Naming Organic Compounds

Naming Organic Compounds

The increasingly large number of organic compounds identified with each passing day, together with the fact that many of these compounds are isomers of other compounds, requires that a systematic nomenclature system be developed. 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:

Name	Methane	Butane	Acetone	Toluene	Acetylene	Ethyl Alcohol
Formula	CH4	C4H10	CH3COCH3	CH3C6H5	C2H2	C2H5OH

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.

The IUPAC Systematic Approach to Nomenclature

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. 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. 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 sp³ hybridized).
An excellent presentation of organic nomenclature is provided on a Nomenclature Page. created by Dave Woodcock.
A full presentation of the IUPAC Rules is also available.

Alkanes

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.

Examples of Simple Unbranched Alkanes

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 uniformly 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 C_nH_2n+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 using the full name (e.g. cyclopropyl before isobutyl).     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 accommodated, 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. Halogenated alkyl substituents such as bromomethyl, BrCH₂–, and trichloromethyl, CCl₃–, may be listed and are alphabetized according to their full names.
For additional examples of how these rules are used in naming branched alkanes, and for some sub-rules of nomenclature   .
 

Cycloalkanes

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 C_nH_2n. Although a cycloalkane has two fewer hydrogens than the equivalent alkane, each carbon is bonded to four other atoms so such compounds are still considered to be saturated with hydrogen.

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