N atom better nucleophile than O

[TimeforDAT]

In the ACS book pg 73 2006 edition, it says that the nitrogen atom is intrinsically better nucleophile (stronger base) than the oxygen atom and preferentially attacks the carbonyl carbon atom.

Why is the nitrogen a better nucleophile? isn't the oxygen a better electron withdrawing atom than the nitrogen?

---

Members don't see this ad.

 

[levyusupov]

since O is more electronegative then N, the electron can be held stronger in the O then in the N. thus, it will be harder to form O nucleophile then N nucleophile.

 

[poc91nc]

I think the line of reasoning for why N is a better nucleophile is because nitrogen is larger than oxygen.

Therefore, the outer electrons are held more loosley and more easily donated (as a nucleophile/lewis base).

Electronegativity doesn't explain why Nitrogen is a better nucleophile b/c nitrogen is to the right of oxygen. Electronegativity increases left to right.

N|O

This same line of reasoning explains why R-S:- is a better nucleophile than
R-O:- (alkoxide)

 

[jdoc04]

N (-3) charge makes it a stronger nucleophile than O (-2).
What does a nucleophile like??.............Positive charges therefore N is more strongly attracted to a electrophile (+).

Nitrogen is not more EN than O nor is it larger.
EN increases Left to Right across a period and up a group
N- MW 14
O- MW 16

I hope this helps!!

 

[levyusupov]

ok i got it.

since O is more electronegative then N, the electron can be held stronger in the O then in the N. thus, it will be harder to form O nucleophile then N nucleophile.

 

[AaBbZz]

I think Poc91nc reasoning is correct larger atoms=loosely held electrons.

Nucleophiles are electron rich and want to donate the electrons, and the loosely held electrons are easier to give up making them the best nucleophiles

 

[War Eagle]

I think Poc91nc reasoning is correct larger atoms=loosely held electrons.

Last I checked on a table N is smaller than O. The real reason has been stated already. N is less electronegative, therefore its electrons are held more loosely and can react more easily with other molecules. You can also think of it in the terms that more electronegative atoms handle a negative charge better (F is more likely to have a negative charge than C). Since N isn't as electronegative as O, it doesn't carry the negative charge as well and is more reactive.

 

[poc91nc]

Last I checked on a table N is smaller than O. The real reason has been stated already. N is less electronegative, therefore its electrons are held more loosely and can react more easily with other molecules. You can also think of it in the terms that more electronegative atoms handle a negative charge better (F is more likely to have a negative charge than C). Since N isn't as electronegative as O, it doesn't carry the negative charge as well and is more reactive.

War Eagle,

Could you please verify your claim? For main group elements, size ALWAYS increases from RIGHT to LEFT. As you go right to left EN decreases as well. Therefore, decreasing electronegativity ALWAYS parallels size for the main group elements.

Which comes first...chicken or the egg? So...does size not contribute to how loosley the outer electrons are held? Is electronegativity a manifesation of size? or vice versa?

Nitrogen is to the left oxygen on the periodic table.

If you still don't believe me:
pg 309 of Chemistry by Silberberg figure 8.12
atomic radii of nitrogen 75
atomic radii of oxygen 73

 

[poc91nc]

Actually, I should be more specific....


As size increases, electronegativity decreases.

 

[AaBbZz]

N is LARGER than O. It has less protons which consequently holds the valence electrons less loosely giving it a larder atomic radius than O which has more protons.

N is also less electronegative than O, what I should have said is that for me it is easiest to think of the best nucleophile as one that can easily give away electrons aka the larger atom

 

[poc91nc]

If size increases...electronegativity decreases.

So, is nitrogen less electronegative b/c it is larger, or is nitrogen larger b/c it is less electronegative?

That is what I am confused about....which explains which? size or electronegativity....chicken or egg?

 

[AaBbZz]

I think what helps most is understanding the definitions of both electronegativity and atomic size. All the periodic trends relate to each other in some way so it just depends on how you look at it.

Electronegativity= the ability an atom has to attract electrons

so Flourine the smallest element to the right of the periodic table wants that extra electron to make it a noble gas. The small size here is one of the reason electronegativity is so high because it alows the flourine atom to pull the electrons close to it.

Atomic Size: How big an atom is

So big atoms have far away electrons that are held loosely (aka definition of weak electronegativity)

Nucleophilicity=electron rich species which will give its electrons to electrophiles "electron loving" species

Again for me I personally think it is easier to look at large atoms as the best nucleophiles because large atoms can easily give away electrons which is what a nucleophile wants to do.

You can think of it as weak electronegativity but for me I think of electronegativity as something to do with strength of bonding

 

[poc91nc]

Why do I even care...I took my DAT like 3 weeks ago.

This is sad...oh well.

 

[War Eagle]

War Eagle,

Could you please verify your claim? For main group elements, size ALWAYS increases from RIGHT to LEFT. As you go right to left EN decreases as well. Therefore, decreasing electronegativity ALWAYS parallels size for the main group elements.

Which comes first...chicken or the egg? So...does size not contribute to how loosley the outer electrons are held? Is electronegativity a manifesation of size? or vice versa?

Nitrogen is to the left oxygen on the periodic table.

If you still don't believe me:
pg 309 of Chemistry by Silberberg figure 8.12
atomic radii of nitrogen 75
atomic radii of oxygen 73

Now I see the point you were making. I thought you were saying that N was larger by mass, not radius. my bad. but yeah you were rigth what you said. miscommunication

 

[levyusupov]

N is less Electronegative thus will hold the electrons less tightly,,,with loose electrons, we can use them to ATTACK an electrophile !!!!! N is more polarizeable than O !!!!!!!

NO ARGUE ABOUT IT !!!!!!!

 

[bkim]

I think it depends on whether we are talking about Sn1(protic) or Sn2(aprotic).
In a protic solvent, the size is the more important factor. Bigger size-> more polarizable. N- is better for Sn1.

As for Sn2, the key is the base strength. Which is more willing to give the electrons? This is inversely related to size (F- > Cl-> Br-> Cl-) if we are looking down the periodic table vertically. However, if we look at the periodic table from left to right, N- > O- in base strength, even though N- is bigger. Since N is less electronegative than O, it is more willing to give up the electron -> stronger base -> better nucleophile for Sn2. We cannot always use electronegativity as a sole factor, since F- > Cl- even though F is more electronegative than Cl.

Compare it with Cl- vs F-.

Cl- > F- for Sn1 because Cl- is bigger, and F-> Cl- for sn2 because F- is stronger case.

 

[levyusupov]

N is less Electronegative thus will hold the electrons less tightly,,,with loose electrons, we can use them to ATTACK an electrophile !!!!! N is more polarizeable than O !!!!!!!

No Argue about it !!!!!!!!!

 

[joonkimdds]

Since N is less electronegative than O, it is more willing to give up the electron -> stronger base -> better nucleophile for Sn2.

If it is willing to give up the electron, shouldn't it be weaker base since it tends to become + meaning more acidic?

 

[phungy]

I think the line of reasoning for why N is a better nucleophile is because nitrogen is larger than oxygen.

Therefore, the outer electrons are held more loosley and more easily donated (as a nucleophile/lewis base).

Electronegativity doesn't explain why Nitrogen is a better nucleophile b/c nitrogen is to the right of oxygen. Electronegativity increases left to right.

N|O

This same line of reasoning explains why R-S:- is a better nucleophile than
R-O:- (alkoxide)

You are right that N is better than O. But wrong, electroegativity DOES explain why N is better nucelophile than O. Charges/sterics being equal, a lone pair on a less electronegative element will make for a better nucleophile than a more electronegative one. If you have a really electronegative nucleophile, those lone pair electrons will be pulled closer to the atom instead of serving as a nucleophile.

 

[poc91nc]

You are right that N is better than O. But wrong, electroegativity DOES explain why N is better nucelophile than O. Charges/sterics being equal, a lone pair on a less electronegative element will make for a better nucleophile than a more electronegative one. If you have a really electronegative nucleophile, those lone pair electrons will be pulled closer to the atom instead of serving as a nucleophile.

I was correcting someone who said that b/c N is more electronegative it is a better nucleophile.

You're right, electronegativity does explain it.

However, so does size.

Read the rest of the thread to see where I am coming from.

I am not disputing the effect of electronegativity....but which is a manifestation of which?

Both can explain the phenomena

 

[phungy]

I was correcting someone who said that b/c N is more electronegative it is a better nucleophile.

You're right, electronegativity does explain it.

However, so does size.

Read the rest of the thread to see where I am coming from.

I am not disputing the effect of electronegativity....but which is a manifestation of which?

Both can explain the phenomena

I did only read half the thread so my bad! But yes, I agree that both electronegativity and radii both play a role in determining how good the nucleophile is. 🙂

 

[poc91nc]

You are right that N is better than O. But wrong, electroegativity DOES explain why N is better nucelophile than O. Charges/sterics being equal, a lone pair on a less electronegative element will make for a better nucleophile than a more electronegative one. If you have a really electronegative nucleophile, those lone pair electrons will be pulled closer to the atom instead of serving as a nucleophile.

I already understand that line of reasoning.

Can you explain which is a manifestation of which? Which is earlier in the causal chain?

While you can explain strength of nucleophicity in terms of both electronegativity and size, which is more fundamental, and therefore a more sound and absolute explanation?

 

[phungy]

From Wade's Organic textbook 5th edition pg. 230

Trends in Nucleophilicity:
1. A species with a negative charge is a stronger nucleophile than a similar neutral species. In particular, a base is a stronger nucleophile than its conjugate acid.

2. Nucleophilicity decreases from left to right in in the periodic table, following the increase in electronegativity from left to right. The more electronegative elements have more tightly held nonbonding electrons that are less reactive toward forming new bonds.

3. Nucleophilicity increases down the periodic table, following the increase in size and polarizability.

I understand what you are asking. So since N is better than O and S is better than O. Which is better? N or S? S right?
What about B or S?

Atomic radii periodic table