Exercise 3: comparing Rf values of compounds
The pairs of products below are considered.
Select those with the correct relative frontal ratios
Note: "<" means "Rf smaller than" and ">" means "Rf larger than".
2-nitrotoluene has an intramolecular hydrogen bond (between the phenol and nitro functions) which decreases the interaction with the stationary phase; its migration is then faster than that of phenol which establishes hydrogen bonds with the stationary phase, via its phenol function.
2-nitrotoluene has an intramolecular hydrogen bond which decreases the interaction with the stationary phase;
4-nitrotoluene has two hydrogen bonds with the stationary phase, which slows down its migration. The frontal ratio of 4-nitrophenol is thus smaller than that of 2-nitrophenol (and not larger).
Benzaldehyde establishes hydrogen bonds with the stationary silica phase, which slows down its migration.
The aromatic hydrocarbon does not establish such hydrogen bonds: its frontal ratio is greater than that of benzaldehyde (and not smaller).
Maleic acid (Z stereoisomer) has a dipole moment while fumaric acid (E stereoisomer) has no overall dipole moment.
The Z stereoisomer thus establishes stronger interactions with the polar stationary phase (silica): it migrates less rapidly than the E isomer. The frontal ratio of maleic acid Z is thus actually smaller than that of its E diastereoisomer.