[kemilss]

Hey everyone, first, thanks for helping out. I'm an adult taking grade 12 chemistry as self study, and I'm taking a diploma exam in January.

I'm able to find most answers in my book after looking long enough, but I'm pretty thankful for sources on the internet, and sites like this.

Any way, I understand the naming of hydrocarbons quite well, but there is one situation that I'm not sure about.  That is when, if all else is equal and I have 2 branches coming off a parent chain where I could start numbering left-right, or right-left, but one way would have for example an ethyl branch, while the other might have a methyl branch, which comes first?  Is it alphabetical, so Ethyl is first, or is Methyl first because it is smaller?

Something such as [ CH3-CH2-CH2-CH(-CH2CH3)-CH(-CH3)-CH2-CH2-CH3 ]

So would this be 4-ethyl-5-methyloctane or 5-ethyl-4-methyloctane ?

[Archer]

With branched chain alkanes the numbering is based on the first substituent has the lowest number and the substituents are listed in alphabetical order.

Something such as [ CH3-CH2-CH2-CH(-CH2CH3)-CH(-CH3)-CH2-CH2-CH3 ]

So would this be 4-ethyl-5-methyloctane or 5-ethyl-4-methyloctane ?

So in this case it would be 4-ethyl-5-methyloctane.

This is an extremely useful reference book which I use regularly, you may find it helpful.

 http://www.rsc.org/shop/books/2011/9781849730075.asp

[Archer]

Just for clarity, if you extended the ethyl branch to a propyl chain,



then it would be 4-methyl-5-propyloctane

[kemilss]

Just for clarity, if you extended the ethyl branch to a propyl chain,



then it would be 4-methyl-5-propyloctane

Awesome, that really clears it up.  Thanks.  So basically, when the lowest number are equal ( in this case 4,5 ) I choose the lowest number according to alphabetical order.  So 4-ethyl-5-methyl,  or 4-methyl-5-propyl, or  4-ethyl-5-propyl (if that were the case).

[Archer]

So basically, when the lowest number are equal ( in this case 4,5 ) I choose the lowest number according to alphabetical order.  So 4-ethyl-5-methyl,  or 4-methyl-5-propyl, or  4-ethyl-5-propyl (if that were the case).

That is my understanding of the IUPAC rules, yes.

This is a fairly unusual case where the chain has to have an even number of carbons and the branches have to be on the middle two carbons. Otherwise the numbering is based on the substituent with the lowest number.