MCAT Biology : Isomerism and Stereoisomers

Study concepts, example questions & explanations for MCAT Biology

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Example Question #1487 : Mcat Biological Sciences

(R)-2-butanol rotates plane-polarized light at an angle of . A racemic mixture of (R)-2-butanol and (S)-2-butanol is created in a beaker. A burner is placed under the beaker and begins to boil the mixture. The flame evaporates half of the mixture, which then condenses into a separate beaker. The flame is turned off when the amounts are equal in both beakers. Which of the following statements is true? 

Possible Answers:

The sample in the original beaker will rotate plane-polarized light at 

Both beakers rotate plane-polarized light, but the degree of rotation cannot be determined

Neither sample will rotate plane-polarized light

One beaker will rotate plane-polarized light at , the other at 

Correct answer:

Neither sample will rotate plane-polarized light

Explanation:

The original racemic mixture will not rotate plane-polarized light. Rotation by each enantiomer will be equal in magnitude and opposite in direction, cancelling each other out and leading to zero rotation. Enantiomers have the same physical properties, meaning they will boil at the same temperature. As a result, boiling half of the mixture into another beaker does not separate the mixture into two distinct compounds; we have simply created two beakers containing a racemic mixture. This means that neither sample will rotate plane-polarized light. 

Example Question #1 : Isomerism And Stereoisomers

A student adds hydrogen cyanide (HCN) to the compound shown below. After equilibration and isolation of the products, he measures the degree to which light is polarized. What degree of rotation is he likely to find?

Racemic

Possible Answers:

0°

90°

180°

45°

-45°

Correct answer:

0°

Explanation:

Cyanide (CN-) is an excellent nucleophile. As such, it will likely attack the aldehyde, a fairly strong electrophile. Cyanide has two routes of entry and can attack from either above or below in equal proportions. Given this, two stereoisomers will be made that each polarize light equal amounts in opposite directions. Thus, the net rotation of light will be 0°. Mixtures such as these, are known as "racemic."

Example Question #2 : Isomerism And Stereoisomers

Which compound(s) below is(are) not optically active?

Meso

 

Possible Answers:

IV only

II only

I and IV

I only

II and III

Correct answer:

I and IV

Explanation:

This question is indirectly asking "Which of these compounds are meso?" Meso compounds contain an internal line of symmetry and therefore produce no optic rotation. A good way to judge whether a compound is meso or not is to determine R and S configurations of each chiral center. If a compound is meso and one end has an R chiral center, then on the other end must be S, given that each center has the same substituents. Choice I can immediately be seen as meso because its conformation shows an internal line of symetery at carbon #3. Choice IV is also meso, due to free rotation at the terminal carbons. Choices I and IV will not be optically active.

Example Question #3 : Isomerism And Stereoisomers

Which of the following compounds is optically active?

Possible Answers:

Trans-1,4-dichlorocyclohexane

1,1,5,5-tetrachloropentane

4-ethylheptane

2,3-dibromopentane

Correct answer:

2,3-dibromopentane

Explanation:

Chiral compounds are optically active, and will rotate plane-polarized light. 2,3-dibromopentane has two chiral carbons, making it a chiral compound.

The other compounds are symmetric and achiral, and will not rotate plane-polarized light.

Example Question #3 : Isomerism And Stereoisomers

(R)-2-butanol rotates plane-polarized light at an angle of . A mixture of (R)-2-butanol and (S)-2-butanol is created in a beaker, and contains 60% R enantiomer and 40% S enantiomer. This will result in a sample that will __________.

Possible Answers:

rotate light at  

More information is needed in order to answer the question

not rotate light

rotate light in between  and 

Correct answer:

rotate light in between  and 

Explanation:

Enantiomers rotate plane-polarized light in opposite directions, with the same magnitude. An equal (racemic) mixture of enantiomers will result in a solution that does not rotate plane-polarized light; however, if there is a larger percentage of one enantiomer, plane-polarized light will be rotated in the direction of the greater enantiomer. Our mixture has a greater portion of the R enantiomer, which rotates light when it is pure. We know that our rotation will be less than zero degrees (a racemic mixture), but greater than (pure R enantiomer).

Example Question #4 : Isomerism And Stereoisomers

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

A scientist is studying a reaction that uses the same mechanism as reaction 1. In his experiment, the reactant has a chiral central carbon. His reactants were dextrorotary at . If all of his reactants are converted to product, what is true of the solution following completion?

Possible Answers:

It is levorotary at

It is levorotary at

It is dextrorotary at some value between  and

It is levorotary at some value between  and

It is dextrorotary at

Correct answer:

It is levorotary at

Explanation:

Reaction 1 involves an inversion of stereochemistry. If the central carbon is optically active due to its chirality, we would expect an inversion of relative conformation; thus, a dextrorotary rotation at would become levorotary to the same degree.

Example Question #5 : Isomerism And Stereoisomers

How many additional isomers exist for the molecule shown below?

Heptane

Possible Answers:

8

4

6

9

7

Correct answer:

8

Explanation:

If a problem like this is encountered on the MCAT, the best approach is to draw out each isomer as quickly as you can. In a short amount of time you will see that this molecule, an isomer of heptane, has 8 additional isomers, giving a total of 9. All 9 are shown below.

 Isomers

*Note: The question asks for additional isomers, not including the one already shown.

Example Question #6 : Isomerism And Stereoisomers

Which of the following types of compound is proof for the following statement: "A compound can be achiral, yet still have chiral carbons?"

Possible Answers:

Enantiomers

Meso compounds

Diastereomers

Epimers

Correct answer:

Meso compounds

Explanation:

Meso compounds are compounds that contain two or more chiral carbons, however, the chiral carbons offset each other resulting in an achiral compound. You can recognize meso compounds because they will have a plane of symmetry.

Example Question #7 : Isomerism And Stereoisomers

 

 

Besides the enantiomer shown below, how many other possible stereoisomers of ephedrine are possible?

Mcat_problem_set_1

Possible Answers:

Correct answer:

Explanation:

Ephedrine has two stereocenters (carbons 1 and 2), meaning there would be , or , total possible stereoisomers. One is already shown, so there would be three others.

Example Question #8 : Isomerism And Stereoisomers

Two alkenes differ only in the spatial orientation of two atoms around a double bond. One of the alkenes is in the E configuration while the other is Z. What kind of isomers are these two alkenes?

Possible Answers:

Diastereomers

Geometric

Constitutional

Enantiomers

Correct answer:

Geometric

Explanation:

When analyzing isomers, the first step is to decide whether the molecules differ in the connectivity of their atoms. If the atoms differ in their linkage to each other, the isomers will be constitutional; if they have the same connectivity, the isomers will be some type of stereoisomer. In this case, the molecules have the same connectivity, but differ in their orientation around a double bond. These are geometric isomers, which is a general name for E-Z or cis-trans isomers.

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