Chapter 4

Halogenation

  • Addition of halogen to an alkane
  • Only works on sp3-hybridized carbons
  • Works best with chlorine and bromine
  • Iodine is too slow
  • Fluorine blows up (too fast)
  • Goes through radical intermediate, so putting halogen on more substituted carbon is better
  • Bromine is way more selective than chlorine
  • Initiation
  • Homolytic cleavage of X2
  • One molecule splits into two radicals
  • Propagation
  • Start with one molecule and one radical; end up with one molecule and one radical!

  • Termination
  • Any two radicals stick together

Reactive Intermediates

  • Carbocations
  • Carbon with three bonds and no lone pairs
  • Trigonal planar geometry
  • sp2-hybridized
  • empty p-orbital
  • stability - 3°>2°>1°> methyl
  • electron-deficient
  • only intermediate which can undergo rearrangement
  • We’ll see a lot of this later in the course.
  • It’s a good idea to think “carbocations can rearrange” every time you see a carbocation intermediate for the rest of your life!
  • Radicals
  • Carbon with three bonds and one unpaired electron
  • Trigonal planar geometry
  • sp2-hybridized
  • one electron in the p-orbital
  • stability - 3°>2°>1°> methyl
  • electron-deficient
  • Carbanions
  • Carbon with three bonds and a lone pair
  • sp3-hybridized
  • Tetrahedral geometry
  • Stability- methyl>1°>2°>3°
  • Strong bases/nucleophiles
  • Carbenes
  • Carbon with two bonds and one lone pair
  • Neutral
  • Rare
  • sp2-hybridized
  • empty p-orbital
  • can be electrophile or nucleophile
  • Radicals, carbocations, and carbanions are stabilized by resonance
  • One away from a double bond is called “allylic”
  • Being directly on the double bond is particularly unstable and this position is called “vinyl”
  • One away from an aromatic ring is called “benzylic”
  • Being directly on the aromatic ring is particularly unstable and this position is called “aryl”

The rest of chapter 4 that you should know from Gen Chem

  • Equilibrium Constant
  • Keq=
  • If Keq> 1, then more products than reactants
  • If 0<Keq<1, then more reactants than products
  • He may ask you to propose a Keq for a reaction based on some information.
  • If he tells you a reaction is highly exergonic, then your Keq should be really large.
  • Think 103 or higher
  • If he tells you a reaction is highly endergonic, then your Keq should be really small.
  • Think 10-3 or smaller.
  • If he wants one of these extremes and you give a value like 1.5 or .7, he won’t give full credit.
  • ∆G=∆H-T∆S
  • If ∆G<0, then spontaneous
  • This is called exergonic.
  • If ∆G=0, then at equilibrium
  • If ∆G>0, then non-spontaneous
  • This is called endergonic.
  • Bond-Dissociation Energies
  • If ∆H<0, then the reaction is exothermic.
  • If ∆>0, then the reaction is endothermic.
  • ∆H of a reaction can be estimated by
  • ∆H=bonds broken-bonds formed
  • Kinetics
  • For A + B → C + D
  • rate=k[A]m[B]n
  • The overall reaction order is m + n
  • A is of the mth order and B is of the nth order
  • Activation Energy
  • The difference in energy from the reactants to the highest-energy transition state
  • Transition States
  • Chemical species which exist fleetingly
  • Structure or species when bonds are breaking and/or forming all at once
  • Occur at the peaks of a reaction coordinate diagram
  • Intermediates
  • Chemical species which occur at the troughs of a reaction coordinate diagram
  • Exist long enough to be isolated
  • This means that he will not use the word fleetingly when describing them.
  • Reaction coordinate diagrams
  • The number of humps is the number of steps in a reaction.
  • The valleys are intermediates.
  • At the top of each hump is a transition state.
  • The highest hump is the rate-determining step.