Q 11-11-77 E - Organic Chemistry (Mcmurry)

Question

Compound X is optically inactive and has the formula ${\mathrm{C}}_{16}{\mathrm{H}}_{16}{\mathrm{Br}}_2$. On treatment with strong base, X gives hydrocarbon Y, ${\mathrm{C}}_{16}{\mathrm{H}}_{14}$. Compound Y absorbs 2 equivalents of hydrogen when reduced over a palladium catalyst and reacts with ozone to give two fragments. One fragment, Z, is an aldehyde with formula ${\mathrm{C}}_7{\mathrm{H}}_6\mathrm{O}$. The other fragment is glyoxal, ${\left(\mathrm{CHO}\right)}_2$. Write the reactions involved, and suggest structures for X, Y, and Z. What is the stereochemistry of X? 

Step-by-Step Solution

Answer

Optically inactive

Optically inactive compounds are the compounds which can’t rotate plane polarized light. Mostly they are achiral and meso compounds. 

Alkyl halide reaction with a strong base

Alkyl halides on reaction with strong base undergo elimination. The elimination of a beta-hydrogen and the halide occurs to produce an alkene.

Alkene reaction with ozone

Ozone react with alkenes and do oxidative cleaving of the alkene. The carbon-carbon double bonds are replaced with carbon-oxygen double bonds, giving carbonyl compounds.

Alkene reaction with dihydrogen in Pd presence

Alkene reacts with dihydrogen in Pd presence to reduce alkene to alkanes.

C 16 H 16 Br 2 possible structures

Here using the given molecular formula of X, two optically inactive structures of X are possible. Both are meso compounds having symmetry within the molecule so optically inactive.

C 16 H 16 Br reaction with a strong base

On reaction with strong base elimination will occur. Both of the possible structures of X will give same alkene on reaction.

                                                 Structure of Y

Reaction of Y with dihydrogen/Pd

In question it is given that two equivalents of hydrogen are used and Y obtained is alkene. It shows the presence of two double bonds in the molecule which on reaction reduce to alkane.

Reaction of Y with ozone

In question it is given that glyoxal formation occur so the double bond must be present consecutively and the structure of Z is given below: 

                                            2 molecules of Z are formed