Constitutional isomers, which share an identical chemical formula, present a frequent challenge in chemical processing and purification. Distillation is a separation technique that exploits the physical differences between compounds to isolate them. Because the differing atomic arrangements inherently lead to measurable differences in physical properties, constitutional isomers can almost always be separated by distillation.
Defining Constitutional Isomers
Constitutional isomers, also known as structural isomers, are compounds that share the exact same molecular formula but possess different arrangements in the way their atoms are connected. The atoms themselves are the same in number and kind, but the sequence and bonding pattern are distinct. For instance, the alkane with the formula \(\text{C}_4\text{H}_{10}\) exists as two constitutional isomers: the straight-chain n-butane and the branched 2-methylpropane, also commonly called isobutane.
This difference in connectivity results in two entirely different molecular shapes. Another example is the pentane family, where \(\text{C}_5\text{H}_{12}\) can be the linear n-pentane or the highly-branched neopentane (2,2-dimethylpropane). The fundamental chemical identity of each isomer is unique because the structural blueprint is not the same, leading to varying physical and chemical characteristics.
How Distillation Separates Compounds
Distillation is a process used to separate liquid mixtures based on the principle of differential volatility. The mixture is heated to encourage the components to transition into a gaseous state, or vapor. The ease with which a substance vaporizes is directly related to its vapor pressure.
The compound with the higher vapor pressure, which also has the lower boiling point, will vaporize more readily than the others. This enriched vapor is then channeled away from the liquid mixture and cooled, causing it to condense back into a liquid form. This collected liquid, known as the distillate, is thus purified and separated from the remaining liquid mixture.
Structural Impact on Boiling Points
The unique atomic connectivity of constitutional isomers is the reason they exhibit different physical properties, making separation by distillation possible. A molecule’s boiling point is determined by the strength of the intermolecular forces (IMFs) holding the individual molecules together in the liquid phase. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
In nonpolar constitutional isomers like alkanes, London dispersion forces are the only relevant IMFs, and their strength is highly dependent on the molecular surface area. A linear isomer, such as n-pentane, has an elongated structure that maximizes the surface contact area with neighboring molecules. This extensive contact allows for stronger cumulative London dispersion forces, meaning more energy is required to break the attractions and boil the liquid.
In contrast, a highly branched isomer with the same formula, such as neopentane, adopts a more compact, spherical shape. This shape significantly reduces the available surface area for intermolecular contact, which weakens the London dispersion forces between molecules. Consequently, the branched isomer requires less energy to boil and will have a substantially lower boiling point.
The boiling point of n-pentane is approximately \(36.1^\circ\text{C}\), while its isomer neopentane boils much lower at about \(9.5^\circ\text{C}\). This difference of over \(26^\circ\text{C}\) is a direct result of the difference in molecular geometry and is more than sufficient for effective separation by distillation.
Constitutional isomers can also differ in polarity due to the placement of functional groups, which introduces stronger intermolecular forces. For example, ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) and dimethyl ether (\(\text{CH}_3\text{OCH}_3\)) are isomers with the formula \(\text{C}_2\text{H}_6\text{O}\). The presence of the hydroxyl (\(\text{OH}\)) group in ethanol allows for hydrogen bonding, a powerful IMF, giving it a much higher boiling point (\(78.4^\circ\text{C}\)) than the non-hydrogen-bonding dimethyl ether (\(-24.8^\circ\text{C}\)).
Factors Influencing Separation Efficiency
While constitutional isomers possess different boiling points, the practical efficiency of their separation by distillation depends on the magnitude of that difference (\(\Delta\)BP). For mixtures where the boiling points are separated by a large margin, generally greater than \(25^\circ\text{C}\), a simple distillation apparatus is often sufficient for a reasonably pure separation. This is because the vapor produced is already highly enriched in the lower-boiling component.
When the boiling point difference between two isomers is small, typically less than \(25^\circ\text{C}\), a more sophisticated technique called fractional distillation becomes necessary. Fractional distillation utilizes a fractionating column placed between the heating flask and the condenser. This column provides a large surface area for repeated cycles of vaporization and condensation, effectively performing numerous simple distillations in sequence.
The efficiency of a fractional column is described by its number of theoretical plates, where each plate represents one complete vaporization-condensation cycle. Achieving high purity for isomers with extremely small \(\Delta\)BP requires a column with a high number of theoretical plates, which can increase the cost and complexity of the operation.