MCAT Biology : Isomerism and Stereoisomers

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Molecular Properties

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

Which statement accurately describes the chirality of the compounds in the depicted reactions?

Possible Answers:

None of the carbon atoms are chiral centers

The central carbon atom in the reactant of reaction 2 is a chiral center

The central carbon atoms of the reactant and product in reaction 1 are chiral centers

The carbon atoms on either side of the double bond in the product of reaction 2 are chiral centers

Only the central carbon in the product of reaction 1 is a chiral center

Correct answer:

None of the carbon atoms are chiral centers

Explanation:

A chiral carbon is bound to four different substituents; none of the carbon atoms in the passage have this property. There is an inversion of stereochemistry in reaction 1, but it is largely irrelevant because the carbon is not chiral.

Example Question #3 : Stereochemistry

 Compounds that are mirror images of each other are called __________.

Possible Answers:

enantiomers

conformers

stereoisomers

diastereomers

Correct answer:

enantiomers

Explanation:

Stereoisomers are isomers that differ in the orientation of atoms in space, but have the same bonding patterns and structures. Enantiomers are a specific class of stereoisomers that differ in orientation around a chiral center to create mirror image molecules.

Diastereomers are a type of stereoisomer that are not related through a reflection operation, and may differ at more than one chiral center. Conformers have the same structural formula, but different shapes due to bond rotation.

Example Question #111 : Organic Chemistry, Biochemistry, And Metabolism

Carbohydrates can be classified as either aldoses or ketoses. Glucose is an example of an aldose. How many chiral centers are present in a molecule of linear glucose (C6H12O6)?

Possible Answers:

Five

One

Four

Two

Correct answer:

Four

Explanation:

Glucose is formed by a chain of six carbons. Carbon 1 participates in the aldehyde functional group, and cannot be chiral due to its double bond with oxygen. The following four carbons, carbons 2-5, are bound to a hydrogen, a hydroxy group, and two R chains, extending toward carbon 1 and toward carbon 6, respectively. These four carbons each have four different constituents, making them chiral centers. Carbon 6 is bound to the rest of the molecule, two hydrogens, and a hydroxy group; because it is bound to two hydrogens, it cannot be chiral.

In glucose, carbons 1 and 6 are achiral, while carbons 2, 3, 4, and 5 are chiral centers.

Example Question #11 : Isomerism And Stereoisomers

Which of the following differences would never describe isomers?

Possible Answers:

Different absolute configurations

Different rotations of plane-polarized light

Different molar masses

Different boiling points

Correct answer:

Different molar masses

Explanation:

Isomers are molecules that have the same molecular formula, but are different overall compounds due to molecular organization and/or orientation. As a result, isomers will always have the same molar mass.

Constitutional isomers will have the same molecular formula, but different binding patterns, resulting in different absolute configurations and often different chemical properties, such as boiling point. Enantiomers are a special class of stereoisomers that will rotate plane-polarized light.

Example Question #2 : Stereochemistry

A molecule has three chiral centers. How many stereoisomers of this compound will have different boiling points compared to the original molecule?

Possible Answers:

One

Six

Seven

Two

Correct answer:

Six

Explanation:

The first step is to determine how many stereoisomers there are for this molecule. Since the number of stereoisomers is dependent on the number of chiral carbons, we can solve according to the equation , where  is the number of chiral centers. Since there are three chiral centers, we determine that there are eight stereoisomers for this molecule. Keep in mind that this number includes the original molecule.

Next, we need to compare the different stereoisomers to the original molecule. The original molecule will have one enantiomer and six diastereomers. Remember that enantiomers have the same physical properties, so we will not include this isomer in the final answer. Diastereomers, on the other hand, have different physical properties compared to the original molecule. As a result, six stereoisomers will have different boiling points compared to the original molecule.

Example Question #11 : Isomerism And Stereoisomers

  Compounds A and B are __________.

Mcat_9

Possible Answers:

meso

structural isomers

diastereomers

not isomers

enantiomers

Correct answer:

enantiomers

Explanation:

Both molecules contain only one stereocenter, bound to the alcohol, methyl, hydrogen, and a carbon chain. Since this stereocenter is inverted between the two molecules, they are enantiomers.

The cyclopentyl carbon to which the methyl group and sidechain are attached is not a stereocenter, because it contains a plane of symmetry within the ring. Remember that enantiomers are mirror images, and differ in stereochemistry at only one stereocenter. Diastereomers differ in stereochemistry at multiple stereocenters. Meso compounds are identical. Structural isomers, unlike stereoisomers, have different bonding patters for the same molecular formula.

Example Question #13 : Isomerism And Stereoisomers

Which of the following methods is best for separating two enantiomers?

Possible Answers:

Column chromatography

Treating a mixture with aqueous acid, followed by selective extraction with ethyl acetate

Recrystallization

Treating a mixture with a chiral salt, followed by separation of the resulting diastereomers

Distillation

Correct answer:

Treating a mixture with a chiral salt, followed by separation of the resulting diastereomers

Explanation:

Enantiomers have the same physical properties, and thus would have the same polarities and boiling points. Since column chromatography relies on differences in polarity and distillation relies on differences in boiling point, these methods would be ineffective for separation. Likewise, recrystallization would be ineffective, as it relies on differences in solubility, and would treatment with aqueous acid would react equally with both enantiomers

Treating enantiomers with a chiral molecule, however, would result in two diastereomers, which would have different physical properties and could be readily separated.

Example Question #61 : Isomers

The molecules shown below are best described as __________.

Molecules

Possible Answers:

isomers

epimers

enantiomers

diastereomers

Correct answer:

isomers

Explanation:

The molecules in this problem are isomers because they each have unique configurations and do not share the same funcitonal groups at the same carbon positions. Enantiomers are reflections of each other. Diastereomers are stereoisomers that differ at one or more stereocenters, while epimers are stereoisomers that differ at only one stereocenter.

Example Question #1 : Aldehyde Chemistry

Grinard

What reagent(s) will successfully complete the synthesis reaction shown above?

Possible Answers:

Isopropyl-MgBr; hydronium ions

Methyl-MgCl; hydronium ions

Ethyl-MgI; hydronium ions

N-propyl-MgBr; hydronium ions

Correct answer:

N-propyl-MgBr; hydronium ions

Explanation:

This is an example of a Grignard reagent reaction. Because we are adding three carbons to our chain, the Grignard reagent we need must have three carbons on it. We can therefore rule out methyl grignard and ethyl grignard.

N-propyl is the straight-chained 3-carbon alkane, while isopropyl is branched. Looking at our final product, we can see the carbon chain we have added is straight-chained, and thus N-propyl Grignard is the best option. Because Grignard reagents are relatively basic, we must add an hydronium ion workup to protonate our alcohol.  

Example Question #1 : Help With Enantiomers

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

Mcat_1

Possible Answers:

Two

Three

One

Zero

Four

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.

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