Tertiary alcohols reaction with hydrogen halides

Alcohols react with hydrogen halides to yield al kyl halides The reaction is useful as a synthesis of al kyl halides The reactivity of hydrogen halides de creases in the order HI > HBr > HCI > HF Alcohol re activity decreases in the order tertiary > secondary > primary > methyl... 

It is important to note that although methyl and primaiy alcohols react with hydrogen halides by a mechanism that involves fewer steps than the conesponding reactions of secondary and tertiary alcohols, fewer steps do not translate to faster reaction rates. Remember, the order of reactivity of alcohols with hydrogen halides is tertiary > secondary > primary > methyl. Reaction rate is governed by the activation energy of the slowest step, regardless of how many steps there are. 

Secondary and tertiary alcohols undergo S l reactions with hydrogen halides. The reaction of an HX with 3° alcohol proceeds readily at room temperature, whereas the reaction of an HX with a 2° alcohol requires heat. 

Secondary and tertiary alcohols undergo S g1 reactions 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. 

Alcohols react with hydrogen halides to form haloalkanes. The rate of the reaction differs for primary, secondary, and tertiary alcohols. 

As carbocations go CH3" is particularly unstable and its existence as an inter mediate m chemical reactions has never been demonstrated Primary carbocations although more stable than CH3" are still too unstable to be involved as intermediates m chemical reactions The threshold of stability is reached with secondary carbocations Many reactions including the reaction of secondary alcohols with hydrogen halides are believed to involve secondary carbocations The evidence m support of tertiary carbo cation intermediates is stronger yet... 

The SnI mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to but different from S l... 

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... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the caibocation stability order and is readily accommodated by the mechanism we have outlined. 

We have established that the nucleophile is not important in the rate of an S l reaction. We need now to discuss two ways in which it is important. Both concern the nature of the product. A better nucleophile will not accelerate the Sjsjl reaction but it may determine which product is formed. In the reactions of tertiary alcohols with concentrated HCl or HBr there is always more water than halide ion present and yet the t-alkyl halide is formed in good yield, reaction of tertiary alcohols with hydrogen halides... 

Unlike tertiary and secondary carbocations, primary carbocations are too high in energy to be intermediates in chemical reactions. Since primary alcohols are converted, albeit rather slowly, to aUcyl halides on treatment with hydrogen halides, they must follow some other mechanism that avoids carbocation intermediates. This alternative mechanism is believed to be one in which the carbon-halogen bond begins to form before the carbon-oxygen bond of the alkyloxonium ion is completely broken. 

Preparation Alkenes when treated with hydrogen halides (HCl, HBr, and HI) or halogens (Cl and Br ) yield alkyl halides and dihaloalkanes respectively. Another useful method of preparing alkyl halides is by treating an alcohol with a hydrogen halide (HX = HCl, HBr, or HI). The reaction works best for tertiary alcohols. Primary and secondary alcohols can be converted to alkyl halides by treating them with thionyl chloride (SOCy or phosphorus tribromide (PBr ). 

Different reagents such as HX and PX3 may be used to prepare alkyl halides from primary and secondary alcohols. However, because elimination reactions predominate when tertiary alcohols are treated with phosphorous trihalides, preparing tertiary alkyl halides from tertiary alcohols proceeds with good yields only if concentrated hydrogen halides, HX, are used. The reaction of 2-methyl-2-butanol with hydrochloric acid to produce 2-chloro-2-methylbutane (Eq. 14.17) illustrates this transformation. 

When studying reactions that are believed to involve carbocations as intermediates, it is common to test this proposal by assessing the stereochemical relationship between the organic reactant and its product. For example, if a carbocation is an intermediate in the reaction of tertiary alcohols with hydrogen halides, both stereoisomers of 4-tert-butyl-l-methylcyclohexanol are converted to the same carbocation ... 

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