Acidity

[rainashley]

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EK explains how HCl is very polar and a proton is more easily removed from HCl than methane, therefore HCl is more acidic. The book explains that weak bonds are more acidic. Isnt this contradictory, or am i wrong?
I thought HCl being more polar holds the proton stronger than methane?
Arent polar molecules stronger than nonpolar ones? Isnt HCl stronger if its more polar? What am i missing?
Also, could you please explain how a larger molecule is more acidic... how does being larger allow it to be more acidic?
And what makes a conjugate stable? How is stability measured?
Thanks!

 

[RogueUnicorn]

it's not contradictory. you are wrong. not sure where to start with this, so why don't you take the lead. what does polar mean?



also, if you are indeed registered for the jan mcat, you need to push it back alot. seriously, i am saying this for your benefit alone.

 

[sixpence]

EK explains how HCl is very polar and a proton is more easily removed from HCl than methane, therefore HCl is more acidic. The book explains that weak bonds are more acidic. Isnt this contradictory, or am i wrong?
I thought HCl being more polar holds the proton stronger than methane?
Arent polar molecules stronger than nonpolar ones? Isnt HCl stronger if its more polar? What am i missing?

The H-Cl bond than say an H-H bond because Cl is larger than H. Longer bonds = weaker bonds = stronger acid. Similarly, Cl is bigger than F so the H-F bond is stronger and thus a weaker acid. I've also heard that when you have two atoms of different sizes, like H and Cl, the overlap of orbitals is uneven, creating a weaker bond.
The H-Cl bond is polar yes because of electronegativity differences, which means unequal sharing of electrons within a bond. If the more electronegative atom is holding a larger portion of the electrons then the H is barely holding on (bonded).

Also, could you please explain how a larger molecule is more acidic... how does being larger allow it to be more acidic?

It should be larger atom and not molecule? Acids are often measured by the stability of the conjugate base. A larger atom (in this case Cl-) allows the negative charge to be spread out evenly. Thus a proton would have a hard time finding the negative charge. If the atom is tiny, the negative charge is compact and strong, ready to deprotonate.

And what makes a conjugate stable? How is stability measured?

maybe you should use tbr books instead of ek because this is a pretty basic concept thats important in ochem as well.

 

[rainashley]

it's not contradictory. you are wrong. not sure where to start with this, so why don't you take the lead. what does polar mean?



also, if you are indeed registered for the jan mcat, you need to push it back alot. seriously, i am saying this for your benefit alone.

really? even though im scoring around 34s on my practice AAMCs?
thanks for your help

 

[rainashley]

it's not contradictory. you are wrong. not sure where to start with this, so why don't you take the lead. what does polar mean?



also, if you are indeed registered for the jan mcat, you need to push it back alot. seriously, i am saying this for your benefit alone.

polar is the unequal sharing of electrons by molecules... unequal distribution... a high electronegativity atom pulls the electrons from a low electronegativity atom... whats your point??

 

[RogueUnicorn]

really? even though im scoring around 34s on my practice AAMCs?
thanks for your help

that's nice for you, though it's hard to believe with the questions you've been asking.

polar is the unequal sharing of electrons by molecules... unequal distribution... a high electronegativity atom pulls the electrons from a low electronegativity atom... whats your point??

very good. so if a covalent bond is the sharing of electrons, and a highly polar bond is a very unequal sharing, the bond very easily broken.

 

[rainashley]

that's nice for you, though it's hard to believe with the questions you've been asking.

never assume
im not here to judge
then why are highly polar molecules stronger than nonpolar molecules? i thought since its highly polar it should be stronger... and less likely to be pulled apart... what am i missing?
thanks

 

[RogueUnicorn]

the answer is in your question. polar molecules and polar bonds are not the same. sixpence has also posted a nice detailed response.

 

[werd]

think of it like this... more polar means electrons spend more time around one atom than another --> the bond itself has less electron density and is weaker --> weaker bond means more easy to let go of the hydrogen to make H+.

alternatively, the more polar a bond, the more like a salt the compound will act --> dissociation of the atoms in aqueous solution --> more H+ and Cl- and less HCl's.

alternatively, how pissed is chlorine to be "Cl-" in solution?... not at all, we see it all the time. how pissed is methane to be "CH3-" in solution? ... pretty darn pissed, it's not gonna give up that hydrogen.

this is pretty basic stuff (no pun intended). alright, pun intended...

 

[loveoforganic]

@OP: The book is talking about two different things. Deprotonation and bond dissociation aren't the same.

HCl => H+ + Cl-

This is deprotonation.

HCl => H. + Cl.

This is bond dissociation. When you hear about ionic bonds being stronger than covalent bonds, this is what is being referred to.

 

[rainashley]

thanks for your help
i am still confused... and ill explain why
cis molecules (compared to trans) are more polar, have stronger intermolecular forces, and have higher BP
so if cis is more polar than trans and it is stronger... then why does have a weak bond if the polar bond makes it weaker? what am i missing here?
thanks

 

[demayette]

thanks for your help
i am still confused... and ill explain why
cis molecules (compared to trans) are more polar, have stronger intermolecular forces, and have higher BP
so if cis is more polar than trans and it is stronger... then why does have a weak bond if the polar bond makes it weaker? what am i missing here?
thanks

Chemistry is confusing man...I think polarity of cis/trans molecule has more to do with net dipole moment.

 

[rainashley]

why is alcohol more polar than an aldehyde/ketone?

 

[RogueUnicorn]

what's the acidic H attached to in an alcohol. what about aldehyde.

 

[rainashley]

what's the acidic H attached to in an alcohol. what about aldehyde.

on the Oxygen
on alpha carbon
so?

 

[Geekchick921]

on the Oxygen
on alpha carbon
so?

I want to tell you the answer but it would be better if you figured it out for yourself.

 

[StarryNights]

OP, it just seems to me like you rub quite a few people the wrong way. I feel that it is a bit unfair towards you because we all learn at our own pace. However, and I say this honestly for your own benefit, you don't really seem like you want to figure things out for yourself. A lot of your follow-up questions are quick-draw style as if you want people to lead you step by step. I don't think a lot of people have the patience for this because it seems like you can probably answer your own questions if you put in the effort. I think on one of the other threads someone posted up a site for you to get more information from and you responded by telling him to just answer your question? You can figure this out and you obviously care about doing well on the MCAT. I think you will get a lot more out of this whole process if you modify your learning approach a bit. Best of luck on your upcoming MCAT though!

 

[wanderer]

Homolysis. Heterolysis. Look them up.

 

[kentavr]

rainashley, I'll try to scetch the explanation, but I am afraid it is beyond MCAT.
1. I would say that your question is absolutely valid. Indeed, by Coulomb law the more electric charge involved in interaction the more attractive force is. F~Q*q/r^2. Polarity ~ charge ~ Force (You right)
2. I guess that all electro negativity explanations are not clear, since there is no fundamental law of electronegativity. It is just a shortcut (mnemonic) due to our inability to solve Schredinger equation for muli electrons atoms.
3. However, the denumerator of Coulomb law has distance(or length of the bond) squared. In case of unequal atoms the distance between atoms probably increase due to unequal electric field which seriously affect the resulting force. Also the polar solvent (water) can squeeze in between ionic bonded atoms and change dielectric constant which also decrease the attractive force.
4. All in all, it looks like the length of the bond is more important factor for acidity then charge itself. Decreasing the attractive force makes proton free.
5. I left out self-inductive effect for simplicity.