Three quotations arrive for a "99 percent phenolic intermediate". One is a liquid that freezes in winter, one a powder melting at 55 degrees, one an aqueous solution only. All began as the same benzene ring; where a single group sits on it decides the downstream product, the registration and the price. Buyers who check only the purity number discover this after the material arrives.

The Ring Is Cheap; the Pattern Is the Product

Phenol (CAS 108-95-2) itself is a commodity: C6H6O, molecular weight 94.11, melting point 43 degrees, boiling point 181.7 degrees, isolated originally from coal tar. Almost nobody buys it for its own sake; the margin lives in its derivatives. Add a methyl and you have the cresols. Add a second hydroxyl and you enter the diols. Add an amino group and a sulfonic acid group, as in 4-aminotoluene-3-sulfonic acid (CAS 88-44-8), and the same hydrophobic ring becomes a water-soluble azo-dye coupling component with a melting point near 300 degrees and an assay quoted at 98.5 percent. One ring, one assay number, three industries.

Alkyl Phenols: Cresols and a CAS That Outranked Its Name

m/p-Cresol (CAS 84989-04-8) is sold as a mixed-isomer liquid, C7H8O, melting range 11 to 12 degrees, boiling range 201 to 203 degrees, slight water solubility and a shelf life of 24 months. The isomers depress each other's freezing point, but the ratio is not cosmetic detail: the para isomer feeds antioxidant and resin chemistry while the meta isomer routes into pesticides and specialty resins. A quotation that omits the m/p ratio is quoting a eutectic, not a raw material.

Then there is a lesson from our own catalogue. The page titled 4-tert-butylphenol (CAS 80-46-6) carries the formula C11H16O and the synonym 4-tert-amylphenol. Count the carbons: tert-butylphenol is C10H14O, CAS 98-54-4. The CAS number and the C11 formula both describe p-tert-amylphenol. Both molecules alkylate rings for resins, but they are different registrations, and a formulator ordering the butyl compound for that CAS would receive the amyl one. When a name and a CAS disagree, the CAS plus the molecular formula win.

Chlorinated phenols repeat the lesson in miniature. 2,4-dichloro-3,5-dimethylphenol (CAS 133-53-9), C8H8Cl2O, molecular weight 191.05, melting point 91 to 96 degrees, is DCMX: the two-chlorine sibling of PCMX with the second chlorine in a different ring position. Same phenol backbone, one extra atom, one different position, and you have a new CAS number, a new biocide dossier and a new price.

Diols: Resorcinol, Toluquinol and What an Alkyl Chain Buys

Resorcinol (CAS 108-46-3) places two hydroxyls meta to each other: C6H6O2, molecular weight 110.11, white needles that blush pink on air oxidation, melting point 110 degrees, density 1.28 grams per cubic centimetre, a 1 percent solution reading pH 5.0. The meta geometry drives the resorcinol-formaldehyde condensates that bind rubber to steel cord and carries into pharmaceutical uses.

Alkylating that ring changes the customer. 5-heptylresorcinol (CAS 500-67-4), C13H20O2, molecular weight 208.30, keeps the two hydroxyls but hangs a seven-carbon chain beside them, turning a water-soluble diol into a beige crystalline powder built for cosmetic work where the ring must dissolve in oils, not water. Same core chemistry, opposite solubility profile.

Toluquinol (CAS 95-71-6), C7H8O2, molecular weight 124.14, melting point 124 degrees, is a methylated hydroquinone, and it inherits the family habit: hydroquinone-type rings oxidise to quinones and darken on standing, which is why the storage line matters as much as the assay. A caution on reading datasheets: this page's synonym list includes 2,5-xylenol, which is a different compound with the formula C8H10O. Synonym lists are typed by hand; the CAS and the formula are the identity.

Benzaldehydes: para, 3,4,5 and the ortho Oddity

On the aldehyde side, position does the same work. p-Hydroxybenzaldehyde (CAS 123-08-0), C7H6O2, molecular weight 122.12, melting point 112 degrees, carries its hydroxyl para to the formyl group, and every paraben preservative is an ester built on that pair.

3,4,5-trimethoxybenzaldehyde (CAS 86-81-7), C10H12O4, molecular weight 196.2, melting point 72 to 74 degrees, stacks three electron-donating methoxy groups on one ring, which is exactly the substitution pattern the antibiotic trimethoprim inherits. Keep it dark: benzaldehydes oxidise to their benzoic acids, and an acid value or a benzoic-acid limit belongs on the specification next to the assay.

2,5-dimethoxybenzaldehyde (CAS 93-02-7), C9H10O3, molecular weight 166.17, melting point 46 to 48 degrees, sits just above summer warehouse temperatures, so the powder that flows in November arrives as consolidated flakes in August; packaging is part of the quotation. And o-phthalaldehyde (CAS 643-79-8), C8H6O2, molecular weight 134.13, melting point 55 degrees, is the ortho oddity: two aldehydes on adjacent carbons close onto primary amines to form fluorescent isoindoles, which makes OPA the working reagent in amine and amino-acid assays, while a separate high-level-disinfectant market replaced glutaraldehyde with it. One molecule, two unrelated industries, both traced to the ortho geometry.

Two Oddballs: Phenoxyethanol and Glyoxylic Acid

2-Phenoxyethanol (CAS 122-99-6), C8H10O2, molecular weight 138.16, boiling point 245.2 degrees, density 1.094, carries a phenol ring and a glycol-ether tail, and its older name, phenyl cellosolve, gives the game away: it evaporates slowly, dissolves across water and oil phases and doubles as a preservative in its own right. The same molecule serves both jobs, but the grade, the tests and the certificate differ, so state the intended use when you enquire.

Glyoxylic acid (CAS 298-12-4), C2H2O3, molecular weight 74.04, is stranger still: the free acid barely exists outside water, hydrating and oligomerising the moment it is born, so commerce knows only the 50 and 70 percent aqueous solutions, boiling near 111 degrees, with a shelf life of just 12 months. Compare offers per kilogram of active C2H2O3 at the stated strength, not per kilogram of drum, or the cheaper 50 percent solution quietly costs more.

Eight Lines for a Phenolic Enquiry

First, CAS and molecular formula, checked against the name, because the amylphenol page above shows how the two diverge. Second, isomer ratio wherever the product is a mixture, cresols included. Third, melting point with its window, the cheapest purity gate a solid has. Fourth, for aldehydes, an acid value or benzoic-acid limit alongside the assay. Fifth, strength basis for any solution, glyoxylic acid being the standing example. Sixth, storage and packaging: dark and sealed for the hydroquinone types, heated or drum-lined handling where the melt point dips below the warehouse. Seventh, the regulatory identity spelled out for chlorinated phenols, DCMX and PCMX being different dossiers. Eighth, a sample with a batch CoA before scale, because on a substituted ring the position you cannot see is the product you are buying.