Naming alkanes based on multiple different side groups?

Presumably the lack of a 3 in the suggested name would be because if the ethyl group was anywhere else, the longest continuous carbon chain would have more than 5 carbons and so the name wouldn’t be {blah blah}pentane.
So let’s suppose we are to now ditch the 3 as surely it’s pretty unambiguous where the ethyl group is, right? What do we call it now?
Let’s try ethyl-2-methylpentane first.
This is problematic because this correctly suggests the ethyl group is a substituent, but substituents must always have a number assigned before, which hasn’t happened and so this name is wrong.
Now how about 2-methylethylpentane?
This name is rather ambiguous. Does it mean the ethyl group is also on carbon 2, or is that just the methyl group?
So the least ambiguous name would be 3-ethyl-2-methylpentane, so I’d stick with it.

Yeah, they had stated that it’s ethyl-2-methyl-pentane, but as you stated and as I thought, this doesn’t specify the location of ethyl.

I shall take a photo of the text they followed with, trying to explain why it doesn’t have a number. I’ll upload it after breakfast.


See their explanation below:

There shouldn’t be a space or hyphen between the methyl and pentane - it becomes one word as per the naming convention.
The explanation itself isn’t wrong that the only place you can put the ethyl group on is carbon 3 and its rationale for why this is the case isn’t wrong - but ethyl-2-methylpentane isn’t accepted by IUPAC (you can look up the pubchem page for 3-ethyl-2-methylpentane and you’ll see under the synonyms that ethyl-2-methylpentane is not listed and so it isn’t accepted).

Mmm, I’m not sure I understand why it can only go on the third carbon though. Why couldn’t it go on, say the first or fourth from left carbon?
Does it have something do with the number of carbons within ethane? so, as ethane is C₂H₆… there must be at least the same number of carbons either side on the chain which the placement of the side group is?
So, if I put ethane on the second carbon, it would mean another carbon would have to be to the left of it meaning that the main group chain would then have 6? So, it would still be 3-ethyl and ignoring methyl it would be 3-ethylhexane ??

I have underlined the ethyl group of interest in all structural formulae below for ease of understanding.
Suppose we put the ethyl group on the first carbon atom in the chain of 5
e.g CH2(CH2CH3)CH(CH3)CH2CH2CH3
Or alternatively CH3CH2CH2CH(CH3)CH2CH2CH3
The ethyl group essentially extends the parent chain of the molecule. It would be 7 carbons long and so would be 4-methylheptane. Of course, this isn’t the case as the parent chain must be 5 carbon atoms long for “pentane” to form part of the name.
Now suppose we put it on carbon 2
e.g (CH3)2C(CH2CH3)CH2CH2CH3
Or alternatively CH3CH2C(CH3)2CH2CH2CH3
Much like in the first example, the ethyl group would now become the starting point of the parent chain as it would be the start of the longest possible string of carbon atoms.
We can now see that the longest continuous chain of carbon atoms is 6 long and that we would have two methyl groups on carbon 3, so it would be 3,3-dimethylhexane. A similar problem to the first example.
Finally, let us consider the case where the methyl group is on carbon 3
e.g CH3CH(CH3)CH(CH2CH3)CH2CH3
Or alternatively CH3CH(CH3)CH(CH2CH3)2
Regardless of which ethyl group you consider a part of the parent chain, it’s still 5 carbons long. This is the only case where it is a pentane (carbons 4 and 5 have similar problems to carbons 1 and 2).

I am still extremely confused.

Let me see if I can make sense of it. By extension of the main chain, are the carbons within the side group extending the chain?

So, below ethyl is on carbon-2. The longest chain is now 6 carbons, making this hexane and methyl is on the 3rd atom, so this evolves to 3-Methylhexane?..




This can be mistaken for 2-ethylpentane by forgetting that fact that it has extended the main chain?

This is correct if you are taking ordinary pentane and plonking an ethyl group onto carbon 2.
However, in the example I have gone over, I’ve taken 3-ethyl-2-methylpentane and moved the ethyl group from carbon 3 to carbon 2, which makes what would be in theory “2-ethyl-2-methylpentane”
“2-ethyl-2-methylpentane” will have two methyl groups on carbon 2 as well as the ethyl group.

This is new to me, but based on what I have read.
See below: is this described as 3-ethyl-2-methyl pentane?
The course suggests that ethyl doesn't need a number assigned to it, but other sources provide conflicting information.

That ethyl group could be at either 3 or 4 so I think ur right and the explanation is wrong.

It can only be at 3.
If you put it at 4, it becomes part of the longest continuous chain and it’s no longer a “pentane”.
But yes, the name should still specify which carbon the ethyl group is on.

Ah yes, it becomes hexane at that point. Silly me.

So, is this what you mean?

I’m not sure if I named it correctly. I think 3,3 states that there are two methyl’s on carbon 3. I was thinking 2,3 but that tell me that there’s a methyl on carbon 2 and carbon 3.

Happens to the best of us. It’s a surprisingly common mistake to make even after getting plenty of practice at naming organic compounds at A level.