Organic Chemistry : Help with Cis-Trans Isomers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Isomers

Which of the following lists the product(s) of the presented reaction?

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Possible Answers:

Only cis-1-4-dimethylcyclohexane

Methylcyclohexane

None of the other answers is correct.

Only trans-1-4-dimethylcyclohexane

Both trans and cis-1-4-dimethylcyclohexane

Correct answer:

Both trans and cis-1-4-dimethylcyclohexane

Explanation:

Hydrogen and a catalyst like paladium reduce the double bond to a single bond. There is no equal steric hinderance on each side. The hydrogen can bond from either side. That means the methyl group can either be oriented into the page or out of the page. One form is cis, and one form is trans.

Example Question #1 : Isomers

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II.Untitled

III.Untitled

Which of the given molecules is(are) chiral?

Possible Answers:

II and III

I only

I and III

I, II, and III

I and II

Correct answer:

I and III

Explanation:

For a molecule to be chiral, it must have a stereocenter and no axis of symmetry. An atom with a stereocenter has no identical bonds; it is a carbon atom with four unique substituents. There are two stereocenters in each of the three molecules. Notice that if you take the second molecule and draw a line connecting the top carbon and the point between the the two carbons with hydroxy groups, it has an axis of symmetry and therefore cannot be chiral. There is no way to draw that axis of symmetry for molecules one and three.

Example Question #1 : Help With Cis Trans Isomers

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How many stereocenters does the given molecule have?

Possible Answers:

Two

Three

One

Five

Four

Correct answer:

Two

Explanation:

A stereocenter exists when the central atom is bound to four unique substituents. In the given molecule, the carbons are numbers from left to right. Carbons 1, 3, 5, 6, and 8 are all bound to at least two hydrogen atoms and cannot be stereocenters. Similarly, the carbons in the two methyl groups (bound to C4 and C7) do not qualify as stereocenters. Carbon 7 has two identical methyl substituents. This leaves only C2 and C4. The molecule has two stereocenters.

Example Question #4 : Isomers

What is the IUPAC name of the molecule shown?

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Possible Answers:

E-5-carboxy-2-pentene

Z-5-carboxy-2-pentene

E-pent-3-enoic acid

Z-pent-3-enoic acid 

Correct answer:

Z-pent-3-enoic acid 

Explanation:

Carboxylic acid is highest priority, so carbon chain labelled from right to left. Since highest priority groups are on the same side of the double bond, it's given the "Z" designation.

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