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Hammett Law: Reaction Mechanism Science Posted: 09 Nov 2011 04:05 PM PST Reaction constant p is the (rate-limiting step) to reflect the polarity of the reaction transition state. Reaction  constant example, p> 0 is the reaction that the reaction is accelerated by electron withdrawing groups have said before, that it has a negative electric charge transition state suggests that. p <0 is that the positively charged as possibly. A good example of this fact helps to clarify the reaction mechanism which give nucleophilic substitution reaction. Consider the following substitution reaction such as virtually. The only reaction is S N 1 and S N I do not know either of two mechanisms proceed. Reaction Mechanism Process Picture SN an / SN 2 transition states consider. As shown in the figure below, SN 1 transition state is positively charged, SN 2 transition state shows that the negative electric charge. The nature of the reaction constant p> 0 in the negatively charged transition state (SN 2 mechanism) takes, p<0 then the positively charged transition state (SN 1 mechanism) can be considered to take. Hammett rule plot for this reaction, orto, as a general under-reaction is reversed the polarity of the boundary of the constant 0. This substitution reaction, X substituent would change the type of reaction mechanism is understood. Hammet Rule Mechanism Reaction Another example would change the rate-limiting step to reaction mechanism substituents at the imine formation reaction represented by the semicarbazone. When X is electron withdrawing reaction speed is limited ?, when will the rate-limiting electron-donating reaction ?. Hammet also be explained by a science discussion in the plot as described above. Hammett rule is a rule of thumb, however, the reaction can be applied very often a few hundred cases. This also contributed Hammett has taken a position to enhance Hammett law.

Extended Science of Hammett Law Posted: 09 Nov 2011 04:05 AM PST Hammet was assembled to consider only the theory of substituent inductive effect. In fact, an element can not be ignored as the substituent resonance effect. The requirement of a transition state resonance contribution is different for each reaction. The Extended Hammett Equation Was Proposed log (K X / K X0 ) = ? {? 0 + R (? + – ? 0 )} (the case of electron withdrawing groups are ? + ? instead of – ) Requirement r in extended equation of Hammet law is a science of parameter that represents the resonant contribution of the reaction. Effect of reaction rate r is higher the greater the resonance effect. Taft Proposed After Hammet Equation Science Aliphatic compounds should also consider the steric effect of substituents. RWTaft is RCOOEt ester hydrolysis under acidic conditions is scarcely polar effects (0 ? ?), while alkaline hydrolysis is subject to polar effects Kina Hiroshi (0 ? ?) especially focusing on the difference between the reaction rate quantified based on three-dimensional effects, Taft equation was proposed. log (K / Ko) = ? * ? * + ?es ? * ? * polar effect, indicating the degree of three-dimensional effect ?Es. Science Representation is being made by two completely separate the effect of the expression that is characteristic of Taft different from Hammett rule.

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