MCAT Organic Chemistry Review
1.1 IUPAC Naming Conventions
Nomenclature is one of the most important prerequisites for answering organic chemistry questions on Test Day; if you don’t know which chemical compound the question is asking about, it’s hard to get the answer right! That’s why it’s so important to understand both IUPAC and common nomenclature. Once you have a handle on these systems, you can easily translate question stems and focus on finding the correct answer. Let’s begin by examining the IUPAC naming conventions before we examine specific compounds and functional groups.
Nomenclature is often tested on the MCAT by providing the question stem and the answer choices in different formats. For example, the question stem will give you the IUPAC name of the reactant, and the answer choices will show product structures—leaving you to figure out both the structure of the reactant and the reaction taking place.
The primary goal of the International Union of Pure and Applied Chemistry (IUPAC) naming system is to create an unambiguous relationship between the name and structure of a compound. With the conventions established by IUPAC, no two distinct compounds have the same name. The IUPAC naming system greatly simplifies chemical naming. Once we understand the rules, we can match names to structures with ease.
1. Identify the Longest Carbon Chain Containing the Highest-Order Functional Group
This will be called the parent chain and will be used to determine the root of the name. Keep in mind that if there are double or triple bonds between carbons, they must be included. If one of the functional groups would provide a suffix for the compound (for example, an alcohol, which will be discussed later), then the parent chain must contain this functional group. We’ll examine priorities of functional groups throughout this chapter, but keep in mind that the highest-priority functional group (with the most oxidized carbon) will provide the suffix. This step may sound easy, but be careful! The molecule may not be drawn so that the longest carbon chain is immediately obvious. If there are two or more chains of equal length, the more substituted chain gets priority as the parent chain. Figure 1.1 shows a hydrocarbon with the longest chain labeled.
Figure 1.1. Finding the Longest Carbon Chain
2. Number the Chain
In order to appropriately name a compound, we need to number the carbon chain, as shown in Figure 1.2. As a convention, the carbon numbered 1 will be the one closest to the highest-priority functional group. If the functional groups all have the same priority, numbering the chain should make the numbers of the substituted carbons as low as possible.
Figure 1.2. Numbering the Longest Carbon Chain The highest-priority functional group should have the lowest possible number; if all substituents have the same priority, make their numbers as low as possible.
After we have discussed the functional groups most commonly tested on the MCAT, we’ll review a table of those functional groups in order of priority. For now, keep in mind that the more oxidized the carbon is, the higher priority it has in the molecule. Oxidation state increases with more bonds to heteroatoms (atoms besides carbon and hydrogen, like oxygen, nitrogen, phosphorus, or halogens) and decreases with more bonds to hydrogen.
Just like straight chains, rings are numbered starting at the point of greatest substitution, continuing in the direction that gives the lowest numbers to the highest-priority functional groups. If there is a tie between assigning priority in a molecule with double and triple bonds, the double bond takes precedence.
3. Name the Substituents
Substituents are functional groups that are not part of the parent chain. A substituent’s name will be placed at the beginning of the compound name as a prefix, followed by the name of the longest chain. Remember that only the highest-priority functional group will determine the suffix for the compound and must be part of the parent chain.
Carbon chain substituents are named like alkanes, with the suffix –yl replacing –ane. The prefix n– that we see in Figure 1.3 on n-propyl simply indicates that this is “normal”—in other words, a straight-chain alkane. Because this prefix will not always be present, it is safe to assume that alkane substituents will be normal unless otherwise specified.
Figure 1.3. Common Normal Alkyl Substituents The bond on the right side of each substituent connects to the parent molecule.
In Figure 1.4, we see some examples of what alternative alkyl substituents may look like.
Figure 1.4. Common Alternative Alkyl Substituents The bond on the right side of each substituent connects to the parent molecule.
If there are multiple substituents of the same type, we use the prefixes di–, tri–, tetra–, and so on to indicate this fact. The prefix is included directly before the substituent’s name.
4. Assign a Number to Each Substituent
Pair the substituents that you have named to the corresponding numbers in the parent chain. Multiple substituents of the same type will get both the di–, tri–, and tetra– prefixes that we have previously noted and also a carbon number designation—even if they are on the same carbon.
5. Complete the Name
Names always begin with the names of the substituents in alphabetical order, with each substituent preceded by its number. Note, however, that prefixes like di–, tri–, and tetra– as well as the hyphenated prefixes like n– and tert– are ignored while alphabetizing. Nonhyphenated roots that arepart of the name, however, are included; these are modifiers like iso–, neo–, or cyclo–. Then, the numbers are separated from each other with commas, and from words with hyphens. Finally, we finish the name with the name of the backbone chain, including the suffix for the functional group of highest priority. Figure 1.5 shows an example of an entire hydrocarbon named with IUPAC nomenclature.
Figure 1.5. An Example of a Complete IUPAC Name
MCAT Concept Check 1.1:
Before you move on, assess your understanding of the material with these questions.
1. List the steps of IUPAC nomenclature:
2. Circle or highlight the parent chain in each of the following compounds:
3. How are numbers separated from each other and from words in a compound’s name?