4-Hydroxy-TEMPO Benzoate Free Radical: Properties & Uses
4-Hydroxy-TEMPO Benzoate Free Radical: Properties & Uses
4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-Oxyl Benzoate Free Radical is a stable nitroxide radical compound derived from 4-hydroxy-TEMPO by esterification of its hydroxyl group with benzoic acid, widely used as a spin label, EPR probe, and reagent in free radical chemistry research.
- Chemical Name
- 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-Oxyl Benzoate Free Radical
- SKU
- H157124
- Radical Type
- Stable nitroxide (N–O•) radical
- Key Functional Groups
- Nitroxide (N–O•), ester (benzoate), piperidine ring
- Primary Applications
- EPR/ESR spectroscopy, spin labeling, antioxidant research, controlled radical polymerization
- Stability
- Persistent radical; stable under ambient conditions when stored properly away from reductants and light
What Is 4-Hydroxy-TEMPO Benzoate Free Radical?
4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-Oxyl Benzoate Free Radical (Aladdin SKU H157124) is a member of the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) family of stable nitroxide radicals. The compound is formed by esterification of the C-4 hydroxyl group of 4-hydroxy-TEMPO with benzoic acid, yielding a benzoate ester that retains the paramagnetic N–O• radical center characteristic of all TEMPO derivatives.
Unlike many organic radicals that are highly reactive and short-lived, nitroxide radicals such as this compound are persistent due to steric shielding of the radical center by the four flanking methyl groups on the piperidine ring. This steric protection prevents radical dimerization and renders the compound isolable and handleable at room temperature under normal laboratory conditions.
The benzoate ester modification, compared with the parent 4-hydroxy-TEMPO, increases the lipophilicity of the molecule, which can be advantageous for membrane partitioning studies, polymer compatibility, and certain organic-phase reactions.
Key Chemical Properties and Structural Features
Radical stability: The unpaired electron resides on the nitrogen–oxygen bond of the nitroxide group. The four α-methyl substituents on the piperidine ring create steric bulk that kinetically stabilizes the radical against dimerization or hydrogen-atom abstraction, making it a persistent radical in the Ingold–Fischer sense.
EPR signature: Like all nitroxide radicals, this compound produces a characteristic three-line EPR (electron paramagnetic resonance) spectrum arising from hyperfine coupling of the unpaired electron with the 14N nucleus (I = 1). The hyperfine splitting constant (aN) and g-factor are sensitive to the local environment, underpinning the compound's utility as a spectroscopic probe.
Lipophilicity: The benzoate ester group significantly increases log P compared with 4-hydroxy-TEMPO, facilitating use in hydrophobic matrices, organic solvents, and lipid bilayer studies.
Redox behavior: The nitroxide can be reversibly reduced to the corresponding hydroxylamine (diamagnetic) by mild reductants such as ascorbate, or oxidized to the oxoammonium cation by oxidants. This reversible redox cycling is central to many of its biological and catalytic applications.
Research Applications
EPR and spin-labeling studies: The compound serves as an EPR-active spin label or spin probe. When incorporated into or associated with macromolecules, membranes, or polymer matrices, the EPR lineshape reports on local dynamics, polarity, and order parameters. Its enhanced lipophilicity relative to polar TEMPO derivatives makes it particularly suitable for membrane and polymer contexts.
Controlled radical polymerization (NMP): Stable nitroxide radicals are well-established mediators in nitroxide-mediated polymerization (NMP), a form of reversible-deactivation radical polymerization (RDRP). TEMPO-type radicals reversibly cap growing polymer chains, enabling control over molecular weight and dispersity. The ester functionality may offer additional compatibility with specific monomer systems.
Antioxidant and oxidative stress research: TEMPO derivatives are used as model antioxidants and superoxide dismutase mimetics in studies of reactive oxygen species (ROS). The reversible redox chemistry of the nitroxide group allows these compounds to scavenge radicals catalytically.
Organic synthesis: Oxoammonium species generated in situ from TEMPO derivatives are powerful and selective oxidants for primary alcohols to aldehydes and secondary alcohols to ketones, often used in combination with co-oxidants such as NaOCl or PhI(OAc)2.
Storage, Handling, and Safety Guidelines
Storage: Store in a cool, dry, well-ventilated area away from direct light and heat sources. Keep the container tightly closed. Protect from strong reducing agents (e.g., ascorbate, thiols) and strong oxidizing agents, which can destroy or alter the radical species. Refrigerated storage (2–8 °C) is recommended for long-term preservation of radical integrity.
Handling precautions: Use appropriate personal protective equipment including lab coat, chemical-resistant gloves, and eye protection. Work in a well-ventilated fume hood when handling powders or solutions to avoid inhalation of dust or vapors.
Stability in solution: Solutions should be prepared fresh or stored under inert atmosphere (nitrogen or argon) to minimize adventitious reduction. Protect from prolonged exposure to UV light, which may promote photochemical decomposition.
Waste disposal: Dispose of according to applicable local, regional, and national regulations for chemical waste. Do not dispose of down drains without treatment.
Note: Always consult the Safety Data Sheet (SDS) provided with the product for complete hazard information before use.
Frequently asked questions
What makes 4-Hydroxy-TEMPO Benzoate Free Radical a 'stable' radical?
The compound is classified as a persistent (stable) radical because the four methyl groups flanking the nitrogen–oxygen radical center on the piperidine ring provide sufficient steric shielding to prevent radical dimerization and most bimolecular quenching reactions. This steric protection, rather than electronic delocalization alone, is the primary reason the unpaired electron persists indefinitely under ambient, oxygen-free conditions.
How does the benzoate ester group affect the properties of this compound compared with 4-hydroxy-TEMPO?
Esterification of the 4-hydroxyl group with benzoic acid removes the free hydroxyl, increasing the lipophilicity (log P) of the molecule relative to 4-hydroxy-TEMPO. This makes the benzoate derivative more compatible with hydrophobic environments such as lipid bilayers, organic polymer matrices, and nonpolar organic solvents, while the nitroxide radical character and EPR activity are retained.
Can this compound be used as a spin label in EPR spectroscopy?
Yes. Like all TEMPO-based nitroxides, 4-Hydroxy-TEMPO Benzoate Free Radical is EPR-active and produces a characteristic three-line spectrum due to hyperfine coupling with the ¹⁴N nucleus. It can function as a spin probe to report on molecular dynamics, polarity, and viscosity of its local environment. Its increased lipophilicity relative to more polar TEMPO derivatives may favor partitioning into hydrophobic domains such as lipid membranes or polymer matrices.
What solvents are suitable for dissolving 4-Hydroxy-TEMPO Benzoate Free Radical?
The compound is expected to dissolve well in common organic solvents such as dichloromethane, chloroform, ethyl acetate, acetone, and DMSO due to its lipophilic benzoate ester group. Solubility in water is expected to be low. Users should verify solubility experimentally for their specific application conditions, as exact solubility data should be confirmed from the product specification sheet.
How should I store this compound to maintain radical integrity?
Store the compound in a cool (2–8 °C recommended), dry, dark environment in a tightly sealed container under inert atmosphere if possible. Avoid exposure to strong reducing agents such as thiols or ascorbate, strong oxidants, moisture, and prolonged UV irradiation, all of which can degrade the nitroxide radical. Solutions should be prepared fresh or stored under nitrogen or argon.
References
- Offer, T.; Russo, A.; Samuni, A. The pro-oxidative activity of SOD and nitroxide SOD mimics. FASEB J. 2000, 14, 1215–1223.
- Hawker, C. J.; Bosman, A. W.; Harth, E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem. Rev. 2001, 101, 3661–3688.
- de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. On the use of stable organic nitroxyl radicals for the oxidation of primary and secondary alcohols. Synthesis 1996, 1996, 1153–1174.
- Berliner, L. J. (Ed.) Spin Labeling: Theory and Applications. Academic Press, New York, 1976.
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