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The SN2 reaction: Substitution Nucleophilic Bimolecular

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Several distinct mechanisms are possible for nucleophilic substitution reactions at saturated C atoms, depending on the substrate, nucleophile, leaving group and the reaction conditions. By far the most common are the S N 1 and S N 2 mechanisms. Simple alkyl groups, methyl and primary alkyl groups always react by the S N 2 mechanism and never by S N 1 . Why methyl and primary alkyl groups react by the S N 2 mechanism? What are the steps in an S N 2 mechanism? Partly because the corresponding cations are unstable and partly because it is easy for the nucleophile to attack the C atom since it is surrounded by H atoms – and not by bulky alkyl groups. Experimental results show the following in S N 2 reactions: In this mechanism (Fig. 1)  there is backside attack 1 . The nucleophile attacks the carbon atom on the opposite side from the leaving group – 180 away from the leaving group – and the carbon atom turns inside out as the reaction proceeds The reaction is a one-step proc...
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Carbocations: Factors affecting their Stability

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A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable . Anything which donates electron density to the electron-deficient center will help to stabilize them. Factors that stabilize them are the following:  Neighboring carbon atoms (inductive effect) Neighboring carbon-carbon multiple bonds (resonance effect) Neighboring atoms with lone pairs (resonance effect) How carbocations are stabilized by neighboring carbons atoms? The stability of carbocations decreases as the number of carbons attached to the C+ decreases. That means that tertiary carbocations are more stable than secondary that in turn are more stable than primary (Fig. 1). Fig. 1: Carbocation stability increases as methyl substitution increases around the electron deficient carbon C+. The methyl groups (-CH 3 ) are electron donating and therefore stabilize the positive charge (inductive ...
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How to determine the order of a reaction by the initial rates method?

How to determine the order of a reaction? Determining the form of the Rate Law (Order of Reaction) by the method of Initial Rates Home > Rate of a Chemical Reaction - Chemical Kinetics > Rates of Chemical Reactions and the Collision Model > How to determine the order of a reaction by the initial rates method?   How to determine the order of a reaction by the initial rates method?   A rate law is an expression which shows how the rate of a reaction depends on the concentrations of reactants. For the decomposition of NO 2 we can write: 2NO 2 (g) → 2NO(g) + O 2 (g)  Rate = k [NO 2 ] n      (Rate Law) k is the rate constant and n is the order of reaction (n can be positive or negative, integer or fraction) Both k and n must be determined experimentally. For the general reaction: aA + bB → cC + dD   (1) the rate law is given by:   Rate = k. [A] m [B] n     (2)   There are two types of rate laws: The differential rat...
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