高等有机化学2

Chapter 2  Strain and Stability2.1 Thermochemistry of stable molecules 2.2 Thermochemistry of reactive intermediates 2.3 Basic conformational analysis 2.4 Electronic effects 2.5 Highly strained molecules2.1 Thermochemistry of stable moleculesType of energies & energy changes• Gibbs free energy (ΔG °) • Enthalpy (ΔH °) • Entropy (ΔS °) Δ G ° =   Δ H ° ‐T   Δ S °Gibbs free energy (ΔG°)A Æ B    ΔG °=GB‐GA   ΔG ° <0  ‐‐‐ transformation of A to B is exergonic ΔG ° >0  ‐‐‐ ‐‐‐ transformation of A to B is endergonic lnKeq= ‐ ‐ΔG ° /RT    Keq= [B]/[A] ΔG ° = ΔH ° ‐T ΔS ° （Gibbs‐Helmholtz equation） lnKeq q = ‐ΔH ° /RT + ΔS ° /REnthalpy (ΔH °)• Enthalpy (ΔH °): heat of reaction, including  heat of formation, heat of combustion, heat of  hydrogenation, etc. • ΔH ° <0  reaction is exothermic • ΔH ° >0  reaction is endothermicEntropy (ΔS °)• Entropy (ΔS °): a measure of molecular disorder • Degrees of freedom: translational (bond stretches),  rotational (bond rotations), vibrational (bond angle  vibrations) • In general, the more kinds of motions and the more  unconstrained those motions are are, the more favorable  the entropy.Bond dissociation energies (BDE)• R‐HÆR.+H. (gas phase, phase  homolytic bond cleavage)    ΔH° = BDE • Thermodynamic y  BDE can be used to predict p  the reactivity y of   a molecule • Use BDE to predict exothermicity and endothermicity  CH3‐H + H‐OH Æ CH3‐OH + H‐H BDE 105.1     119          92.3        104.2 ΔHº= (105.1+119) – (92.3+104.2)= 27.6 **Use BDE values from the same data setSome specific bond dissociation energies (kcal/mol)Table 2.1Group incrementTable 2.2Strain energy gyTable 2.32.2 Thermochemistry y of reactive intermediatesCommon reactive intermediates :A: carbocationsB: free radicalsC: carbanionsD: carbenes•Only carbanions have a complete octet around the carbon •There are many other organic ions and radicals with charges and unpaired    electrons l  on atoms other h  than h  carbon bCarbocations2.2 Thermochemistry of reactive intermediates Carbocations•Stability in gas phase ‐‐‐Hydride Ion Affinity (HIA)R HÆR+HΔHº = HIA‐+‐•Stabilization due to alkyl substitution ‐‐‐hyperconjugative interactopn •Stabilization due to increasing delocalization•Heteroatom effects‐‐‐related to πdonating ability and σwithdrawing yability of the heteroatomHIA for selected carbocationsFrom Bowers, M. T. (ed.) (1979), Gas Phase Ion Chemistry,Academic Press,Press New YorkTable 2.4 Gas phase hydride ion affinities for selected carbocations(in kcal/mol)in solutionCarbocations•Stable ion media for carbocations‐‐‐an enviroment devoid of S bl i di f b inucleophiles and bases to make carbocations persistentdeveloped by Olah et. al. as stable ion media•SbF5RX + SbFÆR+ + Sb F X‐5 210Sb2F10X‐‐‐‐very poor nucleophile and very weak base Other stable ion media: HF‐SbF5;FSO3H‐SbF5…Generation of Carbocations （1）（2）（3）（4）Fate of Carbocations2.2 Thermochemistry of reactive intermediates Carbanions HXÆH++X‐p gTable 2.3 pKa values of some organic acidsCarbanionsStability of Carbanionsystablization CarbanionGeneration and Fate of Carbanions Generation：Reactions of carbanions：Exercise1. Find two examples of reactions proceeded via carbocations and carbanions respectively from recent publications. Explain the way for the formation of the reactive intermediates and the reactions they got envolved.envolved2. A stable dialkylphosphinyl radical as shown above was reported recently(J. Am. Chem. Soc. 2011, 133, 12968‐12971).(J Am Chem Soc201113312971)Read the reference carefully and give another 4‐5 examples of phosphorus centered radicals. Describe how were they prepared and explain the reasons for their stability.。