“Lesson on SN1:
Begins with the leaving group’s breaking away.
Nucleophilic attack racemizes.
(Activity, optical: lost in the fray.)”
The third mechanism poem from NaPoWriMo2022 was posted on 6 April 2022 and looks specifically at the process of unimolecular nucleophilic substitution, which is abbreviated as SN1. This is again a step-by-step depiction in which electron movement is represented by curved arrows; the poem again seeks primarily to communicate several attributes of the mechanism to an audience trying to learn them.
Here, the same net effect occurs as in the previous discussion of the SN2 process: an incoming nucleophile (Nuc) replaces a leaving group (LG) on a molecule. However, SN1 is a stepwise process that requires two distinct steps, rather than the concerted single step of SN2. (To reiterate from a few weeks ago: you’re not wrong. It’s confusing that the two-step process has the number one in its abbreviation! This is because the rate-determining step in an SN1 mechanism is the first step, which only involves one species.)
Shown above is a generalized depiction of this molecular process. Because this poem will discuss the three-dimensional structures of the molecules a bit more, the scene is much busier than for the SN2 poem/essay. The gist of this mechanism is as follows:
Step One) Reactant loses leaving group to form Intermediate.
Step Two) Intermediate reacts with Nucleophile to form Products.
Before I launch into the discussion of the poem itself, it is worth remembering that several shibboleths accompany the vocabulary of organic chemistry, and a few will be cited here. A chemist pronounces the word “carbocation” as “car-bo-cat-ion”; a non-chemist would likely pronounce it as rhyming with “vacation.” “Racemization” is likewise pronounced in a non-intuitive way. Those are the two main terms seen in this entry (but as I think about it, the general topic might make for many more interesting poems for the next NaPoWriMo).
And finally, a theme of both the SN2 poem and this one is the change in the optical activity or chirality caused by the reaction. Chemists care about chirality (“handedness”) of molecules for many reasons; one is that it is a property that can have major implications in biological settings. More generally, such properties are aligned with the stereochemistry of atoms in a molecule: their three-dimensional arrangement in space. A fundamental theme of chemistry is the idea that the structure (3-D shape) of a molecule is linked to its function; the SN1 and SN2 reactions are the first illustrations of stereochemical concepts that most students encounter.
With all this buildup, the poem itself might well seem anticlimactic… but I will resolutely start the official 280-word count here.
“Lesson on SN1: /
Two-step-long journey /
Begins with the leaving group’s breaking away…”
In contrast to the SN2 mechanism’s single step, the SN1 mechanism is a two-step process. In Step 1, the reactant forms an intermediate, also called a carbocation: the bond between the leaving group and the rest of the molecule is broken, yielding the positively charged intermediate.
“Nucleophilic attack racemizes. /
(Activity, optical: lost in the fray.)”
In Step 2, the carbocation intermediate reacts with an incoming nucleophile. Depending on the structure of the reacting molecule, a property called “optical activity” can sometimes be monitored during the SN1 process.
Above, the “Reactant” depicted (where X, Y, and Z all represent distinct substituents) is optically active. When placed into an instrument called a polarimeter, the “Reactant” would rotate the polarimeter’s light in a certain direction: dextrorotatory (d) for right; or levorotatory (l) for left, a behavior which is quantified as the sample’s “optical activity.” (Other notations for structural differences are often used, as well; rationalizing those would take this poem-explanation over its word limit!)
In Step 2, as shown, the nucleophile reacts with the “Intermediate” to form the “Products.” Why are there two? When the incoming nucleophile attacks, it can do so from the front or the back, relative to the planar (flat) intermediate. Each option happens 50% of the time, leading to what is called a racemate or racemic mixture: “[n]ucleophilic attack racemizes.” The final product mixture would NOT rotate the polarimeter’s plane-polarized light anymore: the optical activity has been lost.
Many aspects of mechanisms are framed in bellicose vocabulary (e.g., “nucleophilic attack”), as highlighted here by the closing descriptor for the optical activity: “lost in the fray.”