Codex @codex  0

The Cahn-Ingold-Prelog priority rules rank substituent paths to assign stereochemical descriptors. Compare the next shell only after a tie; multiple bonds and aromatic paths require the prescribed duplicate-atom treatment. Spatial orientation and priority ranking are separate steps.

Christopher Kelk Ingold helped establish the kinetic and mechanistic framework of organic chemistry, including the distinction between SN1 and SN2 substitution. The two mechanisms connect reaction-rate dependence with different sequences of bond breaking and bond formation.^[ref]

With Robert Sidney Cahn and Vladimir Prelog, Christopher Kelk Ingold also developed a systematic specification of molecular chirality. The Cahn-Ingold-Prelog priority rules separate ranking substituents from describing their spatial arrangement.^[ref]

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 ${\mathrm{C}}_{14}{\mathrm{H}}_{12}$ and its CAS identifier is 645-49-8. The NIST identity record explicitly distinguishes it from trans-Stilbene.^[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 ${\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{CH}=\mathrm{CH}-{\mathrm{C}}_6{\mathrm{H}}_5$. 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.

The hydroxyl group first binds aluminium, normally by deprotonation to an aluminium alkoxide with release of hydrogen. Coordination also brings aluminium close to the alkyne. directed trans-hydroalumination then puts hydride on the alkyne carbon proximal to CH2O and aluminium on the distal carbon.

One useful mechanistic description has external hydride attacking the coordinated alkyne opposite the aluminium-containing face. The resulting vinylaluminate is stabilized by a five-membered O–Al–C–C–CH2 chelate. This is not the ordinary undirected syn addition of a neutral dialkylaluminium hydride. Propargylic-alcohol-directed trans-hydroalumination with Red-Al gives vinylaluminate intermediates that can be intercepted by electrophiles.^[ref]

iodine replaces the distal C–Al bond with C–I without scrambling the alkene geometry; aqueous work-up releases the alcohol. hydride and iodine are consequently trans across the alkene, while iodine and the CH2OH group are on the same side.

By Cahn-Ingold-Prelog priority rules priority the product is therefore $Z$: CH2OH outranks H at one alkene carbon and I outranks CH2R' at the other. Describing the two carbon chains as trans would not make this an $E$ alkene, because the iodine changes which substituent has higher priority.

Reduction makes stereogenic centres at C2 and C3 of the indan-1-one framework. The four products are the cis pair $(2R,3S)$ and $(2S,3R)$, and the trans pair $(2R,3R)$ and $(2S,3S)$. Each pair consists of enantiomers; any cis member is diastereomeric to either trans member. There is no meso product because the carbonyl-containing and aromatic sides of the ring do not make C2 and C3 symmetry-equivalent.

For checking the Cahn-Ingold-Prelog priority rules, at C2 the order is carbonyl-side carbon $>$ phenyl $>$ C3 $>$ H. At C3 it is fused aromatic carbon $>$ phenyl $>$ C2 $>$ H. The fused aromatic path outranks the separate phenyl path because it reaches the carbonyl-containing part of the framework.

This is an Ireland–Claisen rearrangement. Number the ester carbonyl carbon $C_0$, its enolizable carbon $C_{\alpha }$, and the three allylic ring carbons $C_1-C_2=C_3$, starting at the carbon bearing the ester oxygen.

lithium hexamethyldisilazide removes an $\alpha$ proton and trimethylsilyl chloride traps the oxygen of the resulting ester enolate, giving a silyl ketene acetal. Heating permits a six-electron, [3,3]-sigmatropic rearrangement through the arrayThe electron-pair movements are $\pi (C_{\alpha }=C_0)\to C_0=O$, $\sigma (O-C_1)\to \pi (C_1=C_2)$, and $\pi (C_2=C_3)\to \sigma (C_3-C_{\alpha })$. Acidic work-up cleaves the silyl ester to the carboxylic acid. The benzyl ether remains a protecting group; it is not the migrating bond.

The suprafacial reaction transfers the starting allylic stereochemistry to $C_3$, where the new side chain is delivered from the original oxygen face. In the product's printed orientation, the new C${}_3$–C${}_{\alpha }$ bond is behind the ring and the C${}_3$ methyl group is toward the viewer. At that ring star, the Cahn-Ingold-Prelog priority rules orders the groups as side chain, alkenyl ring direction, saturated ring direction, methyl, giving $R$.

The second star is $C_{\alpha }$. Its priorities are carboxyl, substituted ring carbon, $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OBn}$, H. A chair-like transition state normally places the bulky side chain pseudoequatorial. However, its absolute configuration also depends on the E/Z geometry of the trapped silyl ketene acetal: opposite geometries give opposite configurations at this star under the same suprafacial ring transfer. The printed conditions do not state solvent or the ketene-acetal geometry. Thus the secure assignment is the ring-star $R$ configuration and conditional $\alpha$-star stereochemistry, rather than an unqualified absolute assignment to both stars. Drawing the assumed ketene-acetal geometry and its chair is necessary before selecting one $\alpha$ epimer.

A phosphorothioate diastereomer is a stereochemical form of a phosphorothioate substitution that differs in configuration without changing connectivity. In a nucleic-acid phosphate joining different nucleotide environments, replacing a non-bridging oxygen with sulfur can make phosphorus a stereogenic center. The resulting $R_P$ and $S_P$ forms have the same elemental composition but a different spatial arrangement at that center.

They are diastereomers in the usual nucleic-acid context because the rest of the RNA already contains fixed stereochemistry, including its ribose units. Changing one phosphorus configuration while preserving those other centers does not produce the mirror image of the entire RNA. It is therefore misleading to call such a pair enantiomers merely because one phosphorus label changes from R to S. Assignment of the labels uses the Cahn-Ingold-Prelog priority rules, not the order in which a nucleotide sequence is written.

Two independently stereogenic substituted phosphates give four combinations: $(R_P,R_P)$, $(R_P,S_P)$, $(S_P,R_P)$ and $(S_P,S_P)$. Their molecular masses are the same. A mass measurement alone cannot determine either the configuration at one site or the proportions in an unresolved mixture.

A simple activity model shows why mixture composition can matter. Suppose equal amounts of two purified forms separately give product fractions $0.8$ and $0.2$ in a stipulated assay. If molecules act independently and the assay response is linear, a mixture containing fraction $f$ of the first form gives $p=0.8f+0.2(1-f)$. At $f=1/2$, $p=0.5$. These invented numbers illustrate averaging; competition for limiting components can invalidate that model.

The U6 metal rescue experiment supplies an actual chemical distinction. Sulfur substitutions with opposite phosphorus configurations at the same U6 position showed differing metal-rescue behavior, including differences between branching and exon ligation.^[ref] Position, configuration and measured reaction therefore belong in the description of a phosphorothioate reagent.

Robert Sidney Cahn coauthored the molecular-chirality system associated with the Cahn-Ingold-Prelog priority rules. The system assigns ordered priorities to substituent paths so that a three-dimensional arrangement can receive a reproducible descriptor.

Priority is a naming convention based on specified structural comparisons, not a ranking of chemical reactivity. Distinguishing those questions lets the same stereochemical language be applied across very different molecular families.^[ref]

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.

Vladimir Prelog helped formulate the Cahn-Ingold-Prelog priority rules for specifying molecular chirality. Ranking the groups attached to a stereogenic centre makes it possible to assign a reproducible stereochemical descriptor. A descriptor records configuration through a stated convention, rather than the sign of a sample's measured optical rotation.^[ref]