Question

Which of the following compounds are saturated? Which are unsaturated? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\), (b) \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\), (c) cyclohexane, (d) cyclohexene, (e) benzene.

Short answer

Saturated compounds: (a) butane \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and (c) cyclohexane. Unsaturated compounds: (b) 1-butene \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\), (d) cyclohexene, and (e) benzene.

Step-by-step solution

(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

This compound is butane, which is an alkane with a linear structure: \(\mathrm{CH}_3 - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3\). There are only single bonds between the carbon atoms, so it is a saturated compound.

(b) \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}\)

This compound is 1-butene, which is an alkene with a structure: \(\mathrm{CH}_2 = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_3\). There is a double bond between the first two carbon atoms, so it is an unsaturated compound.

(c) Cyclohexane

Cyclohexane is a cycloalkane with a six-membered ring structure containing only single bonds between carbon atoms. The structure of cyclohexane is: ``` CH2---CH2 | | CH2 CH2 | | CH2---CH2 ``` Since it contains only single bonds, cyclohexane is a saturated compound.

(d) Cyclohexene

Cyclohexene is a cycloalkene with a six-membered ring structure containing one double bond between two carbon atoms. The structure of cyclohexene is: ``` CH2---CH2 | | CH2 CH | | CH2==CH2 ``` As it contains a double bond, cyclohexene is an unsaturated compound.

(e) Benzene

Benzene is an aromatic hydrocarbon with a six-membered ring structure containing alternating single and double bonds between carbon atoms. The structure of benzene is: ``` CH==CH | | CH CH | | CH==CH ``` Since it contains double bonds, benzene is an unsaturated compound. In summary, the saturated compounds are (a) butane and (c) cyclohexane, while the unsaturated compounds are (b) 1-butene, (d) cyclohexene, and (e) benzene.

Key concepts

Alkane and Alkene

Alkanes and alkenes are types of hydrocarbons, which are organic compounds composed of carbon and hydrogen atoms. The simplest way to distinguish between the two is by looking at their chemical bonds.

Alkanes, also known as paraffins, are saturated hydrocarbons. This means that they have only single bonds between their carbon atoms, forming a stable structure. The formula for an alkane is \( C_nH_{2n+2} \), where \( n \) is the number of carbon atoms. A classic example of an alkane is butane (\( C_4H_{10} \)), which consists of only single bonds as shown in the exercise solution.

Alkenes, on the other hand, are unsaturated hydrocarbons. They contain at least one carbon-carbon double bond, which makes them less saturated with hydrogen compared to alkanes. The presence of the double bond also introduces the possibility of isomers, as the bond creates a definitive 'up' or 'down' aspect to the molecule's shape. The general formula for alkenes is \( C_nH_{2n} \). The compound 1-butene (\( C_4H_8 \) with a double bond) illustrates an alkene structure.

Chemical Structure of Hydrocarbons

Understanding hydrocarbons requires knowing about their chemical structure. A hydrocarbon's chemical structure is more than just a list of its component atoms; it's a 3D representation of how those atoms are bonded together and spaced relative to each other.

For example, the difference between saturated and unsaturated hydrocarbons is due to their bond types. The saturated hydrocarbons have all their carbon atoms linked by single bonds, leading to the maximum number of hydrogen atoms they can hold. In contrast, unsaturated hydrocarbons have one or more double (for alkenes) or triple (for alkynes—not covered in this exercise) bonds between carbon atoms, resulting in fewer hydrogen atoms.

The properties of hydrocarbons, such as their reactivity and boiling point, are greatly influenced by their structure. Saturated hydrocarbons tend to be less reactive and have higher boiling points than their unsaturated counterparts of similar molar mass because of the increased Van der Waals forces.

Aromatic Hydrocarbons

Aromatic hydrocarbons are a special category of unsaturated hydrocarbons that have a very stable ring structure with alternating single and double bonds, known as the benzene ring. These compounds are not only named for their unique, ring-like structure but also for the fact that many have distinctive aromas.

The classic example of an aromatic hydrocarbon is benzene (\( C_6H_6 \) which has a hexagonal ring structure with three double bonds interspaced with single bonds. This configuration is often represented by a circle within a hexagon to denote the equal distribution of the electron cloud across the ring—indicating delocalized electrons.

These delocalized electrons provide extra stability to the molecule and are responsible for the unique chemical reactions characteristic of aromatic compounds. Aromatic hydrocarbons are widely used in the chemical industry as starting materials for the synthesis of dyes, drugs, plastics, and explosives. Benzene is just one example, but there are many other aromatic compounds with additional rings or other functional groups attached.