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What's the difference between these two compounds? Well they have same number of carbons, same number of bonds, and both all single bonds. However its pretty clear that they are different due to the position of the methyl group. On the first molecule its on the 2 carbon, on the next molecule its at the 3 spot. These two molecules are what's called isomers. Isomers have the same chemical compounds, yet placed in different arrangements. These arrangements have a great impact on several of the compounds properties, such as reactivity.
In Germany in 1957, researchers created a compound known as Thalidomide. The drug was very effective in helping to alleviate morning sickness in pregnant women. In clinical trials, it worked. However, once the babies of some of these women were born, they had debilitating defects, many of which were fatal. From further testing, researchers found that indeed, in the medicine, there was a drug that alleviated morning sickness with little to no side effects. However, in the process of synthesizing this drug, an isomer formed that reacted incredibly badly with the human body. This is the danger of isomers, seemingly identical compounds with very different results.
The first type of isomer is called a conformational isomer. Unlike double or triple bonds, molecules in single bonds are free to rotate around the axis where the bond is attached. This leads to the compound having a different shape.
Next are structural isomers. The example from the beginning of the lesson, C6H14, is a structural isomer. The same group in a different spot is one way to get this type of isomer. Another way is by having a double or triple bond in a different location. When identifying structural isomers be careful to note that molecules that are merely mirrored may not be isomers, since you number carbons starting from the side closest to the point of interest. For example, the image on the left (or top) is a isomer pair, the right (or bottom) is not.
Similarly, the left (or top) image displays isomers, the right (or bottom) does not.
Additionally, there are functional group isomers. Molecules with functional isomerism will have the same molecular formula, but differing arrangements. These aren't quite in their own category, but its important to know functional group pairs that can form isomers. They are as follows: alcohols and ether, aldehydes and ketones, carboxylic acids and esters. Use this link to investigate models of these pairs.
Lastly are stereoisomers. Stereoisomers leave behind 2D drawings and refer to molecules in 3D space. This type of isomers has all the same connectivity, meaning that the atoms in each compound are attached the same way at each corresponding spot. Stereoisomers can be broken down further, which will be covered in the next lesson.
To keep track of all these terms, study this flowchart. This flowchart will be expanded on next lesson. Once you've done that, use this link to take a quiz.
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