# reaction of alcohol with pcl5 mechanism

Halide ions are good nucleophiles (they are much stronger nucleophiles than water), and since halide ions are present in high concentration, most of the carbocations react with an electron pair of a halide ion to form a more stable species, the alkyl halide product. Note the stereochemistry and also remember that benzyllic carbons are good Sn2 electrophiles.

But with PCl3 a problem arises. Sulfur dioxide and hydrogen chloride are given off.

The first equation shows the dehydration of a 3º-alcohol. This is the basis for the carbon's electrophilicity. The π electrons of the carboxyl group, , migrate to pick up the positive charge. In the dehydration of 1-methylcyclohexanol, which product is favored? The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Note how the carbocation after the rearrangement is resonance stabilized by the oxygen. There are also side reactions involving the $$POCl_3$$ reacting with the alcohol. Layne Morsch (University of Illinois Springfield). The mixture of the iodide and phosphoric(V) acid produces hydrogen iodide, which reacts with the alcohol. The use of thionyl chloride for converting alcohols to alkyl chlorides has the added benefit that both of the by-products, sulfur dioxide and hydrogen chloride, are gases. The carboxyl carbon of the carboxylic acid is protonated. Halide ions are good nucleophiles (they are much stronger nucleophiles than water), and since halide ions are present in high concentration, most of the carbocations react with an electron pair of a halide ion to form a more stable species, the alkyl halide product.

The lone pair of electrons on oxygen atom makes the –OH group weakly basic. Reaction with phosphorus(V) chloride, PCl5. Direct displacement of the hydroxyl group does not occur because the leaving group would have to be a strongly basic hydroxide ion: We can see now why the reactions of alcohols with hydrogen halides are acid-promoted. A tertiary halogenoalkane (haloalkane or alkyl halide) is formed. R-OH + PCl 5 R-Cl + POCl 3 + HCl PI 3 has to be generated in situ via reaction of iodine and phosphorous. If you have a neutral liquid not contaminated with water, and clouds of hydrogen chloride are produced when you add phosphorus(V) chloride, then you have an alcohol group present. The alcohol reactions with thionyl chloride or phosphorus tribromide are discussed in the next section. Mechanisms of the Reactions of Alcohols with HX. write an equation for the conversion of an alcohol to an alkyl halide. These esters are formed by reacting an alcohol with an appropriate sulfonic acid. The order of reactivity of alcohols is 3° > 2° > 1° methyl. For more information contact us at [email protected] or check out our status page at https://status.libretexts.org. Actually, while it s true that PCl5 is commonly used to convert the -OH group of an alcohol or carboxylic acid to -Cl, it also undergoes a reaction with aldehydes and ketones at low temperature (usually use 0 degrees C when writing it on paper), converting RCOR to RCCl2R (ketone to geminal dichloroalkane) and RCHO to RCHCl2 (aldehyde to geminal dichloroalkane) with the reproduced material. Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid, but for primary or secondary alcohols the reaction rates are too slow for the reaction to be of much importance. questions on replacing the OH group by halogens. Oxygen donates two electrons to a proton from sulfuric acid H2SO4, forming an alkyloxonium ion. The electronegativity of oxygen is substantially greater than that of carbon and hydrogen. This procedure is also effective with hindered 2º-alcohols, but for unhindered and 1º-alcohols an SN2 chloride ion substitution of the chlorophosphate intermediate competes with elimination. $CH_3CH_2CH_2OH + SOCl_2 \rightarrow CH_3CH_2CH_2Cl + SO_2 + HCl \label{1.1.9}$. The overall result is an SN1 reaction. The following epoxide can be transformed into an alcohol using a grignard reagent, take for example allylmagnesium chloride.

$3CH_3CH_2CH_2OH + PCl_3 \rightarrow 3CH_3CH_2CH_2Cl + H_3PO_3 \label{1.1.3a}$. Below, an abbreviated mechanism for the reaction is displayed.

identify the alcohol which should be used to prepare a given alkyl halide using one of the reagents specified in Objective 3. select the most appropriate reagent for converting a given alcohol to a given alkyl halide. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion +OH2 (The pKa value of a tertiary protonated alcohol can go as low as -3.8). The water molecule (which is a stronger base than the HSO4- ion) then abstracts a proton from an adjacent carbon, forming a double bond. Alcohols may be converted to alkyl sulfonates, which are sulfonic acid esters. 6. Although halide ions (particularly iodide and bromide ions) are strong nucleophiles, they are not strong enough to carry out substitution reactions with alcohols themselves. contained in this article in third party publications We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Finally, the reading shows the production of an ester from an alcohol and and an acid chloride. Alcohols react with the strongly acidic hydrogen halides HCl, HBr, and HI, but they do not react with non-acidic NaCl, NaBr, or NaI. identify the alkyl halide formed when a given alcohol reacts with thionyl chloride, phosphorus tribromide, or a hydrogen halide. Direct displacement of the hydroxyl group does not occur because the leaving group would have to be a strongly basic hydroxide ion: 17.5: Alcohols from Carbonyl Compounds: Grignard Reagents, Conversion of Alcohols into Alkyl Halides, Mechanisms of the Reactions of Alcohols with HX, Mechanism for the Dehydration of Alcohol into Alkene, Dehydration reaction of secondary alcohol, discuss the reactions of alcohols that have been introduced in previous units. Secondary, tertiary, allylic, and benzylic alcohols appear to react by a mechanism that involves the formation of a carbocation, in an $$S_N1$$ reaction with the protonated alcohol acting as the substrate.. The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation and form a double bond. Following are several examples of the oxidation of secondary alcohols: Carboxylic acid formation. For more information contact us at [email protected] or check out our status page at https://status.libretexts.org. If you choose to follow this link, use the BACK button on your browser to return to this page. Although the oxonium ion is formed by protonation of the alcohol, it can also be viewed as a Lewis acid-base complex between the cation ($$R^+$$) and $$H_2O$$. is available on our Permission Requests page. An alcohol molecule adds to the carbocation produced in Step 1. Because thionyl bromide is relatively unstable, alkyl bromides are normally prepared by reacting the alcohol with phosphorous tribromide (PBr 3).