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Hammett equation : -
The basic equation is:
relating the equilibrium constant K for a given equilibrium reaction with substituent R and the reference K0 constant with R=H to the substituent constant σ which depends only on the specific substituent R and the reaction constant ρ which depends only on the type of reaction but not on the substituent used.
The equation also holds for reaction rates k of a series of reactions with substituted benzene derivatives:
In this equation k0 is the reference reaction rate of the unsubstituted reactant and k that of a substituted reactant.
A plot of log(K/K0) for a given equilibrium versus log(k/k0) for a given reaction rate with many differently substituted reactants will give a straight line.
The mesomeric effect or resonance effect in chemistry is a property of substituents or functional groups in a chemical compound. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter M. The mesomeric effect is negative (-M) when the substituent is an electron-withdrawing group and the effect is positive (+M) when based on resonance the substituent is an electron releasing group.
- Examples of -M substituents: acetyl (IUPAC ethanoyl) - nitrile - nitro
- Examples of +M substituents: alcohol - amine
The net electron flow from or to the substituent is determined also by the inductive effect. It's important to note that the mesomeric effect as a result of p - orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the electronegativity of the atoms and their structural chemistry (which atoms are connected to which).
The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold in 1938 as an alternative to the Pauling's synonymous concept of resonance [1]. Mesomerism is in this context is often encountered in German and French literature but in English literature the term resonance dominates.
all from wiki
simple :
inductive effect operates in c-c (saturated)
mesomeric effect operates in C=c ( unsaturated triple bond tooo)
but when both r present mesomeric dominates inductive
remember which groups r for inductive and which groups r for mesomeric......
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The inductive effect in chemistry is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction (IUPAC definition). The net polar effect exerted by a substituent is a combination of this inductive effect and the mesomeric effect.
The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge(δ-) and the other atom has a slight positive charge(δ+).
If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect.
Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character is indicated by the +I effect.
As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out, and is significant only over a short distance. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.
The inductive effect may be caused by some molecules also. Relative inductive effects have been experimentally measured with reference to hydrogen.
Inductive effects can be measured through the Hammett equation.