[r12214001]

1.please rank the carbanions stability order. and explain

[CH2F]- ,[CH2Cl]- ,[CH2Br]- ,[CH2I]-

2.please rank the carbcations stability order. and explain

[CH2F]+ ,[CH2Cl]+ ,[CH2Br]+ ,[CH2I]+

[sjb]

1.please rank the carbanions stability order. and explain

[CH2F]- ,[CH2Cl]- ,[CH2Br]- ,[CH2I]-

2.please rank the carbocations stability order. and explain

[CH2F]+ ,[CH2Cl]+ ,[CH2Br]+ ,[CH2I]+

What factors do you think may affect stability?

[orgopete]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

[r12214001]

1.please rank the carbanions stability order. and explain

[CH2F]- ,[CH2Cl]- ,[CH2Br]- ,[CH2I]-

2.please rank the carbocations stability order. and explain

[CH2F]+ ,[CH2Cl]+ ,[CH2Br]+ ,[CH2I]+

What factors do you think may affect stability?

electronegativity, field effect

[insertwittyname]

You said electronegativity, now think of how that could stabilise the charged species. Remember more electronegative a species, more would it 'pull' the electrons in a bond towards itself.

[mjc123]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

The relevant compounds would be CH₃F, CH₃Cl etc.

[AromaticAcrobatic]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

The relevant compounds would be CH₃F, CH₃Cl etc.

That's funny, at first I thought the same thing. But in Orgo's defense that actually is relevant, because this trend follows the same trend as the pKa's of the halogenated acids. I.E HF, HCl, HBr, HI.

As for the poster, I like that you said electronegativity because that most definitely does effect stability but also think about what a good leaving group is and why? Maybe consider resonance within an atoms hybridization. Do some of the atoms have some hybridization character that others don't?

 

[r12214001]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

thanks for your reply, it is not a simple question but an important concept. Can you explain ?

[orgopete]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

The relevant compounds would be CH₃F, CH₃Cl etc.

If you have that data, please post it.

Let me give what I assume is the expected answer, the fluoromethyl anion is the most stable. I expect this is due to the theory that fluorine is the most electron withdrawing, and not because there is actual data showing it to be.

The poster may go back to his or her professor and ask why the acidity of the haloforms is as noted in Wikipedia. I know the haloacetic acids are the exception. You must compare all of the substituted acetic acids in order to recognize their acidity does not follow electronegativity either.

[AromaticAcrobatic]

Wikipedia lists HCF3 pKa 25-28, HCCl3 as 15.7, HCBr3 as 13.7, and HCI3 was not reported.

thanks for your reply, it is not a simple question but an important concept. Can you explain ?

I think what the pKas were trying to emphasize was stability. i. e, stronger acid weaker base and vice versa. And seeing how nuclephilicity almost parallels basicity, you can use the pKas to judge the stability of the anion. For example, the numbers agree with leaving group stability.

Orgo,
Do you agree with the expected answer?

 

[orgopete]

Orgo,
Do you agree with the expected answer?

No, it is totally wrong. If you search, you will find I have many comments about the failure of electronegativity. The poster is new and so I put something up that might make the poster think there might be a problem or question. I did not want the poster to put themselves in a position to defend my ideas. I doubt whoever wrote the problem had any actual data to base the question on. The Wikipedia data matches what one should expect based upon electron withdrawing properties.

In my manuscript, I explain the data that Pauling used to create electronegativity theory. If you take the archetypical substituted acetic acids, they do not follow electronegativity. The fluoroacetic acid is very acidic, and I think I have written a correct explanation for this series as well.

[AromaticAcrobatic]

Okay, so we are on the same page about this question and I agree that electronegativity is misleading in this type of problem.
I have seen some of your posts about electronegativity and I'm pretty sure we both debated this topic in a forum before.
I am a firm believer in electronegativity but sometimes there are other properties to consider. Like I said in the other forum: resonance has priority over electronegativity and electronegativity has priority over steric effects. (Not that this order of priority has anything to do with this particular question, its just an example of how other properties are to be considered outside of electronegativity).