Problem 32 - Chemistry: The Central Science

Question

Draw all structural and geometric isomers of butene and name them.

Step-by-Step Solution

Answer

The isomers of butene are: 1. 2-methylpropene ``` H H H | | | H--C-C=C-H | H ``` 2. cis-1-butene or (Z)-1-butene ``` H H | | H-C=C-H | | H H ``` 3. trans-1-butene or (E)-1-butene ``` H H | | H-C=C-H | | H H ``` 4. cis-2-butene or (Z)-2-butene ``` H H | | H-C-C=C-H | | H H ``` 5. trans-2-butene or (E)-2-butene ``` H H | | H-C-C=C-H | | H H ```

Step 1: Identify potential structural isomers of butene

First we need to identify all possible structural isomers of butene. Structural isomers are obtained by rearranging the carbon skeleton or changing the location of double bonds. There are two possible structural isomers for butene with different carbon skeletons. Isomer 1: The carbon skeleton has three continuous carbon atoms and a carbon atom branching off from the middle atom, forming a T-like structure. The double bond can only be between the last two carbons in the straight line (otherwise it would have more than four carbon atoms). Isomer 2: The carbon skeleton has a straight chain of four carbons. The double bond can be between the first and second carbons or between the second and third carbons. These two isomers are different in the position of the double bond and are called positional isomers.

Step 2: Draw the structural isomers of butene

Now we will draw the identified isomers. Isomer 1: ``` H H H | | | H--C-C=C-H | H ``` Isomer 2a (double bond between first and second carbons): ``` H H H H | | | | H-C=C-C-C-H ``` Isomer 2b (double bond between second and third carbons): ``` H H H H | | | | H-C-C=C-C-H ```

Step 3: Identify and draw geometric isomers of butene

Now we will identify and draw any possible geometric isomers. Geometric isomerism occurs when there is restricted rotation around the double bond; thus, geometric isomers are only possible for isomers 2a and 2b. Isomer 2a: - Cis form (when identical groups are on the same side of the bond): ``` H H | | H-C=C-H | | H H ``` - Trans form (when identical groups are on opposite sides of the bond): ``` H H | | H-C=C-H | | H H ``` Isomer 2b: - Cis form: ``` H H | | H-C-C=C-H | | H H ``` - Trans form: ``` H H | | H-C-C=C-H | | H H ```

Step 4: Name the isomers according to IUPAC nomenclature

Finally, we need to name each isomer according to IUPAC rules. The naming focuses on the location of the double bond and the presence of geometric isomerism (cis or trans). Isomer 1: 2-methylpropene Isomer 2a (cis form): cis-1-butene or (Z)-1-butene Isomer 2a (trans form): trans-1-butene or (E)-1-butene Isomer 2b (cis form): cis-2-butene or (Z)-2-butene Isomer 2b (trans form): trans-2-butene or (E)-2-butene