Sample/practice exam december 2015, questions and answers - practice problems on stereochemistry PDF

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Practice problems on stereochemistry...

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Practice Problems: Stereochemistry 1) For each of the following structures, determine whether it is chiral or not. Can you identify all the stereogenic centres(s) and name their absolute configurations? Recall that a molecule may possess stereogenic centres but not be chiral, or vice versa. OH

O O

OH

O H H

HO

C

OH

C

C

H3 C O

CH3

OH

Br

Br

OH

CH3

O

2) Some molecules have stereochemistry despite the fact that they have no stereogenic centres. Usually, this arises because of structural hindrance to internal motion. Can you draw the enantiomer of each of the following chiral molecules? CO2CH3

O

O

CO2CH3 H

H CO2H OCH3 OCH3

H

H H H

3) How many planes of symmetry does each of the following molecules possess? Cl

OH

Br OH Br OH

Cl H

N H

H

N

H

4) Identify heterotopic atoms or groups in each of the following. Indicate whether the group is enantiotopic or diastereotopic. OH HO O O HO OH O

OH NH2

Answers to Stereochemistry Practice Problems 1. OH Achiral - molecule has a plane of symmetry as shown.

3

1 O

Chiral - the priorities around the indicat ed stereogenic centre are shown. Its configuration is thus S. T his is a more subtle application of the CIP rules. For more inf ormation, click here.

O 2 4 OH

O

O O HO

HO

OH

OH O

H C H3 C

OH

HO H

C

OH

H

Chiral - it is not superimposable on its

C

C CH3

C

H3 C

mirror image. We have not discussed C H CH3 how to name the absolute

mir ror i mage

Br

Br

Br

O

Achiral - this structure is meso. You can see this more easily if you rotate the central C-C bond. The stereocentres have the S and R configurations, from left to right.

Br

O

configurations of systems like this, but you should be able to identify whether they are chiral or not.

Chiral - it is not superimposable on its mirror image. Note that it does have a C 2 axis of symmetr y (i.e. if you rotate by 180 o around an axis t hrough t he C=O bond, the result is indistinguishable), but this does not destroy chirality.

mir ror image

OH Achiral - there is a p lane of symmetry in the plane of the image.

CH3

2. O

CO2CH3

O

Notice that in the enantiomer, all CO2CH3 stereogenic centres must be inverted, AND also the rotation of the dimethoxyphenyl H group.

H

OCH3 OCH3

H H HO2C

Since the given structure is a helix, its enantiomer is the helix with the opposite spiral sense.

trans-Cyclooctene is also a helical structure, and again its enantiomer has the opposite spiral sense.

3. OH OH

One symmetry plane

OH Cl Br One symmetry plane (in the plane of the image). Note that there is a point of symmetry at the centre of the ring. Br Cl

N N

Four symmetry planes - the three drawn here plus one bisecting the three C-C bonds.

Adamantane is made up from 4 cyclohexane rings. Each ring has 3 symmetry planes, thus there are 12 planes of symmetry in total (I think I have this right! This is tricky and should be taken as a "thinker" challenge rather than a typical question you would encounter on a test).

4. H

H Enantiotopic H atoms. (NB the face of the carbonyl are also enantiotopic) O H

H

OH HO

Diastereotopic H atoms. O

HO OH Ha Hb

Hb

CH3

Each c,d pair is enantiotopic. Hd

Ha

Each a,b pair is enantiotopic.

Hc Hc

Hd

O

H3C

OH NH2

The methyl groups are diastereotopic...