DAT Ochem question

[panmit]

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Which of the following is the least acidic

CH4

PH3

H2S

NH3

H2O

I get that if you follow the CARDIO mnemonic, CH4 would be the least acidic because it has the most unstable conjugate base. However, does that mean CH4 is more basic than NH3? That doesn't make any sense.

 

[jwan14]

Can CH4 even be considered as a base? By the definitions of a base (proton acceptor and electron donor) it cannot be because it can't form another bond to anything.

 

[jwan14]

At least I have never EVER seen any molecule with 5 bonds to a C or a C with 4 bonds and a lone pair. Could be wrong but even if I was the DAT won't test that haha.

 

[panmit]

No I don't think it can be considered a base. I was just trying to reason out qvaults explanation. Weaker acid means stronger base, so if CH4 is a weaker acid, then shouldn't it be a stronger base than NH3?

Out of the possible choices, NH3 is the strongest base, so I thought it would be the weakest acid, but CH4 turned out to be the answer

 

[discensdentibus]

that is a dangerous way to reason through a question like that OP

This, will get you on the right track of thinking:
Bronsted: a base is a proton acceptor; an acid is a proton donor
Lewis: A base is an electron pair donor; an acid is an electron pair acceptor (i.e an electrophile)

CH4 is already making 4 bonds. It has no electrons pairs to donate and it cannot accept a proton. Ammonia? Hes got an electron pair; he (or she) can accept a proton. CH4 is not a base whatsover. The conjugate BASE, however, would be a -CH3 species. This thing is pretty much illegal it would be so basic. Look over grignard reagents. Using a simple mnemonic won't work 100% of the time.

Carbon does not make 5 bonds; this is probably the biggest no no in all of skeletal structure drawing. It can exist as a carbene and make 2 bonds and bear a lone pair. Possibly look over your acids and base chapters in your ochem textbook.

 

[panmit]

Thanks, yeah I know its the wrong way of thinking. I was just trying to extrapolate from datqvaults explanation. I read more about it on masteroforganicchemistry and I understand now