Hammond postulate, nucleophilic reactions

G Use the Hammond postulate to predict whether a transition state will be leactont-like or product-like, and explain how this distinction alfeclsthe selectivity of a reaction. 0 Draw and describe the slrudurae ol corbooalione, carbonione. Ires radicals, and caibenesand the structural lectures that stabilize them. Explain which are electrophilic and which are nucleophilic. 

When the electrophilic atom is H+ and the alkene is unsymmetrical, H+ adds to the alkene so that the more stable carbocation (almost always the more substituted one) is formed. The nucleophile then adds to the carbocation to give the product. The observation that the nucleophile adds to the more substituted C of the double bond is known as Markovnikov s rule. Markovnikov s rule is simply an application of the Hammond postulate the faster reaction is the one that leads to the intermediate lower in energy. 

The energy difference of the HOMO of the nucleophile and the LUMO of the carbonyl compared to the LUMO of the electrophile and the HOMO n-orbital of the carbonyl will be a factor in establishing whether the reaction is electrophile or nucleophile driven. In the case of a reaction catalysed by acid the reaction is considered to be electrophile driven and attack of the nucleophile occurs to the protonated carbonyl. A carbonyl, coordinated with a Lewis acid or cation (e.g. H, Li , Na", AIH3 [5-7]) or uncoordinated, can be attacked by a neutral or anionic nucleophile. In the former case the nucleophile must bear an acidic hydrogen to allow for the formation of a neutral product [4]. Since reduction of aldehydes and ketones is exothermic the Hammond postulate dictates that the transition state is closer in energy and structure to the reactants than to the products. 

Abstract The control elements that did not find mention in the earlier chapters are dealt with here. The prominent among these elements are spiroconjugation, peris-electivity in pericyclic reactions, torquoselectivity in conrotatory-ring openings, ambident nucleophiles and electrophiles, a-effect in nucleophilicity, carbene addition to 1,3-dienes, Hammett s substituent constants, Hammond postulate, Curtin-Hammett principle, and diastereotopic, homotopic, and enantiotopic substituents. 

II is of interest that the slopes of the Br0nsted plots of these reactions where X and Y are NO2, Cl , Br and I are close to 0.5 (the expected value if the transition state is midway between reactants and products), but smaller for OH and CN which kinetically are poor nucleophiles, i.e., have relatively high energy-barriers and hence product-like transition states (in accordance with the Hammond postulate). A similar pattern is observed for the a-deuterium isotope effects for methyl transfers, for which the cross-relation predicts that the fractionation factors will be related by the equation the view that the theoretical... 

Primary alcohols react by an Sj 2 mechanism in which water is displaced by the nucleophilic halide ion because the primary carbon atom is sterically accessible to the nucleophile. The alternate mechanism would have a high activation energy because the transition state would resemble a highly unstable primary carbocation. We used a similar argument to explain the direction of electrophilic addition to double bonds. According to the Hammond postulate, the structure of a transition state resembles the species that is most similar to it in energy. In the case of the mechanism for the reaction of an al-... 

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