- Identifying nucleophilic and electrophilic centers
- Curly arrow conventions in organic chemistry
- 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
- Sn1 vs Sn2: Summary
Sn1 vs Sn2: Solvent effects
Which types of solvents favor Sn1 and Sn2 reactions. Created by Jay.
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- I've noted that Jay writes ionic compounds like this: Na+-OH (7:32)
Is this the standard way of writing it? I'm used to seeing Na+OH-, with the sign after the anion... (both of course in superscript).(10 votes)
- I'm 7 yrs late but I also have only seen it with the sign after the molecule(4 votes)
- At3:25if he were to finish the mechanisms and show the product, what would it look like? Is the water molecule going to attach to the carbocation as an H2O and replace the Halogen?(5 votes)
- Yes, but that's not the end of the mechanism. The water molecule will attack the carbocation forming a bond with one of the lone pairs of electrons on the oxygen. Because this will create a + charge on the electronegative oxygen (3 bonds, 1 lone pair = 5 electrons, neutral oxygen has 6), a proton transfer must occur. Another H2O molecule could attack one of the hydrogens leaving an OH with a bond to the carbon and 2 lone pairs of electrons (oxygen will be neutral and happy). In other words, this mechanism will form an alcohol.(8 votes)
- If DMSO is more likely to take the left structure as Jay at05:50says, is it probable for it, used as the solvent in a reaction, to take the right, polar aprotic structure and increase the nucleophilicity? And are the polar aprotic solvents generally bulky, so that they, due to steric hindrance, generally do not solvate the anion of the nucleophile?(3 votes)
- It never has either the structure on the right or the structure on the left. It always is a resonance hybrid of the two structures, with a partial negative charge on the O atom and a partial positive charge on the S.
The major requirement for a polar aprotic solvent is that it have a large dipole moment and a large dielectric constant. It does not have to be bulky. Other examples of polar aprotic solvents are acetone, dimethylformamide, and acetonitrile.(4 votes)
- Doesn't acetone also have a resonance structure, similar to dimethylsulfoxide?(3 votes)
- Yes, acetone also has the same resonance structure with a negative charge on the oxygen and a positive charge on the adjacent carbon. This resonance structure is incredibly important when dealing with carbonyl (C=O) compounds.(3 votes)
- Why polar aprotic solvent (that is negative) don't act like a stabilizator of carbocation?(3 votes)
- You have to first consider the fact that in an Sn1 reaction the carbocataion is formed only when the leaving group takes the electron pair and leaves.
This happens because of two reasons
1>The presence of a neucleophile.
2>Stabilization of the negatively charged leaving group by solvation.
In a polar aprotic solvent (that is negative) the region of electron deficiency is usually sterically hindered so it is not able to effectively stabilize the leaving group when it is in its ionic form, therefore the carbocataion is not formed in the first place!(2 votes)
- Does it also make an SN1/SN2 reaction more likely if the solute (alkyl halide) is a polar molecule?(2 votes)
- please explain that how does the strength of nucleophile increase by dissolving it in polar aprotic solvent?(1 vote)
- A polar protic solvent can hydrogen bond to a nucleophile, so that the nucleophile is surrounded by a shell of solvent molecules, just like a Very Important Person is surrounded by a shell of bodyguards. The nucleophile has to push the bodyguards out of the way before it can get at a substrate molecule.
Aprotic solvents can't hydrogen bond to a nucleophile.
The nucleophile does not have a protective shell around it, so it can easily attack a substrate.
So a nonpolar solvent increases the strength of a nucleophile.(3 votes)
- I know the threshold for in difference in electronegativity for finding out whether or not two bonded atoms are polar, but what's the threshold for protic? Since C-H isn't is has to be at least > 0.35 difference...
- At1:15Jay says that we have a planar carbocation. Well, then he shouldn't have used the wedge-dash representation right?(1 vote)
- It’s fine to draw it like that.
Hold your left hand out in front of you, your middle three fingers pointing upwards, thumb pointing towards you and little finger pointing away. That’s how you should be thinking about the molecule at this time in the video.(2 votes)
- Does the nuceophile participates in SN2 reaction has to be negative? And how do we determine if a molecule or an ion is strong, weak nucleophile, or not a nucleophile.(1 vote)
- The nucleophile in an Sn2 reaction does not necessarily have to have a negative charge, but negative-charged nucleophiles are more "effective" than neutral-charged ones in terms of the kinetics of the reaction.
Also, the strength of the nucleophile depends on what kind of molecule you are using, as well as what type of reaction is occurring, and what reactant & solvent is present. So things such as formal charge (where neg. charge is better than neutral) and electronegativity (less EN, the better since electrons are less "tightly held", so the nucleophile will be more readily able to donate electrons) will affect the strength of the nucleophile.(1 vote)
- [Lecturer] The choice of a solvent can have an effect on an SN1 or an SN2 mechanism. 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. HMPA. 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. If we had used something like water, we know that water is a polar protic solvent with the oxygen being partially negative and the hydrogens being partially positive and a polar protic solvent would interact with our nucleophile solvating it and essentially decreasing the effectiveness of our nucleophile. So that's why polar protic solvents don't work as well if you want an SN2 mechanism. A polar aprotic solvent increases the effectiveness of our nucleophile therefore favoring our SN2 mechanism.