1.9 Mechanism of the Reaction of Alcohols With Hydrogen Halides We'll often find it convenient to write chemical equations in the abbreviated form shown here, in which reagents, especially inorganic ones, are not included in the body of the equation but instead are indicated over the arrow. Inorganic products-in this case, water-are usually omitted. These simplifications focus our attention on the organic reac- tant and its functional group transformation PROBLEM 4.12 Write chemical equations for the reaction that takes place between each of the following pairs of reactants (a)2-Butanol and hydrogen bromide (b)3-Ethyl-3-pentanol and hydrogen chloride (c)1-Tetradecanol and hydrogen bromide SAMPLE SOLUTION (a)An alcohol and a hydrogen halide react to form an alkyl halide and water. In this case 2-bromobutane was isolated in 73% yield CH3CHCH2 CH3 HBr ->CH3CHCH2 CH3 H2O 2-Butanol Hydrogen bromide 2-Bromobutane Water 4.9 MECHANISM OF THE REACTION OF ALCOHOLS WITH HYDROGEN HALIDES chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Calling the reaction a substitution tells us the relationship between the organic reactant and its prod- uct but does not reveal the mechanism. In developing a mechanistic picture for a par ticular reaction, we combine some basic principles of chemical reactivity with experi mental observations to deduce the most likely sequence of elementary steps Consider the reaction of tert-butyl alcohol with hydrogen chloride: (CH3)3COH HCl ->(CH3)3CCl H,O The generally accepted mechanism for this reaction is presented as a series of three ele mentary steps in Figure 4.7. We say"generally accepted"because a reaction mechanism can never be proved to be correct. A mechanism is our best present assessment of how a reaction proceeds and must account for all experimental observations. If new experi mental data appear that conflict with the mechanism, the mechanism must be modified to accommodate them. If the new data are consistent with the proposed mechanism, our confidence grows that it is likely to be correct We already know about step I of the mechanism outlined in Figure 4.7; it is an If you have not already writ- example of a Bronsted acid-base reaction of the type discussed in Section 4.6 and formed ten out the solutions to the basis of Problems 4.9 through 4.11 Problems 4.9 to 4.11, you should do so now Steps 2 and 3, however, are new to us. Step 2 involves dissociation of an alky loxonium ion to a molecule of water and a carbocation, a species that contains a posi- tively charged carbon. In step 3, this carbocation reacts with chloride ion to yield tert- butyl chloride. Both the alkyloxonium ion and the carbocation are intermediates in the reaction. They are not isolated, but are form d consumed in another dur- ing the passage of reactants to products. If we add the equations for steps 1 through 3 together, the equation for the overall process results. A valid reaction mechanism must Back Forward Main Menu Study Guide ToC Student OLC MHHE WebsiteWe’ll often find it convenient to write chemical equations in the abbreviated form shown here, in which reagents, especially inorganic ones, are not included in the body of the equation but instead are indicated over the arrow. Inorganic products—in this case, water—are usually omitted. These simplifications focus our attention on the organic reac￾tant and its functional group transformation. PROBLEM 4.12 Write chemical equations for the reaction that takes place between each of the following pairs of reactants: (a) 2-Butanol and hydrogen bromide (b) 3-Ethyl-3-pentanol and hydrogen chloride (c) 1-Tetradecanol and hydrogen bromide SAMPLE SOLUTION (a) An alcohol and a hydrogen halide react to form an alkyl halide and water. In this case 2-bromobutane was isolated in 73% yield. 4.9 MECHANISM OF THE REACTION OF ALCOHOLS WITH HYDROGEN HALIDES The reaction of an alcohol with a hydrogen halide is a substitution. A halogen, usually chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Calling the reaction a substitution tells us the relationship between the organic reactant and its prod￾uct but does not reveal the mechanism. In developing a mechanistic picture for a par￾ticular reaction, we combine some basic principles of chemical reactivity with experi￾mental observations to deduce the most likely sequence of elementary steps. Consider the reaction of tert-butyl alcohol with hydrogen chloride: The generally accepted mechanism for this reaction is presented as a series of three ele￾mentary steps in Figure 4.7. We say “generally accepted” because a reaction mechanism can never be proved to be correct. A mechanism is our best present assessment of how a reaction proceeds and must account for all experimental observations. If new experi￾mental data appear that conflict with the mechanism, the mechanism must be modified to accommodate them. If the new data are consistent with the proposed mechanism, our confidence grows that it is likely to be correct. We already know about step 1 of the mechanism outlined in Figure 4.7; it is an example of a Brønsted acid–base reaction of the type discussed in Section 4.6 and formed the basis of Problems 4.9 through 4.11. Steps 2 and 3, however, are new to us. Step 2 involves dissociation of an alky￾loxonium ion to a molecule of water and a carbocation, a species that contains a posi￾tively charged carbon. In step 3, this carbocation reacts with chloride ion to yield tert￾butyl chloride. Both the alkyloxonium ion and the carbocation are intermediates in the reaction. They are not isolated, but are formed in one step and consumed in another dur￾ing the passage of reactants to products. If we add the equations for steps 1 through 3 together, the equation for the overall process results. A valid reaction mechanism must (CH3)3COH tert-Butyl alcohol HCl Hydrogen chloride (CH3)3CCl tert-Butyl chloride H2O Water HBr Hydrogen bromide H2O 2-Butanol Water CH3CHCH2CH3 OH 2-Bromobutane CH3CHCH2CH3 Br 4.9 Mechanism of the Reaction of Alcohols With Hydrogen Halides 139 If you have not already writ￾ten out the solutions to Problems 4.9 to 4.11, you should do so now. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website