Video: Applying Knowledge of the Water-Solubility of Acetylene and the Nature of Water-Acetylene Interactions

For statements I and II, state for each if they are true or false. I) Acetylene C₂H₂ is very soluble in water. II) Water molecules form hydrogen bonds with acetylene molecules. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) Acetylene C₂H₂ is very soluble in water. II) Water molecules form hydrogen bonds with acetylene molecules. If both are true, state if II is a correct explanation for I.

Acetylene is the smallest example of an alkyne containing a carbon≡carbon triple bond. You might also see it called ethyne. The first thing you might notice about an acetylene molecule is that all the atoms are in a straight line. Carbon is slightly more electronegative than hydrogen. So you could say that the carbons are delta negative and the hydrogens are delta positive. But this effect is very small. But anyway, because acetylene is linear and symmetrical, the individual bond dipoles cancel each other out, meaning that acetylene is nonpolar. As a result of this, acetylene doesn’t have strong intramolecular forces. All we see are London dispersion forces. So it should come as no surprise that acetylene has a relatively low boiling point at negative 84 degrees Celsius.

Statement I suggests that acetylene is very soluble in water. So Let’s have a look at water now. A water molecule has a bent structure because of the two lone pairs on the oxygen. But in contrast with acetylene, oxygen is much more electronegative than hydrogen, making the oxygen delta negative and the hydrogen delta positive. Because of the shape of the molecule, we have one side delta negative and one side delta positive. So we have a permanent dipole. So in addition to the normal London dispersion forces, there are dipole–dipole forces in water and, crucially, hydrogen bonding. This occurs because the hydrogen is bonded to oxygen, which is highly electronegative. And the hydrogens of one water molecule interact with the oxygens of other water molecules, creating a strong bond.

Now, we can think about whether acetylene would be soluble or not in water. Now, an easy way to evaluate this is like dissolves like. So polar solvents dissolved polar solutes, and nonpolar solvents dissolve nonpolar solutes. So on this basis, statement I is false. Acetylene would not be very soluble in water. The next statement is that water molecules form hydrogen bonds with the acetylene molecules. In order to form a hydrogen bond, you need a, a hydrogen atom bonded to electronegative atom and b, a lone pair on an electronegative atom.

The hydrogens on acetylene aren’t bonded to electronegative atom like nitrogen, oxygen, or fluorine. So they won’t act as hydrogen bond donors. And also there are no lone pairs on the carbons, although carbon isn’t electronegative enough in the first place. So water molecules cannot form hydrogen bonds with acetylene molecules. And statement II is false. Even though water has the capacity to both give and receive hydrogen bonds, acetylene has neither. Since neither statement is true, we don’t have to do the last part of the question.

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