GAMSAT Purple Booklet | ACER Practice Test 2 - Q 18-23 | Gamsat Section 3

Worked solutions for Q 18-23 of ACER Practice Test 2 - GAMSAT Purple
Booklet

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[heretocomment]

Without mentioning Chiral carbons since it's not even mentioned in the question or information:

For question 23, neither Structure II or III can be a stereoisomer according to the rules stated, which is that, if the molecule is a mirror images that cannot be superimposed, then it is a stereoisomer. In other words, if it's mirror image is superimposable, then it is not a stereoisomer.

A disubstituted Structure II could have the 2 H connected by single bonds perpendicular to the line of symmetry (the 2 left hand H's in the PDF or the 2 bottom ones in the video at 0:46). The mirror image of this is superimposable and hence not a stereoisomer.

Structure III is a triangular prism. A disubstituted Structure III could have the 2 H's along one side of a triangle substituted, or the 2 H's along one side of the rectangle substituted. A mirror image of this is again, superimposable. This latter version is particularly easy to imagine as you can just spin it around its vertical axis.

So the answer should be C.

[mikeb]

for question 22, why are sterioisomers of the configurations you drew (2 on 1 side and 1 on the other or 3 on a single side) not being accounted for. If they are accounted for there would be double as many isomers, which brought me to the answer of four