Alpha-Isomethyl Ionone: properties, uses, pros, cons, safety, alternatives

Synonyms

α-Isomethyl ionone, isomethyl-α-ionone, 3-methyl-4-(2,6,6-trimethylcyclohex-2-en-1-yl)but-3-en-2-one, “Alpha” isomethyl ionone (trade wording in some catalogues)

The name describes the structure of the molecule:

• Alpha. This term is often used in organic chemistry to indicate the location of a functional or substituent group. In this case, it refers to the location of the isomethyl group in the ionon molecule.
• Isomethyl refers to an isomer of a methyl group. An isomer is a compound with the same molecular formula as another compound but a different arrangement of atoms in space.
• Ionone. This is a term used to describe a variety of fragrant substances found in a wide range of essential oils. Ionones are often used in perfumery and cosmetics for their pleasant scent.

Description of raw materials used in production:

• Citral - An organic compound used as a starting point in the synthesis of ionones. It's present in many essential oils and has a lemon-like fragrance.
• Hydrogen and catalysts - Used for reducing citral to ionones.

The synthesis process takes place in several stages:

• Preparation. Methylheptenone can be synthesized by reacting isoprene with Methacrolein in the presence of an acid catalyst.
• Cyclization. Methylheptenone is cyclized, using an acid catalyst, to form a mixture of isomeric ionons, including the alpha-isomethyl ion.

Definition

Alpha-Isomethyl Ionone is an organic compound belonging to the ionone family (an unsaturated ketone related to terpenoid derivatives), used primarily as a fragrance material to provide floral/violet and powdery nuances in perfumes and functional fragrances. From an industrial standpoint, it is frequently supplied as a fragrance-grade material whose purity and isomeric profile can vary by manufacturer, which is relevant for olfactory consistency and QC specifications.

In use, the compound is typically present at trace to low percentages within a perfume compound (mixture), while its concentration in the finished cosmetic product depends on the fragrance level and on the product type (leave-on vs rinse-off). Therefore, correct management is not only formulation-related but also regulatory, because consumer disclosure becomes mandatory above certain thresholds.

Cosmetics. It is used as a fragrance ingredient across many categories (personal care, haircare, cleansing, fine fragrance). The impact in formula is primarily sensory (olfactory profile). The most relevant technical aspects are: solubility/compatibility with the vehicle, stability of the fragrance profile over time, and compliance with allergen labelling thresholds and any applicable restrictions (including IFRA rules in standard industry practice).

Alpha-Isomethyl Ionone is a restricted ingredient as III/90 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009.

It is a chemical fragrance that is added to products such as liquid soaps, shampoos and others to add a violet scent.

• Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.
• Perfuming. Unlike fragrance, which can also contain slightly less pleasant or characteristic odours, the term perfume indicates only very pleasant fragrances. Used for perfumes and aromatic raw materials.

It is a product that can give allergies.

Other uses

Used as a cold-resistant plasticiser, fixative for gas chromatography, main plasticiser in plastics.

Identification data, specifications, and indicative physico-chemical properties

Functional role and practical “mode of action”

Function: fragrance (perfuming)
What it does in formula: contributes to the bouquet with floral/violet and powdery facets and supports olfactory body and softness.
Technical note: as a fragrance raw material, the “mechanism” is sensory (interaction with the olfactory system). Formulation performance depends on correct solubilisation/distribution in the relevant phase (oil/solvent/fragrance).

Formulation compatibility

Alpha-Isomethyl Ionone is generally compatible with typical fragrance architectures (organic solvents, perfume bases, anhydrous systems) and with many cosmetic bases. The most common watch-outs are:

• Solubility and clarity. In water-rich systems (lotions, gels, cleansers), performance depends on the fragrance vehicle and the solubiliser/surfactant system. Haze or phase separation often indicates sub-optimised solubilisation of the fragrance as a whole (not only this single molecule).

• Interactions with polymers and surfactants. Certain systems (e.g., thickening polymers, high levels of anionic surfactants) can change fragrance availability and perceived intensity, as well as product clarity.

• Stability of the olfactory profile. Rather than macroscopic degradation, it is common to observe shifts in top/heart/base balance over time depending on packaging, light, temperature, and permeability. Appropriate management includes packaging selection, system-level antioxidants (where applicable), and accelerated stability testing on the finished product.

Indicative use guidelines

• Fine fragrance and alcoholic/anhydrous systems: the level is defined by the olfactory brief and by applicable IFRA rules for the category; the substance is typically one component within the fragrance mixture, not the sole driver.

• Leave-on products (creams, non-rinse deodorants, leave-on hair): primary attention to allergen declaration thresholds and to fragrance compliance documentation (IFRA statement for the compound).

• Rinse-off products (shampoos, shower gels, cleansers): similar attention, with different declaration thresholds compared to leave-on.

• Quality approach: define minimum specifications (purity, GC profile, colour, odour, physical parameters) and manage batch-to-batch variability with coherent sensory and instrumental controls.

Quality, grades, and specifications

• Identity: confirmation via GC/GC-MS and comparison with standards; check consistency across naming systems (INCI/CAS/EC).

• Olfactory profile: panel evaluation and comparison to an internal standard (golden sample).

• Physical parameters: density, refractive index, colour (trend over time).

• Purity/impurities: supplier specifications (CoA) and limits relevant to intended use.

• Stability and storage: tightly closed containers, protected from heat and light; monitor colour/odour drift over shelf-life.

• Document compliance: up-to-date SDS; IFRA documentation for the fragrance (if used in a mixture); batch traceability.

Safety, regulatory, and environmental considerations

From a cosmetic toxicology perspective, Alpha-Isomethyl Ionone is treated as a fragrance allergen. The main consumer-relevant risk is skin sensitisation in predisposed individuals. For this reason, EU fragrance allergen rules require on-pack declaration when the concentration in the finished product exceeds the applicable thresholds (commonly 0.001% for leave-on and 0.01% for rinse-off, under the applicable regulatory framework).

On the EU regulatory side, updates introduced by Commission Regulation (EU) 2023/1545 confirmed the threshold-based approach and set transitional timelines: non-compliant products may be placed on the EU market until 31 July 2026 and made available until 31 July 2028, subject to the applicable conditions. Operationally, this requires regulatory/formulation teams to verify: the allergen’s presence in the fragrance, the finished-product concentration by category, and correct label declaration using the required nomenclature and timelines.

In industry practice, fragrances are also commonly managed against IFRA Standards, which may impose category-specific limits based on safety assessments of materials or chemical families. As a result, real-world compliance is typically the combination of: EU Cosmetics rules, IFRA documentation for the fragrance, and finished-product safety assessment under GMP controls.

Formulation troubleshooting

Alternatives

Conclusion

Alpha-Isomethyl Ionone is an ionone-family fragrance material used for floral/violet nuances in perfumery and cosmetics. In product development, the critical points are not only formulation-related (solubilisation, sensory stability, repeatability) but also regulatory, because it is managed as a fragrance allergen with threshold-based declaration requirements and EU updates that demand careful control of compliance, timelines, and supporting documentation (SDS/IFRA/PIF).

Mini-glossary

INCI: international standard nomenclature used for cosmetic ingredient labelling.
Fragrance allergen: odorous substance associated with allergic contact dermatitis in sensitised individuals; above thresholds it may require label declaration.
GC / GC-MS: gas chromatography (and gas chromatography–mass spectrometry), analytical techniques for identity and impurity profiling.
CoA: Certificate of Analysis, supplier batch analytical certificate.
SDS: Safety Data Sheet.
IFRA: International Fragrance Association; industry standards that can restrict use levels by product category.
GMP: Good Manufacturing Practice, manufacturing best practices to ensure quality and process control.
HACCP: Hazard Analysis and Critical Control Points, risk analysis and critical control methodology (a relevant concept in regulated supply chains).

More information:

"Alpha-Isomethyl Ionone, studies"

Typical commercial product characteristics Alpha-Isomethyl Ionone

Appearance	Colorless or Yellow liquid
Boiling Point	285.3±29.0°C at 760 mmHg
Flash Point	122.1±17.5°C
Density	0.9±0.1 g/cm3
Vapor Pressure	0.0±0.6 mmHg at 25°C
Refraction Index	1.508
Specific Gravity	0.925
PSA	17.07000
LogP	4.41
Storage	Room Temperature
Chemical Safety

Price:

50ml/1.70 oz          $14.50

1000ml/33.80oz     $208.00

• Molecular Formula    C₁₄H₂₂O
• Molecular Weight      206.32
• Exact Mass      206.167068
• CAS  127-51-5
• UNII    9XP4LC555B
• EC Number   204-846-3
• DSSTox Substance ID  DTXSID7027047      DTXSID10859248
• IUPAC  (E)-3-methyl-4-(2,6,6-trimethylcyclohex-2-en-1-yl)but-3-en-2-one
• InChl=1S/C14H22O/c1-10-7-6-8-14(4,5)13(10)9-11(2)12(3)15/h7,9,13H,6,8H2,1-5H3/b11-9+
• InChl Key      JRJBVWJSTHECJK-PKNBQFBNSA-N
• SMILES   CC1=CCCC(C1C=C(C)C(=O)C)(C)C
• MDL number  MFCD00034582
• PubChem Substance ID    329751380
• ChEBI  179835
• SCHEMBL     891912
• NSC     66432
• FEMA     2714
• JECFA     404
• NACRES  NA.24
• HS Code       2914299000   

Synonyms :

• 3-methyl-4-(2,2-dimethyl-6-methylenecyclohexyl)-3-buten-2-one
• 4-(2,6-Trimethyl-2-cyclohexen-1-yl)-3-methyl-3-buten-2-one
• Isoraldeine
• Methyl ionone
• Alpha-cetone
• Methyl ionone gamma
• Acetone 99,5%
• α-Cetone
• α−Isomethyl ionone
• Iso-.alpha.-methyl ionone
• Gamma Methyl Ionone
• AC1NS9PL

References__________________________________________________________________________

Yamazaki Y, Hayashi Y, Arita M, Hieda T, Mikami Y. Microbial Conversion of alpha-Ionone, alpha-Methylionone, and alpha-Isomethylionone. Appl Environ Microbiol. 1988 Oct;54(10):2354-60. doi: 10.1128/aem.54.10.2354-2360.1988. PMID: 16347747; PMCID: PMC204258.

Abstract. alpha-Ionone, alpha-methylionone, and alpha-isomethylionone were converted by Aspergillus niger JTS 191. The individual bioconversion products from alpha-ionone were isolated and identified by spectrometry and organic synthesis. The major products were cis-3-hydroxy-alpha-ionone, trans-3-hydroxy-alpha-ionone, and 3-oxo-alpha-ionone. 2,3-Dehydro-alpha-ionone, 3,4-dehydro-beta-ionone, and 1-(6,6-dimethyl-2-methylene-3-cyclohexenyl)-buten-3-one were also identified. Analogous bioconversion products from alpha-methylionone and alpha-isomethylionone were also identified. From results of gas-liquid chromatographic analysis during the fermentation, we propose a metabolic pathway for alpha-ionones and elucidation of stereochemical features of the bioconversion.

Avonto C, Wang M, Chittiboyina AG, Vukmanovic S, Khan IA. Chemical stability and in chemico reactivity of 24 fragrance ingredients of concern for skin sensitization risk assessment. Toxicol In Vitro. 2018 Feb;46:237-245. doi: 10.1016/j.tiv.2017.09.007. 

An S, Lee AY, Lee CH, Kim DW, Hahm JH, Kim KJ, Moon KC, Won YH, Ro YS, Eun HC. Fragrance contact dermatitis in Korea: a joint study. Contact Dermatitis. 2005 Dec;53(6):320-3. doi: 10.1111/j.0105-1873.2005.00720.x. 

Abstract. The purpose of this study is to determine the frequency of responses to selected fragrances in patients with suspected fragrance allergy and to evaluate the risk factors. 9 dermatology departments of university hospitals have participated in this study for the past 1 year. To determine allergic response to fragrances, 18 additional fragrances in addition to the Korean standard and a commercial fragrance series were patch-tested in patients with suspecting cosmetic contact dermatitis. Over 80% of the patients were women, and the most common site was the face. Cinnamic alcohol and sandalwood oil (Santalum album L.) showed high frequencies of positive responses. Of the specific fragrances, ebanol, alpha-isomethyl-ionone (methyl ionone-gamma) and Lyral (hydroxyisohexyl 3-cyclohexane carboxdaldehyde) showed high positive responses. We compared the results obtained during this study with those of other studies and concluded that including additional fragrance allergens may be useful for the detection of fragrance allergy.

Ishizaki S, Itoh M, Komai T, Honda T, Kitahara T. Biotransformation of alpha-isomethylionone to 1-(2,6,6-trimethyl-2-cyclohexen-1-yl)propan-2-one. Biosci Biotechnol Biochem. 2004 May;68(5):1164-6. doi: 10.1271/bbb.68.1164. 

Abstract. The main biodegradation product of (+/-)-alpha-isomethylionone (2) with standard activated sludge was characterized as (+/-)-1-(2,6,6-trimethyl-2-cyclohexen-1-yl)propan-2-one (1) by its analysis and synthesis. Both enantiomers (1a and 1b) of 1 were synthesized by starting from (R)- and (S)-2,4,4-trimethyl-2-cyclohexen-1-ol (3a and 3b), respectively.