An enantiomer is defined as a stereoisomer that is a mirror image of another. In other words, if a molecule was placed in front of a mirror, its mirror image would be the enantiomer. Questions such as these require a bit of visualization, but should be easy points. In addition, if you see the answer choice quickly and are confident in your decision, move onto the next question without spending time on the other choices, as mental geometry and visualization may take more time than anticipated.

A chiral center is defined as a tetrahedral atom with four unique substituents. In the molecule above there exists only one carbon that has four different substituents, which is carbon #4 (counting from the right). Carbon #4 has -chloro and -hydroxy groups, as well as two different longer alkyl chains. No other atom has four unique substituents.

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.

The hydrogenation of compound Cwould 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.

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.

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

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.

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."

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.

Enantiomers are defined as compounds that are mirror images of one another. They have the same basic physical and chemical characteristics, but differ in how they bend plane-polarized light. Although we know that the angle is inverted between configurations, R- and S- does not tell us which direction the light will be bent. This information requires further experimentation.

Note that in some cases, enantiomers can demonstrate different properties, such as different levels of toxicity to the body. For this reason, certain drugs are selected for only a single enantiomer.

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.