Alkyl groups alcohol reactions with hydrogen halides

Alcohols react with hydrogen halides to produce alkyl halides. In these reactions, the —OH group of the alcohol is replaced by the halide ion of the acid. All alcohols react with concentrated HBr and HI solutions to produce alkyl bromide and alkyl iodide. These reactions occur by a specific mechanism as shown below. 

The reactions to be described in the remainder of this chapter use either an alkane or an alcohol as the starting material for preparing an alkyl halide. By knowing how to prepare alkyl halides, we can better appreciate the material in later chapters, where alkyl halides figure prominently in key functional group transformations. Just as important, the preparation of alkyl halides will serve as our focal point as we examine the principles of reaction mechanisms. WeTl begin with the preparation of alkyl halides from alcohols by reaction with hydrogen halides. 

Tertiary alcohols undergo substituhon reactions with hydrogen halides faster than secondary alcohols do, because terhary carbocahons are more stable and therefore easier to form than secondary carbocahons. (Recall that alkyl groups stabilize carbocahons by hyper-conjugahon Section 6.2) As a result, the reachon of a terhary alcohol with a hydrogen halide proceeds readily at room temperature, whereas the reachon of a secondary alcohol with a hydrogen halide has to be heated to have the reachon occur at a reasonable rate. 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

Sulfonic esters are most frequently prepared by treatment of the corresponding halides with alcohols in the presence of a base. The method is much used for the conversion of alcohols to tosylates, brosylates, and similar sulfonic esters. Both R and R may be alkyl or aryl. The base is often pyridine, which functions as a nucleophilic catalyst, as in the similar alcoholysis of carboxylic acyl halides (10-21). Primary alcohols react the most rapidly, and it is often possible to sulfonate selectively a primary OH group in a molecule that also contains secondary or tertiary OH groups. The reaction with sulfonamides has been much less frequently used and is limited to N,N-disubstituted sulfonamides that is, R" may not be hydrogen. However, within these limits it is a useful reaction. The nucleophile in this case is actually R 0 . However, R" may be hydrogen (as well as alkyl) if the nucleophile is a phenol, so that the product is RS020Ar. Acidic catalysts are used in this case. Sulfonic acids have been converted directly to sulfonates by treatment with triethyl or trimethyl orthoformate HC(OR)3, without catalyst or solvent and with a trialkyl phosphite P(OR)3. ... 

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... 

As esters the alkyl halides are hydrolysed by alkalis to alcohols and salts of halogen acids. They are converted by nascent hydrogen into hydrocarbons, by ammonia into amines, by alkoxides into ethers, by alkali hydrogen sulphides into mercaptans, by potassium cyanide into nitriles, and by sodium acetate into acetic esters. (Formulate these reactions.) The alkyl halides are practically insoluble in water but are, on the other hand, miscible with organic solvents. As a consequence of the great affinity of iodine for silver, the alkyl iodides are almost instantaneously decomposed by aqueous-alcoholic silver nitrate solution, and so yield silver iodide and alcohol. The important method of Ziesel for the quantitative determination of alkyl groups combined in the form of ethers, depends on this property (cf. p. 80). 

Thus reaction with alkyl halides such as allyl bromide or pro-pargyl bromide allow for the introduction of oleflnlc2. . or acetylenic side groups onto the phosphazene ring VI, while alcohol leads to the formation of hydrido-phosphazene complexes VII. The hydrogen in these compounds can be replaced with halogen to yield the first series of iodor-phosphazene compounds VIII. 

The reaction of alcohols, alkyl halides or alkenes with alkenes in the presence of anhydrous hydrogen fluoride should lead to conjugated alkylation of the alkenes with simultaneous introduction of an alkyl group and a fluorine atom to give products 1. 

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