Just as each distinct compound has a unique molecular structure which can be designated by a structural formula, each compound must be given a characteristic and unique name. As organic chemistry grew and developed, many compounds were given trivial names, which are now commonly used and recognized. Some examples are:. Such common names often have their origin in the history of the science and the natural sources of specific compounds, but the relationship of these names to each other is arbitrary, and no rational or systematic principles underly their assignments.
A rational nomenclature system should do at least two things. First, it should indicate how the carbon atoms of a given compound are bonded together in a characteristic lattice of chains and rings. Second, it should identify and locate any functional groups present in the compound. Since hydrogen is such a common component of organic compounds, its amount and locations can be assumed from the tetravalency of carbon, and need not be specified in most cases.
The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. As an introduction to the IUPAC nomenclature system, we shall first consider compounds that have no specific functional groups. Such compounds are composed only of carbon and hydrogen atoms bonded together by sigma bonds all carbons are sp 3 hybridized.
An excellent presentation of organic nomenclature is provided on a Nomenclature Page. Hydrocarbons having no double or triple bond functional groups are classified as alkanes or cycloalkanes , depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings.
Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. The alkanes and cycloalkanes are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any aromatic rings in their molecular structure.
A common "ane" suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names.
Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms. Some important behavior trends and terminologies: i The formulas and structures of these alkanes increase uniformly by a CH 2 increment.
Beginning with butane C 4 H 10 , and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C 6 H 14 isomers, shown below as abbreviated line formulas A through E :. Although these distinct compounds all have the same molecular formula, only one A can be called hexane.
How then are we to name the others? The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the "ane" suffix is replaced by " yl " in naming groups. The symbol R is used to designate a generic unspecified alkyl group. Find and name the longest continuous carbon chain.
Identify and name groups attached to this chain. Number the chain consecutively, starting at the end nearest a substituent group. Designate the location of each substituent group by an appropriate number and name. Assemble the name, listing groups in alphabetical order using the full name e. The prefixes di, tri, tetra etc. Halogen substituents are easily accommodated, using the names: fluoro F- , chloro Cl- , bromo Br- and iodo I-.
The group is named by replacing the -ane suffix of the parent hydrocarbon with -yl. For example, the -CH 3 group derived from methane CH 4 results from subtracting one hydrogen atom and is called a methyl group. Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically.
Name alkanes according to the LCC longest continuous chain of carbon atoms in the molecule rather than the total number of carbon atoms. This LCC, considered the parent chain, determines the base name, to which we add the suffix - ane to indicate that the molecule is an alkane.
If the hydrocarbon is branched, number the carbon atoms of the LCC. Numbers are assigned in the direction that gives the lowest numbers to the carbon atoms with attached substituents.
Hyphens are used to separate numbers from the names of substituents; commas separate numbers from each other. Place the names of the substituent groups in alphabetical order before the name of the parent compound. Although this o-, m-, p- system is the common naming system for benzene derivatives, they have been applied broadly in books and literatures. For the following mono-substituted benzene derivatives, phenol, benzoic acid and benzaldehyde, their common names are adopted in the IUPAC system.
When other substituents are introduced into those benzene derivatives, the common name will be used as the parent name of the compound with the base functional group OH for phenol, COOH for benzoic acid and CHO for benzaldehyde given the 1 position.
For example:. An example is given here:. All Rights Reserved. Skip to content With the ability to identify functional groups, next we will learn how to give IUPAC names to compounds containing a few functional groups, by following a set of rules. This chain determines the parent name of the compound.
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