Leaving groups

[globy321]

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Why is Iodine a better leaving group than Fluorine?

 

[COTA2Phys]

Because iodine is larger, and therefore can delocalize electrons more i.e. more stable once it leaves.

 

[sciencebooks]

I learned it like this: The leaving group is the weakest base. Basicity decreases going DOWN the periodic table in the same group. Thus, the better leaving group is iodine.

What COTA2Phys said is also right as size drives basicity.

 

[Anthrax]

I learned it like this: The leaving group is the weakest base. Basicity decreases going DOWN the periodic table in the same group. Thus, the better leaving group is iodine.

What COTA2Phys said is also right as size drives basicity.

This is also how I learned it too without remembering the strength of leaving groups relative to each other.

 

[LoLCareerGoals]

Additionally if I remember correctly, being more electronegative means forming a stronger bond (since the bond is more polar). F is the most electronegative element. This means bonds it forms are highly polar and hence strong. Strong bonds are harder to break -> weaker leaving group.

 

[brownbaglunch]

Additionally if I remember correctly, being more electronegative means forming a stronger bond (since the bond is more polar). F is the most electronegative element. This means bonds it forms are highly polar and hence strong. Strong bonds are harder to break -> weaker leaving group.

This is not right.

Electronegativity is a measure of how much an atom wants to keep both electrons of the bond for itself. This means that increased electronegativity means increased acidity. BUT...

As mentioned above, the larger ions have more space to handle the negative charge. This stabilizes the conjugate base. In the case of the halogens, the effect of this stability is greater than the electronegativity. The net effect is that as you go down the halogens you get a stronger acid.

 

[LoLCareerGoals]

This is not right.

Electronegativity is a measure of how much an atom wants to keep both electrons of the bond for itself. This means that increased electronegativity means increased acidity. BUT...

As mentioned above, the larger ions have more space to handle the negative charge. This stabilizes the conjugate base. In the case of the halogens, the effect of this stability is greater than the electronegativity. The net effect is that as you go down the halogens you get a stronger acid.

Right so then HF is the strongest acid correct? Since we are talking about leaving a carbon, there you go:

C-F: 485 KJ/mol
C-Cl: 328 KJ/mol
C-Br: 276 KJ/mol
C-I: 240 KJ/mol

Just like I said, harder to leave when your bond is tougher to break.

 

[shaboobly]

Right so then HF is the strongest acid correct? Since we are talking about leaving a carbon, there you go:

C-F: 485 KJ/mol
C-Cl: 328 KJ/mol
C-Br: 276 KJ/mol
C-I: 240 KJ/mol

Just like I said, harder to leave when your bond is tougher to break.

HF is relatively weak compared to the other halogen acids. there's many things to consider, but electronegativity, bond length, ability to delocalize negative charge are all factors.

HF is weaker than the rest because there's a higher effective nuclear charge, shorter bond, and not as able to delocalize the electrons over the small area that fluorine is, compared to the other large halogens.

 

[LoLCareerGoals]

You think I don't know HF is weakest of halo-acids? I am posting an answer to an organic chemistry question. This is a 4th week of gen chem factoid every freshman knows.
I just provided a counterexample to his statement. See the highlight. He said I said something incorrect in my earlier post. I put #s to back my point up.

 

[brownbaglunch]

I didn't mean to aggravate you. Electronegativity effects are absolutely not what makes the smaller halides worse leaving groups. I apologize for any hypertension this caused.

Your data is irrelevant, as we agree on the overall trend.

 

[SomethingLatin]

Additionally if I remember correctly, being more electronegative means forming a stronger bond (since the bond is more polar). F is the most electronegative element. This means bonds it forms are highly polar and hence strong. Strong bonds are harder to break -> weaker leaving group.

You are correct in more EN equates to more polarity equates to a stronger bond. As EN difference increases the bond has more ionic character. However, while strong bonds are harder to break than weak bonds, you cant apply this mantra to acid disassociation in water. Think about bond breaking in gas state vs. in water and you will probably figure it out. Entropy for the two are wildly different when you can stabilize the charged products in a diprotic solvent(water).

 

[shaboobly]

You think I don't know HF is weakest of halo-acids? I am posting an answer to an organic chemistry question. This is a 4th week of gen chem factoid every freshman knows.
I just provided a counterexample to his statement. See the highlight. He said I said something incorrect in my earlier post. I put #s to back my point up.

dont get so worked up after you said HF is the strongest acid.

 

[SomethingLatin]

dont get so worked up after you said HF is the strongest acid.

To his defense he was talking about bond strength not acidity strength.

 

[shaboobly]

To his defense he was talking about bond strength not acidity strength.

i clearly see a question pertaining to the acidity of HF...

 

[LoLCareerGoals]

I didn't mean to aggravate you. Electronegativity effects are absolutely not what makes the smaller halides worse leaving groups. I apologize for any hypertension this caused.

If you do and I am wrong, aggravation is worth it as the info will stick better. There is no need to apologize, but there is a need to argue your point.
I don't care if a textbook did not directly list electronegativity as a parameter affecting how good of a leaving group a given group is in a nice bullet format to be mindlessly memorized. I like to use tools I've picked up earlier and connect dots instead of mindlessly recite stuff. Notice the word "additionally" in my first post, I did not contest earlier points, merely trying to add another angle to the same question.

Now back to the science.
When evaluating whether or not a leaving group is a good one, most factors discussed so far focus on the energetics:
"... more i.e. more stable once it leaves. "
1) This is product side of the deltaH equation. More stable -> more energetically favorable.
2) "Because iodine is larger, and therefore can delocalize electron"
This merely means that transition state (of say SN2 reaction when leaving group leaves) is stable and thus Eactivation is low. Does not affect net delta H, but still low Ea is good for the reaction kinetics.

Now I bring up bond weakness on the reactant, which is still part of the energetics discussion only I am focusing on the reactants. My point is still that delta H is more exothermic when weaker bonds are on the reactant side. Why is this incorrect or even dissimilar to the point made in 1) ^ ?

Your data is irrelevant, as we agree on the overall trend.

Why is it irrelevant? You don't like to exercise your brain and mindlessly memorize? As long as conclusion is in accordance with a textbook completeness of reasons for this conclusion is irrelevant?

i clearly see a question pertaining to the acidity of HF...

Look again. I even bolded his imo careless statement and asked a rhetorical question with an obvious counterexample to the statement.
Acidity decreases with el-negativity for binary acids, but it is the opposite for oxyacids and that's just 1 exception I know about. I just dislike incomplete careless statements like "increased electronegativity means increased acidity".