Alcohols, reaction with phosphorus halides

Phosphorus halides react with alcohols to yield alkyl halides at low temperature (0 °C). Primary and secondary alcohols undergo Sn2 reactions with PX3. This type of reaction does not lead to rearranged products, and does not work well with 3° alcohols. PI3 has to be generated in situ via reaction of iodine and phosphorus. 

The reaction is performed most simply by the addition of the propargylic alcohol to a solution of the phosphorus halide. Rearrangement of the phosphorus ester proceeds at ambient temperature or with mild heating. When phosphorus trihalides are used, the product can be isolated as the phosphonic dichloride.168169 Aqueous workup provides the phosphonic acid.162 In most instances, however, a dialkyl phosphorochloridite with only a single halogen on phosphorus available for reaction with alcohol has been used.165 170 174... 

Reaction with alcohols and phenols. Horner and co-workers introduced use of the reagents for the preparation of alkyl and aryl halides from alcohols or phenols, and Wiley and co-workers established that the reagents have considerable advantage over phosphorus pentahalides in the C—OH—C—X conversion and discussed the mechanism. For the preparation of n-butyl bromide the Wiley group added bromine under nitrogen to a solution of n-butanol and triphenylphosphine in dimethyl-formamide until 2 drops persisted in giving the solution an orange tint. Volatile... 

A general method for conversion of alcohols to chlorides involves reaction with PCI3 or PCI5. Similarly, alcohols and PBrs yield alkyl bromides. These reactions presumably involve initial attack on the phosphorus halides by the alcohol to form a trialkyl phosphite ester. Introduction of the halide then occurs, with inversion of... 

Reaction with phosphorus trihalides (Section 4 13) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides... 

Both reactants m the Williamson ether synthesis usually originate m alcohol pre cursors Sodium and potassium alkoxides are prepared by reaction of an alcohol with the appropriate metal and alkyl halides are most commonly made from alcohols by reaction with a hydrogen halide (Section 4 7) thionyl chloride (Section 4 13) or phosphorus tri bromide (Section 4 13) Alternatively alkyl p toluenesulfonates may be used m place of alkyl halides alkyl p toluenesulfonates are also prepared from alcohols as their imme diate precursors (Section 8 14)... 

Another general method for converting alcohols to halides involves reactions with halides of certain nonmetallic elements. Thionyl chloride, phosphorus trichloride, and phosphorus tribromide are the most common examples of this group of reagents. These reagents are suitable for alcohols that are neither acid sensitive nor prone to structural rearrangement. The reaction of alcohols with thionyl chloride initially results in the formation of a chlorosulfite ester. There are two mechanisms by which the chlorosulfite can be converted to a chloride. In aprotic nucleophilic solvents, such as dioxane, solvent participation can lead to overall retention of configuration.7... 

In a similar vein, the keto bridge in 5 can be replaced by oxygen with retention of activity. Reduction of acetophenone derivative 19 by means of sodium borohydride leads to the corresponding alcohol (20). Reaction with phosphorus tribromide with cyanide gives... 

The mechanism for the reactions with phosphorus halides can be illustrated using phosphorus tribromide. Initial reaction between the alcohol and phosphorus tribromide leads to a trialkyl phosphite ester by successive displacements of bromide. The reaction stops at this stage if it is run in the presence of an amine which neutralizes the hydrogen bromide that is formed.9 If the hydrogen bromide is not neutralized the phosphite ester is protonated and each alkyl group is successively converted to the halide by nucleophilic substitution by bromide ion. The driving force for cleavage of the C—O bond is the... 

Alkyl halides are almost always prepared from corresponding alcohols by the use of hydrogen halides (HX) or phosphorus halides (PX3) in ether (see Section 5.5.3). Alkyl chlorides are also obtained by the reaction of alcohols with thionyl chloride (SOCI2) in triethylamine (Et3N) or pyridine (see Section 5.5.3). 

Propargyl alcohols may be converted to allenes by several methods, for example, (a) through the intermediate formation of propargyl halides which are not isolated but react directly with cuprous salts and hydrogen halide [60, 72-73] or cyanide [71] (b) typical alcohol reactions with thionyl chloride [74a-d] phosphorus halides [75-77], and miscellaneous reagents (see Scheme 3). 

Other halides that are useful in converting alcohols to alkyl halides are PC15, PC13, PBr3, and Pl3, which are acid halides of phosphorus oxyacids. As with thionyl chloride, a weak base often is used to facilitate the reaction. The base acts to neutralize the acid formed, and also to generate bromide ion for SN reactions ... 

Replacement of the hydroxyl group in a phenol by halogen cannot be accomplished by reaction with the hydrogen halides as in the case of alcohols, and reaction with phosphorus halides gives only low yields of halogenobenzenes (except in the case of nitrophenols), the main product being a phosphite or phosphate ester. 

Reaction of phosphorus and sulfur halides with alcohols... 

Alcohols and phenols are also weak bases. They can be protonated on the oxygen by strong acids. This reaction is the first step in the acid-catalyzed dehydration of alcohols to alkenes and in the conversion of alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols by reaction with thionyl chloride or phosphorus halides. 

Ethane occurs in natural gas, from which it is isolated. Ethane is among the chemically less reactive organic substances. However, ethane reacts with chlorine and bromine to form substitution compounds. Ethyl iodide, bromide, or chlorides are preferably made by reaction with ethyl alcohol and the appropriate phosphorus halide. Important ethane derivatives, by successive oxidation, are ethyl alcohol, acetaldehyde, and acetic acid. Ethane can also be used for the production of aromatics by pyrolysis (Fig. 1). 

Reactions of Alcohols with Phosphorus Halides 484 Reactions of Alcohols with Thionyl Chloride 485 11-9 Mechanism 11-3 Reaction of Alcohols with PBr3 485 11-10 Dehydration Reactions of Alcohols 487... 

Mechanism of the Reaction with Phosphorus Trihalides The mechanism of the reaction of alcohols with phosphorus trihalides explains why rearrangements are uncommon and why phosphorus halides work poorly with tertiary alcohols. The mechanism is shown here using PBr3 as the reagent PC13 and PI3 (generated from phosphorus and iodine) react in a similar manner. 

Phosphites can be very easily prepared by the reaction of a phosphorus halide with alcohol and an organic base, typically triethylamine or pyridine.36 It is generally necessary to use low temperatures and to keep the reaction free from moisture and air. If diols are used, reaction conditions can be adjusted so that oxygen either... 

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