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A compound containing a carbon–carbon double bond.

An alkenyl group is a monovalent carbon–hydrogen substituent derived from an alkene by removing one hydrogen atom. The attachment can be at a double-bonded carbon or elsewhere in the carbon skeleton. For an acyclic parent with one double bond, the group composition is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n-1}$.^[ref]

For example, a vinyl group attaches as $\mathrm{R}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_2{\mathrm{H}}_3$, whereas an allyl group attaches as $\mathrm{R}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_3{\mathrm{H}}_5$. The attachment atom in the first example belongs to the double bond; in the second it is adjacent to the double bond. A prenyl group is another alkenyl group, with five carbon atoms.^[ref]

The missing hydrogen identifies an attachment site in a larger molecule; it does not mean that the completed attached group must contain an unpaired electron. Formulae alone also do not specify connectivity. For $n=3$, the composition ${\mathrm{C}}_3{\mathrm{H}}_5$ does not by itself tell where the attachment or double bond lies. A structural formula supplies information that the atom count omits.

All-trans-retinol is the form of retinol whose four acyclic double bonds have trans, or $E$, geometry. Its molecular formula is ${\mathrm{C}}_{20}{\mathrm{H}}_{30}\mathrm{O}$, and it retains the alcohol group characteristic of retinol. The prefix fixes double-bond geometry; it does not specify one frozen conformation about every single bond.^[ref]

The distinction from 11-cis-retinol illustrates why elemental composition is incomplete structural information. Both have the same formula, while one specified double bond has different geometry. A mass measurement consistent with that formula cannot on its own identify which isomer is present.

A second distinction concerns the alcohol group. Lecithin retinol acyltransferase can convert retinol into a retinyl ester by transferring an acyl group. Changing ester status and changing cis–trans geometry are separate chemical operations.^[ref]

For example, an assay that pools all vitamin-A-related signals could register material before and after esterification without resolving the conversion. An interpretable experiment names the chemical form and the structural distinction being measured, rather than inferring identity solely from a shared retinoid classification.

An allylic halide has a halogen atom bonded to a carbon adjacent to a carbon-carbon double bond. The halogen-bearing carbon is not itself one of the two alkene carbons.

The adjacent π system can stabilize developing charge or participate in delocalized intermediates. Allylic halides can therefore undergo substitution with patterns different from an otherwise comparable saturated alkyl halide.

Allyl isothiocyanate has the connectivity $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHC}{\mathrm{H}}_2-\mathrm{N}=\mathrm{C}=\mathrm{S}$. The organic group is attached through nitrogen to the isothiocyanate unit, distinguishing it from an alkyl thiocyanate.

It is a major pungent constituent of black-mustard volatile oil. The structure combines an allyl double bond with a separate nitrogen-carbon-sulfur reactive group, so these sites must be distinguished when discussing a reaction.

Alpha-tocotrienol is the tocotrienol with the additional ring methylation pattern 5,7,8. “Alpha” specifies that pattern, while the tocotrienol part specifies the unsaturated side chain.^[ref] ChEBI records the molecular formula ${\mathrm{C}}_{29}{\mathrm{H}}_{44}{\mathrm{O}}_2$ and average molar mass $424.669\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.^[ref]

Two comparisons distinguish the naming axes. Alpha-tocopherol shares the alpha ring pattern but has six more hydrogen atoms in its saturated chain. Other tocotrienols retain three side-chain double bonds while changing the ring methylation. Thus an analytical statement about “alpha” alone does not identify side-chain saturation.

For an original preparation calculation, suppose 2.000 mg of alpha-tocotrienol is present in a final volume of 20.00 mL. Ignoring impurities and volume uncertainty,An equal mass of alpha-tocopherol contains a different number of molecules because its molar mass differs. Identity therefore enters a concentration calculation before any comparison of biological response.

An experimental comparison can separate identification from function: chromatography with appropriate standards establishes the amount of the selected form, while a specified oxidation assay examines chemical activity. The latter observation depends on the reaction medium and endpoint. Membership in the vitamin E family does not make those two measurements interchangeable or establish a clinical outcome.

An amide bond, in the carboxamide sense used here, is the carbon–nitrogen covalent bond in the unit $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{R}}^{\prime }{\mathrm{R}}^{\prime \prime }$. The nitrogen is directly attached to the carbonyl group's carbon. Merely finding a carbon–nitrogen bond elsewhere in a molecule does not identify an amide bond. The broader IUPAC class name amide also covers other acid-derived compounds; this article concerns the carbonyl linkage.^[ref]

For example, acetamide has connectivity $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{H}}_2$. Its amide C–N bond is the bond from the carbonyl carbon to nitrogen, rather than the adjacent C–C bond. A peptide bond is a biologically important instance of this connectivity in a chain of amino acids.

Chemical resonance describes electron sharing through the nitrogen and carbonyl group. Two useful contributing structures areThese are descriptions contributing to one electronic structure, rather than two molecules rapidly alternating. The amide bond consequently has partial double bond character, and the ordinary amide group favors approximate planarity. Linus Pauling's lecture connects that structural picture to measured bond lengths and explains why hypothetical resonance forms remain useful.^[ref]

This supplies a useful modeling constraint: a chain containing peptide bonds cannot be treated as if every backbone single bond rotates equally freely. Approximate amide planarity constrains the conformations available without asserting that every amide is perfectly planar.

Arachidonic acid is a twenty-carbon polyunsaturated fatty acid with four cis carbon–carbon double bonds, at positions 5, 8, 11 and 14 when counted from the carboxyl group. Its molecular formula is ${\mathrm{C}}_{20}{\mathrm{H}}_{32}{\mathrm{O}}_2$. The shorthand $20:4n-6$ records twenty carbons, four double bonds and the first double bond six carbons from the methyl end.^[ref]

The indexing conventions describe the same structure from opposite ends: the bond between carbons 14 and 15 from the acid end is the first encountered from the methyl end. The cis geometry also distinguishes this molecule from stereoisomers with the same formula. Composition, double-bond positions and geometry all belong to its chemical identity.

A cage isomer of benzene with one carbon–carbon double bond, conventionally described as tricyclo[3.1.0.0(2,6)]hex-3-ene. Its ideal cage geometry belongs to the C2v molecular point group. Molecular symmetry depends on the full three-dimensional cage rather than the apparent symmetry of a planar skeletal projection.

Buta-1,3-diene is $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$, a four-carbon diene with a conjugated system. Adjacent p orbitals allow electron delocalization across the two double bonds and the intervening single bond.

A suitable cisoid conformation can react with a dienophile in a Diels-Alder reaction. The need for the reactive conformation connects internal rotation with cycloaddition reactivity.

A normal mode involving substantial stretching of a carbon–carbon double bond. conjugated systems alter its force constant and infrared intensity.

A carboxyl group is the functional group $-\mathrm{C}(=\mathrm{O})\mathrm{OH}$: one carbon has a double bond to oxygen and a single bond to a hydroxyl group. It gives a carboxylic acid its characteristic acid functionality. In acetic acid, the group is attached to a methyl group, giving $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$.^[ref]

Loss of the acidic hydrogen produces a carboxylate, the conjugate base:The pH influences the balance between these forms. A structural drawing of the neutral acid therefore need not show the predominant charge state in every solution. The carboxyl group is a part of the molecule, while acetic acid names the entire compound.

cis-Resveratrol is the resveratrol stereoisomer with the two aryl groups on the same side of the central double bond, corresponding to the $Z$ configuration. It has the same neutral molecular formula, ${\mathrm{C}}_{14}{\mathrm{H}}_{12}{\mathrm{O}}_3$, and formula-based molar mass as trans-Resveratrol, but a different spatial arrangement.^[ref]

The isomers need different optical response factors. Brent C. Trela and Andrew L. Waterhouse measured an ethanol absorption maximum at $288\mathrm{nm}$ with molar absorption coefficient $12600\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ for the cis form; their stability tests also depended on light exposure and pH.^[ref]

At that maximum, an original $10\mu \mathrm{mol}{\mathrm{L}}^{-1}$ solution in a $1\mathrm{cm}$ path has ideal absorbanceThis is lower than the trans-isomer's corresponding example at its own maximum, but the two maxima occur at different wavelengths. Dividing their peak coefficients does not supply the cis/trans response ratio at one common detector wavelength.

For a mixture at a chosen wavelength $\lambda$, an additive Beer-Lambert law model isOne reading generally cannot identify both concentrations. Two wavelengths help only if their response rows are independent and the coefficients are appropriate to the solution. Multivariate calibration makes that requirement explicit; physical separation by chromatography supplies another route. Neither equal molecular formulas nor a single colour measurement establishes isomeric purity.

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.

A conjugated ester has an ester carbonyl group participating in a conjugated electronic system, commonly an adjacent carbon-carbon double bond. An α,β-unsaturated ester is an important example.

Conjugation connects reactivity at the carbonyl carbon with reactivity at the more distant β carbon. Distinguishing direct carbonyl addition from conjugate addition is consequently essential when predicting products.

A six-membered carbocycle with one carbon–carbon double bond.

A five-carbon cyclic conjugated diene. Cycloaddition across its two double bonds produces a norbornene framework while retaining the original five-membered carbon ring.

Dimethylallyl diphosphate is a five-carbon isoprenoid building block carrying an activated diphosphate group. Its carbon skeleton is $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{C}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-$, attached through oxygen to diphosphate. The neutral-acid molecular formula is ${\mathrm{C}}_5{\mathrm{H}}_{12}{\mathrm{O}}_7{\mathrm{P}}_2$. ChEBI calls this specific compound prenyl diphosphate; the title here distinguishes it from longer prenyl diphosphates.^[ref]

The placement of its double bond differs from that in isopentenyl diphosphate. Matching molecular formulas therefore does not make these substrates interchangeable: an enzyme encounters a specific arrangement of bonds, not just five carbons and two phosphorus atoms.

In the reaction catalysed by dimethylallyltranstransferase, dimethylallyl diphosphate supplies the donor group, isopentenyl diphosphate is the other substrate, and ten-carbon geranyl diphosphate is produced with release of diphosphate.^[ref] Mixing $1\mu \mathrm{mol}$ of each substrate can produce at most $1\mu \mathrm{mol}$ of geranyl product in the ideal isolated reaction. Summing both substrate amounts and predicting two micromoles of product would count molecules without respecting the two-substrate reaction stoichiometry.

A double bond is a covalent bond of bond order two. Restricted rotation and substituent geometry at a carbon–carbon double bond can affect acyl-chain packing in a lipid bilayer.

An enol carbonate is an enol derivative whose oxygen bears a carbonate ester group, represented schematically by ${\mathrm{R}}_2\mathrm{C}=\mathrm{CR}-\mathrm{O}-\mathrm{C}(=\mathrm{O})-\mathrm{O}{\mathrm{R}}^{\prime }$. The enol-derived carbon-carbon double bond and the carbonate group are both part of its reactivity.

Masking an enol as a derivative allows its structure and reactions to be controlled separately from those of the parent carbonyl compound. The exact substitution pattern therefore matters when using an enol carbonate in synthesis.