Q40E - Organic Chemistry (Mcmurry)

Question

Explain why each of the following names is incorrect:

1. 2,2-Dimethyl-6-ethylheptane
2. 4-ethyl-5,5-dimethylpentane
3. 3-ethyl-4,4-dimethylhexane
4. 5,5,6-Trimethyloctane
5. 2-Isopropyl-4-methylheptane

Step-by-Step Solution

Answer

1. 2-Ethyl-6,6-Dimethyl heptane 
2. 3-Ethyl-2-Methylhexane 
3. 3-Ethyl-4,4-Dimethylhexane
4. 3,4,4-Trimethyloctane
5. 2,3,5-Trimethyloctane

Step 1: IUPAC (International Union for Pure and applied Chemistry)

IUPAC system is a method of naming various chemical compounds systematically. It is constituted by 4 parts,

IUPAC name=  $\mathbf{Locant}\mathbf{+}\mathbf{Prefix}\mathbf{+}\mathbf{Parent}\mathbf{+}\mathbf{Suffix}$

Locant tells us the position of the substituent, prefix tell us what the substituent is, parent suggest the length of the carbon chain (2,3,4 and so on) and the primary functional group is identified by the suffix part.

Step 2: Drawing structures from names

a.

2,2-Dimethyl-6-ethylheptane

First, as give heptanes, make a 7 carbon chain and number them.

Now, place 2 methyl substituent over C-2 and an ethyl substituent over C-6. The structure becomes-

2,2-Dimethyl-6-ethylheptane

This name for the structure is incorrect, as we know from IUPAC nomenclature rules, that priority of substituent is decided alphabetically, so ethyl should have come before methyl in the name and also it should have been on carbon atom number 2 (C-2).  The correct name is 2-Ethyl-6,6-Dimethyl heptane.

b.

4-ethyl-5,5-dimethylpentane

First draw a five carbon chain and number them.

Place an ethyl group on C-4 and 2 methyl substituents on C-5.

Now the structure obtained is-

                                                   4-ethyl-5,5-dimethylpentane

But, this name for the structure is wrong, as in this case the longest parent carbon chain that should have been selected, is a 6 carbon chain i.e., hexane and not pentane and that too which is having more branching. The correct name is 3-Ethyl-2-Methylhexane.

c.

3-ethyl-4,4-dimethylhexane

For the given name, the structure, obtained following the steps given above comes out to be

3-ethyl-4,4-dimethylhexane

The name is incorrect for the structure, as when we put substituents, we get two chains of 6 carbon atoms, but we need to select the one with maximum number of branching as per the IUPAC rules.The correct name for this should be 3-Ethyl-4,4-Dimethylhexane.

d.

5,5,6-Trimethyloctane

The structure obtained for this name is,

5,5,6-Trimethyloctane

The name given for this structure is incorrect, because according to IUPAC nomenclature, the numbering should be started from the end to which substituents are near. Therefore, the correct name should be 3,4,4-Trimethyloctane.

e.

2-Isopropyl-4-methylheptane

The structure obtained for the name is-           

2-Isopropyl-4-methylheptane

But, the name given is incorrect, because after putting substituents  as per the name given, the longest chain selected should be 8 carbon chain and not 7 carbon chain. The correct name for the structure is 2,3,5-Trimethyloctane.