Phosphorus trihalides, alcohol

Reaction with phosphorus trihalides Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit 10, Class XII). 

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

Alcohol Phosphorus trihalide Alkyl halide Phosphorous acid... 

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... 

Elder, R.C., Florian, L.R., Kennedy, E.R., and Macomber, R.S., Phosphorus containing products from the reaction of propargyl alcohols with phosphorus trihalides. II. The crystal and molecular structure of 2-hydroxy-3,5-di-ferf-butyl-l,2-oxaphosphol-3-ene 2-oxide, /. Org. Chem., 38, 4177, 1973. 

The propargyl alcohols react with trivalent phosphorus halides to give allenic phosphorus esters as described in Scheme 3 and Table VI. In the case of aryl-substituted alkynols or highly hindered t-propargyl alcohols which contain no free acetylenic —H, thionyl halides or phosphorus trihalides yield bromo- or chloroallenes [74d], Thionyl chloride also reacts in a similar fashion with a wide variety of unhindered secondary alcohols (structure XV) to give a mixture of the chloroallene and chloroalkyne [74a-d]. 

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. 

Evaporation of volatiles from PClj-HjO yields crystalline H3PO3 quantitatively. Hydrolysis of phosphorus trihalides at 0°C in an NaHC03 buffer forms mixtures, including [HP03J2-, [H2PO3]-, [HjPOjl -, [PjOJ- -, [POJ5-, and [PjO,]" -. From the PBtj reaction, upon addition of HBr and alcohol, crystalline Na3[HP205]-12 HjO is obtained. 

Alcohols can also be converted to alkyl halides using thionyl chloride or phosphorus trihalides. 

Primary, secondary, and tertiary alcohols undergo nucleophilic substitution reactions with HI, HBr, and HCl to form alkyl halides. These are Sn2 reactions in the case of primary alcohols and SnI reactions in the case of secondary and tertiary alcohols. An alcohol can also be converted into an alkyl halide by phosphorus trihalides or thionyl chloride. These reagents convert the alcohol into an intermediate with a leaving group that is readily displaced by a halide ion. 

React the following alcohols with a phosphorus trihalide and show the products. ... 

Solution Alcohols react with phosphorus trihalides to produce alkyl halides with the same carbon skeleton as that of the alcohol. As seen in previous problems/ alkyl halides can also be prepared from alcohols with a hydrogen halide, but phosphorus halides are often preferred because they tend less to bring about rearrangement of the carbon skeleton. 

This reaction is simple substitution where the halogen, either bromine or iodine will replace the 0H group, and the phosphorus forms the by-product phosphorus acid (HaPOa). With the above facts and the general synthesis at hand, the products of each reaction can be obtained after reacting the alcohol with a phosphorus trihalide. 

Alkyl halides can be prepared from alcohols by use of phosphorus trihalides ... 

Examination indicates that the carbon skeleton remains intact in this synthesis. All that must be done is to alter the carboxylic acid to the alkyl bromide of the same carbon number. Now, if one had n-Ci1H23CH2OH (the corresponding alcohol), 1-bromododecane could be produced by reaction with the phosphorus trihalide PBr3. To obtain the alcohol, use the fact that lithium aluminum hydride, LiAli, is one of the few reagents that reduce a carboxylic acid to an alcohol of the same carbon length. Overall, then, the synthesis becomes ... 

Hence, one needs an alcohol of 4 carbons that possesses a straight carbon chain. N-butyl alcohol fits this description. N-butyl alcohol is reacted with a phosphorus trihalide, say PBra. 

Phosphorus trihalides can in some cases be used to halogenate alcohols (12.282) and to convert nitro compounds into cyanides (12.283). Carboxylic acids are converted to acyl halides (12.284) and deoxygenations can sometimes be effected (12.285). The triiodide is a useful deoxygenating agent. 

The most widespread method of producing complete esters of phosphorous acid, in which R = R2 = R3, is the treatment of alcohols or alkylated phenols, taken in small excesses (0.1-0.5 mole) with phosphorus trihalide (most often trichloride) at a temperature of 150-200°C [70, 158, 285, 294] ... 

We have just seen that an alcohol can be converted into an alkyl halide by treating it with a hydrogen halide. Better yields of the alkyl halide are obtained and carbocation rearrangements can be avoided if a phosphorus trihalide (PCI3 or PBr3) or thionyl chloride (SOCI2) is used instead. 

Primary and secondary alcohols, which react only slowly with HBr and HCl, react readily with thionyl chloride and phosphorus trihalides, such as phosphorus tribromide, to give the corresponding alkyl hahdes. The products of these reactions are easily separated ftom the inorganic by-products. Thionyl chloride produces hydrogen chloride and sulfur dioxide, which are released from the reaction as gases. The chloroaUtane remains in solution. 

Zinc di-isobutyl is a liquid, B.pt. 165° to IGT Tl., less readily oxidised than zinc dipropyl. It reacts vigorously with trihalides of phosphorus and arsenic, yielding corresponding phosphines and arsines. With acetaldehyde it forms principally ethyl alcohol, but also a little isobutyl alcohol, and with isovaleryl chloride it gives di-isobutyl ketone. ... 

The interaction of phosphorns trihalides with alcohols involves a series of consecntive and concnr-rent reactions. In the case of phosphorus trichloride, these can be represented by Eqnations 6.306 through 6.309 in which triesters are first formed, followed by stepwise dealkylation to phosphorous acid. 

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