IUPAC Nomenclature for Substituted Alkanes

IUPAC Nomenclature for Substituted Alkanes

A.    Alkyl Halides

            Alkyl halides are compounds which contain carbon-halogen bonds. They are classified as primary, secondary, or tertiary, depending upon whether the halogen atom is bonded to a primary, secondary or tertiary carbon atom. The common names of alkyl halides are obtained by naming the alkyl group attached to the halogen and adding the name of the halide as a separate word. Halo alkanes follow the same rules regarding numbering and alphabetizing as simple-chain alkanes.

CH3-CI                       CH3CH2-Br

Methyl chloride          Ethyl bromide

IUPAC Rules for Naming Alkyl Halides

The IUPAC names of alkyl halides are obtained by using the following rules:

Rule1. Select the longest chain to which the halogen is attached and give it the name of the corresponding alkane. .

Rule2. Prefix the name of the alkane by chloro, bromo, iodo or fluoro.

Rule3. Number the chain so as to give the carbon carrying the halogen atom the lowest possible number.

Rule4. Other substituents are numbered, named and placed as prefixes in alphabetic order. If there are two or more identical halogen substituents, the prefixes di-, tri-, tetra-, etc. are used. For example.

B. Alcohols

            Alcohol containing molecules replace the -e at the end of an alkane name with    -ol. The chain is numbered so that the hydroxyl group (-OH) obtains the lowest number even if the molecule contains multiple bonds. If the two hydroxyl groups are on the same carbon are known as geminal and on adjacent carbons it is referred to as vicinal.

Example:       CH3CH2- OH - Ethanol

IUPAC Rules for Naming Alcohols

In the IUPAC system, alcohols are named as Alkanols. Following rules are used:

Rule 1. Select the longest chain to which the hydroxyl group is attached.

Rule 2. Name the longest chain. The name is obtained by dropping the final-e from the name of the alkane that contains the same number of carbon atoms, and adding the ending -ol.

Alkane -e + ol= Alkanol

Rule 3. Number the chain to give the lowest number to carbon attached to the hydroxyl group.

Rule 4. Indicate the position of the hydroxyl group by the number of the carbon attached to the hydroxyl group.

Rule 5. Other substituents are numbered, named, and placed as prefixes in alphabetic order. For example,

Rule 6. Alcohols containing two or three -OH groups are named as Alkane diols and Alkane triols respectively. Notice that -e of the corresponding alkane name is retained.

C. Ethers

            Ethers are compounds in which an oxygen atom is bonded to two organic groups (R-O-R). In the common system, the two alkyl groups attached to the oxygen atom are named in alphabetic order and the word ether is added. If the groups are same, the prefix di- is used.

CH₃OCH₃ Dimethyl ether

CH₃CH₂OCH₃ Ethyl methyl ether

In the IUPAC system, ethers are named as Alkoxy alkanes. The smaller alkyl group plus the

oxygen atom is called an alkoxy substituent.

D. Aldehydes

            Aldehydes are compounds in which the carbonyl group (C=O) is bonded to an organic group and hydrogen (or two hydrogens). Their common names are related to those of carboxylic acids.

IUPAC Rules for Naming Aldehydes

In the IUPAC system, aldehydes are named as Alkanals. Following rules are used:

Rule 1. Select the longest chain containing the aldehyde group.

Rule 2. Name the longest chain. The name is obtained by dropping the final-e from the name of corresponding alkane, and adding the ending -al.

Alkane - e + al = Alkanal

Rule 3. Number the chain by assigning the number 1 to the aldehyde carbon. The number 1 is used to indicate the position of the carbonyl carbon, since it is always located at the end of the chain.

Rule 4. Other substituents are numbered, named, and placed as prefixes in alphabetic order. For example,

When there are two aldehyde groups in a molecule, it is named as Alkanedial, Notice that of the corresponding alkane name is retained.

E. Ketones

            Ketones are compounds in which the carbonyl group (C=O) is bonded to two organic groups. Their common names are obtained by naming the alkyl groups attached to the carbonyl group and adding the word ketone.

IUPAC Rules for Naming Ketones

In the IUPAC system, ketones are named as Alkanones. Following rules are used:

Rule 1. Select the longest chain containing the carbonyl carbon atom.

Rule 2. Name the longest chain. The name is obtained by dropping the final- from the name of corresponding alkane, and adding the ending -one.

Rule3. When there are two carbonyl groups in a molecule, it is named as Alkanedione. Notice that –e of the corresponding alkane name is retained.

F. Carboxylic acids

Carboxylic acids are compounds which contain the carboxyl group (-COOH). Their common names end in -ic acid.

IUPACRules for Naming Carboxylic Acids

            In the IUPAC system, carboxylic acids are named as Alkanoic acids. Following rules are used:

Rule1. Select the longest chain containing the carboxyl group.

Rule2. Name the longest chain. The name is obtained by dropping the final-e in the name of the corresponding alkane, and adding the ending -oic acid.

Alkane - e + oic acid = Alkanoic acid

Rule3. Number the chain starting with carboxyl carbon as number 1. The number 1 is not used to indicate the position of the carboxyl carbon, since it is always located at the end of the chain.

Rule4. Other substituents are numbered, named, and placed as prefixes in alphabetic order. For example,

Rule 5. When there are two carboxyl groups in a molecule, it is named as Alkanedioic acid. Notice that -e of the corresponding alkane name is retained.

G. Carboxylic acid derivatives

            Carboxylic acid derivatives are compounds in which the hydroxyl part of the carboxyl group is replaced by various other groups.

(a) AcidChlorides. Acid chlorides are derived from acids by replacing OH group by Cl atom

They are named by changing the ending -ic acid of the corresponding carboxylic acid to -yl chloride.

(b) Esters. Esters are derived from acids by replacing the OH group by OR group. Their name consist of two words. The first word is the name of the alkyl group attached to the oxygen atom. The second word is derived from the carboxylic acid name with -ic acid changed to –ate

(c) Anhydrides. Anhydrides are derived from acids by replacing OH by OCOR. They are named by changing the suffix acid of the parent acid to anhydride.

(d)Amides. Amides are derived from acids by replacing OH by NH₂. They are named by replacing the ending -oic acid (of the IUPAC name) or -ic acid (of the common name) of the corresponding carboxylic acid with the ending -amide.

H. Amines

            Amines are derivatives of ammonia (NH₃) in which one or more hydrogen atoms have been replaced by alkyl or aryl groups. They are classified as primary, secondary or tertiary depending on the number of groups attached to the nitrogen atom. Abbreviations for these terms are 1°,2° and 3°respectively.

R-NH2  Primary amines have one alkyl group attached to the nitrogen atom.

R₂NH  Secondary amines have two alkyl groups attached to the nitrogen atom.

R₃N  Tertiary amines have three alkyl groups attached to the nitrogen atom.

            The R groups in secondary and tertiary amines may be same or different. In the common system, amines are named by adding the suffix amine to the name of the alkyl group or groups. The entire name is written as one word. The names of the alkyl groups are arranged in alphabetic order.

            In the IUPAC system, primary amines are named by replacing the final -e of the parent alkane-amine. If necessary, a number is added to indicate the position of -NH₂ group.

            Secondary or tertiary amines are named as N-substituted derivatives of primary amines. The largest of the groups attached to nitrogen is chosen as the organic group of the primary amine. The remaining alkyl groups are named as substituents by using the prefix N- to indicate that the group attached to nitrogen.

NOMENCLATUREOF POLYFUNCTIONAL COMPOUNDS

            The functional group present in a molecule determines its class. If it contains an --OH group, the molecule is an alcohol. If it contains a -COOH group, it is carboxylic acid. Now let us consider a structure which contains both --OH and -COOH functional groups. It can be named as an alcohol or a carboxylic acid. The IUPAC system gives higher priority to -COOH over -OR, and the structure is named as a carboxylic acid. The --OH group is considered as a substituent.

            When a compound contains two or more different types of functional groups (Poly functional compound), the functional group which specifies its class is called the Principal functional Group. The other functional groups are referred to as Substituents. For example, in the following structure:

The -COOH group is the principal functional group, while --OH group is a substituent. The IUPAC name is 3-Hydroxybutanoicacid.

Selection of the Principal Functional Group

            The IUPAC system has laid down the priority of functional groups for determining the class of a poly functional compound. Table 8.4 gives a list of functional groups in decreasing order of priority for citation as the principal functional group. That is, the functional group which occurs higher up in the priority table is the principal functional group and specifies the class. Therefore, by having a look at the priority table, you can at once know the class of poly functional structure.

IUPAC Rules for Naming Poly functional compounds .

The IUPAC names of poly functional compounds are obtained by using the following rules:

Rule 1. Identify the principal functional group. This gives the class name of the structure. For

example in the following structure:

The two functional groups in the above structure are -COOR and -OR. Since –COOH ranks higher in the priority table, the structure is named as a carboxylic acid.

Rule 2. Number the longest chain containing the principal functional group from the end closer to it.

Rule3. Write the parent name corresponding to the number of carbons in the longest chain.

Longest chain = 5 carbons

Parent name = pentanoic acid

Rule4. Arrange the substituent names with position numbers in alphabetic order.

4-Hydroxy-2-methyl

Rule5.  Prefix substituent names with the parent name.

4-Hydroxy-2-methylpentanoicacid

Rule6. The following functional groups are always named as substituents. Their names are prefixed with the parent name.

            -Cl   Chloro     -Br   Bromo    -I   Iodo           -F   Fluoro       -R   Alkyl

            -OR  Alkoxy   -NH2  Amine  -NO2  Nitro     -CN  Cyano     -NO  Nitroso

Rule7. Carbon-Carbon double or triple bonds are usually indicated by integrating-en- or -yn-into the suffix, as in -enoic acid and -ynol.

Rule8.Compounds containing a double bond (C=C) and a triple bond (C=C) in the main chain are named as alkenynes. The position number of the double bond is inserted before –alken and that of triple bond before -yne.