Question

Carbonic acid A with an overall formula of C₅H₈O₂ yields two geometric isomers, cis (A') and trans (A").On hydrogenation with Pt/H₂ the same racemic carboxyl acid B is obtained from both stereoisomers that can be separated into enantiomers (+)-B and (-)-B. A' and A" rapidly react with one mole of bromine in CCl₄ in the dark at 20°C to yield C. Problems: 

(a) What is the constitution of A and B? 

(b) Write the stereo formulae for A' and A" and the Fischer projection formulae for the enantiomer B(not considering the signs (+) or (-)).

(c) How many stereo isomers of C are simultaneously formed when A' and A" are treated with bromine? 

(d) Briefly, give reasons for your answer to c).

(e) Write the Fischer projection formulae and one Newman projection formula (conformation) for all the stereoisomers of C. Denote those that are mutually enantiomeric and diastereoisomeric. 

Answer 1

(c) Always two (see e): 1 to 4 

(d) The addition of bromine to the alkene gives trans compound under the given conditions. On the addition, two (non-identical) asymmetrical C atoms (chirality centres) are formed yielding together 22 = 4 stereo isomers of which always two are mutually enantiomeric. 

1 and 2 or 3 and 4 are enantiomeric.1 to 3 and 4, and 2 to 3 and 4 are diastereomeric