Read through \"Organic Chemistry as a second Language\" through in completion. F
ID: 840655 • Letter: R
Question
Read through "Organic Chemistry as a second Language" through in completion. Felt comfortable with the material however applying it to the orgo 1 class is not going well. Could some one please help explain the over all mechanism Sn1, Sn2, E2, E1 ect. I understand that the SN1 and SN2 reactions are based on iether the LG will leave on its own in SN1 or at the same time as the NUC: attacks the carbonyal in SN2. However when puttting it together with E2 and E1 it gets fuzzy. Please also if you ar able to explain the effects of various reagents such as the delta symbol underthe arrows as well was what justifies a good nucliophile or electrophile. In adition what is the overall objective with resonance. (I get that all resonance structure are representative of the entire molecule at any given time not one excluxivley hence the need for resonance hybrid structures I just get carried away and dont know when to stop or doubt my self ocasionally when I go to pick a starting place). Sorry I know this is alot. Also if any one could offer advice on synthesis reactions, epoxides, alcolhols, halohydron formaton and halo etherifercation. I good with brief explinations for concepts its the constistancy that is struggle with from problem to problem such as maintaning regiochemistry with reactions that gets me lost and often drawing to many strucures always close but never the right answer.
Explanation / Answer
Determining the mechanism of reaction, whether it be SN2, SN1, E1, or E2, can be extraordinarily difficult at first. Many texts do a poor job of explaining it, and if your teacher is not helpful that can make it worse. It seems that you are a bit confused on the difference between these mechanisms, and so I will explain them one at a time.
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The SN2 mechanism
This mechanism occurs in basic conditions. It is where a nucleophile substitutes for a leaving group. It does indeed require a good leaving group and more importantly, the mechanism occurs in one step. The "2" part of SN2 stems from the rate law of the reaction. An SN2 reaction is considered "bimolecular" according to its rate law--that is, it depends on the relative concentrations of nucleophile and electrophile. You can also look at this in terms of a rate law:
rate = k[Nucleophile][Electrophile]
Now, I notice you mentioned a carbonyl in your question. At this point I feel the need to state that carbonyl reactions are examples of nucleophilic addition--not substitution. So for the time being, don't think of carbonyls and SN2 together.
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The SN1 mechanism
This too is a type of substitution reaction, where a nucleophile displaces a leaving group. The difference here is how the substitution occurs. SN1 mechanisms occur in acidic conditions, and operate in a two-step mechanism. The reasoning is the same as it is for SN2; the "1" means "unimolecular" which stems from the rate law. This rate law is:
rate = k[Electrophile]
Notice that the rate is now only dependent upon the concentration of electrophile. Since the rate is only as fast as its slowest step, the rate of the SN1 mechanism is dependent upon how quickly a leaving group leaves to form a carbocation.
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The E2 mechanism
E2 mechanisms occur in basic conditions, just like SN2 mechanisms do. The naming convention is the same--the "2" comes from the bimolecular nature of the mechanism, and like the SN2 mechanism, the E2 mechanism occurs in one step.
The major difference here is the product which is formed. You remove an H atom from the beta carbon (the carbon which is NEXT to the carbon with the leaving group), form a pi bond, and remove the leaving group.
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The E1 mechanism
The product of an E1 mechanism is the same product of an E2 mechanism; the difference is simply how you get there. E1 mechanisms are similar to SN1 mechanisms--they're unimolecular, so the first step is the leaving group leaving to form a carbocation. Instead of substituting on the carbocation, though, you eliminate from a carbon next to the carbocation. Like SN1 mechanisms, E1 mechanisms occur in acidic conditions.
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With all that being said, one way to classify these four mechanisms is by what conditions they occur in.
In basic conditions, SN2 and E2 can occur (but SN1 and E1 cannot).
In acidic conditions, SN1 and E1 can occur (but SN2* and E2 cannot).
*I star this statement because under certain circumstances, you can get SN2 in acidic conditions, but this is an exception to the rule. If you're curious, I can explain further.
The next question is: how do I differentiate between the two mechanisms if I know what conditions I am in? This is a skill worth learning. And in order to tell the difference, you must know the difference between a good nucleophile and a good base. Bases attack protons and thus do elimination reactions; nucleophiles attack carbons and thus do substitution reactions.
With that, I am going to provide you a small table with how to predict the products of a reaction given basic conditions:
Good base
Poor nucleophile
Good base
Good nucleophile
Poor base
Good nucleophile
So that table might seem a little confusing at first but it should help. A few things to remember:
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A brief explanation for the rest of what you're asking about. Synthesis reactions you need to break apart reaction by reaction so that you can determine how to make a target. I would emphasize making sure you get the product of the reaction right, rather than what you "want" to happen.
Epoxides are kind of strange. They contain a leaving group as a strained ring, so when you attack the carbon of an epoxide, you open the ring to release the oxygen atom. How the ring opens (Whether it's protonated first or not) depends on the conditions you're in. If the conditions are acidic, you protonate the epoxide first and then attack the more substituted carbon. If the conditions are basic, you attack the epoxide carbon at the less substituted carbon.
When doing addition reactions you form the more substituted carbocation whenever it's relevant. I realize that's a vague answer, but it simply depends on the compound and what you're adding.
Type of ElectrophileGood base
Poor nucleophile
Good base
Good nucleophile
Poor base
Good nucleophile
Primary (1o) E2 SN2 SN?2 Secondary (2o) E2 E2 > SN?2 SN?2 Tertiary (3o) E2 E2 N.R. Example reagents LDA, t-BuOK RO-, HO-, R2N-, R-, etc PR3 , RS-, R2SRelated Questions
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