Organic Chemistry, Biochemistry, and Metabolism - MCAT Biological and Biochemical Foundations of Living Systems

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 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.

We can easily identify the carbons based on the IUPAC name. Carbon 1 will be bound to the phenyl and hydroxy groups, while carbon 2 will be bound to the methyl and amino groups.

For carbon 1 (the carbon attached to the -OH group), the lowest priority group (hydrogen) is already pointing away. The other three groups descend in decreasing priority in a clockwise manner,, resulting in an R designation.

For carbon 2 (attached to the -N-CH3 group), the stereochemical assignment is S. The lowest priority constituent (hydrogen) is pointing out of the page. The other three groups are in descending priority in a counterclockwise manner, .

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.

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.

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.

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.

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 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.

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.

Infrared (IR) spectroscopy takes advantage of the electrical difference between atoms in a polar bond. These dipole moments, when exposed to infrared radiation, stretch and contract in what appears to be a vibrating motion between the atoms. The different vibrational frequencies in the molecule allow for the compound to be "read" using IR spectroscopy.

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.

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).

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.

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.

Absolute configuration tells us the orientation of atoms around a central chiral carbon. Enantiomers are compounds that have the opposite absolute configuration at every chiral carbon in the compound. If only half of the centers are opposite of one another, the two compounds are diastereomers. Diastereomers are isomers in which chiral configuration differs at at least one chiral center, but may differ at several.

Both ammonia and methane display sp3 hybridization, however, methane is surrounded by four hydrogens, while nitrogen is surrounded by three hydrogens and a lone electron pair. Lone electron pairs in a molecule require more room compared to bonding atom pairs, as they generate more electron repulsion. As a result, the lone pair in ammonia will make the bond angles in ammonia smaller than the bond angles in the methane molecule.

*Extra information: the bond angles in methane are 109.5o, while ammonia has bond angles of 107o.

Recall that electronic structure refers to the geometry of the atoms and lone pairs around the central atom, while the molecular structure refers strictly to the geometry of the atoms of the molecule. , , and all adopt a tetrahedral electronic geometry; however, when considering just the atoms, only NH3 has a trigonal pyramidal geometry. is trigonal planar, while is linear.

Aromatic compounds are highly stable due to resonance stability. The two aromatic choices are benzene and 2,4-cyclobutadiene. Because a six-carbon ring is more stable than a four-carbon ring, benzene is the best answer. Ring strain will distort a four-carbon ring, increasing its bond energy and decreasing its stability.