CHM 101: Detailed Guide to IUPAC Nomenclature and Organic Compounds

Introduction and Fundamental Principles of IUPAC Nomenclature

The purpose of the IUPAC system of nomenclature, as outlined by Prof Ogundajo and Mr Elesho for CHM 101, is to establish a rigorous international standard for naming chemical compounds. This system facilitates clear communication among scientists globally by ensuring that every unique chemical structure is assigned a unique and unambiguous name, and conversely, that every name correlates precisely to a single structure. The foundational principle of this system involves identifying the longest continuous chain of carbon atoms connected by single bonds, whether that chain exists in a linear (acyclic) or cyclic (alicyclic) form. Any deviations from this basic skeleton—such as the presence of multiple bonds, branches, or atoms other than carbon and hydrogen—are indicated using prefixes or suffixes that follow a strict hierarchy of priorities.

Nomenclature of Alkanes and Cycloalkanes

Alkanes represent the family of saturated hydrocarbons, meaning they consist entirely of carbon and hydrogen atoms connected by single bonds ( $C-C$ and $C-H$ ). The root names of organic compounds are derived from these alkanes based on the number of carbons in the primary chain. For compounds containing five or more carbons, the names utilize Greek or Latin prefixes. In the case of cyclic structures, the prefix "cyclo-" is added to the alkane name. In geometric representations of these rings, every apex represents a carbon atom saturated with the necessary number of hydrogen atoms to satisfy carbon's tetravalence.

The progression of alkane names based on carbon count is as follows:

1 Carbon: Methane ( $CH_4$ )
2 Carbons: Ethane ( $CH_3CH_3$ )
3 Carbons: Propane ( $CH_3CH_2CH_3$ )
4 Carbons: Butane ( $CH_3[CH_2]_2CH_3$ )
5 Carbons: Pentane ( $CH_3[CH_2]_3CH_3$ )
6 Carbons: Hexane ( $CH_3[CH_2]_4CH_3$ )
7 Carbons: Heptane ( $CH_3[CH_2]_5CH_3$ )
8 Carbons: Octane ( $CH_3[CH_2]_6CH_3$ )
9 Carbons: Nonane ( $CH_3[CH_2]_7CH_3$ )
10 Carbons: Decane ( $CH_3[CH_2]_8CH_3$ )
11 Carbons: Undecane ( $CH_3[CH_2]_9CH_3$ )
12 Carbons: Dodecane ( $CH_3[CH_2]_{10}CH_3$ )
13 Carbons: Tridecane ( $CH_3[CH_2]_{11}CH_3$ )
14 Carbons: Tetradecane ( $CH_3[CH_2]_{12}CH_3$ )
20 Carbons: Icosane ( $CH_3[CH_2]_{18}CH_3$ )
21 Carbons: Henicosane ( $CH_3[CH_2]_{19}CH_3$ )
22 Carbons: Docosane ( $CH_3[CH_2]_{20}CH_3$ )
23 Carbons: Tricosane ( $CH_3[CH_2]_{21}CH_3$ )
30 Carbons: Triacontane ( $CH_3[CH_2]_{28}CH_3$ )
31 Carbons: Hentriacontane ( $CH_3[CH_2]_{29}CH_3$ )
40 Carbons: Tetracontane ( $CH_3[CH_2]_{38}CH_3$ )
50 Carbons: Pentacontane ( $CH_3[CH_2]_{48}CH_3$ )

Common cycloalkanes mentioned include cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane, and cyclooctane.

Prioritization of Substituents and Functional Groups

IUPAC categorizes functional groups and substituents into three tiers (Groups A, B, and C) to determine seniority in naming. Group A contains the highest priority groups, which dictate both numbering and the suffix of the molecule. Group B contains unsaturated carbon bonds that affect the suffix. Group C includes groups that are always treated as substituents and named only as prefixes.

Group A—Functional Groups (Ordered from highest to lowest priority):

Carboxylic Acid: Structure $R-COOH$ ; Prefix: carboxy-; Suffix: -oic acid (or -carboxylic acid).
Aldehyde: Structure $R-CHO$ ; Prefix: oxo- or formyl-; Suffix: -al (or -carbaldehyde).
Ketone: Structure $R-C(=O)R$ ; Prefix: oxo-; Suffix: -one.
Alcohol: Structure $R-OH$ ; Prefix: hydroxy-; Suffix: -ol.
Amine: Structure $R-NH_2$ ; Prefix: amino-; Suffix: -amine.

Group B—Functional Groups (Indicated by Suffix Only):

Alkene: Structure $C=C$ ; Suffix: -ene.
Alkyne: Structure $C\equiv C$ ; Suffix: -yne.

Group C—Substituents (Indicated by Prefix Only, equivalent priority):

Alkyl: $R-$ ; Prefix: alkyl-.
Alkoxy: $R-O-$ ; Prefix: alkoxy-.
Halogens: Fluorine ( $F-$ : fluoro-), Chlorine ( $Cl-$ : chloro-), Bromine ( $Br-$ : bromo-), Iodine ( $I-$ : iodo-).
Nitro: $NO_2$ ; Prefix: nitro-.
Vinyl: $CH=CH_2$ ; Prefix: vinyl-.
Allyl: $CH_2CH=CH_2$ ; Prefix: allyl-.
Phenyl: Benzene ring as a substituent.

Naming Alkanes and Cycloalkanes with Group C Substituents

The naming of organic compounds containing only Group C substituents follows a systematic five-step sequence. First, identify the longest continuous carbon chain to determine the root name. If a ring is attached to a longer chain, the chain is the parent; otherwise, the ring is usually the parent. If two chains are of equal length, the one with more substituents is chosen. Second, number the chain so the first substituent encountered receives the lowest possible position number. If there is a tie, move to the second substituent. Third, determine the name and position of each substituent. Substituents on a nitrogen atom are designated with an "N" instead of a number. Fourth, use multiplier prefixes (di-, tri-, tetra-) for identical groups. Fifth, assemble the name by placing substituents in alphabetical order before the root name. Prefixes like sec- and tert- are ignored for alphabetization, but iso- and cyclo- are included. Position numbers must be included for every substituent, even if redundant.

Representative examples include:

3-bromo-2-chloro-5-ethyl-4,4-dimethyloctane.
3-fluoro-4-isopropyl-2-methylheptane.
1-sec-butyl-3-nitrocyclohexane (where numbering is determined by the alphabetical priority of "sec-butyl" over "nitro").

Naming Molecules Containing Group B Functional Groups

When naming alkenes and alkynes, the primary chain must include the multiple bond, and numbering must give the multiple bond the lowest possible position number regardless of other substituents. The "-ane" ending of the root alkane is changed to "-ene" for double bonds or "-yne" for triple bonds. Geometrical isomerism in alkenes is designated with "cis-", "trans-", "E-", or "Z-" prefixes.

If a double and triple bond are present in the same molecule, the bond closer to the end of the chain determines the numbering direction. If they are equidistant from the ends, the double bond is given the lower number. In the final name, "-ene" precedes "-yne" alphabetically. Specific vowel/consonant rules apply: the terminal "e" is dropped if the next letter is a vowel (e.g., pent-3-en-1-yne), and an "a" is added if a multiplier prefix creates consecutive consonants (e.g., buta-1,3-diene).

Examples include:

4,4-difluoro-3-methylbut-1-ene.
1,1-difluoro-2-methylbuta-1,3-diene.
5-methylcyclopenta-1,3-diene.
3-vinylcyclohex-1-ene (used when the double bond cannot be included in the parent chain numbering).
pent-3-en-1-yne and pent-1-en-4-yne.

Naming Molecules Containing Group A Functional Groups

For molecules with Group A groups, the group with the highest priority is assigned the suffix and the lowest possible carbon number. Other Group A groups of lower priority are named as prefixes. The carbon chain must include the highest priority group. If multiple bonds exist (Group B), they change the root to "-ene" or "-yne" before the Group A suffix is added.

Detailed rules for specific Group A families:

Amines: Functional groups on the nitrogen itself are prefixed with "N-" (e.g., N,N-diethylbut-3-en-2-amine).
Alcohols: Highest priority uses "-ol," lower priority uses "hydroxy-" (e.g., 3-hydroxybutan-2-one).
Ketones: Use "-one" suffix or "oxo-" prefix. An example is 4-(N,N-dimethylamino)pent-4-en-2-one.
Aldehydes: Since aldehydes are at the end of a chain, the position "1" is omitted. Standard suffix is "-al," but "-carbaldehyde" is used if the aldehyde carbon cannot be numbered as part of the ring/chain (e.g., cyclohexanecarbaldehyde). "Formyl-" is used for an aldehyde substituent when the carbon cannot be part of the chain numbering.
Carboxylic Acids: Position "1" is omitted from "-oic acid." Common names like formic acid (methanoic) and acetic acid (ethanoic) are IUPAC-accepted. "-carboxylic acid" is used for cases where the acid carbon isn't included in the main chain numbering.

Examples of complex assignments:

4-hydroxybut-2-enal (aldehyde priority over alcohol and alkene).
2-amino-3-phenylpropanoic acid.
2,2-dimethyl-3,4-dioxobutanoic acid.
2-formyl-4-oxocyclohexanecarboxylic acid.

Nomenclature of Aromatic Compounds

Aromatic compounds are derived from benzene ( $C_6H_6$ ) or similar fused ring systems like naphthalene or anthracene. The process involves identifying the parent ring, determining priorities of substituents (using the same hierarchy as aliphatic compounds), and assembling the name alphabetically.

In monosubstituted benzenes, many substituents simply use the prefix followed by "benzene" (e.g., chlorobenzene, nitrobenzene, ethylbenzene). However, certain substituents change the root name of the ring entirely. IUPAC-accepted root names, in decreasing order of priority, include:

Benzoic acid ( $-COOH$ )
Benzene-sulfonic acid ( $-SO_3H$ )
Benzaldehyde ( $-CHO$ )
Phenol ( $-OH$ )
Aniline ( $-NH_2$ )
Anisole ( $-OCH_3$ )
Toluene ( $-CH_3$ )

For disubstituted benzenes, three substitution patterns exist, which can be named numerically or using common prefixes: 1,2- (ortho-), 1,3- (meta-), and 1,4- (para-). Examples include 1,4-dibromobenzene, 3-aminobenzoic acid, and 2-methoxybenzaldehyde. Polysubstituted examples include 3,4-dichloro-N-methylaniline and 2,4,6-trinitrotoluene (TNT).

Aromatic Ketones (Phenones)

A specific subclass of aromatic compounds consists of ketones where the carbonyl group is directly attached to at least one benzene ring. These are named as "phenones," with prefixes indicating the size of the non-aromatic carbon group. Common phenones include:

Acetophenone: Carbonyl attached to a methyl group.
Propiophenone: Carbonyl attached to an ethyl group.
Butyrophenone: Carbonyl attached to a propyl group.
Benzophenone: Carbonyl attached to two benzene rings.