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. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features: ? A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
? A suffix or other element(s) designating functional groups that may be present in the compound.
? Names of substituent groups, other than hydrogen, that complete the molecular structure.Alkanes
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.
The following table lists the IUPAC names assigned to simple continuous-chain alkanes from C-1 to C-10. 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.
| Name | Molecular
Formula | Structural
Formula | Isomers |
| Name | Molecular
Formula | Structural
Formula | Isomers |
|---|
| methane | CH4 | CH4 | 1 |
| hexane | C6H14 | CH3(CH2)4CH3 | 5 |
| ethane | C2H6 | CH3CH3 | 1 |
| heptane | C7H16 | CH3(CH2)5CH3 | 9 |
| propane | C3H8 | CH3CH2CH3 | 1 |
| octane | C8H18 | CH3(CH2)6CH3 | 18 |
| butane | C4H10 | CH3CH2CH2CH3 | 2 |
| nonane | C9H20 | CH3(CH2)7CH3 | 35 |
| pentane | C5H12 | CH3(CH2)3CH3 | 3 |
| decane | C10H22 | CH3(CH2)8CH3 | 75 |
Some important behavior trends and terminologies:
(i) The formulas and structures of these alkanes increase uniformally by a CH2 increment.
(ii) A uniform variation of this kind in a series of compounds is called homologous.
(iii) These formulas all fit the CnH2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon.
(iv) Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen).
Beginning with butane (C4H10), and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C6H14 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.
| Group | CH3? | C2H5? | CH3CH2CH2? | (CH3)2CH? | CH3CH2CH2CH2? | (CH3)2CHCH2? | CH3CH2CH(CH3)? | (CH3)3C? | R? |
| Name | Methyl | Ethyl | Propyl | Isopropyl | Butyl | Isobutyl | sec-Butyl | tert-Butyl | Alkyl |
IUPAC Rules for Alkane Nomenclature 1. Find and name the longest continuous carbon chain.
2. Identify and name groups attached to this chain.
3. Number the chain consecutively, starting at the end nearest a substituent group.
4. Designate the location of each substituent group by an appropriate number and name.
5. Assemble the name, listing groups in alphabetical order.
The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing. |
For the above isomers of hexane the IUPAC names are: B 2-methylpentane C 3-methylpentane D 2,2-dimethylbutane E 2,3-dimethylbutane
Halogen substituents are easily accomodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-). For example, (CH3)2CHCH2CH2Br would be named 1-bromo-3-methylbutane. If the halogen is bonded to a simple alkyl group an alternative "alkyl halide" name may be used. Thus, C2H5Cl may be named chloroethane (no locator number is needed for a two carbon chain) or ethyl chloride.
Cycloalkanes
Cycloalkanes have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The IUPAC names of the first five members of this series are given in the following table. The last (yellow shaded) column gives the general formula for a cycloalkane of any size. If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is CnH2n.
Examples of Simple Cycloalkanes
| Name | Cyclopropane | Cyclobutane | Cyclopentane | Cyclohexane | Cycloheptane | Cycloalkane |
Molecular
Formula | C3H6 | C4H8 | C5H10 | C6H12 | C7H14 | CnH2n |
Structural
Formula |  |  |  |  |  | (CH2)n |
Line
Formula |  |  |  |  |  |  |
Substituted cycloalkanes are named in a fashion very similar to that used for naming branched alkanes. The chief difference in the rules and procedures occurs in the numbering system. Since all the carbons of a ring are equivalent (a ring has no ends like a chain does), the numbering starts at a substituted ring atom.
IUPAC Rules for Cycloalkane Nomenclature 1. For a monosubstituted cycloalkane the ring supplies the root name (table above) and the substituent group is named as usual. A location number is unnecessary.
2. If the alkyl sustituent is large and/or complex, the ring may be named as a substituent group on an alkane.
3. If two different substituents are present on the ring, they are listed in alphabetical order, and the first cited substituent is assigned to carbon #1. The numbering of ring carbons then continues in a direction (clockwise or counter-clockwise) that affords the second substituent the lower possible location number.
4. If several substituents are present on the ring, they are listed in alphabetical order. Location numbers are assigned to the substituents so that one of them is at carbon #1 and the other locations have the lowest possible numbers, counting in either a clockwise or counter-clockwise direction.
5. The name is assembled, listing groups in alphabetical order and giving each group (if there are two or more) a location number. The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing. |
Small rings, such as three and four membered rings, have significant angle strain resulting from the distortion of the sp3 carbon bond angles from the ideal 109.5º to 60º and 90º respectively. This angle strain often enhances the chemical reactivity of such compounds, leading to ring cleavage products. It is also important to recognize that, with the exception of cyclopropane, cycloalkyl rings are not planar (flat).
Hydrocarbons having more than one ring are common, and are referred to as bicyclic (two rings), tricyclic (three rings) and in general, polycyclic compounds. The molecular formulas of such compounds have H/C ratios that decrease with the number of rings. In general, for a hydrocarbon composed of n carbon atoms associated with m rings the formula is: CnH(2n + 2 - 2m). The structural relationship of rings in a polycyclic compound can vary. They may be separate and independent, or they may share one or two common atoms. Some examples of these possible arrangements are shown in the following table.
Examples of Isomeric C8H14 Bicycloalkanes
| Isolated Rings | Spiro Rings | Fused Rings | Bridged Rings |
| No common atoms | One common atom | One common bond | Two common atoms |
 |  |  |  |
Moderation Team
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