Why is this not NH3? Chad said in his video that Nitrogen is an exception, and would be more basic

[JohnTulisa]

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[Mrhyde]

I think your confused about the "exception" The only exception I can think of is not with bases/acids ranking but with stereo chem. And its called "pyramidal inversion"

http://www.ochempal.org/index.php/alphabetical/o-p/pyramidal-inversion/
http://www.chem.ucla.edu/~harding/tutorials/nitrogen_inversion.pdf

And as far as acids / bases pka ranking , I recommend memorizing this table http://www.masterorganicchemistry.com/2010/06/18/know-your-pkas/

 

[DreamingOfDentistry]

Nah John, i remember Chad saying the the CARDIo rules would lead you astray on this one and I do believe that NH2 would be the strongest base and thus least acidic. On his quiz answer explaining it, he shows you a reaction as to why we should know that it is a stronger base than the C triple bond carbanion. Also the other exception was carboxylic acids were more acidic than Phenols. just a heads up incase you forgot the other exception.

 

[klownzo]

The exception you're thinking of is with a terminal alkyne compared to NH2. NH2 is stronger because with a terminal alkyne and NaNH2, the NH2 is the base in the reaction

 

[tommyngu]

Normally for these types of questions I would look at electronegativity and size. Across a period, acidity follows electronegativity. Down a group, acidity follows size.
But when you have to compare between choices that are both in different periods and groups, I believe you can look to effective nuclear charge (with the lower period number taking priority in case of ties). To calculate effective nuclear charge, just take the atomic number (which is equivalent to the number of electrons) and subtract the number of shielding electrons: Zeff = Z - S. The higher the Zeff, the stronger the acid.
For example, between C and P:
C is higher on the periodic table (it's in a lower period) than P so C must be more electronegative. But P is further to the right than C so P must be more electronegative? So which one is it? Electronegativity/size isn't reliable here. But C has Zeff = 4 and P has Zeff = 5 making P more acidic.
Same thing with O vs P. O is more electronegative but P is bigger. You can't tell which will be more acidic, but with Zeff, O=6 and P=5 so the O is more acidic.
(in order of increasing acidity: CH4<NH3<PH3<H2O<H2S)
Correct me if I'm wrong as this is just an idea.

 

[workofshart]

Ignoring exceptions or w.e. I think this one can just be thought of as common sense. A lone carbon is not going to want to give up a proton and deal with a negative charge. The instability is going to lead it to quickly reprotonate as a strong base and thus weak acid. Don't overthink it.

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[tommyngu]

Ignoring exceptions or w.e. I think this one can just be thought of as common sense. A lone carbon is not going to want to give up a proton and deal with a negative charge. The instability is going to lead it to quickly reprotonate as a strong base and thus weak acid. Don't overthink it.

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With all due respect, I don't think any of this is common sense. Your rationale is based on something whether it be assumptions, the fact that you haven't seen it happen often, or a combination of something along with what you know about acids/bases.
It sounds like you're using the strength of the conjugate base for your reasoning, though.
So going with that, comparing atoms in the same period, base strength is opposite electronegativity. Comparing atoms in the same group, base strength is opposite size.
CH4, NH3, and H2O are all in the same period so that makes CH4 the strongest base.
Between PH3 and H2S, PH3 would be the stronger base. Since NH3 is above PH3 on the periodic table, NH3 would be the stronger base since N is smaller in size. But we've already determined that CH4 was stronger than NH3 so the strongest base between all choices would be CH4.
Even though you arrive at the correct answer following these rules, you wouldn't be able to correctly rank all of them since PH3 is actually a stronger base than H2O. So that leaves me back at my previous post...

 

[DreamingOfDentistry]

I think this one is as straightforward as deprotonating it to get the conjugate base, and the less electronegative the atom is with the negative charge, the better the base, thus C would be the least electronegative and therefore the stronger the base and than the weaker acid.

 

[clutchfans]

CH3- is not stable at all

 

[DreamingOfDentistry]

it would form CH3- when deprotonated, not CH3+

 

[clutchfans]

it would form CH3- when deprotonated, not CH3+

that's why i edited my post

 

[tommyngu]

I think this one is as straightforward as deprotonating it to get the conjugate base, and the less electronegative the atom is with the negative charge, the better the base, thus C would be the least electronegative and therefore the stronger the base and than the weaker acid.

How do you know which atom is the least electronegative from this list? If you look at a Pauling scale, you'll find:
C=2.55
N=3.04
O=3.44
P=2.19
S=2.58
Which shows P would be the least electronegative here.

 

[tommyngu]

Anyone wanna chime in on this? I'd like to get learned