MCAT Organic Chemistry Review

Nomenclature

1.2 Hydrocarbons and Alcohols

Hydrocarbons are compounds that contain only carbon and hydrogen atoms. Alcohols, on the other hand, contain at least one –OH group, which lends them additional reactivity. In this section, we’ll explore the naming of hydrocarbons and alcohols.

ALKANES

Alkanes are simple hydrocarbon molecules, with the formula CnH(2n+2). The names for the first four of these compounds are methane (one carbon), ethane (two carbons), propane (three carbons), and butane (four carbons). Alkanes with more than four carbons have a simpler naming pattern in which the name is the Greek root describing the number of carbons followed by –ane. These Greek roots from 5 to 12 are: pent–, hex–, hept–, oct–, non–, dec–, undec–, and dodec–. Some examples of alkanes are shown in Table 1.1.

Table 1.1 Examples of Alkanes

Halogens are common substituents on alkanes. Alkyl halides are indicated by a prefix: fluoro–, chloro–, bromo–, or iodo–.

ALKENES AND ALKYNES

The MCAT does not explicitly test reactions of alkenes or alkynes, but you may still see the suffixes –ene and –yne, which signify double and triple bonds, respectively. Keep in mind that many of these compounds will also have common names, and it is vital to know these common names as well. On Test Day, you are most likely to encounter double bonds in the context of unsaturated fatty acids or other biochemical compounds. The double or triple bond is named like a substituent and is indicated by the lower-numbered carbon involved in the bond. The number may precede the molecule name, as in 2-butene, or it may be placed near the suffix, as in but-2-ene; both are correct. If there are multiple multiple bonds, the numbering is generally separated from the suffix, as in 1,3-butadiene.

ALCOHOLS

Alcohols are named by replacing –e in the name of the corresponding alkane with –ol. The chain is numbered so that the carbon attached to the hydroxyl group (–OH) gets the lowest possible number—even when there is a multiple bond present. The hydroxyl group takes precedence over multiple bonds because of the higher oxidation state of the carbon. If the alcohol is not the highest-priority functional group, then it is named as a hydroxyl substituent (hydroxy–). Figure 1.6 demonstrates a few alcohols and their IUPAC names.

Figure 1.6. Naming Alcohols Alcohols are more oxidized than multiple bonds, so they take priority in nomenclature and are indicated with the suffix –ol.

Alcohols are often referred to by their common names, rather than their IUPAC names. In this version of naming, the name of the alkyl group is followed by the word alcohol. Examples include ethyl alcohol (rather than ethanol) and isopropyl alcohol (rather than 2-propanol.)

MCAT EXPERTISE

The MCAT may use the common names for some alcohols, as well as some of the molecules we will see later. For alcohols, these include ethyl alcohol and isopropyl alcohol—know what these common names refer to!

Alcohols with two hydroxyl groups are called diols or glycols and are indicated with the suffix –diol. The entire hydrocarbon name is preserved, and –diol is added. When naming diols, one must number each hydroxyl group. For example, ethane-1,2-diol is an ethane molecule that has a hydroxyl group on each carbon. This molecule is also known by its common name, ethylene glycol. Diols with hydroxyl groups on the same carbon are called geminal diols; diols with hydroxyl groups on adjacent carbons are called vicinal diols. Geminal diols, or hydrates, are not commonly seen because they spontaneously dehydrate (lose a water molecule) to produce carbonyl compounds with the functional group C=O.

MNEMONIC

Vicinal diols are in the vicinity of each other, on nearby carbons. Geminal diols—like the Gemini twins—are the same, or in this case, on the same carbon.

MCAT Concept Check 1.2:

Before you move on, assess your understanding of the material with these questions.

1.    Fill in the correct names for the alkanes listed below. If more than one compound can be described with a given molecular formula, name the straight-chain alkane to which the formula refers, and draw one alternative.

Molecular Formula

IUPAC Name (Straight-Chain Alkane)

Alternative Structure

CH4

   

C2H6

   

C3H8

   

C4H10

   

C5H12

   

C6H14

   

C7H16

   

C8H18

   

C9H20

   

C10H22

   

2.    In a molecule with two double bonds adjacent to each other and an alcohol, which functional group would take precedence in naming?

3.    What is the difference between a geminal and a vicinal diol?

·        Geminal diol:

·        Vicinal diol:

4.    What are the common names for 2-propanol and ethanol?

·        2-Propanol:

·        Ethanol: