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Chapter08notes - 1 Addition Reactions The characteristic...

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1 The characteristic reaction of alkenes is addition—the  bond is broken and two new  bonds are formed. 1. Addition Reactions Alkenes are electron rich, with the electron density of the  bond concentrated above and below the plane of the molecule. Therefore, alkenes act as nucleophiles and react with electrophiles. Simple alkenes do not react with nucleophiles or bases, reagents that are themselves electron rich.
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2 C C E Nu δ + δ - C C E + Nu 1. Addition Reactions
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3 Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides. 1. Syn and Anti Addition to Alkenes
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4 1. Addition Reactions (some examples)
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5 2. Hydrohalogenation—Electrophilic Addition of HX
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6 The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. 2. Mechanism of Hydrohalogenation C C A C C E C C Nu Nu E Nu δ + δ -
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7 2. Energy Diagram
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8 C C H H H H E Nu E C C H H H H E C C H H H H E C C H H H H Nu E C C H H H H Nu Nu same  as same  as 3. Regioselectivity – Markovnikov’s Rule For symmetrical substrates, no problem for regiochemistry
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9 Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with. 3. Markovnikov’s Rule E Nu C C H H 3 C H H E C C H H CH 3 H E C C CH 3 H H H E C C CH 3 H H H Nu E C C H H CH 3 H Nu Nu Nu or different  from For unsymmetrical substrates:
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10 The basis of Markovnikov’s rule is the formation of a carbocation in the rate-determining step of the mechanism. In the addition of HX to an unsymmetrical alkene, the H atom is added to the less substituted carbon to form the more stable, more substituted carbocation. 3. Carbocation Stability and Markovnikov’s Rule
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11 According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process. Thus, the transition state to form the more stable 2° carbocation is lower in energy. The E a for formation of the more stable 2° carbocation is lower than the E a for formation of the 1° carbocation; the 2° carbocation is formed faster. 3. Hammond Postulate and Markovnikov’s Rule
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12 + (1) H Cl Cl H H Cl (95     :     5) + (2) H Br (98     :     2) H Br Br H 3. Markovnikov’s Rule - examples
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13 Recall that trigonal planar atoms react with reagents from two directions with equal probability.
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