Effect Of Solvent On Sn1 And Sn2 Reactions Pdf
File Name: effect of solvent on sn1 and sn2 reactions .zip
This study investigated the nucleophilic substitution reaction mechanisms of 5 oxatriquinane derivatives, namely: oxatriquinane OTQ , 1,4,7-trimethyloxatriquinane TMO , 1,4,7-triethyloxatriquinane TEO , 1,4,7-tri-iso-propyloxatriquinane TIO and 1,4,7-tri-tert-butyloxatriquinane TTO. In addition to the G 3 conformation one with the substituent groups at 1,4 and 7 positions pointing into the plane of the paper originally proposed by the previous workers, Mascal et al. Geometry optimization and determination of transition state properties of the conformers corresponding to each molecule in the presence of azide ion, N 3 - provided theoretical evidences on the possible reaction mechanisms.
- Designing a “good” nucleophilic substitution
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- Solvent Effects and SN2 and SN1 reactions: Nucleophilic Substitution
- Oxatriquinane Derivatives: A Theoretical Investigation of SN1-SN2 Reactions Borderline
Polar Protic? Polar Aprotic? All About Solvents.
The reaction potential energy surface PES , and thus the mechanism of bimolecular nucleophilic substitution S N 2 , depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the S N 2 mechanism. The number and nature of the substituents around the central atom play a major role in determining reactivity. After proceeding over the barrier of the transition state TS, a similar product complex PC is formed where the leaving group is still weakly bound to the substrate. Finally, the products P are obtained.
Designing a “good” nucleophilic substitution
Intro to organic mechanisms. Alkyl halide nomenclature and classification. Sn1 mechanism: kinetics and substrate. Sn1 mechanism: stereochemistry.
Carbocation stability and rearrangement introduction. Carbocation rearrangement practice. Sn1 mechanism: carbocation rearrangement. Sn1 carbocation rearrangement advanced. Sn2 mechanism: kinetics and substrate. Sn2 mechanism: stereospecificity. Sn1 and Sn2: leaving group. Sn1 vs Sn2: Solvent effects. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter.
Let's start with polar protic solvents. A polar protic solvent is a solvent that has at least one hydrogen connected to an electronegative atom. For example if you look at water here, you can see we have a hydrogen directly connected to an electronegative atom which is oxygen. Water is an example of a polar protic solvent. Next we have methanol which again has a hydrogen directly connected to an electronegative atom and oxygen and finally acetic acid which has the same thing here.
Here is our hydrogen and here is our oxygen. So these polar protic solvents favor an SN1 mechanism. Let me write that in here. So an SN1 mechanism is favored by a polar protic solvent and let's look at why.
So down here I have tert butyl bromide and for an SN1 mechanism the first step here would be loss of a leaving group so these electrons come off on to the bromine to form our bromide anion and we are gonna form a carbocation as well. So let me draw in the carbocation first. So we have a carbon that is bonded to three methyl groups and this is a plainer carbocation so I'm trying to show that.
Our carbon has a plus one formal charge and we are also gonna have our bromine here which we have three lone pairs of electrons.
I'll put those in. And then we're gonna get one more lone pair of electrons on the bromine that came from this bond in here. So highlighting those electrons in magenta. Here are those electrons in magenta and bromine has a negative one formal charge as the bromide anion. So we have this carbocation and this anion in our SN1 mechanism and we know this is right determining step of our SN1 mechanism loss of a leaving group.
If we are using a polar protic solvent such as water, water can stabilize both the cation and the anion. For example for our carbocation we know that carbon has a positive charge on it.
And if we look at water we know that this oxygen here is a partial negative charge since oxygen is more electronegative than hydrogen. This hydrogen would have a partial positive charge so the negative portion of this molecule, the oxygen would interact with this positive charge on our carbocations. Let's go ahead and show a water molecule here and the partially negative oxygen with its three lone pairs of electrons here on the oxygen will help to stabilize our carbocation.
And for our negative anion for our bromide anion here, which is negatively charged, it would be the other end of the water molecule. So if I draw in my water molecule right here so two lone pairs of electrons on the oxygen our partial positive hydrogens would interact and help to stabilize that anion.
So polar protic solvents help to stabilize both the carbocation and the anion and that solvation of both cations and anions helps the SN1 mechanism proceed. So that's why polar protic solvent will favor an SN1 mechanism. Now let's look at polar aprotic solvents.
So first lets look at dimethyl sulphoxide. So more commonly known as DMSO. So here's the DMS and O. Oxygen is more electronegative than sulfur. So the oxygen is going to withdraw some electron density and become partially negative. And the sulfur would be partially positive. A polar aprotic solvent does not have a hydrogen directly connected to an electronegative atom. So we think about the hydrogens on DMSO.
So let me just sketch them in here real fast, there'll be three on this carbon and there'll be three on this carbon. So here we have hydrogens directly connected to a carbon and of course carbon is not very electronegative. So that's why this is a polar aprotic solvent.
Next let's look at DMF. DMF is the short way of writing this one here. Again no hydrogen directly connected to an electronegative atom. This hydrogen is directly connected to this carbon and then this carbon would have three hydrogens on it and then this carbon would have three hydrogens on it. So DMF is a polar aprotic solvent. And finally let's look at this last one here. So the abbreviation would be HMPA. So let me write that down here.
Again no hydrogen is directly connected to an electronegative atom. Polar aprotic solvents favor an SN2 mechanism. So let's look at why. Down here I have an SN2 reaction. On the left we have this alkyl halide. Let's say we have sodium hydroxide. We could use DMSO as our solvent so let me write that in here. So we are gonna use DMSO. And we know in an SN2 mechanism the nucleophile attacks our alkyl halide at the same time our leaving group leaves. So our nucleophile is the hydroxide ion.
It is going to attack this carbon and these electrons are gonna come off on to the bromide to form our bromide anion. So our OH replaces our bromine and we can see that over here in our product.
In an SN2 mechanism we need a strong nucleophile to attack our alkyl halide. And DMSO is gonna help us increase the effectiveness of our nucleophile which is our hydroxide ion.
So let's look at some pictures of how it helps us. So we have sodium hydroxide here. So first let's focus in on the sodium, our cation. So here is the sodium cation. DMSO is a good solvator of cations and that's because oxygen has a partial negative charge. The sulfur has a partial positive charge and these lone pairs of electrons on the oxygen help to stabilize the positive charge on our sodium.
So same thing over here. Partial negative, partial positive and again we are able to solvate our cation. So the fact that our polar aprotic solvent is a good solvator of a cation means we can separate this ion from our nucleophile.
That increases the effectiveness of the hydroxide ion. The hydroxide ion itself is not solvated by a polar aprotic solvent. So you might think, okay well if the oxygen is partially negative and the sulfur is partially positive.
The partially positive sulfur could interact with our negatively charged nucleophile. But remember we have these bulky methyl groups here. And because of steric hindrance that prevents our hydroxide ion from interacting with DMSO. So the hydroxide ion is all by itself which of course increases its effectiveness as a nucleophile. It is better able to attack the alkyl halide.
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SN1 • Polar solvent stabilizes transition state and carbocation intermediate. group. SN2 • Need polar solvent to dissolve nucleophile. Protic solvent slows rate by solvating nucleophile • Aprotic solvent increases rate by binding cation and thus freeing nucleophile.
Solvent Effects and SN2 and SN1 reactions: Nucleophilic Substitution
An S N 1 reaction speeds up with a good leaving group. This is because the leaving group is involved in the rate-determining step. A good leaving group wants to leave so it breaks the C-Leaving Group bond faster. Once the bond breaks, the carbocation is formed and the faster the carbocation is formed, the faster the nucleophile can come in and the faster the reaction will be completed.
The main focus here was at the substrate and the strength of the nucleophile. This, as you know, is more complicated and there are separate posts devoted to this subject. There are so many factors to consider when choosing between S N 1, S N 2, E1 and E2 that the solvent is often overlooked. The solvent is what we use to carry out the reaction so, the main requirement for it is to dissolve the reactants.
Recall that there are two important types of solvents to consider: polar protic solvents and polar aprotic solvents. Polar protic solvents are capable of hydrogen bonding.
Oxatriquinane Derivatives: A Theoretical Investigation of SN1-SN2 Reactions Borderline
Thanks for sharing this information regarding the topic of solvent effects. Really useful and understandable. I really like reading this post and I actually enjoyed reading this. Keep sharing this post more and more! Polar organic solvents. Very helpful.
If you want to do well in this class, there are several things you need to work hard at: Being attentive in class, studying the notes and this textbook especially before exams , practicing problems, and completing the quizzes and homeworks. So there are many different factors that can affect your grade. In the same way, the outcome of a reaction such as nucleophilic substition depends on many different things — reactants, solvent, etc.
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