Question

You are given a mixture of 1 -butyne (bp \(8^{\circ}\) ) and 2 -butyne (bp \(27^{\circ}\) ). How could you separate, and then recover, each of these compounds using chemical means?

Short answer

To separate a mixture of 1-butyne and 2-butyne, perform fractional distillation by setting up an apparatus with a distillation flask, heating source, fractionating column, condenser, and collection flasks. Heat the mixture gently and gradually, allowing for 1-butyne with a boiling point of \(8^{\circ}\) to vaporize first. Collect the condensed liquid in a separate flask. When the temperature reaches \(27^{\circ}\), vaporize and collect 2-butyne. This method effectively separates and recovers both compounds by utilizing the difference in their boiling points.

Step-by-step solution

Understand the compounds in the mixture

First, we need to know more about the compounds given in the problem. 1-butyne and 2-butyne are both alkynes, which are unsaturated hydrocarbons with a triple bond between carbon atoms. The difference between them lies in the position of the triple bond. 1-butyne has the triple bond between the first and second carbons, while 2-butyne has the triple bond between the second and third carbons.

Utilize the difference in boiling points for separation

Since the two compounds have distinct boiling points (\(8^{\circ}\) for 1-butyne and \(27^{\circ}\) for 2-butyne), we can use a method called fractional distillation to separate them. The principle behind this technique is that the components of a mixture with different boiling points will vaporize at different temperatures, allowing for their collection as separate products.

Set up the fractional distillation apparatus

To perform a fractional distillation, we need to set up an apparatus with the following components: 1. A distillation flask: This will hold the mixture to be separated. 2. A heating source: This provides heat to vaporize the compounds in the mixture. 3. A fractionating column: This provides a temperature gradient for the separation of the compounds. 4. A condenser: This cools the vaporized compounds back to liquid form. 5. Collection flasks: These collect the separate, distilled compounds.

Heat the mixture and begin the separation

Heat the mixture gently and gradually. As the temperature increases, the component with the lowest boiling point (1-butyne) will vaporize first. The vapor will rise up the fractionating column and then condense back to liquid form in the condenser. After this, it will drip into the collection flask. Keep track of the temperature during this process, as it will increase until the first compound is fully distilled.

Collect the second compound

After 1-butyne has been distilled and collected, the temperature will continue to rise. When the temperature reaches the boiling point of 2-butyne (\(27^{\circ}\)), it will start to vaporize and go through the same process as 1-butyne. The vapor will rise up the fractionating column, condense in the condenser, and then be collected in a separate flask.

Recover the separated compounds

Once both compounds have been distilled and collected in their respective flask, you now have successfully separated and recovered 1-butyne and 2-butyne from the mixture. This method, fractional distillation, allows you to separate and collect the two compounds effectively by utilizing the difference in their boiling points.

Key concepts

1-butyne

1-butyne is an organic compound, more specifically, an alkyne with the chemical formula C₄H₆. Alkynes are hydrocarbons that have at least one triple bond between two carbon atoms, and in the case of 1-butyne, this triple bond is located between the first and the second carbon atoms in the chain. This particular positioning of the triple bond gives 1-butyne unique properties.

Due to the triple bond's electronegativity, 1-butyne has a relatively linear structure, which minimizes the interactions between molecules. This structural feature contributes to a lower boiling point compared to compounds with more complex interactions. Understanding the molecular structure of 1-butyne is key when separating it from other substances, such as 2-butyne, as the distinct boiling point plays a critical role in the fractional distillation separation process.

2-butyne

Like 1-butyne, 2-butyne is an alkyne, embodying a triple bond that defines its reactions and physical properties. However, the key difference lies in the location of the triple bond, which for 2-butyne is between the second and third carbon atoms. Its chemical formula is also C₄H₆, but the variation in structure results in different chemical behaviors and a higher boiling point than 1-butyne.

The placement of the triple bond in 2-butyne causes a slight deviation in the linear structure compared to 1-butyne. As a result, the interactions between 2-butyne molecules are slightly more complex, which contributes to a higher boiling point of 27 degrees Celsius. The knowledge of 2-butyne’s structure and boiling point is crucial for separating it from 1-butyne using fractional distillation.

Alkynes

Alkynes are a group of unsaturated hydrocarbons that include at least one carbon-to-carbon triple bond. They are known for their reactivity, which is largely due to the high electron density of the triple bond. The triple bond is composed of one sigma bond and two pi bonds, with the pi bonds being responsible for the characteristic reactions of alkynes, such as addition reactions.

Alkynes, including 1-butyne and 2-butyne, are generally less dense than water and possess lower boiling points compared to alkanes and alkenes with the same number of carbon atoms. This is because the linear geometry of alkynes leads to fewer points of contact between molecules, reducing Van der Waals forces, and thus, lowering the boiling points. The simplicity and linearity make them ideal candidates for separation through fractional distillation, leveraging their boiling point differences.

Boiling Point

The boiling point of a compound is the temperature at which it transforms from a liquid to a gas. This physical property is crucial for separating mixtures via fractional distillation, as different substances typically have unique boiling points.

Factors affecting the boiling point include molecular weight, structure, and intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and Van der Waals forces. In the context of 1-butyne and 2-butyne, the difference in boiling points is because the triple bond's location affects the molecule's shape and hence, the intermolecular forces. Boiling points provide a systematic approach to separate compounds in a mixture like in the textbook exercise, allowing the separation of 1-butyne and 2-butyne without the need for chemical reactions, but by their physical properties.