Question

Question. Glycerol (propane-1,2,3-triol) is a viscous syrup with molecular weight 92g/mol, boiling point 290^oC, and density 1.24g/mol. Transforming the three hydroxy groups into their trimethylsilyl ethers (using chlorotrimethylsilane and a tertiary amine) produces a liquid that flows easily, has molecular weight 309g/mol, boiling point approximately 180^oC, and density 0.88g/mol. Draw the structures of these two compounds and explain why glycerol has a lower molecular weight but a much higher boiling point and density.

Short answer

Glycerol Trimethylsilyl ether of glycerol

Glycerol has high boiling point due to strong intermolecular hydrogen bonding that exists due to its structure and its high density is due to extensive packing of molecules which helps in effective hydrogen bonding. There is no hydrogen bonding in trimethylsilyl ether of glycerol.

Step-by-step solution

Step-1. Factors affecting boiling point and density:

Strong intermolecular hydrogen bonding can increase the boiling point of the compound as energy required to overcome these strong intermolecular forces is more and that raises the boiling point of the compound. Extent of packing of molecules determine the density of compound. More effective the packing of molecules, more compact and denser the structure will be.

Step-2. Comparison of boiling point and density between glycerol and TMS ether of glycerol:

Glycerol has extremely strong intermolecular hydrogen bonding because of three hydroxyl groups per molecule. Overcoming these intermolecular forces requires a lot of energy and hence, glycerol has higher boiling point despite having low molecular weight. The density is also high because these glycerol molecules pack together tightly to maximize hydrogen bonding.

TMS ether of glycerol has no hydrogen bonding and on each end of molecule, there exists non-polar and spherical group, thus, these molecules tend to stay apart and have low density and low boiling point.

Glycerol Trimethylsilyl ether of glycerol