= cis-Stilbene
<cis-Stilbene> is the Z geometric isomer of 1,2-diphenylethene, with two <phenyl groups> on the same side of the central carbon–carbon <double bond>. Its <molecular formula> is $\mathrm{C_{14}H_{12}}$ and its CAS identifier is 645-49-8. The NIST identity record explicitly distinguishes it from <trans-Stilbene>.<cis-Stilbene in the NIST Chemistry WebBook>[Formula, CAS, Z-isomer structure and stereoisomer cross-reference.]{ref}
Each carbon of the central double bond bears a <phenyl group> and a <hydrogen> atom. Under the <Cahn-Ingold-Prelog priority rules>, the carbon-attached phenyl group has higher priority than hydrogen at each end. The two higher-priority groups lie on the same side, giving Z. This explains why the cis and Z descriptions agree for this molecule; it does not make the two naming systems interchangeable for every substituted alkene.
The central connectivity can be written $\mathrm{C_6H_5-CH=CH-C_6H_5}$. Changing its geometric arrangement does not alter the count of fourteen carbons and twelve hydrogens. A molecular mass measurement consistent with that formula therefore cannot by itself distinguish the cis and trans isomers.
For an original amount calculation, $0.100\,\mathrm{mmol}$ corresponds to approximately 18.0 mg using the recorded <molar mass>. A pure cis sample, a pure trans sample and a cis/trans mixture can all have that same total amount and elemental composition. Determining the mixture requires an observation sensitive to structure or a separation that resolves its components, rather than simply weighing it more precisely.
The important scientific distinction is between identifying a formula and identifying a stereoisomer. An experimental assignment should connect its measured response to appropriate reference information and stated sample conditions. The registry's specific structural identifier fixes which isomer the name denotes; it does not independently establish the purity of a particular laboratory sample.
Back to article page
