[confusedstud]

https://ibb.co/jbcTSy

In my notes here, for a secondary substrate is straight chained with a weak base like water, Sn2 will dominate. And as the base strength increases E2 will take over. Why is this the case?

If the base is weak, it will consequently be a weak nucleophile as well. So why would Sn2 be preferred over E2 now?

[Babcock_Hall]

Is iodide a weak base?  Is it a weak nucleophile?

[spirochete]

I'm not sure you're getting at what OP is asking Babcock. Iodide ion is a a great nucleophile and a weak base because of its polarizability, among other things. There are many other examples of this category, such as thiolate ions.

But water is an example of a weak base that is also a weak nucleophile. So iodide is not the best example to use here.

Your chart says that an unbranched secondary halide with water will give Sn2 as major product. I'm not sure there's an obvious or compelling explanation for this. I have also read in many places that reactions like this will give a difficult to predict mixture that depends on the structure of the halide as well as the temperature. So probably the table is over simplifying things.

I would ask your teacher the reason. Ultimately for the sake of your grade, you must follow what your teacher says.

[Babcock_Hall]

spirochete,

The OP wrote, "If the base is weak, it will consequently be a weak nucleophile as well."  This is only partially true (meaning that there is some correlation, especially among closely related species).  However, some chemical species are weak bases but strong nucleophiles (iodide and thiolate ions, as you noted), and some a strong bases but weak nucleophiles.  I wanted to cover this point before coming to the question of water.  I agree that asking the teacher is a good thing to do in this case.  The case in which there is a 2° substrate and a weak base, weak nucleophile is covered in Chapter 8 in Klein.

[confusedstud]

Hi thanks for the replies.

I wish I could ask my teacher, but I'm fresh out of the military and waiting for university to start so I'm recapping on my polytechnic notes which I guess would be around A levels or slightly harder than that.

Your chart says that an unbranched secondary halide with water will give Sn2 as major product. I'm not sure there's an obvious or compelling explanation for this. I have also read in many places that reactions like this will give a difficult to predict mixture that depends on the structure of the halide as well as the temperature. So probably the table is over simplifying things.

I remember my teacher saying something like that a few years back.

So I'd guess for secondary substrates it'll be best to leave it by a case by case basis, whereas for primary and tertiary substrates the chart given will hold true to a better degree?

Is there an online resource regarding nucleophilicity vs basicity? I was referred to this https://www.masterorganicchemistry.com/2012/06/06/nucleophilicity-vs-basicity/ but it doesn't really explain in terms of polarizability. In my understanding, a nucleophile donates an electron pair to a carbon while a base donates an electron pair to a hydrogen atom. So I don't really see what makes one thing a good nucleophile but a bad base or vice versa. I thought that the property that makes it a good nucleophile/base would also make it a good base/nucleophile.

[spirochete]

spirochete,

The OP wrote, "If the base is weak, it will consequently be a weak nucleophile as well."  This is only partially true (meaning that there is some correlation, especially among closely related species).  However, some chemical species are weak bases but strong nucleophiles (iodide and thiolate ions, as you noted), and some a strong bases but weak nucleophiles.  I wanted to cover this point before coming to the question of water.  I agree that asking the teacher is a good thing to do in this case.  The case in which there is a 2° substrate and a weak base, weak nucleophile is covered in Chapter 8 in Klein.

You're response was reasonable and appropriate based on what he said. I'm sorry that I was rude. Given that OP's question didn't seem like cheating, I got a bit impatient, and wanted to answer every possible interpretation of the question.

Hi thanks for the replies.

I wish I could ask my teacher, but I'm fresh out of the military and waiting for university to start so I'm recapping on my polytechnic notes which I guess would be around A levels or slightly harder than that.

Your chart says that an unbranched secondary halide with water will give Sn2 as major product. I'm not sure there's an obvious or compelling explanation for this. I have also read in many places that reactions like this will give a difficult to predict mixture that depends on the structure of the halide as well as the temperature. So probably the table is over simplifying things.

I remember my teacher saying something like that a few years back.

So I'd guess for secondary substrates it'll be best to leave it by a case by case basis, whereas for primary and tertiary substrates the chart given will hold true to a better degree?

Is there an online resource regarding nucleophilicity vs basicity? I was referred to this https://www.masterorganicchemistry.com/2012/06/06/nucleophilicity-vs-basicity/ but it doesn't really explain in terms of polarizability. In my understanding, a nucleophile donates an electron pair to a carbon while a base donates an electron pair to a hydrogen atom. So I don't really see what makes one thing a good nucleophile but a bad base or vice versa. I thought that the property that makes it a good nucleophile/base would also make it a good base/nucleophile.

The basic idea is that larger atoms, below row 2 in the periodic table, like sulfur, phosphorous and bromine are better nucleophiles, despite the fact that this same property (larger atom size) makes them weaker bases. Polarizability and higher energy HOMO both have something to do with it.

You should read about nucleophilicity in an organic chemistry textbook. I'm not sure of the exact curriculum for A-levels.

[confusedstud]

The basic idea is that larger atoms, below row 2 in the periodic table, like sulfur, phosphorous and bromine are better nucleophiles, despite the fact that this same property (larger atom size) makes them weaker bases. Polarizability and higher energy HOMO both have something to do with it.

You should read about nucleophilicity in an organic chemistry textbook. I'm not sure of the exact curriculum for A-levels.

I managed to find this http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-5.html which states

"Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I- > Br- > Cl- > F-. The electron density of larger atoms is more readily distorted i.e. polarised, since the electrons are further from the nucleus.

Note that is the opposite order to basicity (acidity increases down a group) where polarisability is much less important for bond formation to the very small proton."

Does the increase in polarisation refer to the anion being bigger so it becomes more distorted by the δ+ C so it becomes easier to kick out the leaving group making it a better nucleophile?

But for basicity, since we're attacking a hydrogen. We don't care about its polarisation down the group but instead, we care about the charge density. And since down the group, the charge density decreases its basicity decreases.

Are these statements conceptually correct?

I don't really understand how being more polarisable equates to being able to extract the carbon from the C-Leaving Group (Why does the increased distortion down the group allow the new C-Nu bond to be formed more easily), and why this same factor doesn't apply when talking about attacking a hydrogen instead of a carbon.

These concepts weren't part of the syllabus for me in the past. But I'd like to learn about this if possible.

[Babcock_Hall]

On the question of nucleophilicity versus basicity, one thing to bear in mind is that basicity is a thermodynamic concept, whereas nucleophilicity is a kinetic one.  Basicity is defined in terms of attack on a proton.  Nucleophilicity can be defined for sp³ carbon atoms, sp² carbon atoms, or other atoms such as phosphorus.  Because nucleophilicity is a kinetic concept, good nucleophiles react more quickly than poor ones by definition.  Reactions that are favorable thermodynamically may or may not be fast kinetically.

An example of a strong base which is often a poor nucleophile is tertiary-butoxide, owing to its size.  Another example of a strong base but a poor nucleophile is the hydride ion.  What you wrote about polarization with respect to nucleophilicity sounds correct.

To get back to your original question, I think that a weak base, weak nucleophile could react via several mechanisms, one of them being S_N1.  Which one predominates might depend partially on additional factors, such as temperature.

[confusedstud]

On the question of nucleophilicity versus basicity, one thing to bear in mind is that basicity is a thermodynamic concept, whereas nucleophilicity is a kinetic one.  Basicity is defined in terms of attack on a proton.  Nucleophilicity can be defined for sp³ carbon atoms, sp² carbon atoms, or other atoms such as phosphorus.  Because nucleophilicity is a kinetic concept, good nucleophiles react more quickly than poor ones by definition.  Reactions that are favorable thermodynamically may or may not be fast kinetically.

A strong base which is often a poor nucleophile is tertiary-butoxides, owing to its size.  Another example of a strong base but a poor nucleophile is the hydride ion.  What you wrote about polarization with respect to nucleophilicity sounds correct.

Thanks, but I still don't quite understand how being more polarisable makes it a better nucleophile (makes it react faster from what you mentioned about it being a kinetic concept). And for basicity, why don't we take into account the polarisability just like we would when comparing nucleophilicity?

I think it has something to do with the small size of a hydrogen atom, but I don't get why it matters when we are talking about a carbon but not when we're talking about a hydrogen atom.

[spirochete]

Regarding first question:

To be sure, I checked Modern Physical Organic Chemistry, which says "Atoms with higher polarizability form the incipient covalent bond to the electrophile more easily because its electron cloud more readily accommodates changes in shape."

In the larger/more polarizable atom, the valence electrons, which are used to form bonds, are further from the nucleus and there are more core electrons shielding the valence electrons from the positive charge of the nucleus. So they are more easily pulled toward another nucleus to form a bond.

Polarizability is something that may correlate with higher Nucleophilicity. Relative nucleophilicity, especially for anions, often depends on the solvent. There are many articles online about solvent effects on nucleophilicity. Polar aprotic vs. polar protic is the main thing.

Regarding second question:

Basicity is a thermodynamic concept, so it refers to the relative stability of the acid and base form. A base is strong because it's an unstable anion and/or because it forms a strong covalent bond to hydrogen. Generally it's some combo of those two things, although some neutral bases containing nitrogen can be rather strong also.

Polarizable atoms tend to be large. Large atom monatomic anions are supposedly stable, I guess because the negative charge is spread out. Just as importantly, large atoms also make weak bonds to hydrogen, because the orbital overlap is bad between the 1S hydrogen orbital and the higher energy orbitals of the large atom.

[confusedstud]

Regarding first question:

To be sure, I checked Modern Physical Organic Chemistry, which says "Atoms with higher polarizability form the incipient covalent bond to the electrophile more easily because its electron cloud more readily accommodates changes in shape."

In the larger/more polarizable atom, the valence electrons, which are used to form bonds, are further from the nucleus and there are more core electrons shielding the valence electrons from the positive charge of the nucleus. So they are more easily pulled toward another nucleus to form a bond.

Polarizability is something that may correlate with higher Nucleophilicity. Relative nucleophilicity, especially for anions, often depends on the solvent. There are many articles online about solvent effects on nucleophilicity. Polar aprotic vs. polar protic is the main thing.

Regarding second question:

Basicity is a thermodynamic concept, so it refers to the relative stability of the acid and base form. A base is strong because it's an unstable anion and/or because it forms a strong covalent bond to hydrogen. Generally it's some combo of those two things, although some neutral bases containing nitrogen can be rather strong also.

Polarizable atoms tend to be large. Large atom monatomic anions are supposedly stable, I guess because the negative charge is spread out. Just as importantly, large atoms also make weak bonds to hydrogen, because the orbital overlap is bad between the 1S hydrogen orbital and the higher energy orbitals of the large atom.

Hmm so can we say that basicity decreases down the group because "large atoms also make weak bonds to hydrogen, because the orbital overlap is bad between the 1S hydrogen orbital and the higher energy orbitals of the large atom" so basically this factor outweighs the point on "In the larger/more polarizable atom, the valence electrons, which are used to form bonds, are further from the nucleus and there are more core electrons shielding the valence electrons from the positive charge of the nucleus. So they are more easily pulled toward another nucleus to form a bond"?

[Babcock_Hall]

Another way to look at this question is to focus on basicity:  larger atoms are more able to stabilize negative charge; therefore, their conjugate base form has less affinity for a proton.

[spirochete]

Regarding first question:

To be sure, I checked Modern Physical Organic Chemistry, which says "Atoms with higher polarizability form the incipient covalent bond to the electrophile more easily because its electron cloud more readily accommodates changes in shape."

In the larger/more polarizable atom, the valence electrons, which are used to form bonds, are further from the nucleus and there are more core electrons shielding the valence electrons from the positive charge of the nucleus. So they are more easily pulled toward another nucleus to form a bond.

Polarizability is something that may correlate with higher Nucleophilicity. Relative nucleophilicity, especially for anions, often depends on the solvent. There are many articles online about solvent effects on nucleophilicity. Polar aprotic vs. polar protic is the main thing.

Regarding second question:

Basicity is a thermodynamic concept, so it refers to the relative stability of the acid and base form. A base is strong because it's an unstable anion and/or because it forms a strong covalent bond to hydrogen. Generally it's some combo of those two things, although some neutral bases containing nitrogen can be rather strong also.

Polarizable atoms tend to be large. Large atom monatomic anions are supposedly stable, I guess because the negative charge is spread out. Just as importantly, large atoms also make weak bonds to hydrogen, because the orbital overlap is bad between the 1S hydrogen orbital and the higher energy orbitals of the large atom.

Hmm so can we say that basicity decreases down the group because "large atoms also make weak bonds to hydrogen, because the orbital overlap is bad between the 1S hydrogen orbital and the higher energy orbitals of the large atom" so basically this factor outweighs the point on "In the larger/more polarizable atom, the valence electrons, which are used to form bonds, are further from the nucleus and there are more core electrons shielding the valence electrons from the positive charge of the nucleus. So they are more easily pulled toward another nucleus to form a bond"?

They don't necessarily need to outweigh each other. I'm not sure if polarizability of the other atom increases the strength of a bond to the hydrogen.   

The large atom size is associated with a more stable anion, like Babcock said. So the polarizable atom is stable in its negative form.

Polarizability, as we're talking about it here, is all about the rate of bond formation. So in that sense it has nothing to do with (bronsted-lowrey) basicity.