Question

Which of the following isomers of octane should have the higher viscosity? CCCCCCCC CC(C)(C)C(C)(C)C

Short answer

The straight-chain isomer of octane ('CCCCCCCC') should have the higher viscosity compared to the highly branched isomer ('CC(C)(C)C(C)(C)C').

Step-by-step solution

Understand the Concept of Viscosity

Viscosity refers to the resistance of a fluid to flow. For molecules of a similar size, the more branching in their structure, the lower the viscosity. This is because more branching results in molecules that can't pack as closely together, leading to less intermolecular interaction and easier flow.

Analyze the Molecular Structure of the Isomers

Examine the structure of the given octane isomers. The first isomer with the SMILES notation 'CCCCCCCC' represents the straight-chain octane, which has no branches. The second isomer with the SMILES notation 'CC(C)(C)C(C)(C)C' represents an octane with a highly branched structure.

Compare Viscosity Based on Molecular Structure

Since straight-chain molecules can pack more closely than branched molecules, they exhibit stronger intermolecular forces, leading to higher viscosity. Therefore, the straight-chain isomer ('CCCCCCCC') should have a higher viscosity than the highly branched isomer ('CC(C)(C)C(C)(C)C').

Key concepts

Molecular Structure of Isomers

When discussing isomers, such as those of octane, it's critical to understand that their molecular structures have significant effects on their physical properties. Isomers are molecules with the same molecular formula but different arrangements of atoms. In the case of octane, the straight-chain isomer has a continuous chain of carbon atoms without any branches, represented by the simplified molecular-input line-entry system (SMILES) notation 'CCCCCCCC'.

On the other hand, the branched isomer, notated as 'CC(C)(C)C(C)(C)C', has carbon atoms that branch off from the main chain. The way these atoms are arranged affects how these molecules interact with each other and how densely they can pack together. A straight-chain molecule can align much more closely with its neighbors than a branched molecule can, due to the latter's irregular shape. This has a profound impact on a range of properties, including viscosity, melting points, and boiling points.

Intermolecular Forces

Intermolecular forces are the attractive and repulsive forces between molecules. They are not as strong as the bonds within molecules (intramolecular forces), but they heavily influence a substance's physical properties. There are several types of intermolecular forces, including van der Waals forces, which can be broken down into London dispersion forces and dipole-dipole interactions.

Importance of Intermolecular Forces in Viscosity

Molecules in a liquid are constantly moving and sliding past one another, and it's the strength of these intermolecular forces that determines how easily molecules can move. The stronger the forces holding the molecules together, the higher the viscosity will be, as more energy is required to make the molecules flow. Since straight-chain isomers allow for more surface contact between molecules, they generally have stronger van der Waals forces than their branched counterparts, resulting in a higher viscosity.

Chemical Nomenclature

Chemical nomenclature is the system of naming chemical compounds, which provides a standardized way of referring to chemical substances. For students grappling with the complexity of chemical names, breaking down the name into its parts can often reveal information about the molecule’s structure itself.

For instance, the term 'octane' comes from the root word 'oct-', indicating that the molecule contains eight carbon atoms, combined with the suffix '-ane', which indicates that the carbons are connected by single bonds (forming an alkane). In chemical nomenclature, the prefixes and suffixes, along with other modifiers, provide insight into the molecular structure, such as whether the chain is straight or branched, as well as functional groups that may be present.

Understanding the nomenclature helps students to visualize the molecular structure and predict the chemical behavior of the compounds, such as reactivity patterns and, as discussed earlier, even physical properties such as viscosity.