A to Z Directory – Virginia Commonwealth University
Chapter 4
Halogenation
▪ Addition of halogen to an alkane
o Only works on sp3-hybridized carbons
▪ Works best with chlorine and bromine
o Iodine is too slow
o Fluorine blows up (too fast)
▪ Goes through radical intermediate, so putting halogen on more substituted carbon is better
o Bromine is way more selective than chlorine
▪ Initiation
o Homolytic cleavage of X2
[pic]
o One molecule splits into two radicals
▪ Propagation
o Start with one molecule and one radical; end up with one molecule and one radical!
[pic]
▪ Termination
o Any two radicals stick together
[pic]
Reactive Intermediates
▪ Carbocations
o Carbon with three bonds and no lone pairs
o Trigonal planar geometry
o sp2-hybridized
o empty p-orbital
o stability - 3°>2°>1°> methyl
o electron-deficient
o only intermediate which can undergo rearrangement
[pic]
▪ 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
o Carbon with three bonds and one unpaired electron
o Trigonal planar geometry
o sp2-hybridized
o one electron in the p-orbital
o stability - 3°>2°>1°> methyl
o electron-deficient
▪ Carbanions
o Carbon with three bonds and a lone pair
o sp3-hybridized
o Tetrahedral geometry
o Stability- methyl>1°>2°>3°
o Strong bases/nucleophiles
▪ Carbenes
o Carbon with two bonds and one lone pair
o Neutral
o Rare
o sp2-hybridized
o empty p-orbital
o can be electrophile or nucleophile
▪ Radicals, carbocations, and carbanions are stabilized by resonance
o One away from a double bond is called “allylic”
[pic]
▪ Being directly on the double bond is particularly unstable and this position is called “vinyl”
[pic]
o One away from an aromatic ring is called “benzylic”
[pic]
▪ Being directly on the aromatic ring is particularly unstable and this position is called “aryl”
[pic]
The rest of chapter 4 that you should know from Gen Chem
▪ Equilibrium Constant
o Keq=[pic]
o If Keq> 1, then more products than reactants
o If 0 ................
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