Leaving Groups

[DrKendrickLamar]

I keep confusing myself with the relationship of Leaving groups with nucleophilicity, electronegativity, and acidity. Help me with the relationship?

The more electronegative the more acidic=a better leaving group? and less nucleophilitic?

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[Kaustikos]

I keep confusing myself with the relationship of Leaving groups with nucleophilicity, electronegativity, and acidity. Help me with the relationship?

The more electronegative the more acidic=a better leaving group? and less nucleophilitic?

You have the right idea. The way I remembered it initially to help me was to think of the groups in terms of acids and bases via how they interact with a proton. Acids love to throw away their proton and bases go at protons like a child molester in Chuck E' Cheese. Going from there, it helped me understand why the bases tended to be the attacking groups and acidic/highly electronegative wanted to detach (ability to hold that negative charge)
Bases want to attack the alkyl groups, not leave them.

Also, this is very important as well; basicity and nucleophilicity determine E2 vs Sn2. A highly basic group will form E2 reactions whereas a highly nucleophilic group will go to a Sn2 mechanism. Highly basic groups will attack the protons on alkyl groups whereas a highly nucleophilic group will attack the carbon.

 

[161927]

Nucleophilicity is a kinetic term, and basicity is a thermodynamic term. Acid/base chemistry involves determining the most stable product. This is why you learned about Ka's and Kb's. There's an equilibrium between acids and their conjugate bases, and the stability of the conjugate pairs is what governs assessments of reactivity. If you think about nucleophiles, you ask what conditions will make this likely to react with an electrophilic center? One answer would be: large and highly polarized (negative charge). A judgment of nucleophilicity does not depend on the resulting products, like you worry about with acid/base chemistry. If you already covered conjugate additions then you know about what it means for a reaction to proceed by kinetics vs. thermodynamics.

 

[olygt]

Is there a simple list that I can look over for good leaving groups vs. bad. I know that basics such as Br, Cl, and I are very good leaving groups. Are there any others that I should keep an eye out for. Also, I am starting to get the acid relationship with rxns. but not so sure about bases. Can someone clarify the strong bases for me...for instance if you add NaOH vs. NH3.


strong acid (HCl) = weak conjugate base = good leaving group
strong bases (NaOH) vs. weak bases= ?????

I think another thing that confuses me with acid/base reactions is sometimes a text says acids are electron pair acceptors and bases are electron pair donors and another text says an acid transfers protons to a base---is this to reach equilibrium w/in a reaction?

I am missing some kind of underlying relasionship with reactions so please if you could clarify....here's just a little comparison that might help me with all reactions:
1) an ester reacts with water/OH- and forms a carboxylate + HOR
2)and ester reacts with Water/H+ and forms a carboxylic acid + HOR
3) A methyl ketone reacts with water/OH- and forms a carboxylate + Iodoform
4) An aldehyde reacts with KMnO4/OH- and forms a carboxylic acid
5) Primary alcohol reacts with K2Cr2O7/H+ and forms a carboxylic acid
6) secondary amide reacts with water/H+/heat and forms a carboxylic acid + RNH3+

why when an ester is treated with water under basic conditions it forms a carboxylate vs. if it were treated under acidic conditions forms a carbxylic acid? How do you know what part of KMnO4 under basic or acidic conditions is going to be in the product or K2Cr2O7 under acid/base conditions or if heat is added as in an amide hydrolysis. And since OH- is a bad leaving group why does it leave when a carboxylic acid is treated with PBr3?

 

[unsung]

I keep confusing myself with the relationship of Leaving groups with nucleophilicity, electronegativity, and acidity. Help me with the relationship?

The more electronegative the more acidic=a better leaving group? and less nucleophilitic?

A leaving group is a good leaving group if it's stable (i.e. unreactive) after it leaves. So, instead of thinking about a bunch of different variables (acidity, electronegativity, etc.), then trying to use those to judge how good a leaving group is, I always try to think of it in terms of the basic concept at the root of it, then go from there.

Okay, so something is a good leaving group if it's stable after it leaves. This brings up the more general question- what makes something stable? Something is stable if it tends not to react. This means that anything that you can think of that would be a good (i.e. strong) base or good nucleophile would make a poor leaving group.

For example, hydroxide -OH is a pretty good base, right? (think NaOH). This means -OH is usually a pretty *bad* leaving group for most reactions (there are a few exceptions, such as base-cat dehydration for an aldol condensation, etc.). But certainly for any Sn2 reaction you can think of, -OH is never going to come off and be the leaving group, since Sn2 is a concerted ONE-STEP reaction, whose transition state depends in part on how good the leaving group is.

Anyway, not to get sidetracked by all the detail, but the main idea to always keep in mind is- how reactive would the thing be if it comes off as a leaving group? If you think it's going to be pretty reactive, then it's likely not going to be a good leaving group. If it's going to be pretty stable, then it'll be a better leaving group. Halogens are good leaving groups.

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Acidity is a different topic, but the theme about stability we considered for leaving groups still applies. How I think about acids is basically: if its conjugate base is stable (i.e unreactive, i.e. weak base), then it's a good (strong) acid. If the conjugate base is a really good (strong) base, then, it's going to be a poor acid.

Why? Things like to be stable. If ripping off a proton from an acid is going to give you something stable and pleasant and unreactive, then it's going to be really easy to rip off that proton. If otoh, the end result of ripping off a proton is something really reactive and aggressive, then it's going to be hard to rip off that proton, meaning what you have is a poor acid. A good acid likes to "give it up", and after it's given it up, it doesn't care, won't text you the next morning. A poor (weak) acid plays hard to get, but after it's given it up, it turns out to be real needy & always wants your attention.

Hmm... I don't know where that metaphor came from.

 

[olygt]

You could disregard my last post. I found a great website that helped me understand how reactions really work and with a little bit more practice I think I can work out just about any reaction. I can't believe I'm saying this, but I'm beginning to like organic chem. I hope this will help any of you who have struggled through this material as I have.

Here's the main site:
http://www.chemguide.co.uk/
If you scroll down the three links that are pertinent to us are "basic organic chem", "properties of organic compounds" and "organic reaction mechanisms"


This link is within the "Properties of organic compounds" link and is really useful with most the reactions we need to know
http://www.chemguide.co.uk/orgpropsmenu.html#top

 

[TJames]

I keep confusing myself with the relationship of Leaving groups with nucleophilicity, electronegativity, and acidity. Help me with the relationship?

The more electronegative the more acidic=a better leaving group? and less nucleophilitic?

The conjugate bases of strong acids will be good leaving groups - but you can't use this alone to tell if something is a good nucleophile. Some conjugate bases of strong acids are great nucleophiles (ie I-) but some are awful (ie MeSO3-)

Here's a way to remember:

Good nucleophiles: Nucleophilicity increases as - charge increases (ie OH- > H2O)

Increases as you go LEFT in the same row (IE N>O>F)

Increases as you go DOWN periodic table (IE Br>Cl>F).

 

[TawMus]

Easy way to remember


Between F,Cl,Br,I --- (I>Br>Cl>F) is the order of strength in Leaving And Nucleophilic groups. EXCEPT for in SN2 reactions, where F>Cl>Br>I describes nucleophilic strength....

 

[talkalot24]

The conjugate base of a strong acid is a good leaving group.

For instance, H2O is a great leaving group. We know this because H30+ is a strong acid.

The lower the pKa, the better the leaving group.

The way I think about it is that a strong acid dissociates completely. It's conjugate base would not, which is what you want in a good leaving group because otherwise it would be reactive. You don't want the leaving group to leave, and then turn around and be a nucleophile to reform the original molecule.