Problem 21 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Can \(\mathrm{C}_{6} \mathrm{H}_{12}\) be a saturated hydrocarbon? (b) Are all molecules containing a benzene ring unsaturated?

Step-by-Step Solution

Answer

(a) No, \(\mathrm{C}_{6} \mathrm{H}_{12}\) cannot be a saturated hydrocarbon since it does not match the general formula for alkanes, \(C_nH_{2n+2}\). (b) Yes, all molecules containing a benzene ring are considered unsaturated due to the presence of double bonds between carbon atoms in the ring.

Step 1: Understanding Saturated and Unsaturated Hydrocarbons

A saturated hydrocarbon is a hydrocarbon where all carbon atoms are bonded with single bonds, and there are no double or triple bonds between carbon atoms. Saturated hydrocarbons are also called alkanes, and their general formula is \(C_nH_{2n+2}\). An unsaturated hydrocarbon, on the other hand, contains at least one double bond or triple bond between carbon atoms. If there is a double bond, the compound is called an alkene and follows the general formula \(C_nH_{2n}\). If there is a triple bond, the compound is an alkyne with the general formula \(C_nH_{2n-2}\).

Step 2: Part (a)

In this part, we have to determine whether the given compound \(\mathrm{C}_{6} \mathrm{H}_{12}\) is a saturated hydrocarbon or not. To do this, let's compare the given compound to the general formulas for saturated and unsaturated hydrocarbons. We have a hydrocarbon with 6 carbon atoms and 12 hydrogen atoms, so its formula can be written as: \(C_6H_{12}\) Now let's compare it with the general formula of alkanes (saturated hydrocarbons), which is \(C_nH_{2n+2}\). Plugging n=6 (the given number of carbon atoms) into the alkane formula gives: \(C_6H_{2(6)+2} = C_6H_{14}\) Since \(C_6H_{12}\) is not equal to \(C_6H_{14}\), it cannot be a saturated hydrocarbon. Thus, the answer to part (a) is no, \(\mathrm{C}_{6} \mathrm{H}_{12}\) cannot be a saturated hydrocarbon.

Step 3: Part (b)

For this part, we need to discuss whether all molecules containing a benzene ring are unsaturated. A benzene ring is a hexagonal ring structure with alternating single and double bonds between adjacent carbon atoms. A molecule containing a benzene ring is considered unsaturated because of the presence of the double bonds between some of its carbon atoms. While the benzene ring may be a part of a larger molecular structure, the presence of these double bonds makes the entire molecule unsaturated. Hence, the answer to part (b) is yes, all molecules containing a benzene ring are considered unsaturated.

Key Concepts

Saturated Hydrocarbons

Saturated hydrocarbons, also known as alkanes, consist solely of carbon and hydrogen atoms. In these molecules, every carbon atom is connected by single bonds, meaning there's no room for any more hydrogen to attach. This is because all the binding spots are taken. This characteristic saturation of bonds makes them quite stable.
These compounds follow the general formula:

• \(C_nH_{2n+2}\)

Here, "\(n\)" symbolizes the number of carbon atoms. If we take methane, for example, with one carbon atom, its formula under this rule is \(CH_4\).
Typically, saturated hydrocarbons appear in straight or branched chains and can sometimes form rings, called cycloalkanes, but without offering double or triple bonds.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons include both alkenes and alkynes and contain carbon-carbon multiple bonds. Alkenes have at least one double bond and adhere to the general formula:

• \(C_nH_{2n}\)

In contrast, alkynes boast at least one triple bond and follow

• \(C_nH_{2n-2}\)

The presence of these double or triple bonds implies that unsaturated hydrocarbons remain open to additional chemical reactions, like hydrogenation, where hydrogen can be added. This property grants them reactivity, particularly in industrial manufacturing, where they serve as pivotal building blocks for many chemicals and plastics. The simplest alkene, ethene, \(C_2H_4\), has one double bond that can react easily to form derivatives like ethanol.

Benzene Ring

The benzene ring is a distinctive hexagonal construction found frequently in chemistry. Considered as an archetype of unsaturated hydrocarbons, it contains six carbon atoms forming a loop, with alternating double bonds forming a sort of continuous ring of electrons above and below the plane of the ring.
This stable configuration stems from a phenomenon called resonance, which spreads electrons over the entire ring, offering stability not seen in other unsaturated compounds. Though it has alternating single and double bonds, the benzene ring as a whole does not behave entirely like other classical unsaturated hydrocarbons, yet is still classified as one due to the presence of double bonds.
Often, benzene rings occur in various complex organic compounds, lending aromatic properties and serving as precursors or structures in pharmaceuticals, dyes, and plastic production.