Video: EC17-18-S2-Q27



Video Transcript

Two organic compounds have the general formula C𝑛H2𝑛, one of which A is saturated with 𝑛 equals six and the other B is unsaturated with 𝑛 equals two. Write the chemical equations for obtaining compound A from benzene and dihydric alcohol from compound B.

This question starts with a puzzle. We’re given some details about some mystery compounds. We have to figure out what those compounds are and then insert those compounds into some chemical reaction equations and add some conditions and reagents. Let’s start with the definition of saturated.

In organic chemistry, saturated molecules do not contain carbon-carbon double bonds or carbon-carbon triple bonds. Looking at it in another way, saturated molecules contain as many hydrogens bound to carbons as possible. Unsaturated means the opposite. An unsaturated molecule contains carbon–carbon multiple bonds or looking at it from the hydrogen prospective, unsaturated molecules can have more hydrogens bound to carbon atoms. Now, what about this general formula we’ve been given?

If we plug in the values for 𝑛, six and two, we get the molecular formulas C6H12 and C2H4. If we look at the general formula, we’ll see that it resembles the general formula for a monoalkene. But an alkene is an example of an unsaturated hydrocarbon, whereas compound A is saturated. The other structure that produces the proportion of hydrogen to carbon in organic molecule is a ring. Therefore, A must be a cycloalkane.

The most sensible option for this compound is cyclohexane. It has the right chemical formula. It has a ring. And crucially, we’re going to have to make it from benzene. Benzene is also a six-membered ring. There are isomers of cyclohexane with the same chemical formula. But these will be very difficult to synthesize from benzene. Since we figured out a likely candidate for molecule A, let’s move on to B.

Molecule B has the formula C2H4 and it’s unsaturated. This means it must at least have a carbon–carbon multiple bond, like a double bond. If we use the remaining bonding slots to fill in the hydrogen, we get ethene. This is the only possible structure for a molecule with formula C2H4. Now that we know the structures of A and B, I’m just going to store them off to the side. Let’s move on to the synthesis of cyclohexane from benzene.

Benzene is an aromatic ring. It has a chemical formula C6H6. Relative to cyclohexane, it is short on hydrogens, six hydrogens to be exact which can be provided by three equivalents of hydrogen gas. By application of heat and pressure in the presence of a catalyst, we can produce cyclohexane from benzene. Now onto the synthesis of dihydric alcohol from ethene.

Dihydric alcohol is in fact a class of compounds, any alcohol that contains two OH groups. We can produce the dihydric alcohol from any alkene using Baeyer’s reaction. In Baeyer’s reaction, water and oxygen from a powerful oxidant can be inserted into a carbon-carbon double bond. This is achieved by treatment with potassium permanganate in alkaline conditions. In the case of the reaction of ethene, the product is ethan-1,2-diol.

So here, we have the chemical equations for the synthesis of compound A, cyclohexane from benzene, and the synthesis of a dihydric alcohol ethan-1,2-diol from compound B, ethene.

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