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Question Video: Predicting the Product of the Hydrogenation of Benzene in Excess Hydrogen Chemistry

What product is produced from the hydrogenation of benzene using a nickel catalyst and excess hydrogen?

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Video Transcript

What product is produced from the hydrogenation of benzene using a nickel catalyst and excess hydrogen?

To determine the product of the hydrogenation of benzene, an aromatic ring consisting of six carbon atoms and six hydrogen atoms, using a nickel catalyst and excess hydrogen, let’s examine the structure of benzene and discuss what the process of hydrogenation means for this chemical structure. In this structural diagram of benzene, the alternating single bonds and double bonds between the carbon atoms indicate that each of the six carbon atoms is bonded to three other atoms and also exhibits the maximum number of bonds a carbon atom can have, which is four.

Hydrogenation is by definition a hydrogen addition reaction, which is a type of addition reaction where a molecule combines with one or more molecules of hydrogen to form a larger molecule, without forming any by-products. However, one might ask, if each of the six carbon atoms in benzene is already bonded four times, which is the maximum number of bonds a carbon atom can form, how can this compound combine with one or more molecules of hydrogen? The answer lies in the fact that between the six carbon atoms of benzene are πœ‹ bonds.

During the hydrogenation reaction of benzene with one molecule of hydrogen, the πœ‹ bond found within the double bond between two carbon atoms is broken, as is the 𝜎 bond between the hydrogen atoms. This allows for two new carbon-hydrogen bonds to be formed, which generates two carbon atoms singly bonded to four atoms and by definition means hydrogenation of benzene has occurred. This particular reaction relies on the nickel catalyst to bring the molecular hydrogen close enough to the carbon-carbon double bonds in benzene to allow for the interaction we’ve just described to occur.

It is important to note this question states the reaction is performed with excess hydrogen. In the presence of excess hydrogen, the interaction we just described between one molecule of hydrogen and the πœ‹ bond between two carbon atoms in benzene would occur throughout the molecule until all πœ‹ bonds have been broken between the carbon atoms in the benzene ring and the new carbon-hydrogen bonds have been formed. This equates to a total of six hydrogen atoms being added to the chemical structure during the hydrogenation process, which means when writing the balanced chemical equation for the hydrogenation of benzene in the presence of a nickel catalyst and excess hydrogen, we will place a coefficient of three in front of the H2 molecule to demonstrate the six hydrogen atoms being added to each benzene molecule.

Looking at the fully hydrogenated structure, the name of this new molecule is called cyclohexane, where cyclo- indicates the carbon atoms are bonded to one another in a ring structure, β€œhex” for the number of carbon atoms in the ring, and -ane to indicate all of the carbon atoms are saturated or singly bonded to only carbon and hydrogen atoms. Therefore, what product is produced from the hydrogenation of benzene using a nickel catalyst and excess hydrogen? The answer is cyclohexane.

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