MCAT Organic Chemistry Review

Nomenclature

Concept Summary

IUPAC Naming Conventions

·        The International Union of Pure and Applied Chemistry (IUPAC) has designated standards for naming chemical compounds. There are five steps in the process.

o   First, find the longest carbon chain in the compound that contains the highest-priority functional group. This is called the parent chain.

o   Second, number the chain in such a way that the highest-priority functional group receives the lowest possible number. This group will determine the suffix of the molecule.

o   Third, name the substituents with a prefix. Multiple substituents of a single type receive another prefix denoting how many are present (di–, tri–, tetra–, and so on).

o   Fourth, assign a number to each of the substituents depending on the carbon to which it is bound.

o   Finally, complete the name by alphabetizing the substituents and separating numbers from each other by commas and from words by hyphens.

Hydrocarbons and Alcohols

·        Alkanes are hydrocarbons without any double or triple bonds. They have the general formula CnH(2n+2).

·        Alkanes are named according to the number of carbons present followed by the suffix –ane.

o   The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10).

o   Larger alkanes use the Greek root for the number (pentane, hexane, heptane, octane, and so on).

·        Alkenes and alkynes contain double and triple bonds, respectively.

o   Alkenes are named by substituting –ene for the suffix and numbering the double bond with the lower-numbered carbon in the bond. Alkynes substitute –yne with the same numbering.

·        Alcohols contain a hydroxyl (–OH) group, which substitutes for one or more of the hydrogens in the hydrocarbon chain.

o   Alcohols are named by substituting the suffix –ol, or by using the prefix hydroxy– if a higher-priority group is present.

o   Alcohols have higher priority than double or triple bonds and alkanes.

o   Common names of alcohols include the name for the carbon chain followed by the word alcohol. For example, ethyl alcohol is the same compound as ethanol.

o   Diols contain two hydroxyl groups. They are termed geminal if on the same carbon or vicinal if on adjacent carbons.

Aldehydes and Ketones

·        Aldehydes and ketones contain a carbonyl group—a carbon double-bonded to an oxygen.

·        Aldehydes have the carbonyl group on a terminal carbon that is also attached to a hydrogen atom.

o   Aldehydes are named with the suffix –al, or by using the prefix oxo– if a higher-priority group is present.

o   Common names of aldehydes include formaldehyde for methanal, acetaldehyde for ethanal, and propionaldehyde for propanal.

·        Ketones have the carbonyl group on a nonterminal carbon.

o   Ketones are named with the suffix –one, and share the prefix oxo– if a higher-priority group is present. Ketones can also be indicated by the prefix keto–.

o   The common names of ketones are constructed by naming the alkyl groups on either side alphabetically and adding ketone. For example, 2-butanone is called ethylmethylketone.

o   Acetone is significant as the smallest ketone. Its IUPAC name is propanone.

·        Carbonyl-containing compounds (aldehydes, ketones, carboxylic acids, and derivatives) also create a lettering scheme for carbons. The carbon adjacent to the carbonyl carbon is the α-carbon.

Carboxylic Acids and Derivatives

·        Carboxylic acids are the highest-priority functional group because they contain three bonds to oxygen: one from a hydroxyl group and two from a carbonyl group.

·        Carboxylic acids are always terminal, although their derivatives may occur within a molecule.

o   Carboxylic acids are named with the suffix –oic acid. They are very rarely named as a prefix.

o   Common names for carboxylic acids follow the trend for aldehydes. Formic acid is methanoic acid, acetic acid is ethanoic acid, and propionic acid is propanoic acid.

·        Esters are carboxylic acid derivatives where –OH is replaced with –OR, an alkoxy group.

o   Esters use the suffix oate or the prefix alkoxycarbonyl–.

o   Common names for esters are derived from the alcohol and the carboxylic acid used during synthesis.

·        Amides replace the hydroxyl group of a carboxylic acid with an amino group that may or may not be substituted.

o   Amides use the suffix –amide or the prefix carbamoyl– or amido–. Substituents attached to the amide nitrogen are designated with a capital N–.

·        Anhydrides are formed from two carboxylic acids by dehydration. They may be symmetric (two of the same acid), asymmetric (two different acids), or cyclic (intramolecular reaction of a dicarboxylic acid)

o   Anhydrides are named using the suffix anhydride in place of acid. If the anhydride is formed from more than one carboxylic acid, both are named in alphabetical order in the name before the word anhydride.

Summary of Functional Groups

·        Functional groups are arranged in order of priority as follows: Carboxylic acid > anhydride > ester > amide > aldehyde > ketone > alcohol > alkene or alkyne > alkane