Question

Write structural formulas for all the constitutionally isomeric \(\mathrm{C}_{4} \mathrm{H}_{9}\) alkyl groups. Check your answers with Table \(25.1,\) and note the names of these groups.

Short answer

Four isomers: n-butyl, isobutyl, sec-butyl, and tert-butyl.

Step-by-step solution

Understanding Constitutional Isomers

Constitutional isomers have the same molecular formula but different connectivity of atoms. For the formula \( \mathrm{C}_{4}\mathrm{H}_{9} \), we need to arrange four carbon atoms and nine hydrogen atoms in different ways to form distinct isomers.

Drawing Linear Isomer

Start by drawing the longest carbon chain with four carbon atoms (butane). Identify the \( \text{n-butyl} \) group by removing one terminal hydrogen: \( \text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_2 \cdot \). This is the straight-chain isomer.

Drawing Branched Isomer - Isobutyl

Create a three-carbon chain with one branch. The isobutyl group is formed by removing a hydrogen from the carbon that branches off from the main chain: \( \text{(CH}_3)_2\text{CH} - \text{CH}_2 \cdot \).

Drawing Branched Isomer - Sec-butyl

Draw a straight three-carbon chain, attach the fourth carbon to the middle carbon. The sec-butyl group is produced by removing a hydrogen from the second carbon: \( \text{CH}_3 - \text{CH}_2 - \dot{\text{CH}} - \text{CH}_3 \).

Drawing Branched Isomer - Tert-butyl

Draw a central carbon connected to three methyl groups. The tert-butyl group involves removing a hydrogen from the central carbon: \( \dot{\text{C}}(\text{CH}_3)_3 \).

Verification with Table 25.1

Verify the drawn structures with the provided Table 25.1 and confirm the names: 1) \( \text{n-butyl} \), 2) \( \text{isobutyl} \), 3) \( \text{sec-butyl} \), and 4) \( \text{tert-butyl} \). All structures should match the descriptions and names given in the table.

Key concepts

Isobutyl

The isobutyl group is a type of branched alkyl group derived from butane by removing one hydrogen atom. In the structure of isobutyl, you will find a three-carbon chain with a branch off the second carbon atom.

Here’s how it looks:

• It includes three carbons forming a straight chain.
• The fourth carbon branches off from the middle carbon in the chain.
• A hydrogen atom is removed from this branching point to create the isobutyl group.
• The structural formula for this group is \((\text{CH}_3)_2\text{CH} - \text{CH}_2\cdot\).

This unique branching allows isobutyl to distinguish itself from the straight-chain or other branched isomers. It's often encountered in organic chemistry tasks about structural isomers and helps in understanding isomeric diversity.

Tert-Butyl

Tert-butyl, short for 'tertiary-butyl', is perhaps the most distinct of the butyl isomers due to its symmetrical structure. It involves a central carbon atom connected to three other carbon atoms, making it a highly branched isomer.

To picture tert-butyl:

• A central (tertiary) carbon is linked to three identical methyl groups \((\text{CH}_3)\).
• Removal of a hydrogen atom happens from this central carbon, leading to its unique configuration.
• The structural formula can be represented as \(\cdot\text{C}(\text{CH}_3)_3\).

Tert-butyl's unique shape makes it an interesting subject in chemical synthesis and studies involving steric effects, as it tends to create a significant space around its central core, affecting how molecules behave and interact.

Sec-Butyl

The sec-butyl, or secondary-butyl, group is another variant among the isomers of butane. It features a straight or continuous chain three-carbon arrangement, with the fourth carbon branching from the second carbon in this chain.

Understanding the sec-butyl structure involves seeing it as:

• A three-carbon straight chain.
• The secondary (middle) carbon atom is where the branching occurs.
• A hydrogen atom is removed from this secondary carbon to form the group's active site, noted as \(\text{CH}_3 - \text{CH}_2 - \cdot\text{CH} - \text{CH}_3\).

Sec-butyl’s configuration offers a balanced approach between linear and fully branched structures. It is quite functional in organic synthesis, particularly for introducing secondary carbon branching in larger molecules.