halogen inductive effect

[regeneration]

HCCl3 is more acidic than CH4. I've always thought this was the case because Cl- is able to stabilize the carbocation that forms when H+ is lost by sharing its negative charge to the carbon. However, if Cl- donated electrons to the carbon before the hydrogen was lost, wouldn't the C-H bond become stronger?

Can someone clarify if/when halogens are pulling away negative charge or donating? I guess in general, would an electronegative atom want to pull away charge or contribute charge? And then why is Cl- donating charge to stabilize the cation?

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[Meredith92]

The way I understand it is not that the cl's stabilize the carbonation.... I think of it as the cl's draw electron density from the C-H bond making the bond weaker and therefore more acidic. Cls are electronegative which means they're electron withdrawing not electron donating ( like the way you describe)

Does that help?

 

[ouroboros39]

It forms a carb-ANion (negative charge, as opposed to positive of carocation). And yes, halogens are typically electron withdrawing inductively, the exception you are likely thinking of is the Electron donating/directing associated with Benzene. The inductive effect is still withdrawing, but the resonance is donating at the Ortho and Para positions (i.e. those spots can act as nucleophiles). Also worth mentioning is oxygen (or nitrogen), which although typically inductively withdrawing, can also resonance donate a lone pair and take on a positive charge if "need be".

This wont be tested probably ever, but you should not equate adding electron density to a bond to strengthening it, since adding electrons to anti-bonding orbitals reduce bond strength which is often what starts reactions.

 

[SPTtoMD]

I think you are confusing acid dissociation with SN1/E1 reactions. Those do form carbocations but any time an acid loses a proton, an anion is formed. Remember an H+ always pops off, thus the extra electron in the covalent bond goes to the conjugate base.

When an acid loses a proton, its conjugate base takes on a negative charge (an example is CH3COOH becoming an acetate anion, CH3COO-). If you have a deactivating/electron withdrawing group attached to whatever atom from which the hydrogen dissociates, that deactivating group can stabilize the negative charge induced by the sudden abundance of electrons.

Additionally, like the previous poster said, the electronegative character draws electron density away from the C-H bond, making it weaker.

Hope that helps!

 

[regeneration]

Ah, whoops! Looks like I might've gotten confused with carbocations and anions.

So, to make sure:

+CCl3 would actually be very very unstable/impossible to form, right? Halogens don't stabalize carbocations, but rather destabalize them, right?

 

[ouroboros39]

Yes, that ion would be very unstable, because the halogens are in close proximity (and like we learned from physics, electric field is much stronger in close range) - the farther the halogens are, they less they de-stablize.

 

[Laureatebarrel]

Halogens are interesting because they can withdraw electronegativity due to their high electronegativity. Also because they have spare electron pairs, they donate electrons via resonance. So, it withdraws electron via sigma bond (single bond) and donate electrons via pi double bond. Ultimately, the electron withdrawing effect is stronger, thu generally halogens are electron withdrawing. In this case since there is no pi bond, there is only electron withdrawing effect here so the carbanion will be stabilized thus stronger acid