Chemical identity for the product is defined by CAS Registry Number 33941-98-9, molecular formula C₈H₁₂O₂, and molecular weight 140.18. The structural model is a seven-membered unsaturated cyclic acetal in which an n-propyl substituent is located at C2 and a single C5–C6 olefinic centre differentiates the ring from saturated 1,3-dioxepane, 1,3-dioxane, and 1,3-dioxolane frameworks. Commercial material is the neat, stabilizer-free compound; no antioxidant, solvent, or polymer-bound carrier is specified. The standard synthesis is acid-catalyzed acetalization of butyraldehyde with cis-2-butene-1,4-diol, followed by neutralization, drying, and fractional vacuum distillation. The double-bond geometry is cis because the starting diol is cis-2-butene-1,4-diol; no trans-isomer grade is assigned. The product is supplied as a low-viscosity liquid in nitrogen-flushed, epoxy-phenolic lined steel drums. Regulatory status includes FEMA GRAS 4285 for flavour use; published exposure data for fragrance applications of this specific configuration is limited.

On a 50 L glass-lined vessel, the acetalization is driven by azeotropic water removal with toluene or cyclohexane at a reflux ratio of 3:1 to 5:1. Equilibrium is typically reached within 4–6 h when water collection ceases and the acid catalyst is neutralized. The principal source of batch-to-batch variance is not crude conversion but the vacuum distillation cut point. A 20-plate Oldershaw column at 10–15 mmHg is used, and the accepted main fraction is collected within a narrow head-temperature window after the forerun. Batches falling outside the refractive index or GC-FID assay limits are redistilled or blended only after confirming that unreacted butyraldehyde does not exceed 0.5%. The n-propyl substituent and the C5–C6 double bond increase calculated logP relative to 2-propyl-1,3-dioxolane, which reduces water solubility and slows hydrolytic aldehyde release in aqueous matrices.

What Limits Hydrolytic Stability in Citrate-Buffered Matrices?

Hydrolysis is the controlling instability pathway for this product. In acidic aqueous media, protonation of an acetal oxygen initiates ring opening and releases butyraldehyde plus cis-2-butene-1,4-diol. The C5–C6 double bond alters ring conformation and stabilizes the alkene-containing leaving group, producing a release profile that is less abrupt than an acyclic acetal but more matrix-sensitive than a saturated 1,3-dioxane analog. For structurally related cyclic acetals, half-lives in citrate buffer at pH 3.0 and 40 °C can fall below 2 h, whereas half-lives at pH 6.0–7.0 and 25 °C can exceed 24 h. Published data for this specific compound under fixed buffer conditions is limited. The material should be pre-dried when ambient handling exceeds 60% relative humidity, and contact with strong mineral acids, Lewis acids, or oxidizing agents must be avoided. Released butyraldehyde can form Schiff bases with primary amines; therefore, amine-based additives are incompatible when aldehyde release must be controlled.

Because the C5–C6 olefin is oxidizable, long-term storage under air may generate trace hydroperoxides. Production drums are nitrogen-blanketed. If bulk storage is opened repeatedly, a peroxide value check before distillation is standard for unsaturated cyclic acetals. No stabilizer system is required for the neat product when the water content remains below 0.5% and the closed container is kept below 30 °C. The flash point of 63 °C places the material in the combustible-liquid class, but vapour accumulation in unventilated mixing rooms must still be controlled. Oxidative degradation products can shift the refractive index and produce resinous high-boiling residues during distillation.

Comparison with saturated low-molecular-weight acetals clarifies why this substance cannot be treated as a direct drop-in replacement for shorter-chain aldehyde precursors. The seven-membered unsaturated ring adds molecular weight, raises the atmospheric boiling point, and introduces a double bond that modifies both sensory release and storage oxidizability.

The unsaturated dioxepine is more sensitive to oxygen than the saturated dioxane analog, as expected from the olefinic position. Unlike saturated 1,3-dioxolanes, it is not recommended for systems requiring a rapid aldehyde burst at neutral pH because hydrolysis is delayed and water activity-dependent. Published comparative sensory data is limited; the use-level difference is therefore established by threshold testing in the target matrix rather than by direct substitution at equal weight.

Boiling Range, Flash Point, and Stabilizer-Free Storage Requirements

Commercially representative certificate-of-analysis ranges for the neat product are shown in the specification table. Values are for the stabilizer-free liquid. If a stabilizer-free status is not required, buyers should not add BHT or tocopherol without evaluating their effect on oxidative stability and sensory neutrality.

Batch-to-batch variance is controlled primarily by distillation cut rather than by post-distillation blending. The refractive index window is narrow because unsaturated acetal isomers and ring-opened diol impurities alter the value. Water content above 0.5% is not corrected by adding desiccant directly to the drum; the batch is redistilled after treatment with molecular sieves. Storage below 30 °C in closed, nitrogen-blanketed containers is the standard operational boundary. Avoid exposure to strong oxidizers such as concentrated nitric acid and peroxide initiators. The product is not a monomer or crosslinking feedstock; use in polymer systems may result in hydrolytic ring opening and generation of butyraldehyde under acidic or high-temperature processing conditions.

If the Application Requires Extended Aldehyde Release, Saturated Acetals Are the Wrong Benchmark

In beverage, confectionery, and baked-goods flavour systems, the compound functions primarily as a release-form of butyraldehyde. The typical compounding procedure is to prepare a 0.1%–1.0% solution in ethanol or propylene glycol, then dose the solution into the process stream to a final concentration in the finished matrix of 1–10 mg/kg. Final levels must be established by triangle testing according to ISO 4120:2021, because partition behaviour differs between clear beverages, emulsions, and fat-containing matrices. Published FEMA GRAS 4285 average maximum use-level data for common flavour categories fall below 20 mg/kg; published data for dairy and high-fat systems is limited.

For fragrance evaluation, a 15% ethanol solution is applied to a smelling strip and compared against a reference standard of the free aldehyde. The product is typically evaluated at 0.5–5.0% of the fragrance concentrate. The unsaturated ring provides a softer aldehyde impression than free butyraldehyde. Because hydrolysis releases cis-2-butene-1,4-diol, the drydown may show a faint winey or lactonic note that saturated acetals do not produce. This distinction is the principal reason for selection in compositions where a green–fruity aldehyde character is required without the sharpness of the free aldehyde.

Process compatibility constraints include avoiding aqueous phases at pH below 4.0 for long holding times, avoiding high-shear homogenization with hot water above 40 °C, and adding the diluted acetal after the heating stage of the production sequence whenever possible. In spray-dried flavour carriers, the inlet temperature should be adjusted so that particle surface moisture does not exceed 5% by thermogravimetric analysis; published data for this specific configuration in spray-dried matrices is limited. The product is not a primary fixative, and it does not replace high-molecular-weight musks or aldehyde stabilizers in concentrated perfume oils.