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Markownikoff’s Rule in Unsaturated Hydrocarbon Reactions

Markownikoff’s Rule explains how hydrogen halides react with unsymmetrical alkenes, determining which carbon the hydrogen attaches to based on the number of hydrogens and carbons present. This rule governs addition reactions of unsymmetrical alkenes, like the addition of HBr to propene, leading to major and minor product formation based on carbocation stability. Learn about the types of carbocations and reasons behind their stability in this fundamental concept of organic chemistry.

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Markownikoff’s Rule in Unsaturated Hydrocarbon Reactions

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  1. Markownikoff’s Rule

  2. Markownikoff’s Rule “When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon of the alkene bearing the larger number of hydrogens and smaller number of carbons"

  3. Addition reactions of unsymmetrical alkenes Addition of HBr to propene CH3CH=CH2: • H–Br can add in two different ways • Two products are possible: Contains more H atoms CH3CH=CH2 CH3CH=CH2 H–Br Br–H CH3CH2–CH2Br CH3CHBr–CH3 Minor product Major product

  4. It’s all controlled by carbocations Secondary carbocation Two C joined to C+ Primary carbocation One C joined to C+ Major product Minor product

  5. Types of carbocation Least stable Primary carbocation • One carbon chains attached to C+ Secondarycarbocation: • Two carbon chains attached to C+ Tertiarycarbocation: • Three carbon chains attached to C+ Most stable

  6. Reasons for carbocation stability Least stable Electron-donating ability of alkyl groups. • Each alkyl group pushes electrons towards C+ charge of carbocation. • Positive charge is spread over alkyl groups The more alkyl groups attached to the C+ • The more the positive charge is spread out • The greater the stability of the carbocation Primary Secondary Most stable Tertiary

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