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The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can therefore be determined by inspection, from the molecularity. [1] The kinetic order of a complex (multistep) reaction, however, is not necessarily equal to the number of molecules involved.
The bimolecular nucleophilic substitution (S N 2) is a type of reaction mechanism that is common in organic chemistry. In the S N 2 reaction, a strong nucleophile forms a new bond to an sp 3 -hybridised carbon atom via a backside attack, all while the leaving group detaches from the reaction center in a concerted (i.e. simultaneous) fashion.
An example of a simple chain reaction is the thermal decomposition of acetaldehyde (CH 3 CHO) to methane (CH 4) and carbon monoxide (CO). The experimental reaction order is 3/2, [4] which can be explained by a Rice-Herzfeld mechanism. [5] This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step :
For example, in the hydrogenation reaction of ethylene the H 2 molecule must approach the bonding zone between the atoms, and only a few of all the possible collisions fulfill this requirement. To alleviate this problem, a new concept must be introduced: the steric factor ρ .
For the NO 2 –CO reaction above, the rate depends on [NO 2] 2, so that the activated complex has composition N 2 O 4, with 2 NO 2 entering the reaction before the transition state, and CO reacting after the transition state. A multistep example is the reaction between oxalic acid and chlorine in aqueous solution: H 2 C 2 O 4 + Cl 2 → 2 CO 2 ...
Diffusion control is more likely in solution where diffusion of reactants is slower due to the greater number of collisions with solvent molecules. Reactions where the activated complex forms easily and the products form rapidly are most likely to be limited by diffusion control. Examples are those involving catalysis and enzymatic reactions.
The rate equation for S N 2 reactions are bimolecular being first order in Nucleophile and first order in Reagent. The determining factor when both S N 2 and S N 1 reaction mechanisms are viable is the strength of the Nucleophile. Nuclephilicity and basicity are linked and the more nucleophilic a molecule becomes the greater said nucleophile's ...
The reaction mechanism of the Biginelli reaction is a series of bimolecular reactions leading to the desired dihydropyrimidinone. [14]According to a mechanism proposed by Sweet in 1973 the aldol condensation of ethylacetoacetate 1 and the aryl aldehyde is the rate-limiting step leading to the carbenium ion 2.