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Qube
Gold Member
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What are your thoughts on this excerpt from my chem prof's textbook?
I have a few problems with the beginning of the text and a huge problem with the bolded statement.
I understand the intentions of my teacher - structure - is highly important in chemistry. Not so much emphasized in general chemistry but very important in organic chemistry (i.e. how do you know whether you're getting a nasal decongestant or an illegal drug?)
However, I find his emphasis on formal charge analysis a bit misguided. He has continually stated that one of his main goals is to ready us for the rigors of organic and analytical chem.
The problem is that the central conceit of orgo is: nucleophile attacks electrophile.
When you write hydroxide ion the way he does, that's okay, because the formal charge - which is nothing more than what its name suggests - a mere formalism - is mostly born by the oxygen (not the hydrogen). Keeping this in mind allows one to understand why, say, potassium hydroxide can absorb carbon dioxide and give a false positive test when testing for carboantes - the nucleophilic oxygen in hydroxide ion attacks the electrophilic carbon in carbon dioxide.
But when he says something like nitrogen bears the "+" charge - that's just creating a chemistry unicorn in place of killing another chemistry unicorn. Yes, nitrogen in ammonium ion bears the positive formal charge. But no, the nitrogen in ammonium ion is NOT electrophilic. That would be the hydrogens in the ammonium ion. Those are the ones with the partial positive charge.
Same thing with hydronium ion. Why write one formula "correctly" while leaving the other implying the wrong things - which can be problematic - especially in organic class? Bottom line is that if one uses formal charge to determine nucleo/electrophilicity, one will be screwed over on a regular basis.
Now, I understand his argument for writing the electropositive element first. But this is simply more tradition, some of which he has already damned. He even breaks his own rule: he actually writes:
[itex]NC^{-}[/itex]
rather than
[itex]CN^{-}[/itex]
which he once derided as "lousy" chemistry.
Here, however, the textbook have done it right, according to one of his rules - write the most electropositive element first!
But he still derides this example of formula writing as WRONG because hey, the negative formal charge rests on the carbon! True! And it just so happens that in the cyanide ion, the partial negative charge also rests on the carbon!
But what about the hydronium ion? He insists on writing
[itex]H_{3}O^{+}[/itex]
instead of
[itex]H^{+}[/itex]
because it is chemical fact that the bare proton is hydrated in water solution - that's just Coulombically (very highly) favorable.
True, there is a positive formal charge on the oxygen! But why not write:
[itex]OH_{3}^+[/itex], which keeps even more with structure? This actually shows that the hydrogens on the hydronium ion are electrophilic! If anything, at least
[itex]H^{+}[/itex]
Makes it clear that there is a positive charge on the hydrogen proton and that it is electrophilic!
The point of my post is:
1) I'm about to take this up with my former general chemistry professor. So are my points factually accurate at the very least?
2) Who do you side with?
3) Bottom line: I appreciate his ridding us of those pesky chemical unicorns that populate the pages of general chemistry textbooks but at the same time, he's creating more misconceptions himself. Take ammonium ion: the hydrogens are the ones that are bearing the partial positive charge, and in organic chemistry, no one cares about formalisms! You can think that the nitrogen bears the positive charge but in reality the nitrogen isn't going to be as electrophilic as the hydrogens. And as he says himself, the only thing that matters is experimental fact. Not what one might write down on paper or what one might think. He's not even consistent with his own damn rules (cyanide example).
4) I don't have a personal beef with him; I just want to show him that I've been thinking about this stuff just like he has. His course has been 10% philosophy, 10% myth busting, and 80% good chemistry.
ON FORMULA WRITING
In this writing chemical formulas are used which keep with structure of species. This is of particular importance since structure is the most important property of any chemical species which has covalent bonds (i.e., molecule and polyatomic ion). For example, consider the following common names/formulas (which keep with structure):
hydroxide ion, HO-
sulfuric acid, (HO)2SO2
hydrogen peroxide, HOOH
ammonia, H3N
ammonium ion, H4N+
hydrated aluminum ion, Al(OH2)63+
Likely you have seen the formulas for these species written as:
OH-
H2SO4
H2O2
NH3
NH4+
and Al(H2O)63+, respectively.
However, these “traditional” formulas are wrong with respect to structure and should not be written as such in the chemical literature – tradition be damned!
To illustrate, hydroxide ion is produced by the deprotonation of water (HOH) which cleaves an H-O bond heterolytically to produce H+ and & OH-. In other words, this heterolytic bond-breaking process converts the originally-shared bonding electron pair between H and O into a nonbonding electron pair residing in the valence environment of the oxygen atom thereby creating a “-” charge. This can be readily verified by formal charge analysis (see Ch. 9, p. 11) and is expected since O is much more electronegative than H (see Ch. 9, p. 6). Furthermore, standard formula-writing practice dictates that a chemical formula has the symbol for the more electropositive element written before the symbol for the more electronegative element. Thus, for hydroxide ion, HO-is correct and OH- is grossly incorrect.
Similarly, ammonia should be written as H3N and ammonium ion should be written as H4N+ since N is more electronegative than H and since the nitrogen atom bears the “+” charge created by addition of H+ to H3N.
I have a few problems with the beginning of the text and a huge problem with the bolded statement.
I understand the intentions of my teacher - structure - is highly important in chemistry. Not so much emphasized in general chemistry but very important in organic chemistry (i.e. how do you know whether you're getting a nasal decongestant or an illegal drug?)
However, I find his emphasis on formal charge analysis a bit misguided. He has continually stated that one of his main goals is to ready us for the rigors of organic and analytical chem.
The problem is that the central conceit of orgo is: nucleophile attacks electrophile.
When you write hydroxide ion the way he does, that's okay, because the formal charge - which is nothing more than what its name suggests - a mere formalism - is mostly born by the oxygen (not the hydrogen). Keeping this in mind allows one to understand why, say, potassium hydroxide can absorb carbon dioxide and give a false positive test when testing for carboantes - the nucleophilic oxygen in hydroxide ion attacks the electrophilic carbon in carbon dioxide.
But when he says something like nitrogen bears the "+" charge - that's just creating a chemistry unicorn in place of killing another chemistry unicorn. Yes, nitrogen in ammonium ion bears the positive formal charge. But no, the nitrogen in ammonium ion is NOT electrophilic. That would be the hydrogens in the ammonium ion. Those are the ones with the partial positive charge.
Same thing with hydronium ion. Why write one formula "correctly" while leaving the other implying the wrong things - which can be problematic - especially in organic class? Bottom line is that if one uses formal charge to determine nucleo/electrophilicity, one will be screwed over on a regular basis.
Now, I understand his argument for writing the electropositive element first. But this is simply more tradition, some of which he has already damned. He even breaks his own rule: he actually writes:
[itex]NC^{-}[/itex]
rather than
[itex]CN^{-}[/itex]
which he once derided as "lousy" chemistry.
Here, however, the textbook have done it right, according to one of his rules - write the most electropositive element first!
But he still derides this example of formula writing as WRONG because hey, the negative formal charge rests on the carbon! True! And it just so happens that in the cyanide ion, the partial negative charge also rests on the carbon!
But what about the hydronium ion? He insists on writing
[itex]H_{3}O^{+}[/itex]
instead of
[itex]H^{+}[/itex]
because it is chemical fact that the bare proton is hydrated in water solution - that's just Coulombically (very highly) favorable.
True, there is a positive formal charge on the oxygen! But why not write:
[itex]OH_{3}^+[/itex], which keeps even more with structure? This actually shows that the hydrogens on the hydronium ion are electrophilic! If anything, at least
[itex]H^{+}[/itex]
Makes it clear that there is a positive charge on the hydrogen proton and that it is electrophilic!
The point of my post is:
1) I'm about to take this up with my former general chemistry professor. So are my points factually accurate at the very least?
2) Who do you side with?
3) Bottom line: I appreciate his ridding us of those pesky chemical unicorns that populate the pages of general chemistry textbooks but at the same time, he's creating more misconceptions himself. Take ammonium ion: the hydrogens are the ones that are bearing the partial positive charge, and in organic chemistry, no one cares about formalisms! You can think that the nitrogen bears the positive charge but in reality the nitrogen isn't going to be as electrophilic as the hydrogens. And as he says himself, the only thing that matters is experimental fact. Not what one might write down on paper or what one might think. He's not even consistent with his own damn rules (cyanide example).
4) I don't have a personal beef with him; I just want to show him that I've been thinking about this stuff just like he has. His course has been 10% philosophy, 10% myth busting, and 80% good chemistry.
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