weak acid vs. weak base

[PlatinumPenny]

I don't understand why HCN is a weak acid but H3CNH2 is a weak base. I understand that since HCN is not on the list of strong acids it must be a weak acid. But I really dont' get why H3CNH2 is a weak base. I guess I assume since it has CN in it it will also be a weak acid like HCN.

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[Arctangent]

In HCN, the carbon is triple bonded to the nitrogen, and the hydrogen is singly bonded to the carbon. Since nitrogen is more electronegative than carbon, it can draw away electron density from carbon, which favors a deprotonation. If you've learned of alkynes in chemistry, you may know that likewise, with two triply bonded carbons, there is enough electron density so that a carbanion (deprotonated carbon) can more readily form.

For H3CNH2, I usually just recognize that R-NH2 is an amine group. Another weak base of the form R-NH2 is ammonia, where the R group is the hydrogen. If you compare amine groups to ammonia, it may help you remember that given nitrogen's lone pair, it can somewhat readily accept a proton.

So as for the overall difference between the two, I would look to recognize that in HCN, the proton is attached to a carbon attached to a fairly electronegative atom, whereas for H3CNH2, the nitrogen group represents an amine group, which is in general a weak base.

 

[PingPongPro]

To put it simply... H3CNH2 is a weak base because it has a lone pair on the N that can be donated. Therefore it acts as a lewis base.

HCN is a weak acid on the otherhand. You have to realize the bonds between CN in H3CNH2 and HCN are different. HCN has a triple bond between C and N (satisfies octet rule). It can act as an acid because it donates H+ and its conjugate base is CN-.

 

[plzNOCarribbean]

I don't understand why HCN is a weak acid but H3CNH2 is a weak base. I understand that since HCN is not on the list of strong acids it must be a weak acid. But I really dont' get why H3CNH2 is a weak base. I guess I assume since it has CN in it it will also be a weak acid like HCN.

HCN is a weak acid because the Sp hybridized carbon is similar to an alkyne, which has a pka= 25, which is a VERY weak acid. Nontheless, alkynes are still acidic and can be deprotonated to form acetylides, which is the same thing here but you just have to recognize that they are weak acids since their conjugate bases are rather strong ( negative charge on a electropositive carbon is rather strong/reactive)

as for the H3CN2H, this is a WEAK base. because it is the conjugate base of ammonium or any other nitrogen with a positive charge. A nitrogen atom that has a (+) formal charge is weakly acidic (pka = around 9), and so its CB, a nitrogen with a 3 bonds and a lone pair is a weak base. Yes, you can have an acid that is weak and still have its CB be weak. (remember, the saying is the STRONGER THE ACID, the WEAKER THE C.B) but this does not mean that we cant have a weak acid/weak base conjugate pair.

lastly, to clarify, it makes sense that a N with a lone pair is a weak base. Since the compound is not charged, then it is relatively stable and wont act as a strong base. Strong bases are always charged, and generally have negative charge on a not so electronegative atom (ie this is why carbanions act as such strong bases). hope this helps

 

[PlatinumPenny]

WOW guys and gals I would have NEVER considered any of the points you are making. Its gonna take a bit for me to absorb it all but thanks a bunch!!