The Handy Chemistry Answer Book. Justin P. Lomont

a favorable manner. Radical species can have any charge or be neutral.

      What are isomers?

      Isomers are chemical compounds with the same molecular formula, but which are different in some way. The major types are constitutional isomers, stereoisomers, and enantiomers (the last one is actually a subset of the second-to-last one, but we’ll get there in a minute).

      Constitutional, or structural, isomers have the same number of atoms, but they are arranged in a different order. For example, four carbon atoms and ten hydrogen atoms can be arranged in two different ways.

      What is a geometric isomer?

      Geometric isomers are molecules containing the same set of atoms and bonding arrangements, but with a different spatial arrangement of the atoms or groups. For example, cis and trans isomers are an example of geometric isomers.

      What are stereoisomers?

      Stereoisomers have the same number of atoms connected in the same order, but differ in their arrangement in space. There are two major types of stereoisomers: enantiomers and diastereomers.

      What is chirality?

      Chiral objects have nonsuperimposable mirror images. What does that mean? Superimposable means one object can be placed over another, or less technically, that they’re identical. So enantiomers are not identical, but they are mirror images. Take a look at your hands—they are enantiomers. If you put one hand up to a mirror, it looks like your other hand (so they are mirror images). But if you try to put one hand on top of your other (no, not palm to palm, that’s cheating), you see they’re not identical (therefore nonsuperimposable).

      What are enantiomers?

      Enantiomers are molecules that are chiral. In organic chemistry, if a carbon atom is bonded to four different atoms (or groups of elements), then we can draw two enantiomers of the molecule. Remember that the connectivity does not change, just the arrangement of the atoms in space.

      Wait—what do those dashed and wedged bonds mean?

      Up to this point we’ve mostly been representing molecules as flat objects, where chemical bonds are just shown as straight lines. But molecules are not flat. In the previous question, the four halogen atoms around the central carbon form a tetrahedron. Chemists use dashed bonds to indicate that they are behind the plane of the paper, and wedged bonds come toward you, above the plane of the paper.

      What are diastereomers?

      This is going to sound like a cop-out, but diastereomers are stereoisomers that are not enantiomers. That’s the real, technical definition. One type of diastereomers show up when carbon forms a double bond. Recall from previous chapters that when there are three groups bonded to a carbon atom, it will be planar (sp² hybridized). If the double bond is in the middle of a carbon chain, there are two possible isomers.

      These two molecules are not superimposable, but they’re also not mirror images, so they are called diastereomers. There are many other forms of diastereoisomers, but this form is the easiest to understand.

      What is a racemic mixture?

      A racemic mixture contains equal amounts of both enantiomers of a molecule.

      What does enantiomeric excess measure?

      The enantiomeric excess is a measure of how much more of one enantiomer is present in a mixture. It’s often reported as a percentage. Racemic mixtures have an enantiomeric excess value of 0% because both enantiomers are present in equal amounts. For a solution composed of 75% of one enantiomer, the enantiomeric excess would be 50% (75% – 25% = 50%).

      How and when was molecular chirality discovered?

      Nonracemic mixtures rotate the plane of a beam of polarized light in either a clockwise or counterclockwise direction. Jean-Baptiste Biot, a French physicist, observed this effect in 1815 with quartz crystals, turpentine, and sugar solutions. These were important results in understanding the nature of light, but it was Louis Pasteur in 1848 that figured out that the effect was based on molecular properties. Pasteur painstakingly separated enantiomerically pure crystals from a racemic mixture of tartaric acid and showed that the two enantiomers rotated light in opposite directions.

      Are all the carbon-carbon bonds in benzene the same length?

      Yes, but you might not think so by looking at a single line structure of benzene. The actual structure of benzene is a combination of two structures, shown below. In technical terms, the electrons in the p bonds are delocalized (spread out) by resonance. The drawing convention that chemists use to represent molecular structure just can’t display this properly in a single structure. The electrons do not move from one place to another, and the carbon-carbon bonds do not oscillate between long and short—the structure is an average of these two drawings. After all, a molecule of benzene doesn’t really care that we can’t properly draw it.

      Sometimes, you might see benzene drawn with a single circle in the center, representing the delocalization of the π-electrons.

      What is resonance?

      Resonance is a way that chemists represent delocalized electronic structure. Let’s take that statement apart to understand what it means. “Delocalized” means that an electron, or a pair of electrons, is not located entirely around a single atom or bond. Take a look at the two structures of nitrogen dioxide (NO₂) on the following page. The negative charge is located on one oxygen atom in one resonance structure, but can be found on the other oxygen in the second resonance structure. Notice that we said “electronic structure” and haven’t said anything about atoms moving here—that’s because they don’t. Resonance only deals with electrons, and the atoms are located in the same arrangement in every resonance contributor. This is important, and makes sense if you remember that the electrons aren’t “moving” from one resonance structure to another. These structures are needed because the actual molecule is more complicated than our simple drawing system can represent.

       When was the term “aromatic” first seen?

      It was first seen in a chemistry publication in 1855. The paper was written by August Wilhelm von Hofmann, but he gives no indication why he chose this term. It’s an unusual case since only a few aromatic compounds are actually smelly, while many nonaromatic compounds in chemistry labs do truly stink.

      What is aromaticity?

      Aromaticity is a special kind of resonance delocalization, one we’ve actually already seen. Delocalization of electrons always makes molecules more stable (compared to imaginary molecules where the electrons cannot spread out). If this delocalization takes place in a flat ring of n carbons, and the number of electrons involved is 4n + 2 (i.e., 2, 6, 10, 14, etc.), then the system is called aromatic.

      What is a functional group in organic chemistry?

      Functional groups are groups of atoms that tend to show similar reactivity in different molecules. Chemists use these groupings to help understand and predict how different molecules react with each other, and they’re also used for naming compounds.