For my fellow premed
perverts who are currently dying in Organic Chemistry and are going through hell SN1/SN2.
Let’s make sense of this shit.
DATA BEFORE HYPOTHESIS
Remember when the professor started talking about data and results and you were like WTF BBQSAUCEANDABISCUITWHAT? Shit happened, we got data, and then we have to explain for it. Because chemistry. 3 things to remember about the data that was collected AFTER substitution reactions:
1. The electrophile (or the substrate)
WITH THE FIRST REACTION, less steric hindrance substrates
HAVE FASTER REACTION RATES, BUT
IN THE SECOND REACTION, the MORE steric hindrance there is in the substrate,
THE FASTER THE REACTION RATE IS. HUH, OK. BUT WHY THE FUCK?
2. The rate law
Because we can measure the rate of the reaction (1,2, shit it’s over 9000) we can fuck with the concentration of both the nucleophile AND the substrate and see if that changes the reaction rate at all.
WITH THE FIRST REACTION, you double the concentration of the substrate while leaving the nucleophile the fuck alone, and you DOUBLE the reaction rate. Double the concentration of the nucleophile, and you DOUBLE the reaction rate. Double both concentration, and you increase the rate by four. Triple the concentration of both, and you increase it by nine times. Half the concentration of one of them, and you half the reaction rate. Mess with one, or both of these suckers, and you mess with the reaction rate. AWESOME.
WITH THE SECOND REACTION, the rate ONLY changes when you fuck with the substrate. Touch the nucleophile, and NOTHING CHANGES.
3. I’m bringing back stereochemistry
WE JUST FINISHED THE CHIRALITY CHAPTER, ARE YOU FOR SERIOUS? I THOUGHT WE WERE DONE WITH THAT SHIT. Look, that shit never goes away. Just DEAL okay?
Not only can we measure the rate, we can ALSO measure optimal rotation. (AHURHUR, STINGS DOESN’T IT, YEAH I’M BRINGING IT BACK.) Optimal rotation is based on stereochemistry (is it an isomer? If so, is it a constitutional or a stereoisomer? If so, are they identical or different? If so, are they enantiomers or diastereomers? AH GOOD TIMES).
WITH THE FIRST REACTION, you take a pure enantiomer compound, down that baby in a solvent, and you end up with a pure enantiomer compound - in reverse
. If it started as a (2S)
-2-bromo-butane, then the product will be a (2R)
WITH THE SECOND REACTION, we have a mixture of BOTH the same configuration and the reverse configuration. Because it’s a mixture, it’s a racemization (if have equal portions of both configurations)
THAT’S THE DATA. DATA BEFORE HYPOTHESIS. SO WHAT NOW HOME SKILLET?
They have to explain:
- How the bonds form and how they break
- The dependence on type of substrate
- The dependence on concentration
- The observations of stereochemistry
Think of hypothesis as “mechanisms”. And that’s what they are. Sounds less intimidating to think of them as hypothesis though…
*images from Master Organic Chemistry