2-bromo-2-methylpropane reactions

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

Self-Test 13.3B When the concentration of 2-bromo-2-methylpropane, C4HgBr, is doubled, the rate of the reaction C4H9Br(aq) + OH (aq) - C4HgOH(aq) + P>r (aq) increases by a factor of 2. When both the C4HgBr and the OH- concentrations are doubled, the rate increase is the same, a factor of 2. What are (a) the reactant orders, (b) the overall order of the reaction, and (c) the units of k if the rate is expressed in moles per liter per second ... 

Consider the reaction between 2-bromo-2-methylpropane and water. 

To illustrate the 8, 1 mechanism, consider the reaction between the tertiary haloalkane 2-bromo-2-methylpropane and the nucleophilic hydroxide ion. A study of the kinetics of the reaction reveals that it has the following rate equation rate = ic[(CH3)3CBr]. 

A-10. Using the data in Table B-l of this Study Guide, calculate the heat of reaction (AH0) for the light-initiated reaction of bromine (Br2) with 2-methylpropane to give 2-bromo-2-methylpropane and hydrogen bromide. 

The substitution reaction of bromomethane with hydroxide ion proceeds about 5000 times faster than the reaction of bromomethane with water. However, the substitution reaction of 2-bromo-2-methylpropane proceeds at the same rate with both of these nucleophiles. Explain. 

Mechanism of the SnI substitution and El elimination reactions of tert-butyl bromide (2-bromo-2-methylpropane). 

Another instructive example of electrophilic or H-bonding assistance of protic solvents (or co-solvents) in SnI reactions is the accelerated acetolysis rate of 2-bromo-2-methylpropane upon the addition of phenols to a tetrachloromethane/acetic acid solution of the reactant [582] see reference [582] for further examples. The usefulness of phenol as a solvent for SnI solvolysis reactions, in particular phenolysis of 1-halo-l-phenylethanes, has been stressed by Okamoto [582], In spite of its low relative permittivity (fir = 9.78 at 60 °C), its low dipolarity fi = 4.8 10 Cm = 1.45 D), and its low nucleophihcity, it represents a solvent of high ionizing power due to its electrophilic driving force. 

For example, the E2 reaction of a 1° alkyl halide (1-bromobutane) forms a monosubstituted alkene, whereas the E2 reaction of a 3° alkyl halide (2-bromo-2-methylpropane) forms a disub-stituted alkene. The disubstituted alkene is more stable, so the 3° alkyl halide reacts faster than the 1° alkyl halide. 

Although nucleophilic substitution with acetylide anions is a very valuable carbon-carbon bondforming reaction, it has the same limitations as any Sn2 reaction. Steric hindrance around the leaving group causes 2° and 3° alkyl halides to undergo elimination by an E2 mechanism, as shown with 2-bromo-2-methylpropane. Thus, nucleophilic substitution with acetylide anions forms new carbon-carbon bonds in high yield only with unhindered CH3X and 1 ° alkyl halides. 

Steric hindrance to the Sn2 reaction. As the computer-generated models indicate, the carbon atom in (a) bromomethane is readiiy accessible, resulting in a fast 5 2 reaction. The carbon atoms in (b) bromoethane (primary), (c) 2-bromopropane (secondary), and (d) 2-bromo-2-methylpropane (tertiary) are successively more hindered, resulting in successively slower Sn2 reactions. 

The observation of first-order kinetics for the SnI reaction of (CHsljCi with H2O tells us that the alkyl halide is involved in a unimolecular rate limiting step. In other words, 2-bromo-2-methylpropane undergoes a spoq taneous, rate-limiting reaction without involvement of the nucleophile. "Hi nucleophile must be involved at some other step. The mechanism shown i Figure 11.9 accounts for the kinetic observations. 

Tertiary alkyl halides E2 elimination occurs when a base such a OH" or RO" is used. For example, 2-bromo-2-methylpropane gives 97% elimination product when treated with ethoxide ion in ethanol- By contrast, reaction under neutral conditions (heating in pure I ethanol) leads to a mixture of products resulting from both SnI sub-1 stitution and El elimination. "... 

In contrast, the hydrolysis of tm-butyl bromide (2-bromo-2-methylpropane) occurs in a stepwise manner (reaction 1.1b). In the first slow step, the C-Br bond breaks, with the bromine atom taking both electrons from the bond and leaving as a negatively charged bromide ion. The remainder of the molecule is the positively charged tert-butyl cation (2-methylprop-2-yl cation). This is a highly reactive intermediate, which reacts rapidly with the hydroxide ion to form the corresponding alcohol. 

A third technique monitors a change in conductivity. In the reaction between an organic halide (2-bromo-2-methylpropane) and water,... 

Finally, for the reaction of 2-bromo-2-methylpropane and water that we considered earlier, the rate law has been found to be... 

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