April 16th - Alkenes and Alkynes

As we know by now (or should know), carbon can form double or triple bonds with other carbon atoms. When there are only single bonds present, it is considered a saturated compound, because all possible bonds are bonded to other atoms. On the other hand, double or triple bonds, present in alkynes and alkenes, are unsaturated. When there are multiple bonds, fewer hydrogens are attached to the carbon atom.

The rules for naming alkenes and alkynes are almost identical. However, remember this: The position of the double/triple bonds always has the lowest number and is put in front of the parent chain. For example, in the photo below:

This molecule consists of four carbons. The double bond occurs at 2. Therefore, this is 2-butene. 

Let's review some of the rules, just in case:

1. Find the longest, appropriate parent chain
2. Number the base chain so that side chains have the lowest total value
3. Name each side chain
4. Add some numbers
5. List the side chain alphabetically (if you encounter the same letter, then the one with the lowest number takes precedence)
6. If you have two or more double/triple bonds present, you will need to add a multiplier right before the '–ene'/'-yne' ending.

That is it! Let's try one:

Name this: 

Let's count the parent chain: It consists of four carbons.

Lowest numbering of alkenes: 1,3

Therefore, this is: 1, 3 butadiene

There are also these fancy structures that go by the name of –cis and –trans. This applies only to alkene groups, specially butene (but not restricted to it).

If the adjacent carbons are on the same side, then its cis ___. If it’s on the opposite sides, it is trans ____. There is also a possibility that it is neither cis nor trans. How does that look? Well, try it out yourself.

Posted by Andrew.