Substituents and cis/trans

I decided to break down the "complicated" substituents into a separate chapter. So here it goes:

Remembering what you learned from the last chapter, you should be able to name long strands of hydrocarbon molecules. The problem arises when you encounter a molecule such as this:

nalk6.gif (1438 bytes)

The longest parent chain is 7 carbons (heptane). But how the heck do you name the substituent? NO, it is NOT just "butyl"

4-butylheptane would look like this:

nalk7.gif (1613 bytes)

The little substituent group nalk8.gif (1068 bytes) causes a lot of problems for beginners. Unfortunately, there is no way to reason how to name this. There are a whole class of substituents which you must memorize. Here they are: (in no particular order)

nalk8.gif (1068 bytes)
nalk9.gif (1148 bytes)
nalk10.gif (1141 bytes)

The sec referes to the fact that the carbon that serves as the "bridge" to the main chain is a secondary carbon (before briding to the main chain). A secondary carbon is a carbon which has 2 carbons attached to it directly. Primary refers to only having one carbon directly attached, and tertiary refers to three carbons directly attached to the carbon of interest. See above for examples.

When writing two different substituents, there are two positions possible between them. Cis and trans. Cis is "same side" and trans is "different side." In branched alkanes, there is free rotation. The only alkanes which get the cis/trans prefixes are the cyclic rings since they don't have free rotation. Look at these molecules:

nalk17.gif (1152 bytes)  nalk18.gif (1171 bytes)

The black arrow designates that the methyl- group is sticking out (look at it 3-D). The dashes represent going into the monitor. The one on the left is trans, the one on the right is cis.

nalk16.gif (1017 bytes)

Free rotation: You can rotate that bromine group to become on the same side as chlorine. That is why you never use cis/trans with chained alkanes. They can all rotate. Although it's hard to describe, you'll have to take my word that single bonds like those can rotate. If you want to prove it to yourself, go borrow a kit where you can build atoms. You'll see.


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