Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.
Roller Coasters & Molecules
Are you one of those people that likes riding on roller coasters at the theme park? Have you ever been on one that goes upside down in a loop? In general, a loop can kind of be thought of as a cycle of sorts.
Did you know that some organic compounds can be referred to as cyclic and kind of resemble a loop of sorts? These types of compounds result when we take two ends of a molecule and bond the ends together to create the cycle or loop, very similar to a loop on a roller coaster. The topic of this lesson is a specific kind of loop known as cyclohexane. We will be studying the derivatives and isomers of cyclohexane as well as how they can be synthesized in the lab. Let's go around the loop!
What Is Cyclohexane?
Cyclohexane is an organic compound that is composed of carbon and hydrogen atoms and is a cyclic structure with the ring being constructed of six carbon atoms bonded together. The bonds in cyclohexane are all single bonds, and the shape resembles a hexagon when it's drawn as a bond-line structure.
Isomers of Cyclohexane
Although cyclohexane is an important compound, it gets more interesting when we start to decorate the ring with other substituents (atoms or groups of atoms). This results in derivatives and isomers of cyclohexane. For example, let's consider both 1,2-dichlorocyclohexane and 1,4-dichlorocyclohexane.
Notice how both compounds contain a cyclohexane ring and each have two chlorine atoms. They differ in how the chlorine atoms are bonded to the ring. This type of relationship represents what are called constitutional isomers, which are compounds that have the same type and number of atoms but differ in the way the atoms are bonded to one another. Another way of stating it is that the compounds have the same chemical formulas but different atom connectivity.
What if we had a case in which the cyclohexane ring had two alcohol (-OH) groups that were bonded to the same positions on the ring, but they had a different spatial orientation relative to one another? Isomers of a compound that differ only in their three-dimensional (spatial) orientation are called stereoisomers. This is the case with cis-1,2-dihydroxycyclohexane and trans-1,2-dihydroxycylohexane. Notice how the two alcohol groups are on the same face of the ring in one case (two dashed bonds) but are on opposite faces (one wedged bond, one dashed bond) in the other.
Cyclohexane Isomer Synthesis
Now that we know how to recognize cyclohexane and some of its isomers and derivatives, let's discuss how some of these compounds can be made synthetically in the lab. For example, cyclohexanol can be oxidized to provide cyclohexanone in a very simple reaction that is commonly run in undergraduate organic chemistry labs. The common way to promote the reaction is with an inorganic reagent called sodium dichromate, which also requires an acid source.
Another compound we can consider is 1,2-dihydroxycyclohexane. Interestingly, both the cis and trans stereoisomers of 1,2-dihydroxycyclohexane can be obtained from the same starting material, cyclohexene. If cyclohexene is reacted with osmium tetroxide as the hydroxylating reagent (reagent responsible for adding hydroxyl -OH groups), the only product is cis-1,2-dihydroxycyclohexane.
If we take cyclohexene and first react it with a reagent called meta-chloroperoxybenzoic acid (m-CPBA) followed by sodium hydroxide (NaOH), we get trans-1,2-dihydroxycyclohexane as the sole product. As we previously mentioned, these two compounds (the cis and trans isomers) are related to one another in that they're stereoisomers.
All right, let's take a moment to review. In this lesson, we learned that cyclohexane is a cyclic organic compound composed of six carbon atoms bonded together. Cyclohexane isomers are compounds that contain atoms or groups of atoms bonded to the cyclohexane ring itself. 1,2-Dichlorocyclohexane and 1,4-dichlorocyclohexane represent what are called constitutional isomers, which are compounds that have the same chemical formula, but the atoms are connected differently. Isomers of a compound that differ only in their three-dimensional (spatial) orientation are called stereoisomers, with the example being cis-1,2-dihydroxycyclohexane and trans-1,2-dihydroxycylohexane.
In terms of synthesis, cyclohexanol can be oxidized to provide cyclohexanone, which is usually accomplished by the reaction of sodium dichromate and an acid source. If cyclohexene is first reacted with meta-chloroperoxybenzoic acid (m-CPBA) followed by sodium hydroxide (NaOH), trans-1,2-dihydroxycyclohexane is the sole product. If cyclohexene is reacted with osmium tetroxide as the hydroxylating reagent, the only product is cis-1,2-dihydroxycyclohexane.
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