Chapter 16, Problem 16.41UTC

Interpretation Introduction

Interpretation: “The condensed structural formula and IUPAC name of two structural isomers of carboxylic acid having molecular formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$ should be drawn and explained

Concept introduction:

• Isomers: Compounds which have the same molecular formula, but different physical or chemical properties are known as isomers and the phenomenon is known as isomerism.
• Structural isomerism: The isomers with same molecular formula but different structural formula or different arrangement of atoms in a molecule are known as structural isomers.
• Condensed structural formula: The complete structural formula can be abbreviated by omitting some or all the covalent bonds and by indicating the number of identical groups attached to an atom by a subscript. This is known as condensed structural formula.
• IUPAC: The term IUPAC is International union of pure and applied chemistry. It provides the set of rules for assigning names to organic compounds.
• Rules for determining the IUPAC names
• The longest continuous chain of carbon atom is taken as the parent chain on which the various alkyl groups are considered side chain.
• Numbering is done from that side where functional group is near.
• If more than one functional group is present, then preference is given to that functional group which is senior in the seniority table group

  $\begin{array}{l}-COOH>-COOR>-S{O}_{3}H>-COX>-CON{H}_2>-CHO>-CN>-CO>-OH>-N{H}_2\\ >-O->-N{O}_2>-X>-R\end{array}$

If a side chain occurs more than once, the numerical prefixes di(for two),tri(for three) etc are added before the names of the group.

When there are two identical side chains at one position, numbers are given for each.

To draw and explain: The condensed structural formula, IUPAC name of the isomers of carboxylic acid having molecular formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$

Solutions:

The condensed structural formula and IUPAC name of the structural isomers of carboxylic acid having molecular formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$ are as follows:

  ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{COOH:-}\text{Butanoic}\text{acid}$

  ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{CHCOOH:-}\text{2-methylpropanoic}\text{acid}$

Explanation of Solution

The carboxylic acid having molecular formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$ has two structural isomers having same molecular formula but different structural formula

Let us first draw the two structural isomers of carboxylic acid having molecular formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$ .It has a functional group $\left(-\text{COOH}\right)$ .

a) 

b) 

Now for (a), if we count the $\text{carbon}\text{atom}$ of the longest chain containing the functional group, it has $\text{4}\text{carbon}\text{atoms}$ . So, the word root is ‘but’. Now for carboxylic acid, we use the prefix is ‘oic acid’ therefore, the name is $\text{butanoic}\text{acid}$ . Now according to the definition of condensed structural formula, we will omit the covalent bond attached to the $\text{carbon}\text{atom}$ and we will indicate the number of identical group attached to the atom by a subscript .Therefore the condensed structural formula is ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{COOH}$ .

Again for (b), if we count the $\text{carbon}\text{atom}$ of the longest chain containing the functional group, it has $\text{3}\text{carbon}\text{atoms}$ .So the word root is ‘prop’. Now for carboxylic acid, we use the prefix ‘oic acid’. Again it has a $\text{methyl}$ group attached to the $2^{nd}carbonatom$ .Therefore, the name is $\text{2-methyl}\text{propanoic}\text{acid}$ .Now according to the definition of condensed structural formula, we will omit the covalent bond attached to the $\text{carbon}\text{atom}$ and we will indicate the number of identical group attached to the atom by a subscript. Therefore, the condensed structural formula is ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{CHCOOH}$ .

Conclusion

Therefore, the condensed structural formula, IUPAC name of the structural isomers of carboxylic acid having formula ${\text{C}}_{\text{4}}{\text{H}}_{\text{8}}{\text{O}}_{\text{2}}$ are

a)  ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{COOH:-}\text{Butanoic}\text{acid}$

b)  ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{CHCOOH:-}\text{2-methyl}\text{propanoic}\text{acid}$