Stilbene

Stilbene is 1,2-diphenylethene: two phenyl groups attached to the two carbons of an ethene double bond, with connectivity ${\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{CH}=\mathrm{CH}-{\mathrm{C}}_6{\mathrm{H}}_5$. Its molecular formula is ${\mathrm{C}}_{14}{\mathrm{H}}_{12}$. The connectivity admits two geometric isomers, cis-Stilbene and trans-Stilbene, which the NIST identity records distinguish despite their equal formulas and molar masses.^[ref][ref]

Each central carbon bears one phenyl group and one hydrogen atom. In cis-Stilbene, the two phenyl groups lie on the same side of the double bond; in trans-Stilbene, they lie on opposite sides. The Cahn-Ingold-Prelog priority rules assign these arrangements Z and E respectively. Specifying only stilbene connectivity does not choose between them.

Atom counting provides a simple structural check:Using $M\simeq 180.245\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, an original $18.0245\mathrm{mg}$ sample corresponds to $0.1000\mathrm{mmol}$ of pure stilbene. That mass conversion is unchanged if the sample contains both geometric isomers; it cannot establish their proportions.

A stilbene isomer composition measurement needs information sensitive to geometry. For an ideal two-component optical example, let $c_E,c_Z$ be the two concentrations and let $a_j$ be background-subtracted absorbance divided by path length at wavelength $j$. A calibrated Beer-Lambert law model givesThe two concentrations can be separated by this model only if ${ϵ}_{E1}{ϵ}_{Z2}-{ϵ}_{E2}{ϵ}_{Z1}\ne 0$. Proportional response columns would leave composition unidentifiable even with perfectly repeated readings. The coefficients must be established for the solvent and measurement conditions; the example does not assign experimental spectra.

Resveratrol illustrates how the backbone recurs in related chemistry: replacing three aromatic hydrogens by hydroxy groups adds three oxygens while retaining a central double bond capable of cis-trans isomerism. Molecular composition, connectivity and geometry are distinct layers of chemical identification.