Chapter 12, Problem 12.125EP

Interpretation Introduction

Interpretation:

The pair of compounds that have the same physical state at room temperature and pressure has to be determined from the given options.

Concept Introduction:

Organic compounds are represented shortly by the molecular formula and structural formula.  Each and every compound has its own molecular formula.  Compounds can have same molecular formula but not same structural formula.

Alkanes are linear chain saturated hydrocarbons and cycloalkanes are cyclic carbon chain saturated hydrocarbons.  They both occur naturally.

Alkanes and cycloalkanes are hydrocarbons.  They are nonpolar molecules.  Water is a polar molecule.  Therefore, alkanes and cycloalkanes do not get solubilized in water.  In other words, alkanes and cycloalkanes are insoluble in water.

Regarding density, alkanes and cycloalkanes have density lower than water.  When alkanes and cycloalkanes are mixed with water, two layers are formed which is a result of insolubility.  Alkanes and cycloalkanes are present on top of water layer which is due to lesser density.

Boiling point of alkanes and cycloalkanes increase with an increase in carbon‑chain length or the ring size.  When considering the continuous‑chain alkanes, the boiling point of alkanes increases about $30°\text{C}$ for every carbon atom that is added to the chain.  The continuous alkanes which contain one to four carbon atoms are gases at room temperature.  The continuous chain alkanes that contain five to seventeen carbon atoms are liquids at room temperature.  The continuous chain alkanes that contain more than eighteen carbon atoms are solids at room temperature.

When branching happens in the carbon chain, it lowers the boiling point of alkanes.  In simple words, unbranched alkanes have more boiling point than branched alkanes with the same number of carbon atoms.

Cycloalkanes have higher boiling point compared to noncyclic alkanes with the same number of carbon atoms.  This is due to the more rigid and more symmetrical structures that occur in cyclic systems.  Cyclopropane and cyclobutane are gases at room temperature.  Cyclopentane to cyclooctane are liquids at room temperature.