name of alkyl group

[joonkimdds]

Z-CH2CHCH3
.........|
........CH3
is called isobutane, and I don't understand why.
the Z is connected to the primary carbon so shouldn't it be called n-butyl?
I don't know what mistake I am making here and how to fix my mistake.

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[lk2230]

It's difficult to tell from the way you have represented it in the last diagram. I take it that the straight line, as indicated from the first two diagrams, is meant to indicate the R group. However, in the last diagram, you seem to have placed the R group in between the second carbon and a CH3 group, which would make the CH3 group not part of that chain. I'm sure this isn't what you meant to do, and the CH3 is meant to be part of the chain. Could you find a way to make it more clear where the R group is actually attaching?

Keep in mind that the iso- term, if I'm not mistaken, generally is used for groups that arranged in a symetrical fashion. So Isopropyl (3 carbons) would be (with the dots used for the same purpose as yours, space-holders for SDN)
....CH3CHCH3
.........*

with the asterik meant to represent to bond between the second carbon and the R group. So this is not sec-propyl, but rather isopropyl because the alkyl group is symetrical. A similar scheme works to make isobutyl:

CH3CHCH3
.....CH2
.....*

wherein the 4 C's form a sort of "T", and the tip of the T is bonded (*) to the R group. Again, the alkyl group is now symmetrical, and thus the term isobutane.

Be careful, because if the alkyl is constructed like this:

.....*
CH3CCH3
.....CH3

with the R group bonded to the second C in the top part of the "T" it is called tert-butyl. It is called tert-butyl because though it has a "T" shape like isobutyl, in this case the R group is bonded to a tertiary carbon.

That should help you out, though I would DOUBLECHECK this. This is off the top of my head and I'm kind of tired. If I have made a mistake, it was in describing one as tert and the other as iso; if I'm wrong it is simply the other way around and you can still compare it to the isopropyl.

 

[joonkimdds]

my third drawing
-CH2CHCH3
.......|
.......CH3

is basically
*CH2CHCH3
.......CH3
in your way of drawing. | was between C and CH3 so they r bonded.
- at the left has nothing so basically it's connected to R.

 

[joonkimdds]

so far, i understand that
Neo = c surrounded by 4carbons around it.
ex>
......CH3
.......|
CH3-C-CH2-Z
.......|
......CH3



n- = Z or R at the end of linear carbon chain
ex>
CH3-CH2-CH2-CH2-Z


Sec, Tert = if Z or R is at 2ndary or tertiary Carbon
ex>
CH3-CH2-CH-CH3
.............|
.............Z

ex>
.......CH3
.......|
CH3-C-CH3
.......|
.......Z



iso = ???
ex>
.......CH3
.......|
CH3-CH-CH2-Z
this is isobutyl but I don't know why this isn't n-butyl

CH3-CH-CH3
.......|
.......Z
this is isopropyl but i don't know why this isn't secondary propyl


to.lk2230 // I tried to use ur 'symmetric' method but it doesn't seem to work because if you look at the
Neo and tertiary example i drew above, it has symmetrical Carbons but it's called tertiary instead of iso.

 

[lk2230]

The reason that the functional group is called isobutyl is because it is BRANCHED.

You are correct that n-butyl and isobutyl are similar in that the R group attaches at a primary carbon. However, the key difference that causes the name difference is that n- is used for when the alkyl group is a straight chain, and iso- is used when it is branched.

In the case of isopropyl, the term is used instead of sec-propyl because, in a sense, there is no sec-group. Essentially, one uses the term sec when the chain is long enough that the carbon sec- describes is only sec when in a longer chain. In isopropyl, it is a bit difficult to define it as sec since it is the only non-primary carbon, and the alkyl group is symmetrical. So the sec- term is not used, and the iso-term is.

You are correct that tert-butyl can also be viewed as symmetrical, but the c that the R group attaches to is clearly tertiary in this case.