Williamson synthesis of ethers

The conversion of chlorohydrins into epoxides by the action of base is an adaptation of the Williamson synthesis of ethers. In the presence of hydroxide ion, a small proportion of the alcohol exists as alkoxide, which displaces the chloride ion from the adjacent carbon atom to produce a cycHc ether (2). 

Treatment of a thiol with a base, such as NaH, gives the corresponding thiolate ion (RS-), which undergoes reaction with a primary or secondary alkyl halide to give a sulfide. The reaction occurs by an Sn2 mechanism, analogous to the Williamson synthesis of ethers (Section 18.2). Thiolate anions are among... 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

The so-called Williamson synthesis of ethers is by far the most important ether synthesis because of its versatility it can be used to make unsymmetrical ethers as well as symmetrical ethers, and aryl alkyl ethers as well as dialkyl ethers. These reactions involve the nucleophilic substitution of alkoxide ion or phenoxide ion for halide (equation 70).26°... 

This reaction is similar to the Williamson synthesis of ethers (method 115). Otthofotmates in which the alkyl group is methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and isoamyl have been prepared from chloroform. The yield of ethyl orthoformate is 45%. Mixed esters are obtained from a mixture of sodium alkoxides and chloroform. Benzotrichloride, C,HjCClj, is converted to methyl orthobenzoate in 86% yield by sodium methoxide in methanol, ... 

These ethers are usually formed by the reaction of an alcoholate with an ester of a halogen acid, the reaction being analogous to the Williamson synthesis of ether. 

With the increasing availability of suitable monomers and the desire for polymers having improved heat resistance combined with higher levels of mechanical properties, displacement (or substitution ) reactions have become of increased interest for polycondensation. A suitable example with monofunctional reactants would be the Williamson synthesis of ethers... 

In the presence of sodium hydroxide, thiols react with alkyl halides to form the sulfides (20) the reaction occurs via the sodium thiolate and is analogous to the well-known Williamson synthesis of ethers and can also be applied to obtain unsymmetrical sulfides (Scheme 18). Symmetrical sulfides may be prepared directly by condensation of sodium sulfide with alkyl halides (Scheme 18). These reactions are of the SN2 type, and consequently the optimum yields of sulfides are realised using primary alkyl halides. 

This reaction is usually called the Williamson synthesis of ethers. In a modification of it, the alkyl halide may be replaced by a sulfonic ester or a dialkyl sulfate. 

Even carbowax (a chemically and thermally stable poly(ethylene glycol), when adsorbed on an inorganic salt with no other solid support, may act as a very efficient gas-solid phase-transfer catalyst. This system has been employed, for example, in the Williamson synthesis of ethers and thioethers, starting from alkyl halides and phenols or thiols in the presence of potassium carbonate as a base Gas-solid PTC shows the advantage that pure products are obtained directly, due to the absence of aqueous and organic solvents. 

EXPERIMENT 22 Williamson Synthesis of Ethers 329 300 MHz IH NMR SPECTRUM OF METHYL p-ETHYLPHENYL ETHER IN CDCI3... 

Preparation.—Recent improvements and variations on the traditional Williamson synthesis of ethers from alcohols (as their metal alkoxides) and alkyl halides [equation (15)] include the use of powdered KOH (as the base) in DMSO, ° and... 

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