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2-Nonenal

2-Nonenal is an organic compound classified as a medium-chain unsaturated aldehyde, with the molecular formula C₉H₁₆O and a molecular weight of 140.22 g/mol.^[1] It exists as a colorless to pale yellow liquid that is insoluble in water but soluble in ethanol and oils, featuring a double bond between carbons 2 and 3 and an aldehyde group at carbon 1.^[1] Chemically, it belongs to the family of monounsaturated fatty aldehydes and is commonly encountered in its (E)-isomer form, contributing to its characteristic oily, grassy, and slightly fatty aroma.^[1]^[2] In food and fragrance industries, 2-Nonenal serves as a key flavoring and aroma ingredient, imparting notes reminiscent of cucumber, melon, and aged fats, and it occurs naturally in products such as beer, coffee, buckwheat, and watermelon.^[1] Its sensory profile makes it valuable for enhancing the taste of processed foods and beverages, though it must be used in controlled amounts due to its potential to evoke off-flavors at higher concentrations.^[1] Additionally, 2-Nonenal is a plant metabolite found in species like oats (Avena sativa) and certain fungi, underscoring its role in natural biochemical pathways.^[1] Biologically, 2-Nonenal is notably implicated in human body odor, particularly the "aging odor" associated with elderly individuals, where it arises from the oxidative degradation of ω7-monounsaturated fatty acids, such as palmitoleic acid, in skin surface lipids.^[3] This process intensifies with age due to increased reactive oxygen species (ROS) and slowed metabolism, leading to higher levels of lipid peroxidation products that produce an unpleasant, greasy scent.^[3] Beyond odor, 2-Nonenal exhibits cytotoxic effects, promoting apoptosis in keratinocytes, reducing cell proliferation, and thinning epidermal layers, which may contribute to skin aging and damage.^[3] It has also been identified as a uremic toxin, potentially exacerbating symptoms like nausea and cardiovascular issues in patients with kidney dysfunction, though its precise toxicological impact requires further study.^[1] Safety assessments indicate it can cause skin and eye irritation, classifying it as a mild sensitizer.^[1]

Chemical Identity

Nomenclature and Structure

2-Nonenal is an organic compound commonly referred to by its trivial name, which indicates the position of the double bond and the aldehyde group in the nonane chain. The systematic IUPAC name for its predominant isomer is (2E)-non-2-enal, reflecting the trans configuration at the double bond.^[2] The general designation "2-Nonenal" typically refers to this E-form, which is the most commonly studied and naturally occurring variant.^[4] The molecular formula of 2-Nonenal is C₉H₁₆O, corresponding to a hydrocarbon chain with one oxygen atom incorporated into the aldehyde functionality. Structurally, it features a straight nine-carbon chain with an aldehyde group (-CHO) at position 1 and a carbon-carbon double bond between positions 2 and 3, classifying it as an α,β-unsaturated aldehyde. This arrangement can be depicted as CH₃(CH₂)₅CH=CHCHO, where the trans geometry in the E isomer positions the aldehyde and alkyl chain on opposite sides of the double bond.^[5]^[2] 2-Nonenal exhibits geometric isomerism due to the internal double bond, existing in E (trans) and Z (cis) forms. The E isomer predominates in natural sources, contributing to its characteristic presence in biological and food-related contexts.^[6]^[7] As a derivative of fatty acid oxidation, 2-Nonenal arises from the peroxidation of omega-7 unsaturated fatty acids, such as palmitoleic acid (16:1 n-7), through non-enzymatic breakdown processes.^[6]

Physical and Chemical Properties

2-Nonenal is a colorless to pale yellow liquid with a powerful, penetrating odor.^[8] The following table summarizes key physical properties of 2-Nonenal:

Property	Value	Conditions
Molecular weight	140.22 g/mol	-
Boiling point	188–190 °C	760 mm Hg
Density	0.855–0.865 g/cm³	25 °C
Refractive index	1.454–1.460	20 °C
Solubility in water	Insoluble (204.9 mg/L estimated)	25 °C

2-Nonenal is soluble in organic solvents such as ethanol and ether.^[8]^[9] As an α,β-unsaturated aldehyde, 2-Nonenal exhibits reactivity toward nucleophiles, primarily through Michael addition at the β-position, leading to the formation of addition products.^[10] It is chemically stable under normal storage conditions but is incompatible with strong oxidizing agents, acids, and bases, which may promote decomposition or further reactions.^[11]^[12]

Synthesis and Production

Laboratory Methods

One common laboratory method for synthesizing (E)-2-Nonenal involves ozonolysis of castor oil, which is rich in ricinoleic acid. The process entails bubbling ozone through a solution of castor oil in methanol or acetic acid at low temperature to cleave the double bond in ricinoleic acid, forming an ozonide intermediate. This is followed by a reductive workup using dimethyl sulfide to reduce the ozonide to the aldehyde, and subsequent mild acidification with dilute sulfuric acid to promote dehydration and yield (E)-2-Nonenal.^[13] This one-pot procedure achieves yields up to 80% with 95% purity, leveraging the high ricinoleic acid content (about 90%) of castor oil as an inexpensive starting material.^[13] Another established route employs the Wittig reaction, where heptanal is condensed with formylmethylene triphenylphosphorane (Ph₃P=CHCHO), generated in situ from the corresponding phosphonium salt and a base such as sodium hydride in an aprotic solvent like tetrahydrofuran. The reaction proceeds under inert atmosphere at room temperature, forming the α,β-unsaturated aldehyde (E)-2-Nonenal via stereoselective olefination, with triphenylphosphine oxide as the byproduct. This method typically affords yields of around 66%, offering a direct and stereocontrolled approach suitable for small-scale preparations.^[14] The aldol condensation provides an alternative synthesis by reacting heptanal with acetaldehyde under basic conditions, such as in the presence of sodium hydroxide or a solid base like magnesium oxide in ethanol or aqueous media at 80–100°C. The enolate from heptanal (or acetaldehyde, depending on conditions) adds to the carbonyl of the other aldehyde, forming a β-hydroxy aldehyde intermediate, which undergoes dehydration upon heating or acidification to yield (E)-2-Nonenal as the major product. Laboratory yields for this cross-condensation are typically low, up to 21%, influenced by catalyst choice and temperature control to minimize self-condensation side products.^[15] Across these methods, reactions are conducted under inert atmosphere (e.g., nitrogen or argon) to prevent aerial oxidation of the sensitive aldehyde functionality, with typical overall lab-scale yields of 50–80%. Purification is achieved by distillation under reduced pressure (boiling point approximately 88–90°C at 12 mmHg) to isolate the pure (E)-isomer, often confirmed by spectroscopic analysis.^[1]

Industrial Sources

2-Nonenal is primarily produced industrially through the ozonolysis of ricinoleic acid, the main unsaturated fatty acid component derived from castor oil obtained from castor beans (Ricinus communis). This one-pot process involves treating castor oil with ozone in industrial solvents such as methanol or acetic acid at low temperatures, followed by reductive cleavage of the ozonide using dimethyl sulfide and subsequent dehydration with sulfuric acid to favor the (E)-isomer. The method yields up to 80% (E)-2-nonenal with 95% purity, making it economically viable due to the low cost of castor oil and the simplicity of the scalable procedure, which is commonly employed in the flavor and fragrance sectors for aroma compound synthesis. Ozone handling requires appropriate safety measures due to its reactivity.^[13] Biotechnological production of 2-nonenal, though less prevalent than chemical synthesis, utilizes microbial biotransformation with engineered yeast strains such as Saccharomyces cerevisiae on lipid substrates to generate the aldehyde via enzymatic pathways mimicking lipid peroxidation. These approaches involve whole-cell fermentation systems that convert precursors like unsaturated fatty acids such as linoleic acid into (2E)-nonenal, offering potential sustainability benefits but remaining niche due to lower yields (around 9%) and optimization challenges compared to traditional methods.^[16] Commercially, trans-2-nonenal (CAS 18829-56-6) is supplied by chemical manufacturers including Sigma-Aldrich and Thermo Fisher Scientific for use in food flavoring at concentrations of parts per million (ppm), where it imparts cucumber-like and fatty notes. Industrial grades typically achieve purity levels of 97% or higher for the (E)-isomer, ensuring compliance with regulatory standards for aroma applications.^[5]^[17]

Natural Occurrence

In Foods and Beverages

2-Nonenal occurs naturally in several foods and beverages, primarily as a volatile compound derived from lipid oxidation processes. In aged beer, it forms through the peroxidation of linoleic acid during wort production and is released non-oxidatively during storage, contributing to off-flavors described as stale or cardboard-like. Levels of 2-nonenal in beer typically increase from trace amounts to above its flavor threshold of 0.1 μg/L (0.1 ppb) over time, particularly under accelerated aging conditions such as 6 days at 38°C or extended storage at room temperature.^[18] In roasted buckwheat and related grain products, such as soba tea, 2-nonenal serves as a key aroma component, imparting nutty and green sensory notes that enhance the characteristic cereal-like profile. Studies using gas chromatography-mass spectrometry (GC-MS) have identified (E)-2-nonenal in freshly ground buckwheat flour and roasted samples, with high odor activity values (OAVs) ranging from 219.8 to 489.2, indicating its significant contribution to the overall aroma. These compounds arise from thermal degradation during roasting, distinguishing soba tea's earthy, toasty scent.^[19]^[20] 2-Nonenal is present at low levels in certain vegetables and fruits, notably cucumbers and melons, where it contributes a cucumber-like odor alongside more dominant volatiles. In cucumbers, (E)-2-nonenal concentrations range from 0.024 to 0.132 μg/g fresh weight, yielding aroma values of 48 to 264 relative to its threshold of 0.5 ng/g, supporting green and tallowy notes during fruit development. Similarly, in muskmelons, (E)-2-nonenal appears as a potent odorant at levels of 44 to 189 μg/kg, adding fatty and green nuances to the complex melon flavor profile identified through aroma extract dilution analysis. It also forms in oxidized edible oils, such as soybean oil, during heating or storage, where it emerges from trilinolein degradation at frying temperatures around 190°C, contributing plastic and fatty off-odors.^[21]^[22]^[23] The compound arises in food processing through the peroxidation of linoleic acid, a polyunsaturated fatty acid abundant in many plant-based ingredients, leading to hydroperoxide intermediates that decompose into aldehydes like 2-nonenal during frying or prolonged storage. Typical concentrations in processed foods range from 0.1 to 10 ppb, sufficient to influence sensory quality without overwhelming other flavors. As a flavorant, 2-nonenal is added to dairy and meat products to enhance green and fatty notes, mimicking natural oxidation-derived aromas in items like butter or cooked meats, where it blends with descriptors such as cucumber, citrus, and oily.^[18]^[9]^[24]

In Human Physiology

2-Nonenal is generated endogenously in the human body through non-enzymatic lipid peroxidation of omega-7 monounsaturated fatty acids, such as palmitoleic acid, within sebum on the skin surface.^[25] This process is initiated by reactive oxygen species and lipid hydroperoxides that degrade these fatty acids, leading to the formation of the unsaturated aldehyde, notably the (E)-isomer.^[10] This process can be initiated by hydroperoxides derived from the oxidation of squalene, a major component of skin surface lipids, which propagate the peroxidation of ω7 monounsaturated fatty acids under oxidative stress.^[26] Production primarily occurs in sebaceous glands, where sebum is synthesized and secreted onto the skin surface, with 2-Nonenal subsequently excreted via skin lipids and sweat.^[25] The compound is most notably detected on areas rich in sebaceous glands, such as the nape of the neck. Detectable levels of 2-Nonenal rise after age 40, coinciding with increased lipid peroxidation in skin surface lipids, and were first identified as a component of human body odor in 2001 by Japanese researchers.^[25] This age-related increase stems from declining antioxidant defenses, including reduced concentrations of vitamin E in the epidermis and sebum, which fail to adequately neutralize accumulating lipid peroxides.^[27] In older adults, 2-Nonenal emissions from the skin surface are higher compared to negligible levels in younger individuals. This compound contributes to characteristic changes in body odor associated with aging.^[25]

Sensory Characteristics

Odor Profile

2-Nonenal exhibits a distinctive odor profile characterized as greasy, grassy, and fatty, with subtle nuances of cucumber. At higher concentrations, the scent is often perceived as unpleasant and penetrating, evoking a stale or oxidized quality. When diluted, it shifts to a more nuanced waxy and orris-like aroma, contributing to complex sensory notes in various contexts.^[28]^[25] The olfactory detection threshold for 2-Nonenal is remarkably low, ranging from 0.08 to 0.1 ppb in air, making it highly potent even in trace amounts. Its flavor threshold in water is approximately 6 ppb, allowing it to influence taste perceptions at minimal levels. These thresholds underscore its role as a impactful aroma compound in both gaseous and aqueous media.^[7]^[29] The (E)-isomer predominates in natural sources and demonstrates greater olfactory potency compared to the (Z)-isomer, with lower detection thresholds and stronger contributions to overall scent intensity. This isomer specificity enhances its prevalence in aged products. Compared to related compounds like (E)-2-hexenal, which imparts a fresh green apple or leafy note, 2-Nonenal's longer carbon chain lends a deeper, earthier undertone while retaining some grassy elements.^[30] Historically, 2-Nonenal was identified as a primary contributor to stale flavors in beer during the 1970s, notably through research by Jamieson and Van Gheluwe in 1970, which linked it to cardboard-like off-notes in oxidized brews.^[18]

Detection and Analysis

Gas chromatography-mass spectrometry (GC-MS) serves as the primary analytical technique for detecting and quantifying 2-Nonenal due to its volatility and suitability for trace-level analysis in complex matrices such as air, food, and biological samples.^[26] Headspace sampling is commonly employed to capture volatile compounds like 2-Nonenal without direct matrix interference, followed by separation on a non-polar capillary column and identification via electron ionization mass spectrometry.^[31] Characteristic fragment ions at m/z 41, 55, and 70 facilitate specific identification, corresponding to allylic and alkyl fragments typical of unsaturated aldehydes.^[32] Solid-phase microextraction (SPME) is frequently coupled with GC-MS for sensitive, solvent-free extraction of 2-Nonenal, particularly in trace analysis from skin swabs, textiles, or food products.^[33] This technique involves exposing a coated fiber to the sample headspace, which adsorbs 2-Nonenal for subsequent thermal desorption into the GC inlet, achieving detection limits below 1 ppb (e.g., 0.01 µg/L in beer matrices) and enabling non-invasive sampling in body odor studies.^[34] The method's high preconcentration efficiency supports linearity over ng to µg ranges, with recovery rates exceeding 95% in fortified samples.^[35] High-performance liquid chromatography (HPLC) is applied for the analysis of derivatized 2-Nonenal in biological samples, where the compound is converted to more stable and detectable forms, such as hydrazones, to enhance UV or fluorescence detection.^[36] Pre-column derivatization with agents like 2,4-dinitrophenylhydrazine improves sensitivity in plasma or tissue extracts, allowing quantification at low ng/mL levels after solid-phase extraction cleanup.^[37] This approach is particularly useful for studying lipid peroxidation products in physiological contexts, though it requires careful optimization to avoid artifact formation.^[38] Sensory evaluation complements instrumental methods in the food industry, where trained panels assess 2-Nonenal contributions to off-odors through standardized sniffing and scaling protocols.^[39] Panels of 8–12 experts, calibrated against known concentrations near the odor threshold of 0.05–0.1 µg/L, rate intensity and quality attributes to correlate sensory perception with analytical data in products like beer and oils.^[40] Authentic (E)-2-Nonenal standards (CAS 18829-56-6) are essential for method validation, calibration curves, and spike recovery experiments, ensuring accurate quantification across techniques.^[41] These reference materials, available at purities >95%, provide molecular ions at m/z 140 for MS confirmation and support inter-laboratory reproducibility.^[42]

Biological Significance

Role in Aging and Body Odor

2-Nonenal is the primary compound responsible for nonenal-associated odor (NAO), commonly known as "old person smell," which is a distinct greasy and grassy scent emerging in individuals over 40 years of age.^[43] This odor differs from typical sweat or bacterial-derived smells, as it originates from the oxidative degradation of skin surface lipids rather than apocrine gland secretions or microbial activity.^[43] The mechanism involves the breakdown of ω7 unsaturated fatty acids, such as palmitoleic and vaccenic acids, in skin lipids, which accelerates with age due to increased lipid peroxidation and diminished antioxidant defenses.^[43] Levels of 2-nonenal are undetectable in infants and young adults under 40 but rise significantly thereafter, contributing to the age-specific profile of NAO.^[43] This scent is universally recognizable across cultures, with studies showing that people from diverse backgrounds, including Swedish and American participants, can accurately discriminate elderly body odors from those of younger individuals based on smell alone.^[44] In contrast to the sweet, fruity notes associated with infant body odors—often linked to lactones—or the muskier, urine-like scents of adolescents driven by androgen-related steroids, 2-nonenal imparts a unique musty character to mature adults.^[45] Evolutionarily, this age-related odor may signal maturity or health status, potentially aiding in social or mating cues similar to those observed in other animals, though human-specific implications remain under investigation.^[44]

Health Implications and Mitigation

2-Nonenal, an unsaturated aldehyde derived from the peroxidation of omega-7 unsaturated fatty acids, contributes to localized skin aging and inflammation as a product of oxidative stress.^[10] It promotes apoptosis in keratinocytes and reduces cell viability in a dose-dependent manner, with concentrations as low as 5 μM inducing significant apoptotic effects and higher levels (50 μM) leading to necrosis.^[46] These actions decrease the number of proliferating skin cells and thin epidermal layers, exacerbating age-related skin deterioration, though its primary occurrence on the skin surface results in limited systemic absorption in healthy individuals.^[10] In individuals with chronic kidney disease, 2-Nonenal accumulates as a uremic toxin, potentially contributing to symptoms such as nausea and cardiovascular complications, though its precise toxicological role requires further research.^[1] As a biomarker of oxidative stress, 2-Nonenal levels rise with increased lipid peroxidation, reflecting imbalances in antioxidant defenses common in aging skin.^[10] Mitigation strategies target the underlying oxidative processes and direct neutralization of 2-Nonenal. An antioxidant-rich diet, particularly incorporating polyphenols from eggplant such as N-trans-feruloylputrescine, effectively scavenges 2-Nonenal, with eggplant fruit extracts achieving up to 80% reduction at 10 mg/mL concentrations while also lowering reactive oxygen species by 13-16%.^[10] Enhanced hygiene using soaps formulated to neutralize nonenal, such as those containing persimmon tannins, breaks down the compound on the skin surface, reducing odor persistence more effectively than standard cleansers.^[47] Commercial products for aging odor elimination often incorporate ingredients for broad-spectrum deodorization, available in supplements and topical formulations that support skin health.^[48] As of 2025, research continues to explore the long-term dermal and potential broader health impacts of chronic 2-Nonenal exposure, with recent studies emphasizing its role in skin apoptosis and the efficacy of natural scavengers.^[10]

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30-day returnsFights Odor from All Angles – Zinc + Magnesium + Plant Extracts (Peppermint, Parsley, Aloe Vera) help detoxify, support skin health, and provide a refreshing ...