I'm wondering why the above image is named 5-ethyl-3,3-dimethylheptane rather than 3-ethyl-5,5-dimethylheptane. I thought an ethyl substituent has a higher priority (I know this is true when arranging the final name, but what about numbering the parent chain)?
The process for naming this molecule correctly follows the steps listed here:
- Identify the parent hydrocarbon, which will be the one with the longest chain. It is correctly identified as the 7-carbon chain, and as such is a derivative of heptane.
Identify the side-chain functional groups. We have 2 methyl groups and an ethyl group. When listing them, they will be written alphabetically. So, we have:
X-ethyl-Y,Y-dimethylheptane and not Y,Y-dimethyl-X-ethylheptane
Numbering (which is what you are asking about) is determined by numbering the carbon atoms consecutively from the end of the chain nearer the functional group of the highest priority. In terms of molecule numbering, alkane groups have the same functional group priority. We would only need to consider relative priorities of the ethyl and methyl groups if there was only one methyl substituent, in which case you would have 3-ethyl-5-methylheptane.
In our case, the process is to count in from the ends of the primary chain. Here, the first substituents occur at an equal distance from each end of the chain, both the ethyl and the methyl occur at position 3. We now need to invoke one of the IUPAC sub-rules for nomenclature. If the first substituents occur at an equal distance from each end of the chain, we need to number from the end nearer a second substituent. The second substituent is the extra methyl group, and so this is the end that gains numbering priority.
There are many nice examples of nomenclature on the internet. One very good example, which highlights your question is this page. Have a look at Rule 2 and sub-rule 2
That molecule would be 5-Ethyl-3,3-dimethylheptane based on the first point of difference rule. Starting from the left side of the parent chain, you will have a substituent at the 3,3,5 positions while on the right, you'll have your substituent on the 3,5,5 positions. 3,3,5 is lower than 3,5,5 so it should be the priority in naming the compound.
It is also important to take note that the two substituent (ethyl and methyl) have the same priority when considering functional groups. When alkanes are in equivalent positions, the first point rule overtakes. However, if a double bond is present in the molecule, the double bond gets the priority. This will hold unless the double bond is present in the parent chain and not as a substituent.