The 2 and 3 are just the second and third carbons in the longest carbon chain.

For instance, take the molecule 1-chloro-1-fluoroethane.

When you draw out the molecule, look for all carbons where there are four different $\ce{R}$-groups, or side chains, attached. This means the carbon they are attached to is a chiral center. If a molecule has a chiral center, that means the molecule comes in at least two kinds of isomers called stereoisomers. Two isomers have a molecular formula that is the same, but the structures and arrangements of the two molecules are different. Stereoisomers are isomers where two molecules cannot be superimposed on one another because they are mirror images at the chiral center.

[![1-chloro-1-fluoroethane][1]][1]

In this case, one of the carbons is bonded to four different $\ce{R}$-groups: a fluorine atom, a chlorine atom, a methyl group, and an unmarked hydrogen atom. Therefore, this is a chiral center, meaning it has two different mirror images: *S* and *R*.

Since there is only one chiral center, then there are two chiral forms of this molecule: (1*S*)-1-chloro-1-fluoroethane and (1*R*)-1-chloro-1-fluoroethane. When only one chiral center exists, you can write it like this simply without parenthesis or numbers because we know which chiral center you are referring to since there is only one.


For a harder example, take another molecule: 3-fluoro-2-methyloctane.

[![3-fluoro-2-methyloctane][2]][2]

You identify the longest chain, which is 8 carbons long. Then you look for all carbons with 4 different $\ce{R}$-groups. In this case, they are the second and third carbons. The rest are bonded to at least 2 hydrogens, making them achiral.

This means that there are four forms of this molecule: (2*R*,3*R*), (2*R*,3*S*), (2*S*,3*R*) and (2*S*,3*S*).

If all chiral centers have mirror images of each other, then the two molecules are considered enantiomers, meaning the ENTIRE molecule is a mirror image of the other. This means (2*R*,3*R*) and (2*S*,3*S*) are enantiomers of each other because each carbon has its chirality reversed. The same is true for (2*R*,3*S*) and (2*S*,3*R*): they are also enantiomers.

If not all chiral centers are the same or different, then the two molecules are called diastereomers. There are 4 diastereomer combinations for this example, one of which is (2*R*,3*R*) and (2*R*,3*S*), because the chirality of carbon 2 is the same, but the chirality of carbon 3 is different.

You can also determine the number of stereoisomers by calculating $2^n$, where $n$ is the number of chiral centers. For instance if a molecule has four chiral centers, then there will be ($2^4 = 16$) sixteen different stereoisomers.

Finally, the last major trick with stereochemistry is *meso*-compounds. Two stereoisomers are *meso*-compounds if the molecule has a plane of symmetry or rotational symmetry. Meaning, if you can draw a line anywhere across the molecule in such a way where both sides are symmetrical, or rotate the molecule in such a way where you can get the same molecule without turning 360 degrees, there will be at least one set of *meso*-compounds present. Rotational symmetry is harder to notice and easier to forget to check for. When I took organic chemistry, we had a compound given to us for one of our exams that we had to draw out (at first with no stereochemistry in mind), identify the chiral centers, then draw out the stereoisomers, provide the parenthetic notation for each isomer, then identify a set of enantiomers, a set of diastereomers, and, if present, a set of *meso*-compounds. The compound our professor chose was 1,3-dicyclobutoxycyclopentane, which does have plane of symmetry, and therefore, has a set of *meso*-compounds.


  [1]: https://i.sstatic.net/9u6XY.png
  [2]: https://i.sstatic.net/Py69b.png