Nomenclature of Organic Compounds – IUPAC with examples
Nomenclature is the process of naming living and non-living substances. The word comes from the Latin word ‘nomen’ which means name, and ‘calare’, i.e. call.
Internationally agreed principles are used for naming different compounds. However, it may vary from using common and colloquial terminologies to strict rules.
Nomenclature is used in every branch of science, and in this section, IUPAC nomenclature for organic compounds will be discussed.
What is IUPAC?
IUPAC stands for International Union of Pure and Applied Chemistry. It was founded in 1919 as a replacement for the International Congress of Applied Chemistry. The headquarters of this organisation is located in the USA.
IUPAC is the international governing body for standardising atomic weights, symbols, nomenclature, etc. of various chemicals. It develops and maintains such data to aid chemists and researchers across the globe. This organisation is also responsible for creating the IUPAC nomenclature for inorganic and organic compounds.
IUPAC Nomenclature of Organic Chemistry
IUPAC has a prolonged history of providing nomenclature to organic and inorganic compounds. This naming system has been developed to create a set of standardised names that are accepted across the globe. This avoids the confusion of scientists naming substances in different ways.
Under this IUPAC nomenclature of organic chemistry, there is a set of rules that every chemist must follow to name compounds belonging to particular groups.
Furthermore, IUPAC’s nomenclature of organic compounds has three sections - substituents, the length of the carbon chain, and chemical end. Here, the substituents represent functional groups attached to the primary carbon chain. This main carbon chain is the longest possible continuous one. Additionally, the chemical ending represents the molecule’s type.
IUPAC nomenclature decides the fundamental root name by using the longest continuous chain of carbon. After that, this root name is modified with assistance from various functional groups that substitutes carbon and hydrogen atoms in the main structure.
There are various ways to modify the root name of a compound according to its functional group. These are –
Substitutive: This is probably the most common way of modifying a compound’s root name. Moreover, functional groups of the highest priority modify the suffix. On the other hand, other groups or substituents adds prefixes to root names.
Functional Groups: Here, substances are named according to their highest priority conditional groups. For instance, ketone, alkyl halide, alcohol, etc.
Replacement: This process indicates when a carbon atom is getting replaced by another atom.
Conjunctive: It is used for combining named subunits.
Trivial: Due to their widespread use, some compounds are registered under IUPAC nomenclature according to their trivial names.
Creation of Systematic Names
Creating these systematic names is a complicated process. It requires following a step-by-step method to form a nomenclature of organic compounds.
These steps are –
1. Determine the Functional Group that Will be Cited as the Suffix
2. Locate the Longest Carbon Chain With the Required Functional Group and Add Up the Number of Carbon Atoms. It Will Determine the Prefix of A Compound’s Name.
Find out the senior-most among these structural components joined to the primary characteristic group.
Specify unsaturation, if there is any after naming the parent hydride.
Combine the suffix and parent hydride name to determine the principal characteristic group.
Categorise the substituents, and then organise them in alphabetical order according to their corresponding prefixes.
Add multiplicative prefixes without altering the current order. Then insert locants.
Figure out the chirality centre and other stereogenic units, namely double bonds.
Lastly, add stereo descriptors.
Additionally, complex substituents of any organic compound having a branched structure should be regarded as substituted alkyl groups. However, the carbon attached to this group is numbered 1.
According to IUPAC nomenclature methods, such complex substituents should be in the brackets of their corresponding compounds.
Format used for this purpose is – “locant + prefix + root + locant + suffix”.
Characteristic Groups
Root: This term specifies the total number of carbon atoms that exist in a long carbon chain of an organic compound. For instance, ‘meth’ refers to the chain with one carbon atom, and ‘non’ indicates nine carbon atoms in an organic compound.
Suffix: Suffix in IUPAC nomenclature refers to the functional group it belongs to and follows the root name. It is further segregated into two types - primary and secondary.
The primary suffix is used immediately following the root word. For example, alkanes, where ‘ane’ is the suffix used just after the root word.
A secondary suffix is written after the primary one. For instance, compounds with alcohol and alkane group attached to them will be termed as an alkanol, here ‘ol’ is the secondary suffix for the alcohol group.
To comply with IUPAC norms, a suffix is used as part of the name of a particular compound.
Prefix: Prefix is added in front of the root names of compounds under this nomenclature. The use of prefix is very useful, as it shows the existence of side chains and substituent groups of an organic molecule. Such prefixes provide an insight into the acyclic and cyclic nature of compounds.
Similar to the suffix, a prefix is also divided into two categories - primary and secondary. The primary prefix indicates the acyclic or cyclic nature of any organic compound. For example, ‘cyclo’ prefix is used for cyclic compounds.
A secondary prefix portrays the existence of substituent groups and side chains. A prominent example of its use is the CH3 group, which is known as a methyl group.
Nomenclature of Few Compounds
In this IUPAC name list below, the nomenclature of some common compounds has been discussed.
Alkanes: Alkane is a type of saturated hydrocarbon, and its formula is \[C_{n}H_{2n+ 2}\]. The suffix ‘ane’ is used to describe any alkane compound - for instance, methane, ethane, butane, etc. However, when one hydrogen is removed from the end of a chain, it changes the suffix ‘-ane’ to ‘-yl’.
Alkyl Halides: It is considered as a substituent of alkane chains. Alkyl halides are regarded as an equal of alkyl substituent in the parent numbering chains.
These Halogens are characterised by –
F = fluoro-
Cl = chloro-
Br = bromo-
I = iodo-
Alkene: Double bonds of unsaturated hydrocarbons are regarded as an alkene. Here, ‘ene’ indicates the shift to double bonds.
Thus, if there are any second bonds, then the suffix expands to include a prefix. This prefix indicates the number of bonds.
Formula of alkene is \[C_{n}H_{2n}\]. Examples of this compound are ethene, propane, etc.
Alkynes: Similar to the system mentioned above, when a double bond expands to a triple bond, the suffix ‘-yne’ is used. An example of this group is ethyne. The general formula of an alkyne is \[C_{n}H_{2n-2}\].
Alcohols: Alcohols are named after replacing the ‘–ane’ suffix with ‘-anol’. If there are more than once hydroxyl group, then, this suffix is expanded to include a proper prefix.
A common example of this group is anediol, anetriol, pentanol, butanediol, etc.
Ethers: Ethers are known with their common names. In order to find that, two alkyl groups are attached to an oxygen atom, and then put them in alphabetical order with space between names.
They are trailed by the word ether. Additionally, ‘di-’ prefix is used in case both alkyl groups are the same.
Some examples of this group are diethyl ether, ethyl methyl ether, etc.
FAQs on IUPAC Nomenclature of Organic Compounds
1. What is the IUPAC nomenclature of organic compounds?
IUPAC nomenclature is a systematic method of naming organic chemical compounds, established by the International Union of Pure and Applied Chemistry (IUPAC). Its primary goal is to ensure that every compound has a unique and unambiguous name that reveals its chemical structure. This global standardisation eliminates confusion arising from common or trivial names and allows scientists worldwide to communicate clearly.
2. What are the three main parts of an IUPAC name?
An IUPAC name for an organic compound is typically composed of three main parts:
- Word Root: This indicates the number of carbon atoms in the longest continuous carbon chain (the parent chain). For example, 'eth-' for 2 carbons, 'prop-' for 3 carbons, and 'but-' for 4 carbons.
- Suffix: This part is added after the word root and indicates the type of carbon-carbon bonds (primary suffix) or the principal functional group (secondary suffix). For example, '-ane' for single bonds, '-ene' for double bonds, and '-ol' for an alcohol group.
- Prefix: This part comes before the word root and indicates any substituent groups (side chains) or a cyclic nature of the compound. For example, 'methyl-' for a -CH₃ group and 'cyclo-' for a ring structure.
3. Why is a standardised naming system like IUPAC necessary in chemistry?
A standardised naming system like IUPAC is crucial because millions of organic compounds exist, with more being discovered daily. Without a universal set of rules, a single compound could have multiple names based on its origin, discoverer, or local language, leading to massive confusion in scientific research, education, and industry. The IUPAC system provides a logical and unambiguous structure, ensuring that a specific name corresponds to only one chemical structure, which is vital for scientific accuracy and global collaboration.
4. How does IUPAC nomenclature handle branched-chain alkanes compared to straight-chain ones?
For straight-chain alkanes, the name is simply the word root for the number of carbons plus the suffix '-ane' (e.g., Propane for a 3-carbon chain). For branched-chain alkanes, the process is more detailed:
- First, the longest continuous carbon chain is identified as the parent chain, which gives the base name.
- Second, any carbon groups attached to this parent chain are considered substituents (e.g., a one-carbon branch is a 'methyl' group).
- Third, the parent chain is numbered to give the substituents the lowest possible position numbers (locants).
- Finally, the full name is assembled by placing the substituent name (with its locant) as a prefix to the parent chain name, such as 2-Methylpropane.
5. What is the difference between a primary and a secondary suffix in IUPAC naming?
The primary and secondary suffixes provide different information about a molecule's structure. The primary suffix indicates the nature of the carbon-carbon bonds in the parent chain, specifying its saturation level. The main primary suffixes are:
- -ane for saturated chains (all single bonds).
- -ene for chains containing a double bond.
- -yne for chains containing a triple bond.
The secondary suffix is added after the primary suffix and denotes the principal (highest priority) functional group in the molecule, such as -ol for alcohols, -al for aldehydes, or -oic acid for carboxylic acids.
6. How does the priority of functional groups affect the IUPAC name of a polyfunctional compound?
In a compound with multiple functional groups (a polyfunctional compound), the IUPAC system uses a priority order to determine the principal functional group. The group with the highest priority dictates the secondary suffix and the base name of the compound. All other functional groups are considered substituents and are indicated by specific prefixes. For example, in a molecule containing both a hydroxyl (-OH) and a carboxyl (-COOH) group, the carboxyl group has higher priority. Therefore, the compound is named as a carboxylic acid (with an '-oic acid' suffix), and the hydroxyl group is named using the prefix 'hydroxy-'.
7. What are some examples of IUPAC names for common organic compounds?
Here are some examples that show how the IUPAC system works for different types of compounds:
- Alkane: A three-carbon alkane is named Propane (Prop- for 3 carbons, -ane for single bonds).
- Alkene: A three-carbon alkene is named Propene (Prop- for 3 carbons, -ene for the double bond).
- Alkyne: A two-carbon alkyne is named Ethyne (Eth- for 2 carbons, -yne for the triple bond).
- Alcohol: A two-carbon alcohol is named Ethanol (Eth- for 2 carbons, -an- for single bonds, -ol for the -OH group).
- Halogenoalkane: An ethane molecule with a chlorine atom is named Chloroethane.
8. What are 'locants' and what role do they play in IUPAC nomenclature?
In IUPAC nomenclature, locants are numbers used to specify the exact position of substituents, side chains, or functional groups along the parent carbon chain. Their role is to remove any ambiguity about the compound's structure. For instance, 'Butan-1-ol' and 'Butan-2-ol' are two different compounds. The locants '1' and '2' are essential to distinguish between them by indicating whether the hydroxyl (-OH) group is on the first or second carbon atom of the butane chain.
