(a) Draw all the Kekulé structures of anthracene and phenanthrene.

(b) Propose mechanisms for the two additions shown.

(c) In Chapter 8, most of the additions of bromine to double bonds gave entirely anti stereochemistry. Explain why the addition to phenanthrene gives a mixture of syn andanti stereochemistry.

(d) When the product from (c) is heated, HBr is evolved, and 9-bromophenanthrene results. Propose a mechanism for this dehydrohalogenation.

Kekule structures are commonly observed in aromatic compounds. Here carbon atoms are represented as two ends of a line and each binds is shown as a dash. Hydrogen atoms are not mentioned.

Resonance structure of anthracene and phenanthrene

The mechanism for addition in phenanthrene

(c)The addition of bromide to normal double bonds involve the intermediate bromonium ion. So the nucleophilic attack from the opposite side gives entirely anti stereochemistry.

Formtion of brominium ion

Anti stereochemistry is not followed when the carbonium ions are stabilized. Bromonium ion is not formed in these cases.

When the bromine electrophile attacks phenanthrene the resulting carbonium is highly stabilized because of more conjugation so the nucleophile can attack from either side to give the mixture of both cis and trans isomers.

Resonance stabilized carbonium ion

(d) When 9,10-dibromo phenanthrene is heated HBr is eliminated to form 9-Bromo phenanthrene. This is an elimination that follows first-order kinetics. This reaction takes place in a two-step way:

1:

Formation of carbocation

2.

Formation of 9-bromophenanthrene