The SN1 Reaction

• “Initial Tails” and “Final Heads”
• 3 Ways To Make OH A Better Leaving Group
• A Simple Formula For 7 Important Aldehyde/Ketone Reactions
• Acetoacetic
• Acids (Again!)
• Activating and Deactivating
• Actors In Every Acid Base Reaction
• Addition – Elimination
• Addition Pattern 1 – Carbocations
• Addition pattern 2 – 3 membered rings
• Addition Reactions
• Aldehydes And Ketones – Addition
• Alkene Pattern #3 – The “Concerted” Pathway
• Alkyl Rearrangements
• Alkynes – 3 Patterns
• Alkynes: Deprotonation and SN2
• Amines
• Aromaticity: Lone Pairs
• Avoid These Resonance Mistakes
• Best Way To Form Amines
• Bulky Bases
• Carbocation Stability
• Carbocation Stability Revisited
• Carboxylic Acids are Acids
• Chair Flips
• Cis and Trans
• Conformations
• Conjugate Addition
• Curved Arrow Refresher
• Curved Arrows
• Decarboxylation
• Determining Aromaticity
• Diels Alder Reaction – 1
• Dipoles: Polar vs. Covalent Bonding
• E2 Reactions
• Electronegativity Is Greed For Electrons
• Electrophilic Aromatic Substitution – Directing Groups
• Elimination Reactions
• Enantiocats and Diastereocats
• Enolates
• Epoxides – Basic and Acidic
• Evaluating Resonance Forms
• Figuring Out The Fischer
• Find That Which Is Hidden
• Formal Charge
• Frost Circles
• Gabriel Synthesis
• Grignards
• Hofmann Elimination
• How Acidity and Basicity Are Related
• How Are These Molecules Related?
• How Stereochemistry matters
• How To Stabilize Negative Charge
• How To Tell Enantiomers From Diastereomers
• Hybridization
• Hybridization Shortcut
• Hydroboration
• Imines and Enamines
• Importance of Stereochemistry
• Intermolecular Forces
• Intro to Resonance
• Ketones on Acid
• Kinetic Thermodynamic
• Making Alcohols Into Good Leaving Groups
• Markovnikov’s rule
• Mechanisms Like Chords
• Mish Mashamine
• More On The E2
• Newman Projections
• Nucleophiles & Electrophiles
• Nucleophilic Aromatic Substitution
• Nucleophilic Aromatic Substitution 2
• Order of Operations!
• Oxidation And Reduction
• Oxidative Cleavage
• Paped
• Pi Donation
• Pointers on Free Radical Reactions
• Protecting Groups
• Protecting Groups
• Proton Transfer
• Putting it together (1)
• Putting it together (2)
• Putting it together (3)
• Putting the Newman into ACTION
• Reaction Maps
• Rearrangements
• Recognizing Endo and Exo
• Redraw / Modify
• Robinson Annulation
• Robinson Annulation Mech
• Sigma and Pi Bonding
• SN1 vs SN2
• sn1/sn2 – Putting It Together
• sn1/sn2/e1/e2 – Exceptions
• sn1/sn2/e1/e2 – Nucleophile
• sn1/sn2/e1/e2 – Solvent
• sn1/sn2/e1/e2 – Substrate
• sn1/sn2/e1/e2 – Temperature
• Stereochemistry
• Strong Acid Strong Base
• Strong And Weak Oxidants
• Strong and Weak Reductants
• Stronger Donor Wins
• Substitution
• Sugars (2)
• Synthesis (1) – “What’s Different?”
• Synthesis (2) – What Reactions?
• Synthesis (3) – Figuring Out The Order
• Synthesis Part 1
• Synthesis Study Buddy
• Synthesis: Walkthrough of A Sample Problem
• Synthesis: Working Backwards
• t-butyl
• Tautomerism
• The 4 Actors In Every Acid-Base Reaction
  • Conjugation
• The Claisen Condensation
• The E1 Reaction
• The Inflection Point
• The Meso Trap
• The Michael Reaction
• The Nucleophile Adds Twice (to the ester)
• The One-Sentence Summary Of Chemistry
• The Second Most Important Carbonyl Mechanism
• The Single Swap Rule
• The SN1 Reaction
• The SN2 Reaction
• The Wittig Reaction
• Three Exam Tips
• Tips On Building Molecular Orbitals
• Top 10 Skills
• Try The Acid-Base Reaction First
• Two Key Reactions of Enolates
• What makes a good leaving group?
• What Makes A Good Nucleophile?
• What to expect in Org 2
• Work Backwards
• Zaitsev’s Rule

Now that we’ve looked at the SN2, let’s look at another type of substitution reaction.

This time, we’ll start with the same starting halide as last time (2-bromobutane) and we’ll start with one enantiomer (it’s “optically active”).

A funny thing happens when we dissolve this in water. Look at the bonds formed and bonds broken.

We’re still forming C-O and breaking C-Br, so it’s still a substitution reaction. But there’s some key differences.

• When we measure the rate of this reaction, we find that it only depends on the concentration of the alkyl bromide (even if we correct for the fact that water is present in much greater concentration). In other words, the rate is “unimolecular”
•  Unlike the previous case (the SN2 reaction) where we got one enantiomer, with inversion – here we get a mixture of enantiomers (a mixture of retention and inversion).
• When we compare this to other reactions, we find that the rate increases as we go from 1-bromobutane to 2-bromobutane to t-butyl bromide (in other words, the rate increases as we go from primary to secondary to tertiary).
• Finally, unlike the SN2, which proceeds best in polar aprotic solvents (like DMSO) this one proceeds best in polar protic solvents – like water, alcohols, and carboxylic acids.

This is very different than the SN2! So what’s going on here?

Here’s our best theory.

Unlike the SN2, which happens all at once, this reaction happens stepwise. One reaction at a time.

• First, the leaving group leaves. We break the C-Br in this instance, to form a carbocation, and C-Br.
• Then, the nucleophile (water) attacks, which gives us the alcohol as our product.

Let’s see why this makes sense:

• the rate only depends on the alkyl halide because we’re forming an unstable carbocation, which is the slow – rate determining – step.
• The carbocation is flat (sp2 hybridized) – so when the nucleophile attacks a carbon, it can attack from either face of the flat carbocation.  This means we can get a mixture of retention and inversion.
• Carbocations are unstable, and their stability increases as we go from primary (least stable) to secondary to tertiary (most stable).
• Carbocations are polar, so they’re going to be stabilized by polar solvents – and polar protic solvents are better at stabilization than polar aprotic solvents are.

We call this the SN1 reaction (Nucleophilic substitution, unimolecular).

It all makes sense if you think of one thing: the big barrier for the SN1 is carbocation stability. Anything that will stabilize the carbocation, will speed up the SN1.

Repeat!: The big barrier to the SN1 is carbocation stability. 

Thanks for reading! James

P.S. Further reading: SN1 reaction of alkyl halides with water to form alcohols