Ring of epoxides

The highly strained three-membered ring of epoxides makes them much more reactive toward nucleophilic substitution than other ethers. 

The strained rings of epoxides and oxetanes are susceptible to nucleophilic attack. In this section we discuss the reactions of these oxygen heterocycles with nitrogen oxides and other... 

The three-membered ring of epoxides gives them unique chemical reactivity (Section 18.5). 

Campanella A, Baltanas MA. Degradation of the oxirane ring of epoxidized vegetable oils in liquid-liquid heterogeneous reaction systems. Chem Eng J 2006 118 141-152. 

Unlike most ethers epoxides (compounds m which the C—O—C unit forms a three membered ring) are very reactive substances The principles of nucleophilic sub stitution are important m understanding the preparation and properties of epoxides... 

The following section describes the preparation of epoxides by the base promoted ring closure of vicinal halohydrms Because vicinal halohydrms are customarily prepared from alkenes (Section 6 17) both methods—epoxidation using peroxy acids and ring closure of halohydrms—are based on alkenes as the starting materials for preparing epoxides... 

We saw an example of nucleophilic ring opening of epoxides in Section 15 4 where the reaction of Grignard reagents with ethylene oxide was described as a synthetic route to primary alcohols... 

There is an important difference in the regiochemistry of ring opening reactions of epoxides depending on the reaction conditions Unsymmetncally substituted epoxides tend to react with anionic nucleophiles at the less hindered carbon of the ring Under conditions of acid catalysis however the more highly substituted carbon is attacked... 

Nucleophilic ring opening of epoxides has many of the features of an 8 2 reac tion Inversion of configuration is observed at the carbon at which substitution occurs... 

The experimental observations combine with the principles of nucleophilic substi tution to give the picture of epoxide ring opening shown m Figure 16 5 The nucleophile attacks the less crowded carbon from the side opposite the carbon-oxygen bond Bond... 

As we ve just seen nucleophilic ring opening of ethylene oxide yields 2 substituted derivatives of ethanol Those reactions involved nucleophilic attack on the carbon of the ring under neutral or basic conditions Other nucleophilic ring openings of epoxides like wise give 2 substituted derivatives of ethanol but either involve an acid as a reactant or occur under conditions of acid catalysis... 

The reactivity of epoxides toward nucleophilic ring opening is responsible for one of the biological roles they play Squalene 2 3 epoxide for example is the biological... 

Nucleophilic ring opening of epoxides by ammonia (Section 16 12) The strained ring of an epoxide is opened on nucleo philic attack by ammonia and amines to give 3 ammo alcohols Azide ion also re acts with epoxides the products are p azido alcohols... 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

Other reactions involving the hydrogen atom of the hydroxyl group in ethyl alcohol include the opening of epoxide rings to form hydroxy ethers. 

CRV737) A critical discussion of the mechanisms of ring-opening of epoxides by nucleophiles. 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

The most striking chemical property of epoxides is their far- greater reactivity toward nucleophilic reagents compared with that of simple ethers. Epoxides react rapidly with nucleophiles under conditions in which other ethers are inert. This enhanced reactivity results from the angle strain of epoxides. Reactions that open the ring relieve this strain. 

Acid-catalyzed ring opening of epoxides is discussed in Section 16.13. 

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