Addition of HBr to 2-Methylpropene

This reaction does not occur to any appreciable extent  

With this understanding of the mechanism for the ionic addition of hydrogen halides to alkenes, we can now give the following modern statement of Markovnikov s rule. 

Because addition of the electrophile occurs first (before the addition of the nucleophilic portion of the adding reagent), it determines the overall orientation of the addition. 

Notice that this formnlation of Markovnikov s rule allows us to predict the outcome of the addition of a reagent snch as ICI. Becanse of the greater electronegativity of chlorine, the positive portion of this molecule is iodine. The addition of ICI to 2-methylpropene takes place in the following way and produces 2-chloro-l-iodo-2-methylpropane  

Give the structure and name of the product that would be obtained from the ionic addition of IBr to propene. 

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Figure 6.8 Energy diagram for the two-step electrophilic addition of HBr to 2-methylpropene. The first step is slower than the second step.

Addition of HBr to 2-methylpropene gives mainly tert-butyl bromide, because the product with the more stable carbocation intermediate always predominates in this type of reaction. 

A MECHANISM FOR THE REACTION ] Addition of a Hydrogen Halide to an Alkene 341 [ A MECHANISM FOR THE REACTION ] Addition of HBr to 2-Methylpropene 343... 

Now let s draw the forward scheme. Addition of HBr to irai/is-2-butene produces 2-bromobutane, which is subsequently treated with terf-butoxide to give 1-butene. /ift-Markovnrkov addition of water (via hydroboration/oxidation) followed by PCC oxidation gives the aldehyde. Reaction with the Gngnard reagent (produced by awn-Markovnikov addition of HBr to 2-methylpropene, then magnesium, as shown) gives 2-methyl-4-heptanol after water workup. Oxidation with PCC produces the target ketone. 

When the addition of hydrogen halides to alkenes was first studied systematically in the 1930s, chemists observed that the addition of HBr sometimes gave Markovnikov addition and sometimes gave non-Markovnikov addition. These two modes of addition of HBr are illustrated for 2-methylpropene (isobutylene). 

Addition of HX to an unsymmetrical alkene could, in principle, give two different products. The ionic reaction between hydrogen bromide (HBr) and 2-methylpropene (isobutylene [(CH3)2CH=CH2]), where both l-bromo-2-methylpropane (isobutyl bromide [BrCH2CH(CH3)2]) and 2-bromo-2-methylpropane (r-butyl bromide [(CH3)3CBr]) could have formed is shown in Equation 6.26. 

In 1870, the Russian chemist Vladimir Markovnikov observed that reagents add to imsymmetrical alkenes in a specific way. Markovnikov s rule states that a molecule of the general formula HX adds to a double bond so that the hydrogen atom forms a bond to the unsaturated carbon atom with the largest number of directly bonded hydrogen atoms. This is the less substimted double-bonded carbon atom. The addition reactions of HCl with 2-methylpropene and 1-methylcyclohexene with HBr are two examples of Markovnikov s rule. 

Now let s draw the forward scheme. The 3° alcohol is converted to 2-methylpropene using strong acid. Anti-Markovnikov addition of HBr (with peroxides) produces l-bromo-2-methylpropane. Subsequent reaction with sodium acetylide (produced from the 1° alcohol by dehydration, bromination and double elimation/deprotonation as shown) produces 4-methyl-1-pentyne. Deprotonation with sodium amide followed by reaction with 1-bromopentane (made from the 2° alcohol by tosylation, elimination and anfi -Markovnikov addition) yields 2-methyl-4-decyne. Reduction using sodium in liquid ammonia produces the E alkene. Ozonolysis followed by treatment with dimethylsulfide produces an equimolar ratio of the two products, 3-methylbutanal and hexanal. 

Reaction of 2-methylpropene with HBr might, in principle, lead to a mixture of two alkyl bromide addition products. Name them, and draw their structures. 

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. 

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