sn1

Organic Chemistry: Sn1 & Sn2 Quick Guide

Posted on Updated on

Hello!

I’m putting this piece together for a few readers who have finals coming up, I felt their a pain and decided to try to help them from afar as a former Organic Chemistry tutor — as a former premed, I feel their pain. This article will be part 1 of perhaps 2 or maximum 3 articles related to Sn1/Sn2 & E1/E2. So, if you don’t particularly care about Organic right now, you can put this article away later, print it out and put it on your coffee table to impress your friends with chemistry small talk.

The article will only be in the following format:

  • Introduction about my own Organic Chemistry situation when I was a student, to put my situation into context with yours.
  • Brief prelude into Sn1 and Sn2, the big picture + quick reference chart that should be used in conjunction with the articles 2/3 to come.

The second article will be the following format (tentative):

  • The chief goal of this article will be show how the chart is one of the few things you’d have to memorize to have general intuition about Organic Chemistry. This is especially true when weighing competing reactions.
  • Review Mechanisms of Sn1 versus Sn2, referring to the chart.

The tentative third article would then exist:

  • Wrap up substitutions, and finish with mechanisms of eliminations (E1 & E2).

A fourth article might come to add information.

Introduction: Orgo Hard but Doable 

Organic Chemistry (or more affectionately, Orgo) is probably the leading cause of night terrors among premedical students — premeds have even been known to develop a spontaneous stressed induced eye-twitch around finals. I was a nontraditional premed, and my major didn’t require a year of Orgo, so taking the Orgo series were electives where I received grades B+, A, A, for each quarter respectively. My school was a Polytech university, so the class size was pretty small, there were only about 30 students in the 1st quarter, and it went down to about 20+ for the last quarter. Towards the end of the last quarter however only about 10 people showed up for the final. This allowed for the professor to make a menacing scowl at you each time you failed an exam or quiz, this was often a large motivator to do better — or assume the fetal position. Despite the positive of having a small class size, the course is frequently retaken 2-3 times (it’s not uncommon for our 1 year sequence to turn into 2 years just to pull C’s).

I’m sure it’s a conspiracy, but life seems to get more complicated when you take Orgo.  This is because you’re probably towards the home-stretch in your premed career, and you’re likely to have more on your plate then any other time previously as a consequence. This is especially true if you are a nontraditional. When I was taking Orgo, I had to really work on my premed experiences to catch up with traditional premeds. As a consequence, at any given time during the Orgo sequence I was carrying 14-21 units, and conducting undergraduate research, and working, and still scheduled time for my friends, social drinking, and family. Furthermore, I have a terrible memory, so I knew heavy memorization wasn’t going to do it for me, but sometimes it’s a necessary evil. So, Orgo is doable without becoming an Orgo ogre (I apologize, I couldn’t help myself).

Without further adieu long and behold the Sn1 versus Sn2 table before you. 

The reason why first year Orgo texts introduce substitutions early on is because a lot of reactions end up just being substitutions (or eliminations). The gist of Sn1 and Sn2 are quite straight forward, you start with something (substrate) and you want to take a nucleophile and stick onto your substrate, and in the process of making your product you kick a group off called the leaving group (because it gets kicked off).

That’s actually all there is to it, the tricky party is figuring out when it’s an Sn1 or Sn2, or the dreaded no reaction. In order to become proficient I first took the time to be practice my fluency in the rules of two the two reactions. If you look at the chart that I compiled for us, then you’ll see that after careful introspection, the two reactions are really nothing alike except that they prefer to have good leaving groups.

Before we go on, I encourage you to print this chart, and keep a copy for yourself so you can refer to it even for your real assignments. If you can recreate it from scratch, then you’re on your way to going above and beyond the call of duty.

This concludes the first article, more to come soon.

terms NR = no reaction, RXN = reaction. Organic Chemistry has a lot of short hand, scientists are lazy.

Table

Things to Consider

Sn1

Sn2

Substrate In general the more substituents the better; priority of RXN 3 >2 >, 1 & Me-X is for all intents and purposes will yield NR In general the less substituents the better; priority of RXN Me-X > 1 > 2, NR with 3
Kinetics 1st order kR[substrate]; kinetics depends on only the concentration of the substrate 2nd order kR[substrate][nucleophile], i.e. the kinetics depend on both the concentrations of the substrate and nucleophile
Nucleophile Weak nucleophiles work just fine(e.g.CH3CH2OH, H2O) Strong nucleophiles (e.g. NaOH)
Substrate’s leaving group (LG)*

Good leaving groups only

Solvents + Other Stuff in Solution

Yet Even More Stuff to Consider

Sn1

Sn2

Solvent Solvents should encourage ionization A lot of solvents work*
Misc. Reagents Typically when you see transitions metals tossed in (e.g. AgNO3) chances are its likely encouraging ionization, and it’s a Sn1 No gimmicks required usually
Temperature Heat encourages the reaction Typically excess heat discourages the reaction
Stereochemistry Mixture of products, S and R inversions, retentions, and rearrangements The nature of the reaction requires a 100% inversion of the chiral configuration from substrate to product, i.e. always inverted S -> R or R -> S; rearrangements are impossible

As usual, just ask me something or other on twitter! https://twitter.com/masterofsleep

Advertisements