Nomenclature of Organic Compounds

IUPAC System [International union of pure and applied chemistry]

The most important feature of this system is that any given molecular structure has only one IUPAC name which denotes only one molecular structure.
 

Salient Features of IUPAC System

1.    A given compound can be assigned only one name.

2.    A given name can clearly direct in writing of one and only one molecular structure.

3.    The system can be applied in naming complex organic compounds.

4.    The system can be applied in naming multifunctional organic compounds.

5.    This is simple, systematic and scientific method of nomenclature of organic compounds.
 

Rule For Naming

Prefix (alphabetically) root word (alk) primary suffix (ene, yne) secondary suffix (main functional group)

So IUPAC name of any organic compounds essentially consists of two or three parts.

(i)   Root word        (ii)   Suffix         (iii)  Prefix

(i) Root Words

The basic unit is a series of root words which indicate linear or continuous chains of carbon atoms. Chains containing one to four carbon atoms are known by special root words while chains from C₅ onwards are known by Greek number roots.

In general, the root word for any carbon chain in alk-.

(ii) Primary Suffix

Primary suffix are added to the root words to show saturation or unsaturation in a carbon chain.

(iii) Secondary Suffixes

Suffixes added after the primary suffix to indicate the presence of a particular functional group in the carbon chain are known as secondary suffixes.

Note:

The terminal ‘e’ of the primary suffix is removed when initial letter of secondary suffix is vowel. To illustrate the application of above basic rule, the generic names of few classes of organic compounds are given below:

Prefix:

It should always be kept in mind that alkyl groups forming branches of the parent chain are considered as side – chains. Atoms of groups of atoms such as fluoro (-F), chloro (-Cl), bromo 
(-Br), iodo (-I), nitro (-NO₂), nitroso (-NO) and alkoxy (-OR) are referred to as substituents. Roots words are prefixed with the name of the substituent or side chain.

Arrangement of Prefixes, Root Word and Suffixes

These are arranged as follows while writing the name.

Prefix (es) + Root word + Primary suffix + Secondary suffix

For Example:

Prefix = Bromo (at position 3),

Root word = But,

Primary suffix = -ane

Secondary suffix = -oic acid

Hence, the name of the compound is,

3 – Bromo butanoic acid

Prefix = Methyl (at position 4)

Root word = Pent,

Primary suffix = -ene (at position 2),

Secondary suffix = -ol

Hence, the name of the compound is,

4 – Methyl pent – 2 – en – 1 – ol

The names of simple aliphatic organic compounds containing only straight chains atoms of various homologous series are described in table as to explain the basic rules of IUPAC system. In case of compounds. Other than hydrocarbons, only the saturated compounds have been considered.

• Nomenclature Alkanes

The IUPAC system of alkane nomenclature is based on the simple fundamental principle of considering all compounds to be derivatives of the longest single carbon chain present in the compound. The chain is then numbered from one end to the other, the end chosen as number 1 is that which gives the smaller number at the first point of difference.

When there are two or more identical appendages - the modifying prefixes di-, tri-, tetra-, penta-, hexa-, and so on are used, but every appendage group still gets its own number.

When two or more appendage locants are employed, the longest chain is numbered from the end which produces the lowest series of locants. When comparing one series of locants with another, that series is lower which contains the lower number at the first point of difference.

Several common groups have special names that must be memorized by the student.

A more complex appendage group is named as a derivative of the longest carbon chain in the group starting from the carbon that is attached to the principal chain. The description of the appendage is distinguished from that of the principal chain by enclosing it in parentheses.

When two or more appendages of different nature are present, they are cited as prefixes in alphabetical order. Prefixes specifying the number of identical appendages (di, tri, tetra and so on) and hyphenated prefixes (tert-or t, sec-) are ignored in alphabetizing except when part of a complex substituent. The prefixes cyclo-, iso-, and neo-count as a part of the group name for the purposes of alphabetizing.

When chains of equal length compete for selection as the main chain for purposes of numbering, that chain is selected which has the greatest number of appendage attached to it.

5-(2-ethylbutyl) – 3, 3 – dimethyl decane

When two or more appendages are in equivalent positions, the lower number is assigned to the one that is cited first in the name (that is one that comes first in the alphabetic listing).

• Nomenclature of Alkenes

and have the general formula C_nH_2n. These unsaturated hydrocarbons are isomeric with the saturated cycloalkanes. The IUPAC rules for naming alkenes are similar in many respects to those for naming alkanes.

1. Determine the root word by selecting the longest chain that contains the double bond and change the ending of the name of the alkane of identical length from ane to ene.
2. Number the chain so to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond  by using the number of the first atom of the double bond as prefix : Indicate the locations of this substituent groups by the numbers of the carbon atoms to which they are attached.
   
    
3.  Two frequently encountered alkenyl groups are the vinyl group and the allyl group.

                     CH₂ = CH —              CH₂ = CH CH₂ —

                     The vinyl group            The allyl group (are not included in IUPAC system)

                     The following examples illustrate how these names are employed

                     CH₂ = CH — Br           CH₂ = CH — CH₂ Cl

                     vinyl bromide              allyl chloride                     (are not IUPAC name)

4.  The geometry of the double bond of a disubstituted alkene is designated with the prefixes, cis and trans. If two identical group are on the same side of the double bond, it is cis, if they are on opposite sides; it is trans.

• Nomencalture of Alkyne

In the IUPAC system the compounds are named as alkynes in which the final – ane of the parent alkane is replaced by the suffix – yne. The position of the triple bond is indicated by a number when necessary.

When both a double and triple bond are present, the hydrocarbon is named an alkenyne with numbers as low as possible given to the multiple bonds. In case of a choice, the double bond gets the lower number.

In complex structures the alkynyl group is used as a modifying prefix.

• Nomencalture of Aromatic Hydrocarbons

These compounds consists of at least one benzene ring, i.e., a six – membered carbocylic ring having alternate single and double bonds.

• Nomenclature of Alkyl Halide

The alkyl halides have the general formula C_nH_2n+1X or Rx, where X denotes fluorine, chlorine bromine or iodine.

• Nomenclature of Alcohols

General formula          [C_nH_2n+1.OH], IUPAC name is alkanol.

For the simpler alcohols the common names, are most often used. These consist simply of the name of the alkyl group followed by the word alcohol. For example:

We should notice that similar names do not always mean the same classification; for example, isopropyl alcohol is a secondary alcohol, whereas isobutyl alcohol is a primary alcohol.

Finally, there is the most versatile system, the IUPAC. The rules are:

1. Select as the parent structure the longest continuous carbon chain that contains the  -OH group; then consider the compound to have been derived from this structure by replacement of hydrogen by various groups. The parent structure is known as ethanol, propanol, butanol, etc., depending upon the number of carbon  atoms; each name is derived by replacing the terminal -e of the corresponding alkane name by -ol.
2. Indicate by a number the position of the -OH group in the parent chain, generally using the lowest possible number for this purpose.
3. ndicate by numbers the positions of other groups attached to the parent chain.

      Alcohols containing two hydroxyl groups are called glycols. They have both common names and IUPAC names.

Note:   

          2 or 3 OH group can not present on same carbon atom, decomposes to give aldehyde/ketone or carboxylic acid respectively.

• Nomenclature of Ether (R-O-R)

Ethers are compounds in which two C atoms are connected to a single O atom. In IUPAC nomenclature, name one of the alkyl group plus the O atom (RO-) as an alkoxy and comes as a prefix to the parent hydrocarbon. (Oxygen is to be counted with least number of carbon atom)

IUPAC name of ether is alkoxy alkane

       Methoxy ethane

• Nomenclature of Aldehydes

IUPAC names the longest continuos chain including the C of – CH = O and replaces – e of the alkane name by the suffix – al i.e. alkanal. The C of CHO is number 1. For compounds with two – CHO groups, the suffix – dial is added to the alkane name. When other functional groups have naming priority, – CHO is called formyl.

Common names replace the suffix –ic (–oic or – oxylic) and the word acid of the corresponding carboxylic acids by – aldehyde. Locations of substituents on chains are designated by Greek letters e.g.

The terminal C of a long chain is designated w (omega)

The IUPAC names of aldehydes follow the usual pattern. The longest chain containing the –CHO group is considered the parent structure and named by replacing –e of the corresponding alkane by –al. The position of the substituent is indicated by a number, the carbonyl carbon always being considered C-1. Here, as with the carbonyl acids, the C-2 of the IUPAC name corresponds to alpha of the common name.

• Nomenclature of Ketones

Common names use the names of R or Ar as separate words, along with the word ketone. The IUPAC system replaces the –e of the name of the longest chain by the suffix –one. In molecules with functional groups such as – COOH, that have a higher naming priority, the carbonyl group is indicated by the prefix keto or oxo. Thus, CH₃COCH₂CH₂COOH is 4-ketopentanoic acid.

            The simplest aliphatic ketone has the common name acetone. For most other aliphatic ketones we name the two groups that are attached to carbonyl carbon and follow these names by the word ketone. A ketone in which the carbonyl group is attached to a benzene ring is named as phenone, all illustrated below. The positions of various groups are indicated by numbers.

• Nomenclature of Carboxylic Acid

(i) Common or Trivial names:

The names of lower members are derived from the Latin or Greek word that indicate the source of the particular acid.

(ii) Derived System:

Monocarboxylc acid may be named as alkyl derivative of acetic acid.

(iii) IUPAC System:

Acids are named as alkanoic acids. The name is derived by replacing ‘e’ of the corresponding alkane by oic acid.

            HCOOH                    Methanoic acid

            CH₃COOH                 Ethanoic acid

            CH­₃CH₂COOH             Propanoic acid

In case of substituted acids, the longest chain including carboxyl group is selected and numbering is done from the carbon of carboxylic group.7.   Derivative of Carboxylic acid

• Naming Acyl Groups- Acid halide and Anhydrides

The group obtained from a carboxylic acid by the removal of the hydroxyl portion is known as an acyl group. The name of an acyl group is created by changing the - ic acid at the end of the name of the carboxylic acid to –yl, examples:

 (a) Acid Halide

IUPAC name of acid halide is alkanoyl halide.

  where X may be Cl, Br, I

Acid chlorides are named systematically as acyl chlorides.

 (b) Acid anhydride

An acid anhydride is named by substituting anhydride for acid in the name of the acid from which it is derived.

IUPAC name of acid anhydride is alkanoic anhydride.

Example:

Butanoic, ethanoic anhydride

(ii)  Naming Salts and Esters

General formula of ester RCOOR¢

IUPAC name:  Alkyl alkanoate

Example:

CH₃COOC₂H₅  (Ethyl ethanoate)

The name of the cation (in the case of a salt) or the name of the organic group attached to the oxygen of the carboxyl group (in the case of an ester) precedes the name of the acid.

(iii) Name of Amides and Imides

The names of amides are formed by replacing –oic acid (or –ic acid for common names) by amide or –carboxylic acid by carboxamide.

IUPAC name of acid amide is alkanamide

Example:

If the nitrogen atom of the amide has any alkyl groups as substitutents, the name of the amide is prefixed by the capital letter N; to indicate substitution on nitrogen, followed by the name(s) of alkyl group(s).

If the substituent on the nitrogen atom of an amide is a phenyl group, the ending for the name of the carboxylic acid is changed to anilide

• Nomenclature of Amines

Nomenclature of amines is quite simple. Aliphatic amines are named by naming the alkyl group (or) groups attached to nitrogen , and following that by the word amine.

More complicated amines are often named as prefixing amino - (or-N-methylamino -, N-N, diethyl  amino -, etc) to the name of the parent chain.

Aromatic amines - those in which nitrogen is attached to an aromatic ring - are generally named as derivatives of the simplest aromatic amine, aniline.

Salts of amines are generally named by replacing - amine by - ammonium (or - aniline by - anilinium), and adding the name of the anion.

• Nomenclature of Nitro Alkane

General formual    C_nH_2n+1.NO₂

Nitro alkane

Example:

Alkyl nitrites

Common name is alkyl nitrite there is no IUPAC name of nitrite.

R – O – N = O                 Alkyl nitrite

CH₃CH₂ – O NO    Ethyl nitrite

• Nomenclature of Alkyl Cyanide

R – CN                       Alkane nitrile

Example:

CH₃ – C N                    Ethane nitrile

Alkyl iso cyanide   RNC

Alkyl iso nitrile or alkyl iso cyanide.

There is no specific IUPAC name for alkyl iso cyanide or isonitrile.

CH₃NC                Ethyl isonitrile

When a compound contains more than one functional group, the numbering and the suffix in the name of multifunctional compound are determined by nomenclature priority.

Preference Order

Refer the below mentioned links to get an immediate solution to all queries on Organic chemistry:

JEE Organic Chemistry Syllabus

Organic Chemistry Revision Notes

Reference books of Organic Chemistry