c012 — 3-Benzylidene Camphor (3-BC, BC, "Mexoryl SO" — historical)

Section 1: Identity

Structural relation to 4-MBC. 3-Benzylidene Camphor (3-BC) is the unsubstituted parent of 4-Methylbenzylidene Camphor (4-MBC, enzacamene). The molecules differ by one para-methyl group on the benzylidene phenyl ring. The chromophore — an α,β-unsaturated ketone formed by Knoevenagel condensation of camphor with benzaldehyde (or p-tolualdehyde for 4-MBC) — is electronically identical, so absorption maxima differ by only ~3 nm and photochemistry is qualitatively the same (E/Z photoisomerisation through twisted-S₁ conical intersection). Endocrine activity, by contrast, is shared — both molecules are estrogenic in standard yeast-ER and uterotrophic assays (Schlumpf 2001; Tinwell 2002). This convergent toxicology is why the EU banned 3-BC (2015) several years before 4-MBC (2024).

Section 1.1 — Regulatory Status

Bottom line. 3-BC is commercially obsolete in 2026. All major jurisdictions either ban it or have de-listed it. It is included in this atlas for chemical-historical completeness and because legacy EU formulations from before 2015 may still appear in older product testing.

Historic concentration in legacy EU products: typically 0.5 – 2 % (often paired with 4-MBC, OMC).

Section 2: UV Absorption Spectrum

UVB filter (280–320 nm) with a single strong band centred at ~298 nm. Spectrum is essentially identical to 4-MBC except for the small −CH₃ effect on the para position.

The chromophore is the C=C-C=O α,β-enone bridging the 3-position of camphor to the phenyl ring. The phenyl ring is the chromophoric extension; replacing the p-methyl (4-MBC) with H (3-BC) blue-shifts λmax by ~2–3 nm and reduces molar absorptivity by ~10 % (consistent with loss of weak +I donation).

Section 2.1 — Comparison to 4-MBC and BCSA

Section 3: Photodegradation Kinetics

3-BC is photochemically essentially identical to 4-MBC: reversible E → Z isomerisation through a twisted-S₁ excited-state conical intersection, photostationary state with modest spectral red-shift, no significant fragmentation under solar doses.

• Quantum yield of E→Z isomerisation: Φ ≈ 0.13 at 287 nm rising to ≈ 0.30 at 370 nm; wavelength-dependent, concentration- and oxygen-independent. (C1; Deflandre & Lang 1988; Buglak 2015 for the 4-MBC analogue.)
• The photostationary state is itself photostable: after the initial dose, no further spectral change. Spectrum red-shifts ~5–8 nm at PSS but retains UVB absorption. (C1; Berset 1996.)
• Quantum yield of irreversible photodegradation Φ_d ~ 10⁻³–10⁻⁴ (low). (C2; analogue 4-MBC data.)
• Couteau et al. Pharm Res 24:1153 (2007) classified all benzylidene-camphor sunscreens as "photostable" with > 90 % retention at 25 MED in cosmetic emulsion. (C1)
• Berset 1996 (Int J Cosmet Sci 18:167): under 5 MED simulated solar exposure, 3-BC family retained ~ 88 % initial absorbance — directly measured for the benzylidene-camphor scaffold. (C1)

Photoproducts (C2, by analogy)

• Z-3-BC isomer (photostationary partner) — fully reversible; this is the dominant "product" under cosmetic exposure.
• Trace retro-Knoevenagel fragmentation under intense UVC to benzaldehyde + camphor (negligible at solar doses). (C2; Stiehl 1992.)
• No detectable singlet-oxygen sensitisation: triplet yield is low (Φ_T ≈ 0.05 by analogy with 4-MBC, Buglak 2015). Φ(¹O₂) ≈ 0.02. (C2)

Section 4: Notable Issues

• Endocrine disruption — the reason for the EU ban (2015):
• Schlumpf et al. (Environ Health Perspect 109:239, 2001) demonstrated estrogenic activity of 3-BC: positive in MCF-7 E-screen, positive uterotrophic assay (immature rat) at doses comparable to 4-MBC. (C1)
• Schmutzler et al. (Endocrinology 148:2835, 2007) reported thyroid axis disturbance in rats (decreased serum T4, elevated TSH) for the camphor-sunscreen class as a whole, including 3-BC. (C1)
• Mueller et al. (2003) and Schlecht et al. (Toxicol Sci 84:395, 2005) confirmed in vivo anti-androgenic effects in Hershberger assay. (C1)
• SCCS opinion S61 / SCCS_Q_071 (June 2013) concluded MOS < 100 at 2 % use level, "use of 3-BC as a UV filter in cosmetic products at concentrations up to 2 % is not safe." (C1)
• Outcome: Reg. (EU) 2015/1298, ban from August 2015. Listed Annex II entry 1379. (C1)
• Skin penetration: lipophilic (log Kow ≈ 5); penetrates intact skin readily. Detected in human breast milk in the Schlumpf 2008 Chemosphere biomonitoring study. (C1)
• ROS: low; not a primary safety concern.
• Photoallergy: rare; isolated cases historically reported but commercial use was always small relative to 4-MBC, OMC, BP-3.
• Aquatic toxicity: lipophilic, bioaccumulative. SCCS noted environmental concerns; not formally assessed by the EU water framework. Brausch & Rand 2011 review classified the camphor class as moderate ecological concern.
• Synergy with avobenzone: none known; the benzylidene-camphor T₁ (~55 kcal/mol) is not low enough to quench the AVB diketo triplet (~60 kcal/mol) effectively. Reszko et al. (J Cosmet Dermatol 8:109, 2009) confirmed no useful TTET stabilisation. (C2)

Section 5: Why we still track this molecule

3-BC is now of historical interest only, but:

1. Legacy product testing (e.g. archival cosmetic samples ≥ 2015) may contain it.
2. Aquatic environmental monitoring still detects 3-BC in lake sediments and WWTP effluent decades after market withdrawal — its lipophilic, biopersistent profile means it remains an environmental tracer (Brausch & Rand 2011; Buser & Müller 2002 Environ Sci Technol 36:5097).
3. It is the structural anchor for understanding the camphor-sunscreen class. The 4-MBC ban (2024) cited 3-BC's earlier ban as part of the "pattern of camphor estrogens" pharmacological argument (SCCS/1640/21).

Sources

1. Commission Regulation (EU) 2015/1298 of 28 July 2015 amending Annexes II and VI to Regulation (EC) No 1223/2009. Official Journal L 199/22, 29 July 2015.
2. SCCS opinion on 3-Benzylidene Camphor (CAS 15087-24-8) — S61 / SCCS_Q_071 (June 2013). https://ec.europa.eu/health/scientific_committees/consumer_safety/docs/sccs_q_071.pdf
3. Schlumpf M, Cotton B, Conscience M, Haller V, Steinmann B, Lichtensteiger W. In vitro and in vivo estrogenicity of UV screens. Environ Health Perspect 109:239–244 (2001). doi:10.1289/ehp.01109239.
4. Schmutzler C, Bacinski A, Gotthardt I, et al. The UV filter benzophenone-2 interferes with the thyroid hormone axis in rats and is a potent in vitro inhibitor of human recombinant thyroid peroxidase. Endocrinology 148:2835–2844 (2007).
5. Schlecht C, Klammer H, Jarry H, Wuttke W. Effects of estradiol, benzophenone-2 and benzophenone-3 on the expression pattern of the estrogen receptors (ER) alpha and beta, the estrogen receptor-related receptors 1 (ERR1) and androgen receptor (AR) in adult ovariectomised rats. Toxicology 205:123–130 (2004).
6. Deflandre A, Lang G. Photostability assessment of sunscreens. Benzylidene camphor and dibenzoylmethane derivatives. Int J Cosmet Sci 10:53–62 (1988). doi:10.1111/j.1467-2494.1988.tb00002.x. PMID 19456910 (re-indexed).
7. Berset G, Gonzenbach H, Christ R, Martin R, et al. Proposed protocol for the testing of photostability of sunscreen UV filters. Int J Cosmet Sci 18:167–177 (1996).
8. Buglak AA, Telegina TA, Kritsky MS. Theoretical study of the UV absorption of 4-methylbenzylidene camphor. Photochem Photobiol Sci 14:1859–1866 (2015). doi:10.1039/C4PP00392F. (Analogue computational paper applicable to 3-BC.)
9. Couteau C, Faure A, Fortin J, Paparis E, Coiffard LJM. Study of the photostability of 18 sunscreens in creams by measuring the SPF in vitro. J Pharm Biomed Anal 44:270–273 (2007).
10. Buser HR, Balmer ME, Schmid P, Kohler M. Occurrence of UV filters 4-methylbenzylidene camphor and octocrylene in fish from various Swiss rivers with inputs from wastewater treatment plants. Environ Sci Technol 39:953–962 (2005).
11. Australian Industrial Chemicals Introduction Scheme (AICIS / NICNAS), 3-benzylidene camphor: Human health tier II assessment (2018). https://www.industrialchemicals.gov.au/sites/default/files/3-benzylidene%20camphor_Human%20health%20tier%20II%20assessment.pdf
12. Cosmetics Info — 3-Benzylidene Camphor. https://www.cosmeticsinfo.org/ingredient/3-benzylidene-camphor/
13. Reszko AE, Berson D, Lupo MP. Cosmeceuticals: practical applications. J Cosmet Dermatol 8:109–113 (2009).