IUPAC_NOMENCLATURE

PRINCIPLES OF NOMENCLATURE

            In early days of organic chemistry, each new compound was given an individual name. Such name was based on the source, some property, or some other trivial reason. The common names are like nicknames.

For Ex - Formic acid (HCOOH) was named as it was obtained by distillation of red ants (Latin, formica = ants).  Barbiturates after the name of a woman Barbara.

IUPAC SYSTEM OF NOMENCLATURE

v  With the rapid growth of organic chemistry, the number of compounds increased day by day (now about 6 million).

v  It became impossible to give common names to such a large number of compounds.

v  In 1957, the International Union of Pure and Applied Chemistry evolved a scheme for giving systematic names to organic compounds on the basis of structure. This is known as the IUPAC System.

v  This system has set rules for naming organic molecules from their structures.

v  The systematic name of a compound derived from its structural formula by applying IUPAC rule is referred to as its IUPAC Name.

v  One organic compound can have only one IUPAC name.

v  Knowing the IUPAC name of a compound, we can  write its structural formula.

v  However, common names of the first few members of a homologous series have been retained in the IUPAC system.

v  Naming organic compounds is an essential skill. You must be able to give correct name for a Structure or derive a correct structure from a name.

How to Name Organic Compounds

            The naming of organic compounds are in five parts. You should name molecules starting backwards from this list. For example, determine functional group first, then saturation, then the parent chain, then the substituents, and then the stereoisomer characteristics:

Stereoisomer Characteristics Substituents Parent Chain Bond Types Functional Groups

1. Stereoisomer Characteristics

a. Cis (Z) /Trans (E)

1. Cis – identical groups on same side

2. Trans – identical groups on opposite sides

                  

                       

b. (R)/(S)

To determine whether R or S, find the direction of the priority groups numbered

1-3. Lowest priority group (#4) must be in back when doing this.

1. (R)- priority #1-3 are clockwise

2. (S)- priority #1-3 are anti- clockwise

c. (+)/(-)

i. Identifies rotation under plane-polarized light

ii. To determine whether molecule is (+) or (-), need to do laboratory tests where the molecule is put in plane-polarized light

1. (+) – rotates clockwise

2. (-) – rotates anti-clockwise

2. Substituents

a. After determining the functional group and parent chain, everything else that is left are considered substituents.

b. Carbon chain- Name using following names based on number of carbons. Numbers correlate with amount of carbons in chain:

Number of carbons in substituent chain Name

1 Methyl          2 Ethyl            3 Propyl           4 Butyl            5 Pentyl

6 Hexyl           7 Heptyl          8 Octyl            9 Nonyl           10 Decyl

c. Branched carbon substituents (not connected in straight line like usual):

isopropyl:                    tert-butyl:

d. Halide functional groups- take the halogen name (ex. Chlorine) and remove the “-ine”

and add an “-o” in place for the resulting name (ex. Chloro)

e. Include any other functional groups that were not part of the parent chain. (Ex. –oxy- for

ethers)

f. Rings- If parent chain is in a ring, add cyclo- to the beginning of parent name.

g. Give number to indicate location of substituent on parent chain.

(ex. 3-methylpentane indicates a methyl group on the 3rd carbon of the parent chain of pentane)

3. Parent Chain

a. Name chain with following names in accordance to number of carbons in parent chain.

Number of carbons in substituent chain Name

1 Meth-           2 Eth-              3 Prop-            4 But-              5 Pent-

6 Hex-             7 Hept-            8 Oct-              9 Non-             10 Dec.

To determine parent chain:

i. Try to look for longest chain of carbon in molecule. This is the parent chain.

ii. If there is a functional group, double bond and/or triple bond, include the functional group in the parent chain. Then try to include the double bond, then the triple bond. This may require choosing a shorter parent chain.

4. Bond Types

a. –ane-; alkanes; single bonds only in structure

b. –ene-; alkenes; double bonds exist in structure

c. –yn-; alkynes; triple bonds exist in structure

d. If more than one double or triple bond use prefixes:

2. Di-               3. Tri-              4. Tetra-           5. Penta-          6. Hexa-.

If both double and triple bonds exist, list double bond then triple bond. (ex. 2,3,5-triene- 4,5-diyne)

5. Functional Groups

a. Written as a suffix at the end of compounds name.

b. -3-ol – Give number to indicate the carbon that the functional group is attached to

c. If there is more than one functional group, put the one higher on the hierarchy in the suffix and the others in the substituents.

d. –e; If no functional group attached, end the name with an “e”. (ex) pentane)

e. Some functional groups are carboxylic acid, aldehyde, ketone, amines, alcohols etc.

I.   NOMENCLATURE OF ALKANES

v  Alkanes are hydrocarbons that contain only single bonds. (The compounds that contain only carbon and hydrogen).

v  The first four members of the series are known by their common names: Methane, Ethane, Propane, and Butane.

v  The names of larger alkanes are derived from the Greek prefixes that indicate the number of carbon atoms in the molecule. Thus pentane has 5 carbons, hexane has 6, and so on.

v  General Formula for alkane is CnH2n + 2

v  In the common system all isomeric alkanes have the same parent name.

For example, two isomers in C₄H₁₀ alkanes are known as butanes. The names of various isomers are distinguished by prefixes. The prefix indicates the type of branching present in the molecule.

(1) Meaning of the Prefix n- Prefix n- is used for those alkanes in which all carbons are in a continuous chain. The prefix n- stands for normal or straight-chain.

             CH3-CH2-CH2-CH2-CH3  n-Pentane

 (2) Meaning of the Prefix Iso- Prefix iso- is used for those alkanes which have a methyl group (CH₃-) attached to the second last carbon atom of the continuous chain.

(3) Meaning of the Prefix Neo- Prefix neo- is used for those alkanes which have two methyl

groups attached to the second last carbon atom of the continuous chain.

Classification of Carbon Atoms

The structural formulas of alkanes contain four types of carbons:

(1) Primary Carbon (1°). A carbon atom attached to one other (or no other) carbon atom is called primary carbon.

(2) Secondary Carbon (2°). A carbon atom attached to two other carbon atoms is called secondary carbon.

(3) Tertiary Carbon (3°). A carbon atom attached to three other carbon atoms is called tertiary carbon.

(4) Quaternary Carbon (4°). A carbon atom attached to four other carbon atoms is called quaternary carbon.

Alkyl Groups

An alkyl group is formed by removing one hydrogen atom from an alkane.

            The symbol R- is often used to represent an alkyl group. The grouping R- (e.g.CH3CHr) is a compound and must be bonded to another atom or group of atoms. Alkyl groups are named dropping -ane from the name of the corresponding alkane, and adding the ending-yl.

Non alkyl Groups

A number of non alkyl groups are used in naming organic compounds. For example

-Cl       = Chloro                      -Br       = Bromo                     -I         = Iodo

-F         = Fluoro                     -NO2   = Nitro                       -NO     = Nitroso        

-NH2   = Amino                      -OH     = Hydroxy

Straight Chain Alkanes

# Carbon	Name	Molecular Formula	Structural Formula
1	Methane	CH4	CH4
2	Ethane	C2H6	CH3CH3
3	Propane	C3H8	CH3CH2CH3
4	Butane	C4H10	CH3CH2CH2CH3
5	Pentane	C5H12	CH3CH2CH2CH2CH3
6	Hexane	C6H14	CH3(CH2)4CH3
7	Heptane	C7H16	CH3(CH2)5CH3
8	Octane	C8H18	CH3(CH2)6CH3
9	Nonane	C9H20	CH3(CH2)7CH3
10	Decane	C10H22	CH3(CH2)8CH3

A) IUPAC Rules for Naming Alkanes

            The IUPAC system is the same for all classes of organic compounds. The IUPAC rules for naming alkanes are given below.

Rule1. Select the longest continuous carbon chain. Remember that this chain does not have to be that portion of the molecule that is written horizontally.

Rule2. Name the longest chain. The longest carbon chain is chosen as the basis for the name.

Rule3. Number the longest chain. The carbon atoms in the longest chain are numbered. The numbering is started from that end which will give numbers having the lowest value to carbons carrying substituents.

Rule4. Identify the substituent. Name the substituent. Indicate its position by the number of the carbon atom to which it is attached.

Rule5. Prefix the position number and name of the substituent onto the parent name. The whole name is written as one-word. Notice that the number and name of the substituent are separated by a hyphen.

Rule6  Identify the substituents by names and position numbers. When the same substituent present two or more times in the molecule, prefixes di, tri-, tetra-, penta-, etc. are used. Position of each substituent is indicated by a separate number. These position number separated by commas, are put just before the name of the substituent, with the hyphen before and after the numbers when necessary.

        

Rule7. When two or more different substituents are present, their names are arranged in alphabet order and added to the name of the parent alkane, again as one word

B. Alkanes: Branched-Chains

            The straight simple-chain alkanes have simple names and can be easily memorized, but the branched-chain alkanes require a set of simple rules derived by the IUPAC.

Basic Rules:

1. Identify the parent chain, longest carbon chain in the molecule.

             If there is more than one carbon chain of equal length, then identify the chain that is more substituted.

2. Number the carbon atoms in the longest carbon chain, from the end that gives the substituents as low a number as possible.

3. Name all the substituents, groups that are attached to the parent chain.

            The substituents are named by their respective prefix and end with -yl. Example: CH3- methyl, CH3CH2-ethyl

            There are some common "special" names for branched alkane substituents which should be memorized. A few of them are:

4. Assign numbers to substituents to identify where the attachment of the substituent is to the parent chain.

            You must still assign a number to each substituent along with the prefixes di-, tri-tetra- etc., even if there are the same substituents in the molecule.

Example: 1,2-dimethyl, 1-methyl

5. Add the prefixes, suffixes together, remembering to alphabetize substituents in the complete name.

i. The prefixes cyclo-, iso-, and neo- are considered part of the group name so they are alphabetized.

ii. Ignore the prefixes di-, tri-¸tetra-, tert-, sec-, etc., when alphabetizing.

iii. Use commas between numbers and dashes between numbers and words. Example: 1, 3-dimethyl

iv. If you are required to describe the isomer, you may also need identify whether the molecule is R or S, cis or trans, E/Z, etc.