BHT radical

Examine the spin density surface for BHT radical. Is the unpaired electron localized or delocalized Examine BHT radical as a space-filling model. What effect do the bulky tert-butyl groups have on the chemistry of the species (Hint BHT radical does not readily add to alkenes or abstract hydrogens from other molecules.)... 

Compare the spin density surface for vitamin E radical to those of phenoxy and BHT radicals (see also Chapter 16, Problem 2). Are there significant differences among the three If so, elaborate. What is the function of the long alkyl chain in vitamin E Examine an electrostatic potential map for vitamin E radical. Do you expect it to be soluble in aqueous (polar) or non-aqueous (non-polar) environments, or both ... 

FT-IR results also showed that one new (small) absorption at 1659 cm"1 appeared, which could not be attributed to peroxide decomposition products. This absorption also appeared when the peroxide-curing experiments were carried out using an amorphous EPM, indicating that the absorption did not relate to rearrangement of the third monomer moiety (ENB in this case). It is tentatively concluded that the absorption at 1659 cm 1 is related to EPDM main-chain modifications, resulting from disproportionation reactions of EPDM macroradicals with BHT radical fragments. 

Why do these molecules inhibit peroxide formation We see for BHT in Figure 5.3.2.3 below that the O—H in BHT is susceptible to cleavage and the hydrogen is transferred to the carbon radical to form R—H. Due to resonance stabilization and considerable steric hindrance from the adjacent tert-butyl groups, the BHT radical is exceptionally stable and unreactive. This terminates the chain reaction mechanism of radical propagation and each BHT molecule therefore inhibits the formation of thousands of other peroxides. Thus, only trace amounts of BHT are necessary. 

BHT Radical Reaction. BHT scavenges aUcyl radicals, and the BHT radical is very stable... 

Butylated hydroxytoluene (BHT, also formerly known as lonol, E321) is 3,5-di-tert-butyl-4-hydroxytoluene (11-12). In comparison with BHA, BHT is somewhat more effective as an antioxidant of animal fats. The formation of a BHT radical and its reactions are shown in Figure 11.10. 

Flqure 11.10 Reactions of BHT radical with hydroperoxyl radicals (R = fert-butyl). 

MAA and EAA are stable Hquids, and are shipped in nonretumable 208-L (55-gal) polyethylene-lined dmms. For bulk shipments, insulated stainless steel tank containers and tmcks provide secure protection. 2-Acetoacetoxyethyl methacrylate is a Hquid stabili2ed with radical inhibitors such as BHT [128-37-0] and has a shelf life of approximately three months. Shipment is in 60- or 208-L polyethylene-lined dmms. Acetoacetaryhdes are nicely crystalline, stable soHds and are shipped in 208-L dmms with polyethylene liners. 

Butadiene reacts readily with oxygen to form polymeric peroxides, which are not very soluble in Hquid butadiene and tend to setde at the bottom of the container because of their higher density. The peroxides are shock sensitive therefore it is imperative to exclude any source of oxygen from butadiene. Addition of antioxidants like /-butylcatechol (TBC) or butylated hydroxy toluene (BHT) removes free radicals that can cause rapid exothermic polymerizations. Butadiene shipments now routinely contain about 100 ppm TBC. Before use, the inhibitor can easily be removed (247,248). Inert gas, such as nitrogen, can also be used to blanket contained butadiene (249). 

Phenol is a radical scavenger . Other radical scavengers include 3,5-di-tert-butyl-4-hydroxytoluene (butylated hydroxytoluene or BHT) and vitamin E. 

BHT is an antioxidant. It reacts with oxygen free radicals. It can thus slow down the rate at which ingredients in a product oxidize in direct contact with air, a process that can cause changes in the taste or color. BHT can be added to the food itself, or to the packaging material, and it is used primarily to prevent fats from becoming rancid. 

The potency of a chain-breaking antioxidant, which scavenges peroxyl radicals, will decrease as the concentration of lipid peroxides in the LDL particle increases (Scheme 2.2). This is illustrated in the experiment shown in Fig. 2.3 in which the antioxidant potency of a peroxyl radical scavenger (BHT) decreases as a function of added exogenous hpid hydroperoxide. If the endogenous lipid peroxide content of LDL were to vary between individuals, this could explain the observed diferences in the effectiveness of a-tocopherol in suppressing lipid peroxidation promoted by copper. 

SCHEME 10.5 Proposed pathway for the nonenzymatic conversion of BHTOOH to BHT-QM in keratinocytes. BHTOOH is oxidized to a peroxy radical that spontaneously loses oxygen. Two BHT phenoxy radicals then undergo disproportionation. 

A major problem with BHT is that at elevated temperatures volatilisation may occur. Higher molecular weight hindered phenols can be used. Aromatic amine radical scavengers that are derived from 1,4 di-aminobenzene or diphenylamine are also used. 

The commonest synthetic antioxidants are butylated hydroxyanisole (BHA) and butylated hydroxy toluene (BHT). Other synthetic antioxidants are w-propyl gallate and n-octyl gallate. Any substance that can act as a radical trap will have antioxidant properties. There are strict rules governing the use of antioxidants in foods. Only those substances that are on the permitted list can be used. 

BHT (2,6-di-tert-butyl-4-methylphenol), a phenolic antioxidant, on reaction with NO under neutral conditions, results in scavenging of the potentially harmful NO via radical reactions [143]. Sodium phenolate under basic conditions undergoes a Traube-type reaction at the ortho-position to produce a cupferron derivative [144]. When the ortho-positions are sterically blocked and the para-position does not bear a proton, cyclohexadienone diazeniumdiolates may be formed (Scheme 3.12) [145]. 

Analogously, 5-tributylstannylimidazole 29 was easily obtained from the regioselective deprotonation of 1,2-disubstituted imidazole 28 at C(5) followed by treatment with tributyltin chloride [24]. In the presence of 2.6 equivalents of LiCl, the Stille reaction of 29 with aryl triflate 30 afforded the desired 1,2,5-trisubstituted imidazole 31 with 2,6-di-tert-butyl-4-methylphenol (BHT) as a radical scavenger. Reversal of the nucleophile and electrophile of the Stille reaction also provided satisfactory results. For example, the coupling reaction of 5-bromoimidazole 33, derived from imidazole 32 via a regioselective bromination at C(5), and vinylstannane 34 produced adduct 35 [24],... 

It should be mentioned that the commercially available VC usually contained a small percentage (<2%) of prohibitors that stabilized the reactive VC from polymerizing upon storage. These compounds were usually radical scavengers such as 2,6-di-tert-butyl-p-cresol (DBC) or butylated hydroxy toluene (BHT). Recently, reports pointed out that the presence of these prohibitors actually had a negative impact on the anodic stability of VC on various cathode materials, and VC of high purity was of vital importance in obtaining reproducible performance in lithium ion cells. 

Many phenols, especially hindered phenols such as butylated hydroxy toluene (BHT), are good antioxidants. They act as radical scavengers by readily reacting with stray radicals to give very stable radicals via resonance. 

In 1989, Kato et al. reported the isolation of carazostatin (247) from Streptomyces chromofuscus. Carazostatin represents a novel radical scavenger more active than butylated hydroxytoluene (BHT) (226). Moreover, it exhibits strong inhibitory activity against the free radical-induced, lipid peroxidation in liposomal membranes, and shows stronger antioxidant activity than ot-tocopherol (227). 

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