Organic Chemistry : Isomers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Isomers

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

Untitled

Possible Answers:

Methylcyclohexane

None of the other answers is correct.

Only cis-1-4-dimethylcyclohexane

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 #2 : Isomers

I.Untitled

II.Untitled

III.Untitled

Which of the given molecules is(are) chiral?

Possible Answers:

I, II, and III

I and II

I only

II and III

I and III

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 : Isomers

Untitled

How many stereocenters does the given molecule have?

Possible Answers:

Three

Five

One

Two

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 #1 : Isomers

What is the IUPAC name of the molecule shown?

Screen shot 2015 11 09 at 5.27.40 pm

Possible Answers:

E-5-carboxy-2-pentene

Z-pent-3-enoic acid 

Z-5-carboxy-2-pentene

E-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.

Example Question #1 : Isomers

How many stereoisomers would be obtained by the hydrogenation of compound C?

Mcat_1

Possible Answers:

Three

Four

One

Two

Zero

Correct answer:

Two

Explanation:

The hydrogenation of compound C would add two hydrogen atoms across the double bond, but would generate only one new stereocenter. This stereocenter would be found on the third carbon in the chain (from the right), which would be bound to the phenyl substituent, a methyl group, a hydrogen atom, and the remaining branched carbon chain.

The hydrogenation reaction would create a racemic mixture of both possible orientations of this stereocenter, with both enantiomers present in equal amounts. There would this be two stereoisomer products obtained from the hydrogenation of compound C.

Example Question #1 : Isomers

How many stereoisomers are possible for the compound 2,3,4-trimethylpentane?

Possible Answers:

Two

Three

Four

Five

One

Correct answer:

One

Explanation:

2,3,4-trimethylpentane does not contain any stereocenters. The structure is a five-carbon chain, with the end carbons bonded to three hydrogens each. The three central carbons each carry a methyl group and a hydrogen atom.

Remember that a stereocenter is only present when a carbon is bound to four different substituents. The 2 and 4 carbons both have two methyl groups (the end carbons and the added groups), so they would contain a plane of symmetry and would not be stereocenters. Likewise, a plane of symmetry exists at the 3 carbon; the substituents toward the 1 carbon and 5 carbon are the same. This compound, therefore, would have no stereocenters and could only exist as one stereoisomer.

Example Question #21 : Isomerism And Stereoisomers

Penicillin

Shown above is the chemical structure for penicillin, a common prescription drug. How many chiral carbons does penicillin have?

Possible Answers:

Zero

Three

Five

Two

One

Correct answer:

Three

Explanation:

The correct answer is three. The key to finding chiral carbons is to look for carbons that are attached to four different substituents. We can immediately eliminate any carbons that are involved in double bonds, or that have two hydrogens attached. Given this, we find that there are three chiral carbons. Note that carbon chains of varying content will qualify as different substituents, allowing chiral carbons to bond to two other carbons.

Penicillin_chiral

 

Example Question #1 : Isomers

Screen shot 2015 07 19 at 10.48.54 am

Which two of the molecules shown are enantiomers?

Possible Answers:

II and IV

I and III

I and II

II and III

III and IV

Correct answer:

I and III

Explanation:

The enantiomer of a molecule with multiple chiral centers is formed through configurational inversion at every chiral center. By rotating the ring of molecule III 180 degrees about the bond connected to its carbon chain, it is seen that molecules I and III are constitutionally identical with opposite configurations at every chiral center. These compounds are enantiomers. Tip: mental visualization of bond rotations and other transformations is among the most common difficulties experienced in organic chemistry courses. The use of a molecular modeling kit may greatly assist in molecular visualization. Molecules I and II are the same, just rotated 180 degrees.

Example Question #1 : Help With Enantiomers

Which of the following best describes an S-enantiomer?

Possible Answers:

None of these

Levorotatory

Dextrorotatory

Rotates light clockwise

Correct answer:

None of these

Explanation:

S configuration deals with the arrangement of atoms around a chiral center. Levorotatory and dextrorotatory refer to the rotation of light (either clockwise or counter clockwise), which can only be calculated experimentally using a polarimeter to create plane polarized light. "Rotates light clockwise" is simply another way to say dextrorotatory. Therefore, none of the answer choices are correct.

Example Question #32 : Stereochemistry

Stereoisomers

How many stereoisomers exist for the given compound?

Possible Answers:

Correct answer:

Explanation:

In this question, we're presented with the structure of a compound and we're asked to determine how many stereoisomers for this compound exists. It's important to remember that stereoisomers are compounds that have the same chemical formula and the same connectivity between its atoms, but what sets them apart is how their atoms are oriented in space.

When looking at the structure of the molecule in the question, we can see that there are two chiral carbons (carbons with four different substituents bound). A chiral carbon can have its substituents bound in two different ways, either R or S. Since each chiral carbon has two possible configurations of its atoms, the total numbers of possible stereoisomers is equal to , where  is equal to the number of chiral carbons. Thus, we can conclude that the number of stereoisomers is equal to .

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