Nomenclature of Organic Compounds
Organic nomenclature is a stepwise process - simple elements need to be learned before we get into complicated functional groups.The core skills are identifying the central carbon chain and numbering each carbon atom in relation to the priority of attached functional groups. Please read these first two sections before continuing on to learn about the nomenclature of various functional groups.
Each section below has a brief description and links to full descriptions and examples of nomenclature. Clicking on the heading of each section will give take you to relevant descriptions of the properties of that functional group (where applicable).
Carbon Chains and Functional Groups
The core of every organic compound is the carbon chain from which other functional groups branch out. To construct the basic name of the compound, the longest carbon chain in the compound must be identified and given a name that matches the number of carbons. Some numbers can have two labels: IUPAC names and "common" names. IUPAC conventions are very specific and used in pure scientific context; common names are usually seen in industry. Below is a list of the first ten prefixes, with IUPAC and common names. Common names are increasingly used with readily available chemicals; for example, a popular preservative in IUPAC standards is called "methanal", but is almost universally known as "formaldehyde".
|Number of carbons||IUPAC prefix||Common prefix|
|5||Pent||Am- / Valer-|
In addition to the main hydrocarbon chain, the compound may have functional groups branching off from carbon atoms. Many groups may be present in one compound, but here is always one that takes priority over the others. The suffix of this highest priority is paired with the prefix identifying the number or carbons in the main chain. Other functional groups are represented by prefixes placed before the pairing of the main carbon prefix and highest priority suffix.
Numbering Alkyl Functional Groups
Using the parent carbon chain as a base, you can now begin constructing the name of the compound. The order of atoms in the carbon chain becomes important when functional groups other than the hydrogen atom are attached, so follow these steps when the compound is more complicated than a simple alkane:
- Identify the longest carbon atom chain.
- Identify the position of the functional group with the highest precedence (determined in the priority column of the above table).
- Number the carbon atoms in the parent chain so that the highest precedence functional group is given the lowest number possible. Attach the suffix of the functional group to the longest carbon chain prefix. The end result should be such that the first number should be the least possible. In the event of the first numbers being the same for two methods of numbering, the sum of the numbers of the side chains should be made the least possible; for example, 2,2,5-trimethylhexane (2 + 2 + 5 = 9) is preferred over 2,5,5-trimethylhexane (2 + 5 + 5 = 12), as they both start with '2', but the sum of the numbers of the former is less.
- Identify alkyl side-chains and number them. Include di- and tri- prefixes when necessary.
- Identify the remaining functional groups, if any, and name them using prefixes (hydroxy for -OH, oxy for =O, etc.) and number them.
- Different side-chains and functional groups, with the exception of the defining functional group, are grouped together in alphabetical order. (The prefixes di-, tri-, etc. are not taken into consideration for grouping alphabetically. For example, ethyl comes before dihydroxy or dimethyl, as the "e" in "ethyl" precedes the "h" in "dihydroxy" and the "m" in "dimethyl" alphabetically. The "di" is not considered in either case). All side chains and secondary functional groups are included in this order.
- Identify double and triple bonds. Number them with the number of the carbon atom bonded to it that has the lowest value. For example, a double bond between carbon atoms 3 and 4 is numbered as 3-ene. Multiple bonds of one type (double/triple) are named with a prefix (di-, tri-, etc.). If both types of bonds exist, use "ene" before "yne" e.g. "6,13-diene-19-yne". If all bonds are single, use "ane" without any numbers or prefixes.
- Affix the alphabetically ordered list of side chains and functional groups in front of the suffixed term.
- Add punctuation:
- Put commas between numbers (2 5 5 becomes 2,5,5)
- Put a hyphen between a number and a letter (2 5 5 trimethylhexane becomes 2,5,5-trimethylhexane)
- Successive words are merged into one word (trimethyl hexane becomes trimethylhexane)
The simplest hydrocarbons are composed of a main chain of carbon atoms, which may contain branched alkyl groups or multiple groups. See the sections on Carbon Chain and Prefixes and Alkyl groups for the basic methods on how to name alkanes. Alkenes contain double bonds and are noted with the suffix "-ene", replacing the "-ane of the alkane. Alkynes contain triple bonds and are noted with the suffix "-yne", replacing the "-ane".
Cycloalkanes and Cycloalkenes
For more information on cyclic compounds, please see Cycloalkanes.
Cyclic hydrocarbons are mainly comprised of a chain of carbon atoms that form a continuous "ring", as opposed to hydrocarbons that have defined ends. Cycloalkanes are connected entirely by single bonds and cycloalkenes contain double bonds.
Cycloalkanes are extremely simple to name; the prefix "cyclo" is affixed to the regular alkane name corresponding with the number of carbons in the ring. For example, a cycloalkane comprised of five carbon atoms is named "cyclopentane".
Cycloalkenes follow the same system, but contain numerical prefixes and infixes "di" and "tri" if there are two or three multiple bonds within the ring. If there is only one multiple bond in the ring, numerical prefixes are not needed, as it is assumed that numeration of the carbon atoms would begin at the carbon atom with the multiple bond.
For more information on aromatic compounds, please see Aromatic Hydrocarbons.
Benzene rings are always the central feature of aromatic compounds, but the class as a whole is quite versatile, and may contain several other functional groups linked to benzene. In these cases, different functional groups are separated from each other when writing out the name of the compound. Numeric prefixes are also separated by functional group, and the suffix "-lene" is often used. The prefixes ortho, meta, and para are used to express the spacing of substituents around the benzene ring.
For more information on ethers, please see Ethers.
Ethers contain two R groups, each connected by a single bond to an oxygen atom. Generally, the smaller carbon group is referenced first, and the larger is referenced after the infix "oxy". To clarify, the ether's name begin with the prefix of the smaller carbon chain, followed by "oxy", followed by the alkane name of the larger carbon chain.
For example, an ether with a methyl group and an ethyl group bonded to the central oxygen is named "methoxyethane" or "methyl ethyl ether".
Informally, each R group can be referenced with its alkyl naming and then followed by the word "ether". The example above would informally be named "methyl ethyl ether". If the R groups are the same, the prefix "di" can be added in front of the alkyl name of the R group, then followed by the separate word "ether'. For example, an ether bonded to two R groups with three carbons each is named "dipropyl ether".
|Carboxylic Acids||1||carboxy-||-oic acid|
For more information on carboxylic acids, please see Carboxylic Acids.
Carboxylic acids always contain a carboxyl group (COOH), the carbon of which is usually at an end of the carbon backbone. Therefore, one must simply count the number of carbons in the molecule (including the carbon atom of the carboxyl group) and identify the related prefix. This prefix is attached to the suffix "-anoic acid".
For more information on amides, please see Organic Nitrogen Compounds.
Amides are biologically crucial compounds that do not technically constitute a unique functional group, but rather are defined by containing a carbonyl group whose carbon atom is attached to an amine group. While the most biologically amides, amino acids, polymerize in a condensation polymer reaction to form proteins, these compounds utilize a naming system covered on the Amino Acids page. This page will explain naming unpolymerized amide groups, which do not contain R groups commonly found in biological systems. These amides can be referred to as primary, secondary, or tertiary.
For more information on esters, please see CoreChem:Esters.
Two important components of an ester must be identified before the compound can be named. An alcohol and carboxylic acid participate in a condensation reaction in order to form the ester and retain two separate R groups. The R group from the carboxylic acid is bonded to two oxygen atoms, while the R group from the alcohol is bonded to only one oxygen atom.
The resulting ester's name contains two components, one from each R-group. The R-group from the alcohol comes first and is referred to with the same procedure as an alkyl group. The second and separate word comes from the carboxylic acid's R group, and contains the prefix designating the number of carbons in the chain, as well as the suffix "-oate".
For more information on aldehydes, please see Aldehydes and Ketones.
The aldehyde group is an example of another family of organic compounds defined by the region around the contained carbonyl group, (C=O). The carbon in the carbonyl group is also bonded to a R group and a hydrogen atom, yielding the characteristic formyl group, (-RCOH). This group is almost always found at the end of a carbon chain; so will consider only this case in nomenclature.
Because the defining formyl group is bonded to the end of a carbon chain, aldehydes are relatively easy to name, and nomenclature works much like primary alcohols and carboxylic acids. If numeration is necessary because of other present functional groups, the carbon atom included in the formyl group is always numbered 1. To name aldehydes, drop the e in the alkane group and replace it with al.
For more information on ketones, please see Aldehydes and Ketones.
Ketones contain a carbonyl group (C=O) with the carbon atom bonded to at least two more carbon atoms. Thus, ketones differ from aldehydes and carboxylic acids by containing a carbonyl group in the "middle" of the compound rather than at an "end".
The carbon atom within the carbonyl group is given the lowest number possible when the carbon chain is numbered in order. This numerical prefix is added to a term containing the prefix representing the number of carbon atoms in the chain and followed by the suffix "-one".
For more information on alcohols, please see Alcohols.
Alcohols contain one or more hydroxyl groups, (-OH), along a carbon chain. The identity (primary, secondary, tertiary) of the carbon atoms connected to each alcohol group determine the nomenclature of the compound as a whole. For each case, when a single hydrogen atom is replaced by a hydroxyl group, the basic naming rule is to identify the corresponding alkane, drop the "e", add the suffix "-ol", and add a numerical prefix that identifies where in the carbon chain the alcohol substituent is located.
For more information on amines, please see Organic Nitrogen Compounds.
Amines are derivatives of ammonia, NH3, replacing H atoms with carbon groups. The classification of amines depend on the number of H atoms replaced; primary amines have one -R group substitution, secondary amines have two substitutions, and tertiary amines have three substitutions. The method of nomenclature differs for each of these classifications.