Chapter 4 Reactions of Alkenes and Alkynes

Chapter 4 Reactions of Alkenes and Alkynes - Chapter 4...

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Chapter 4 Reactions of Alkenes and Alkynes 4.1 Addition of HX to alkenes 1. Addition of halogens acids, HX, to alkene yields variety of halo substituted products 2. HCl, HBr, HI all add to alkenes as shown below 2-Methylpropene + HCl --- 2-Chloro-2Methanepropane ` 1-Methylcyclohexane + HBr 1-Bromo-1-Methylcyclohexane Pent-1-ene + HI 2-Iodopentane
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4.2 Orientation of Alkene Addition Reaction : Markonikov’s Rule 1. when an unsymmetrically substituted alkene reacts with HX to give single addition product instead of the expected mixture, the reaction is called regiospecific 2. A regiospecific reaction: 2-Methylpropene + HCl 2-Chloro-2-Methylpropane(sole product) 3. Markovnikov’s Rule: a. In addition of an HX to an alkene, H attaches to the carbon atom with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents b. Example : 2-Methylpropene + HCl 2-Chloro-2-Methylpropane 1-Methylcyclohexene + HBr - 1-Bromo-1methylcyclhexane When both double bond carbon atoms have the same degree of substitution, a mixture of addition reaction is formed
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Pent-2-ene + HBr 2-Bromopentane + 3-Bromopentane c. Since carbocations are always involved as intermediates in these reactions, another way to express Markovnikove’s rule is i. The addition of HX to alkenes, the more highly substituted carbocation intermediate is formed rather than the less highly substituted one ii. Example in the reaction between HCl and 2-Methylpropene leads to the formation of tertiary carbocation than the alternative primary carbocation 4.3 Carbocation Structure and Stabilty 1. Carbocations are planar 2. the positively charged carbon cation is sp 2 hybrid and the three substituents bonded to it are oriented to the corners of an equilateral triangle a. There are only 6 electrons in the carbon valence shell and all six are used to form 3 σ bond and so the p orbital extending above and below the plane is vacant 3. Carbocation stability increases with increasing alkyl substitution: more highly substituted carbocations are more stable than less highly substituted ones a. This is because alkyl groups tend to donate electrons to the positively charged carbon atom b. The more alkyl groups there are the more electron donation there is and the more stable the carbocations c. Less stable - more stable
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Methyl - primary secondary tertiary 4. How Markonikov’s rule works a. More stable carbocation is formed rather than the less stable one. Therefore carbon cation is formed from double bond carbon with more alkyl substituents which formed a more stable carbocation than the one with less alkyl substituents b. For example in the reaction of 1-Methylcyclohexene with HBr, the intermediate carbocation might have either three alkyl substituent ( tertiary cation) or 2 alkyl substituents (secondary cation). Since tertiary cation is more stable than secondary cation, it’s the tertiary cation that’s formed as reaction intermediate leading to tertiary alkyl bromide product
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This note was uploaded on 07/16/2011 for the course CHE 391 taught by Professor Mckague,b during the Spring '08 term at University of Toronto.

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Chapter 4 Reactions of Alkenes and Alkynes - Chapter 4...

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