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Science Poetry

Determining Factors

“Consider a scheme mechanistic; 
Apply explanation simplistic:
The rate of step slowest
Determines how ‘goest’
The rate of whole process logistic.”  

The 19 October 2021 Twitter limerick expressed the concept of the “rate-determining step,” a common approach used by chemists to simplify analyses and determine kinetic information.   

“Consider a scheme mechanistic; / 
Apply explanation simplistic…”

In chemistry, a mechanism is a description of how a starting chemical species (reactant) is transformed into a different one (product).  Often, a “scheme mechanistic” takes multiple steps, so its kinetic rate law would be tricky to monitor precisely.  Simplifying approaches provide more efficient determinations of rate laws; this poem will describe one such “explanation simplistic.”  

“The rate of step slowest /
Determines how ‘goest’ /
The rate of whole process logistic.” 

One simplifying approach assumes that the rate of the slowest step of a multi-step chemical process is roughly equivalent to the rate of the process overall. 

For instance, if a commute takes ten minutes, and nine of them are devoted to waiting for an infuriatingly slow stoplight that cannot be bypassed, then the stoplight step would be considered “rate-determining.”  The  rate of the commute could be approximated as the rate of that step: the rate of the “step slowest,” here caused by the unavoidable stoplight, determines how “goest” (adjusting vocabulary slightly to fit the rhyme!) the rate of the commute.     

For a chemical example, in a reaction called a unimolecular nucleophilic substitution (abbreviated as SN1), two main steps occur.  First, a bond breaks and a “leaving group” originally on the reactant molecule departs, creating a space on the main part of the reactant in which a new bond can form.  Second, an incoming nucleophile (electron donor) forms that new bond: the net result is that the nucleophile substitutes where the leaving group had been.  The bond-breaking step is far slower than the nucleophilic-attack step, so the rate of the entire SN1 reaction is approximated as the rate of the bond-breaking step.