Chapter 4 - Chapter 4: Introduc1on to Alkenes General...

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Unformatted text preview: Chapter 4: Introduc1on to Alkenes General formula : CnH2n Simplest example: ethylene Double bond geometry sp2 hybridiza1on π (Pi) bond Cis/trans isomers Nomenclature 1 ­butene cis ­2 ­butene Trans ­2 ­butene Special names Nomenclature E,Z ­system for stereoisomers (Z) ­1 ­bromo ­1,2 ­dicholroethene Unsatura1on number U: unsatura1on number C: number of carbon atoms H: actual number hydrogen and halogens N: number of nitrogen Physical Proper1es of Alkanes Except for m.p. and dipole moments, alkenes have similar physical proper1es to the corresponding alkanes Heats of forma1on  ­11.6 kJ/mol 4H2+ 4C cis-2-butene (C4H8)  ­7.4 kJ/mol exothermic The standard heat of forma1on of a compound, is the heat change that occurs when the compound is formed from its elements in their natural state at 1 atm. Pressure and 25 °C. cis-2-butene (C4H8) trans-2-butene Hess’s law of constant heat summa1on Rela1ve stabili1es of alkene isomers Rela1ve stability Addi1on Reac1on Regioselec1vity Markovnikov’s Rule: The main product is that isomer in which the halogen is bonded to the carbon of the double bond with the great number of alkyl subs1tuent, and the hydrogen is bonded to the carbon with the smaller number of alkyl subs1tuents Mechanism of addi1on reac1on to alkenes Step 1 Step 2 Carboca1ons 1.Alkyl subs1tu1on stabilize the empty p orbital 2. hyperconjuga1on Carboca1on rearrangement Reac1on Rates Energy Barrier Mul1step reac1on Hammond’s postulate Catalysis Cataly1c hydrogena1on of Alkenes Hydra1on of Alkenes Rearranged hydra1on products Alcohol dehydra1on Principle of microscopic reversibility Enzyme Catalysis Key points •  •  •  •  •  •  •  •  •  •  •  General formula of alkenes, bond length and bond angles; sp2 orbitals (hybridiza1on) and pi bonds (MO theory); Nomenclature of alkenes; E,Z systems; CIP priority; Unsatura1on number (degree of unsatura1on); Heats of forma1on (exothermic vs. endothermic reac1on); Regioselec1vity, Markovnikov’s rule, mechanism of addic1on reac1on to alkenes; Carboca1on, rearrangement; Transi1on state, energy barrier, rate ­limi1ng step Hammond’s postulate Catalysis, cataly1c hydrogena1on ...
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This note was uploaded on 08/24/2010 for the course CHEM 3351 at Colorado.

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