Question

Treatment of cycloheptatriene with one equivalent of bromine gave an oily dibromide that on heating in vacuum at 70° left tropylium bromide, C7H7Br, as yellow crystals, melting point 198-200°. Tropylium bromide is an unusual organic bromide, intrinsically colored and soluble in water but not in non-polar organic solvents like hexane. Draw the structure of tropylium bromide.

Answer 1

Step-by-step explanation:

The given properties of tropylium bromide imply that this should be an ionic compound. Let's see why and how:

• Significantly high melting point compared to molecular substances;
• It is soluble in water, this means we have a polar compound according to the rule like-dissolves-like, polar compounds dissolve in polar solvents. Water is a polar solvent and the polarity of tropylium bromide occurs due to the fact that ionic bonding is involved;
• Just as in the previous point, we should know that hexane is a non-polar organic solvent. It just proves that tropylium bromide is polar.

Tropylium bromide is produced by removing a proton from the cycloheptatriene ring and producing a positively charged carbon. Its charge is balanced by the negatively charged bromide. Thus, we should draw a cycloheptatriene ring (ring with 7 carbons and 3 double bonds) with bromide anion adjacent to it to balance the carbon's positive charge. The image below illustrates this.