Question

Draw structures of alkanes that meet the following descriptions: (a) An alkane with two tertiary carbons (b) An alkane that contains an isopropyl group (c) An alkane that has one quaternary and one secondary carbon

Short answer

2,3-dimethylbutane, 2-methylpentane, and 2,2-dimethylpropane.

Step-by-step solution

Understanding Alkanes and Structural Features

Alkanes are saturated hydrocarbons, containing only single bonds between carbon atoms and filled with hydrogen atoms. A tertiary carbon is bonded to three other carbons; a secondary carbon is bonded to two others. Quaternary carbons have four carbon bonds, and an isopropyl group is a CH(CH3)2 structure.

Drawing an Alkane with Two Tertiary Carbons

To satisfy this condition, consider a structure like 2,3-dimethylbutane. It has two carbon (C) atoms, C-2 and C-3, each bonded to three other carbon atoms, hence both are tertiary carbons.

Drawing an Alkane with an Isopropyl Group

Draw an alkane such as 2-methylpentane. An isopropyl group can be seen as CH(CH3)2. In this molecule, the isopropyl group is attached to the main chain, making it 2-methylpentane as the structure includes the branching isopropyl group.

Drawing an Alkane with One Quaternary and One Secondary Carbon

Consider the structure of 2,2-dimethylpropane. In this structure, the central carbon is quaternary (bonded to four other carbons), and the secondary carbon may not clearly appear unless extended further, but the emphasis remains on the quaternary feature. For a structure clearly showing a secondary carbon, something like neopentane is preferable.

Key concepts

Tertiary Carbon

A tertiary carbon is a carbon atom bonded to three other carbon atoms. This structural feature is crucial in organic chemistry as it impacts the physical and chemical properties of the compound. For example, tertiary carbons might form branching in the carbon chain.

In the alkane, 2,3-dimethylbutane, two such tertiary carbons exist. Here, carbon atoms labeled as C-2 and C-3 are each bonded to three other carbons in the molecule. This means that aside from the methyl groups attached, each of these carbons connects directly to the main carbon chain. This configuration adds stability but also alters the density and boiling points compared to straight-chain alkanes.

When visualizing or drawing alkanes, it’s helpful to:

• Identify straight chains and branching points.
• Note the specific carbons each chain connects to, allowing easier identification of tertiary, secondary, or primary carbons.
• Remember that each tertiary carbon can have different substituent groups, affecting reactivity.

Isopropyl Group

The isopropyl group is a common structural motif in organic chemistry, recognized by its structure CH(CH₃)₂. This group is derived from propane by removing one hydrogen atom from the central carbon, creating a branching group.

An alkane that contains an isopropyl group, like 2-methylpentane, presents a branching structure that changes the overall shape and properties of the molecule. It effectively is a branch adding two methyl groups (CH₃) linked to a single carbon. This branching usually affects molecular interactions such as boiling and melting points.

When analyzing alkanes with isopropyl groups:

• Look for branching from the main carbon chain.
• Identify how the isopropyl group attaches to the main carbon chain, typically increasing complexity and often increasing solubility or altering the molecule's boiling point.
• Examine how such groups impact branching structures and overall stability of the molecule.

Quaternary Carbon

A quaternary carbon is one that is bonded to four other carbon atoms. This configuration results in a fully branching point and is found in molecules with a highly branched structure such as 2,2-dimethylpropane.

In such a molecule, the quaternary carbon is located centrally, connected directly to other carbons which are themselves part of methyl groups. A quaternary carbon can create complex structures depending on the length and type of carbon chains involved.

Understanding these structures can involve:

• Identifying the central carbon bonded to four carbons, creating a node in the structure.
• Recognizing that such configurations will typically alter physical properties like melting or boiling points, due to the compact structure.
• Noting that secondary, primary, and tertiary carbon types may exist alongside quaternary carbons, each influencing the overall molecular geometry and function.

Quaternary carbons are less common in simple alkanes but are essential for complex organic molecules, influencing their behavior and interactions significantly.