Now we must learn additional names for our kill groups with three and four carbons. These molecules have structural or constitutional isomers. Remember, these are isomers that have the same molecular formula but different connectivities. If we take a look here, if we're dealing with a 3 carbon alkyl group, so here we have 1, 2, 3 carbons. This would be a propyl. But who's to say that the connection is at this end carbon? We have a chance that that connection could be on the middle carbon instead.
So here we rearranged our bond and the connection now is on the middle carbon. When this occurs, this group, this alkyl group, now becomes ISO propyl. OK, so when it's on the end, it's propyl, but if it's on the middle carbon, it becomes isolated. For four carbons it comes even more complicated. So if we're looking at the first image, we have 4 carbons 1, 2, 3, 4. We know the prefix is, but here if it's at one of the end carbons, it's butyl. But who's to say that it has to be at that end carbon? And who's to say that the chain has to be a straight chain?
If we move to the left, let's say instead of being at this end carbon or this end carbon, it happens to be on one of the middle ones. In this case, we see it here on this middle carbon. Now this is called SEC butyl SEC because it represents a secondary carbon. This is just a carbon attached to two other carbons. This secondary carbon in blue is connected to 1, 2 other carbons within this alkyl group. That's why it's a secondary carbon. Now, who's to say that it's a straight chain? We could rearrange the four carbons in a different order.
Here we still have 4 carbons here, but it's just rearranged a little bit different. Instead of being a straight chain, we actually have a branching group here. This one here. If it looks like this, then it is called ISO butyl, so in this fashion it's isobutyl. And then finally, in this last one, maybe the four carbons orient themselves in this fashion where this middle carbon here is connected to three of the carbons and that middle carbon is what's attached to something else. In this case, we call this tert butyl.
Why is it called tert butyl? Because that carbon in red is a tertiary carbon. Tertiary carbon means the carbon is attached to three other carbons. If we look, that carbon in red is connected to 1, 2, 3 other carbons. That's why it's a tertiary carbon. So again, for three carbon and four carbon alkyl groups, it can get a little bit tricky because we have different structural slash constitutional isomers that are possible. If you have propyl, you could also have isopropyl, butyl. Even more so there's butyl, sec butyl, isobutyl and tert butyl.
So keep in mind the names that we see here and the way the connections kind of work out when they exist as alkyl groups. This is key to naming them correctly and key to when we start naming more organic compounds.