In normal-phase thin-layer chromatography on silica gel 60 F254, p-xylene is less polar than bromobenzene and will travel farther from the origin. The net molecular dipole moment of p-xylene is 0.00 D because the para-disposed methyl substituents are symmetrically equivalent and their substituent dipole contributions cancel vectorially. Bromobenzene has a single carbon–bromine bond in which the electronegativity difference between carbon and bromine produces a significant permanent dipole; the measured dipole moment is 1.70 D. This difference in permanent dipole is the dominant contributor to the relative adsorption strength on a normal-phase silica sorbent. Silica gel surfaces carry partially ionized silanol groups that engage in dipole–dipole, dipole–induced dipole, and hydrogen-bonding interactions. Bromobenzene can orient its C–Br dipole toward these silanol sites and also present the nonbonding electrons of bromine as a weak hydrogen-bond acceptor. p-Xylene, lacking a permanent dipole and exposing a symmetric electron density distribution, interacts with the stationary phase primarily through dispersive forces. In a low-polarity developing solvent such as n-hexane or an n-hexane/ethyl acetate mixture in a 9:1 volume ratio, the more strongly retained bromobenzene exhibits a lower Rf value, while p-xylene partitions more completely into the mobile phase and migrates to a greater distance.

Mobile-phase polarity can modulate the magnitude of the migration difference but not the qualitative order. In normal-phase development, increasing the ethyl acetate content in hexane raises solvent strength and can reduce retention of both aromatic solutes, producing higher absolute Rf values for both compounds while preserving the relative order. Conversely, a pure saturated hydrocarbon mobile phase such as n-hexane lowers solvent strength and increases retention, again preserving Rf(p-xylene) > Rf(bromobenzene). The difference in migration distance is most pronounced under weakly polar elution and is reduced if a polar modifier such as methanol is introduced at a level sufficient to displace bromobenzene from active silanol sites. Even under those conditions, p-xylene remains the less retained compound on normal-phase silica gel.

The retention order follows from the definition of Rf as the quotient of the analyte migration distance to the solvent front distance. A higher Rf corresponds to farther travel. On a standard normal-phase plate of approximately 250 µm silica gel 60 with 60 Å mean pore diameter, increased analyte polarity increases the residence time in the adsorbed state at the expense of mobile-phase transport. Bromobenzene therefore moves more slowly than p-xylene under identical chamber saturation, plate activation, and solvent development conditions. The statement applies to adsorption thin-layer chromatography on silica gel and to related normal-phase adsorbents such as aluminum oxide. It does not apply to reversed-phase C18 or C8 plates, where the stationary phase is hydrophobically modified silica; in that system the nonpolar p-xylene is more strongly retained by the bonded alkyl phase and bromobenzene would travel farther. Published data for exact Rf shifts in every solvent mixture is limited because Rf values depend strongly on mobile-phase composition, plate activation, development distance, and solvent vapor saturation; however, the qualitative order Rf(p-xylene) > Rf(bromobenzene) on normal-phase silica gel is robust across common nonpolar to moderately polar mobile phases.