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Naming Carboxylic Acids

Today we know several compounds and their properties. On the basis of their origin, compounds can be classified as organic and inorganic compounds. Inorganic compounds do not have carbon but have other atoms such as hydrogen sulphide, metallic compounds etc.

Organic compounds have a certain number of carbon atoms which are arranged in a particular manner to form a definite molecular structure. The organic compounds which are composed of carbon and hydrogen are known as hydrocarbons. Hydrocarbons are one of the most common compounds which are involved in almost all the substances.

They have parent chain of carbon atoms which are bonded with hydrogen atoms to complete their tetravalency. The carbon atoms in the parent chain can be connected with each other through single or multiple covalent bonds. The hydrocarbons with all the single covalent bonds are called as alkanes. Whereas hydrocarbons with double covalent bonds between carbon atoms of parent chain are commonly called as Alkenes and hydrocarbons with triple covalent bonds are named as alkynes. Along with carbon and hydrogen, there may be some functional groups in the molecules which affect the physical and chemical properties of molecule. Functional groups are group of atoms or one atom which impart certain properties to the molecule such as –Cl, -OH, -CHO, -COOH etc. So we can imagine a huge number of organic compounds with different functional groups and carbon atoms. That is the reason; a solid nomenclature is needed to identify these compounds. IUPAC which is International Union of Pure and Applied Chemistry purposed some rules to name all the known compounds on the basis of their structural formula. IUPAC naming helps us to name and identification of any unknown compound.

 

IUPAC Rules for Naming Carboxylic Acids

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IUPAC suggested few rules for naming of organic compounds. For example; in the naming of alkanes, the parent chain of carbon atom reflects the base part of name. Suffix of the name indicates the type of functional groups such as for alkane it is –ane. Other groups of parent chain are called substituents and come as prefix to the name. Alkanes are saturated hydrocarbons with up to a 12 carbon chain. The substituents which are formed by the removal of one –H from the end of the chain reflects with suffix -ane to -yl. So for naming an organic compound, first identify the longest carbon chain (parent chain) then identify all of the substituents bonded on parent chain. Now number the parent chain in such a way that substituents get the lowest numbers.

If two or more side chains are in equivalent positions then chain which will come first in the name will assign the lowest number. The presence of more than one same substituent, the location of each point will come in the name with some prefix like di-, tri etc. Two or more different substituents in the molecule will listed in alphabetical order with the help of their base name. Let’s discuss the naming of carboxylic acids. The general formula of carboxylic acid is RCOOH. Here R- is alkyl group and –COOH is carboxylic functional group. In the IUPAC naming of carboxylic acid, the suffix –ane is replaced by –anoic acid. The name of parent chain will also add the carbon atom of –COOH group and root word will add with suffix. If there are two –COOH groups, the suffix is -anedioic acid. The carbon atom of –COOH group must be numbered as C-1.

Rules in Naming Carboxylic Acids

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The rules for the naming of carboxylic acid are same as for alkanes. The –COOH group (Carboxyl group) takes precedence over other substituent like alkyl groups, halogen substituent, double bonds etc. The presence of double bond in the molecule indicates with –en suffix and –oic acid; combine called as -enoic acid. The position of the double bond in the carboxylic acid is indicated before the parent name.

Examples of Naming Carboxylic Acids

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Let’s have a look on some examples of carboxylic acid. The longest chain gives the base name to the molecule. If there are only single bonds in the molecule, suffix –ane will come. The –COOH group indicates with -oic acid. For example; in HCOOH the base name is meth- as there is only one carbon atom in the molecule. The base name meth- is added with suffix –ane and –oic acid to get methanoic acid. Many of the carboxylic acids are better known with their common names such as HCOOH is called as formic acid as it is found in ant sting.  The carboxylic acid with 2 carbon atoms is CH3COOH. It is found in vinegar therefore also called as acetic acid. There are 2 carbon atoms in the molecule, so the base name would be eth- which must be added with suffix –ane and –oic acid. So name of acid would be ethanoic acid. Similarly name of some other saturated carboxylic acids are listed below.

 Formula 
 longest carbon chain 
  functional group 
 Preferred IUPAC Name  
(alternative name)

 $C_{2}H_{5}COOH$   $C_{3}$ prop
 -COOH (carboxyl) oic acid 
 Propanoic acid
(propionic acid)
 $C_{3}H_{7}COOH$  $C_{4}$ but  -COOH (carboxyl) oic acid
 Butanoic acid
(butyric acid)
 $C_{4}H_{9}COOH$ $C_{5}$ pent  -COOH (carboxyl) oic acid  Pentanoic acid
(valeric acid)
 $C_{5}H_{11}COOH$  $C_{6}$ hex  -COOH (carboxyl) oic acid  Hexanoic acid

Naming Carboxylic Acids and Esters

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The organic compounds with –COOH group are called as carboxylic acids. Here –COOH is functional group in the molecule. The general formula of alkanoic acids is CnH2n+1COOH or R-COOH. The IUPAC rules for nomenclature of carboxylic acid can be summarized as,
  • Find out the longest parent chain with the carbon atom of the COOH functional group.
  • Number the carbon atoms get the base name for the chain, 
  • Drop the "e" from the name of the alkane and add –oic acid as suffix
For example in the given molecule, first number the parent chain in such a way that –COOH group will get C-1 position. 

COOH Group

There are 4 carbon atoms so the name of alkane will be alkane. Now drop the "e" from the name of the alkane, butan- and add –oic acid as suffix. So name would be butanoic acid. The combination of an alcohol and a carboxylic acid form an ester. The general formula for ester is RCOOR in which both R-groups can be same or different. The IUPAC naming of ester is little different from carboxylic acid. It consists of two parts; first part of the name comes from the alcohol. It ends with the letters '-yl'. On the other hand, second part of the name comes from the RCOOH and ends with the letters '-oate'. For example; ethanol and ethanoic acid combine to give ethyl ethanoate. The formula for ethyl ethanoate would be CH3COOCH2CH3. Some other examples of esters are listed below;

 Name of ester 
 Structural formula 
 ethyl propanoate   $CH_{3}CH_{2}COOCH_{2}CH_{3}$ 
 butyl methanoate   $HCOOCH_{2}CH_{2}CH_{2}CH_{3}$ 
 pentyl ethanoate  $CH_{3}COOCH_{2}CH_{2}CH_{2}CH_{2}CH_{3}$

Naming Carboxylic Acids Practice Problems

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The naming of carboxylic acid can be done either with the help of IUPAC rules of by following the common names of molecules. The IUPAC names provide complete information about the structure of molecules. For example (a) and (b) molecules are better named according to IUPAC rules. On the contrary, (c) and (d) are commonly known with their common names like salicylic acid and oxalic acid.

   Structure   Name of carboxylic acid 
 a) 
Methylpentanoic Acid  3-methylpentanoic acid
 b) Benzoic Acid  Benzoic acid
 c) Salicylic Acid
 Salicylic acid  or ortho-hydroxybenzoic acid 
 d)Oxalic Acid
 Oxalic acid or ethanedioic acid
 e)Butenoic Acid
 3-butenoic acid
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