Question

(i) Draw the structures of all isomeric alcohols of molecular formula \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\) and give their IUPAC names. (ii) Classify the isomers of alcohols in question \(11.3\) (i) as primary, secondary and tertiary alcohols.

Short answer

There are 6 isomers: n-pentanol, isopentanol, 2-pentanol, 3-pentanol, 3-methyl-2-butanol, and 2-methyl-2-butanol. Primary: 2, Secondary: 3, Tertiary: 1.

Step-by-step solution

Understanding the Molecular Formula

First, let's understand the molecular formula \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\). It represents a type of alcohol with 5 carbon atoms, 12 hydrogen atoms, and 1 oxygen atom.

Drawing Primary Alcohols

Primary alcohols have the \(\mathrm{OH}\) group attached to a terminal carbon atom. For \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), the primary alcohols are n-pentanol and isopentanol. Draw their structures:- n-Pentanol: CH\(_3\)(CH\(_2\))\(_4\)OH- Isopentanol: (CH\(_3\))_2CHCH\(_2\)CH\(_2\)OH

Drawing Secondary Alcohols

Secondary alcohols have the \(\mathrm{OH}\) group attached to an internal carbon atom. For \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), the secondary alcohols can be drawn as:- 2-Pentanol: CH\(_3\)CH\(_2\)CHOHCH\(_2\)CH\(_3\)- 3-Pentanol: CH\(_3\)CH\(_2\)CH\(_2\)CHOHCH\(_3\)- 3-Methyl-2-butanol: (CH\(_3\))_2CH(OH)CH\(_2\)CH\(_3\)

Drawing Tertiary Alcohols

Tertiary alcohols have the \(\mathrm{OH}\) group attached to a carbon atom bonded to three other carbon atoms. For \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), we can draw:- 2-Methyl-2-butanol: (CH\(_3\))_3COH

Classification by Type

Using the structures from Steps 2-4, classify each alcohol: - Primary Alcohols: n-Pentanol, Isopentanol - Secondary Alcohols: 2-Pentanol, 3-Pentanol, 3-Methyl-2-butanol - Tertiary Alcohols: 2-Methyl-2-butanol

Key concepts

Structural Isomers

Whenever you come across the term "structural isomers" in chemistry, it refers to molecules that share the same molecular formula but differ in how their atoms are connected. Structural isomers are fascinating because they highlight the diversity in molecular architecture even with the same atomic parts. In the case of alcohols with the molecular formula \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), the number and arrangement of the carbon atoms, hydrogen atoms, and the single oxygen atom can vary significantly. This variety is what gives rise to different types of alcohols such as primary, secondary, and tertiary, each with unique physical and chemical properties. The concept of structural isomers is fundamental since it illustrates how tiny changes in structure can lead to dramatically different substances in terms of reactivity and behavior.

IUPAC Nomenclature

The IUPAC nomenclature is an international standard for naming chemical compounds to ensure clarity and consistency worldwide. It is crucial for identifying the specific structure of a compound, especially when dealing with structural isomers like \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\) alcohols. To name these compounds, we follow a systematic method:

• Identify the longest carbon chain containing the \(\mathrm{OH}\) group and base the name on this.
• Number the chain so the \(\mathrm{OH}\) group gets the lowest possible number.
• Note any substituents, such as methyl groups, and their positions on the chain.

By adhering to these rules, we can confidently assign names like n-pentanol, isopentanol, and 2-methyl-2-butanol, each reflecting the precise location of functional groups and carbon atoms.

Primary Alcohols

Primary alcohols have a special feature where the \(\mathrm{OH}\) group is attached to a carbon atom that is, at most, connected to one other carbon atom. This particular arrangement results in alcohols like n-pentanol and isopentanol, which are examples of primary alcohols from our \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\) group. The chemical structures look like this:

• n-Pentanol: \ \(\text{CH}_3(\text{CH}_2)_4\text{OH}\) – the \(\mathrm{OH}\) ending signifies a terminal position.
• Isopentanol: \ \((\text{CH}_3)_2\text{CHCH}_2\text{CH}_2\text{OH}\) – here, the \(\mathrm{OH}\) group is also at the end but the chain branches with a methyl group.

Primary alcohols mainly engage in reactions that target the \(\mathrm{OH}\) group or the strong bond between the \(\mathrm{C}\) and the \(\mathrm{OH}\).

Secondary Alcohols

In secondary alcohols, the \(\mathrm{OH}\) group hooks onto an internal carbon atom – one that's bonded to two other carbon atoms. This setup alters the alcohol's chemistry compared to primary alcohols. For \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), the isomers fitting this description include:

• 2-Pentanol: \ \(\text{CH}_3\text{CH}_2\text{CHOH}\text{CH}_2\text{CH}_3\) – the \(\mathrm{OH}\) rests on the second carbon.
• 3-Pentanol: \ \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CHOH}\text{CH}_3\) – here, the \(\mathrm{OH}\) sits on the third carbon.
• 3-Methyl-2-butanol: \ \((\text{CH}_3)_2\text{CH(OH)CH}_2\text{CH}_3\) – showcasing a branched chain.

Secondary alcohols are characterized by reactions involving oxidation, where they transform into ketones. Their positioning of the \(\mathrm{OH}\) group on an interior carbon greatly affects their reactivity and uses.

Tertiary Alcohols

Tertiary alcohols are unique in that the \(\mathrm{OH}\) group attaches to a carbon atom bonded to three other carbons. This crowded connectivity confers distinct properties. For our formula \(\mathrm{C}_5\mathrm{H}_{12}\mathrm{O}\), an example is 2-methyl-2-butanol:

• 2-Methyl-2-butanol: \ \((\text{CH}_3)_3\text{COH}\) – the \(\mathrm{OH}\) group occupies a central position surrounded by three methyl groups.

Due to their structural design, tertiary alcohols rarely undergo oxidation without breaking carbon-carbon bonds. They are less reactive compared to their primary and secondary counterparts and often resist dehydration. Understanding tertiary alcohols is important for mastering organic chemistry, particularly for syntheses requiring stability against oxidation.