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In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reaction [1] and is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction with effective collision (sufficient energy) and correct orientation. [2]
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.
A reaction can also have an undefined reaction order with respect to a reactant if the rate is not simply proportional to some power of the concentration of that reactant; for example, one cannot talk about reaction order in the rate equation for a bimolecular reaction between adsorbed molecules:
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 :
The rate for a bimolecular gas-phase reaction, A + B → product, predicted by collision theory is [6] = = ()where: k is the rate constant in units of (number of molecules) −1 ⋅s −1 ⋅m 3.
The numbers refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is bimolecular (second-order) while E1 is unimolecular (first-order). In cases where the molecule is able to stabilize an anion but possesses a poor leaving group , a third type of reaction, E1 CB , exists.
An example progress curve for an enzyme assay is shown above. The enzyme produces product at an initial rate that is approximately linear for a short period after the start of the reaction. As the reaction proceeds and substrate is consumed, the rate continuously slows (so long as the substrate is not still at saturating levels).
Biomolecules are an important element of living organisms. They are often endogenous, [2] i.e. produced within the organism, [3] but organisms usually also need exogenous biomolecules, for example certain nutrients, to survive. Biomolecules and their reactions are studied in biology and its subfields of biochemistry and molecular biology.