by treating it with potassium iodide. |
| (d)
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Since both -Cl and -Br are ortho- / para- directing, we cannot just introduce them using the halogen and iron (III) trihalide method. The alternative is via the diazonium reactions and a Sandmeyer reaction. This also means we can use the nitro- group to set up the required meta arrangement. |
| (e)
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Since bromine is an ortho / para director but the large size of the bromine to some degree sterically inhibits ortho substitution, 1,4-dibromobenzene can be obtained by treating benzene with excess Br2 and the Lewis acid catalyst iron (III) bromide. |
| (f)
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In order to complete this synthesis, we need to use aniline.... this powerful activating group allows for the introduction of 3 bromines in the correct arrangement (in fact it is so reactive, a catalyst is not even needed, and the reaction is almost instantaneous at room temperature), then the amino group, -NH2, can be removed by diazotisation and the introduction of a H. |
| Qu2: | Since the bromine is an ortho- / para- director, but is fairly large, the major product of these reactions will be the para- isomers. |
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| Qu3: | A collection of reactions of an aryl Grignard reactions.... you should compare them with those of ethyl magnesium bromide (see chapter 14 questions). In each case remember that there is a aqueous acid work-up. |
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| Qu4: | Yes benzyne is aromatic..... it has a cyclic, planar, conjugated, 6p electron system. |
 Note that
the second p bond (shown as the blue orbitals
in the diagram to the right) of the triple bond is perpendicular and therefore
cannot overlap with the aromatic p system (shown
as cyan orbitals). This means that the two electrons associated with this
bond are not part of the conjugated system.
Benzyne is very reactive due to the strain of the triple bond due to its incorporation into the six membered ring. |
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| Qu5: | Benzyne has a reactive triple bond undergoing addition reactions, including the Diels-Alder reaction: |
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