Guest Post On SN1/SN2/E1/E2 (6): Wrapup

by James

in Organic Chemistry 1

Part 6 of a 6 part series. Previous posts in the series:  1 2 3 4 5 – James


 3 ½ Steps To Any SN1/SN2/E1/E2 Reaction: Wrap Up And Cautions, by @azmanam

Congratulations! You now have all the information you need to help Dr. House diagnose any reaction your professor throws at you. Let’s review what we’ve learned so far. 1) Electrons don’t like to be confined. The more electron density you have in a small volume, the more unstable the molecule will be. 2) The leaving group must be able to accept a pair of electrons and be stable when it leaves. The best leaving groups are weak bases. Beware, the leaving group must be located on an sp3-hybridized carbon atom. 3) Strong nucleophiles and strong bases have lots of electron density concentrated in a very small volume. 4) Electrophiles are classified based on two variables: steric hindrance of the electrophilic carbon atom, and ability to form a relatively stable carbocation. 5) Solvents can either diffuse electron density or concentrate electron density, and can be used as a tie-breaker if needed.

Each piece of the reaction provides us different evidence for or against certain mechanisms. Here is the chart we’ve been building over the last couple of posts.

Classification   Evidence for        Evidence against
Nucleophile    
Strong nuc/strong base     SN2, E2       SN1, E1
Strong nuc/weak base   SN2         SN1, E1, E2
Weak nuc/strong base     E2         SN1, SN2, E1
Weak nuc/weak base            SN1, E1           SN2, E2
     
Electrophile    
Methyl      SN2   SN1, E1, E2
Primary       SN2, E2     SN1, E1
Secondary   SN1, SN2, E1, E2  
Tertiary   SN1, E1, E2      SN2
     
Solvent    
Polar protic     SN1, E1, E2      —
Polar aprotic               SN2     —

 Now it’s just a matter of assessing the evidence from each piece of the reaction and making the diagnosis.

There are two final questions that must be addressed before we leave. What if the leaving group is attached to a stereocenter? And what if the carbocation can rearrange? The answer to the first question depends on which mechanism we are invoking. For the SN2, because the nucleophile must specifically approach the electrophile from a trajectory 180° opposed to the leaving group, the stereocenter will be inverted. For the E2, the leaving group and the β-proton must be anti-coplanar (this can sometimes be best viewed in a Newman projection or chair structure), and will lead to a specific E or Z alkene depending on the other groups on the electrophile. For the SN1 and E1, the intermediate carbocation can be attacked from either face of trigonal plane and has free rotation about all single bonds, so we tend to form a mixture of stereoisomers in the SN1 reaction, and – due to steric reasons – typically the isomer with the large groups ‘trans’ in the E1.

 Adam 6-1What if the carbocation can rearrange? Well, only SN1 and E1 even form carbocations, so we only need to answer this question if we decide were using one of these mechanisms. Carbocations are inherently unstable, and carbocation will only rearrange if we can sacrifice an unstable carbocation to gain a more stable carbocation. Alkyl groups and resonance stabilize carbocations. So we will only rearrange a carbocation if we can increase the number of alkyl groups and/or stabilize the carbocation through resonance. Hydride (H) and alkyl groups are the most common groups to migrate, if rearrangement can occur.

diagram 3

Want some examples? OK! Some of these are more straight forward, and some will force you to make decisions based on conflicting evidence!

 Screen Shot 2013-11-21 at 6.13.12 AM

Screen Shot 2013-11-21 at 6.13.40 AMSo enjoy your newfound expertise with substitution/elimination mechanisms. Remember, if you bring everything back to electron density, it all starts to make sense.

Thanks again to James for letting me contribute for a while. My home blog is chemistry-blog.com, and I hope you’ll join me over there or on Twitter (@azmanam). Happy ochemming :)

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{ 8 comments… read them below or add one }

Matthew

Thanks again, azmanam, very helpful examples!!! I’m so glad you uploaded the final installment before my exam tomorrow (whoops, actually today, now that I look at the time! better get some sleep)

Reply

Alexandra

Is there a post on how to predict the products of substitution reactions?

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Jack

Hey thanks a lot I like thinking about how it all relates back to electron density. Are there answers to the practice problems?

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Mudit

What would be a possible example of a strong nucleophile/weak base?

Reply

Mudit

Never mind, found it in the post on nucleophiles.
Although, shouldnt Cl anion be on that list? As it is a strong acid and has a weak conjugate base?

Reply

Keshav

Why CN anion is consider as a weak base? Its conjugate acid HCN is a weak one. Weak acids have strong conjugate base or vice versa!

Reply

James

Well, acetic acid is a weak acid (pKa 4) and the conjugate base (CH3COO-) is a weak base. Just because something is a weak acid doesn’t necessarily mean its conjugate base is strong.

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Neha

Where can we see the solutions to those questions at the end?

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