acid base question

[morrisol]

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i was just wondering why in this reaction ch3oh is the acid and nanh2 is the base?

ch3oh + nanh2 <-> ch3o- +na + nh3

 

[sapientnarwhal]

Methanol is a weak acid and sodium amide is an extremely strong base. Note that the pKa of NH3 is 38 (making this an extremely weak acid), so in the rxn NH3 + H2O <-> NH2- + H3O+, the conjugate base of NH3 is very strong. Remember a weak acid has a stronger conjugate base and vice versa.

 

[morrisol]

Methanol is a weak acid and sodium amide is an extremely strong base. Note that the pKa of NH3 is 38 (making this an extremely weak acid), so in the rxn NH3 + H2O <-> NH2- + H3O+, the conjugate base of NH3 is very strong. Remember a weak acid has a stronger conjugate base and vice versa.

how would u figure that out without knowing the pkas? thats whats confusing me

 

[sapientnarwhal]

More of an inference question, which would be more stable: CH3O- and NH3, or NaNH 1- and CH3OH2 +? Oxygen can stabilize the negative charge and is more electronegative, and ammonia is formed which is stable (also recall that you really only see ammonia and ammonium acid base rxns and rarely sodium amide). Oxygen has low affinity for and is unstable with + charge, N is unable to accommodate essentially a -2 charge.

Alcohols are typically always weakly acidic.
If you see an anionic salt as a reactant in an acid/base rxn, it's more than safe to assume it is the base.

 

[morrisol]

More of an inference question, which would be more stable: CH3O- and NH3, or NaNH 1- and CH3OH2 +? Oxygen can stabilize the negative charge and is more electronegative, and ammonia is formed which is stable (also recall that you really only see ammonia and ammonium acid base rxns and rarely sodium amide). Oxygen has low affinity for and is unstable with + charge, N is unable to accommodate essentially a -2 charge.

Alcohols are typically always weakly acidic.
If you see an anionic salt as a reactant in an acid/base rxn, it's more than safe to assume it is the base.

oh so it has to do with electronegativity

 

[sapientnarwhal]

Yes to a certain extent, Eneg is the simplest way to explain this but there is a more complicated explanation.

Forgot to mention that the sodium methoxide salt is definitely more stable than the NaNH1- thing (don't even know what to call it).

You should already know that ammonia is basic/nucleophilic, so losing a proton would substantially increase its basicity. You should also know that alcohols are weak acids with strong conjugate bases. This + knowledge that titrations must involve strong acid or base => solve < 5 seconds.

 

[morrisol]

Yes to a certain extent, Eneg is the simplest way to explain this but there is a more complicated explanation.

Forgot to mention that the sodium methoxide salt is definitely more stable than the NaNH1- thing (don't even know what to call it).

You should already know that ammonia is basic/nucleophilic, so losing a proton would substantially increase its basicity. You should also know that alcohols are weak acids with strong conjugate bases. This + knowledge that titrations must involve strong acid or base => solve < 5 seconds.

im just starting to learn acids and bases again so im starting from scratch so i dont know much of that stuff yet

 

[CommyO]

You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases

Acids donate proton
Bases accept the proton

methanol is deprotonated to alkoxide, therefore it is a acid
sodium amide is protonated to ammonia, therefore it is a base

You can also recall:
pKa for alcohols is ~15.7
pKa for amine is ~40
pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction

 

[sapientnarwhal]

You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases

Acids donate proton
Bases accept the proton

methanol is deprotonated to alkoxide, therefore it is a acid
sodium amide is protonated to ammonia, therefore it is a base

You can also recall:
pKa for alcohols is ~15.7
pKa for amine is ~40
pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction

No I would disagree. If OP were only given sodium amide and methanol, how would they determine which is the acid and base? The reaction provided by OP in first post is the answer to a question that was asking for the products of a rxn b/w the NaNH2 and CH3OH.

 

[morrisol]

No I would disagree. If OP were only given sodium amide and methanol, how would they determine which is the acid and base? The reaction provided by OP in first post is the answer to a question that was asking for the products of a rxn b/w the NaNH2 and CH3OH.

You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases

Acids donate proton
Bases accept the proton

methanol is deprotonated to alkoxide, therefore it is a acid
sodium amide is protonated to ammonia, therefore it is a base

You can also recall:
pKa for alcohols is ~15.7
pKa for amine is ~40
pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction

thanks. but if i wasnt given the pkas and on the exam couldnt remember what they were i would look at the equation and think "ch3oh and nanh2 have H" so how would i know which does which job? i just want to figure out how i can answer this question if i forget certain concepts or whatnot on the exam.

 

[CommyO]

thanks. but if i wasnt given the pkas and on the exam couldnt remember what they were i would look at the equation and think "ch3oh and nanh2 have H" so how would i know which does which job? i just want to figure out how i can answer this question if i forget certain concepts or whatnot on the exam.

If you weren't given the products like the other guy said, you might have to think about the valence electrons and how the NaNH2 will readily dissociate into their respective ions Na+(aq) and NH2-(aq).

From there, we can say that the only way the methanol would be the base is if the alcohol was protonated to water which would be a good leaving group and yield water. That is very unlikely considering you have a NH2- ion ready to accept the proton

 

[morrisol]

If you weren't given the products like the other guy said, you might have to think about the valence electrons and how the NaNH2 will readily dissociate into their respective ions Na+(aq) and NH2-(aq).

From there, we can say that the only way the methanol would be the base is if the alcohol was protonated to water which would be a good leaving group and yield water. That is very unlikely considering you have a NH2- ion ready to accept the proton

also, why does Cl- have no affinity for H+ in this reaction
NH4Cl (s) + h2o--> nh4+ + cl-

 

[sapientnarwhal]

Cl- has achieved its most stable electron configuration (noble gas). In other words, it is a poor nucleophile. Review solutions and solubility rules.

btw this is a decomposition rxn, a salt dissolving in water.

NH4Cl <=> [NH4+] + [Cl-]

ammonium salts are highly soluble in aqueous solutions

HCl completely dissociates in water, best example of the stability of Cl-

 

[morrisol]

Cl- has achieved its most stable electron configuration (noble gas). In other words, it is a poor nucleophile. Review solutions and solubility rules.

btw this is a decomposition rxn, a salt dissolving in water.

NH4Cl <=> [NH4+] + [Cl-]

ammonium salts are highly soluble in aqueous solutions

HCl completely dissociates in water, best example of the stability of Cl-

thanks. if you dont mind i have a few questions about how best to approach certain things that i would love opinions on. there are certain concpts that i tend to struggle the most on.

1) how do i know when im dealing with an acid/base or redox reaction separately
2) how do i know if a reaction is multiple steps or not
3) do salts only form when you have a strong acid and strong base reacting? and if not how do i know when a salt forms and how do i identify a salt?
4) also how do i know if something will hydrolyze?
5) if a compound is a weak acid or base then how can it completley dissociate in water?

i know some of this stuff i will learn better when doing orgo but if you had any suggestions that would be great.

 

[morrisol]

whenever you see sodium in a compound, dissociate that molecule and focus on the anion that results. here, it's NH2-

comparing NH2- to CH3OH in solution, think about which one wants a proton more, or in other words think about which one would be more stable holding a negative charge.

since O is more electronegative than N, O will carry a negative charge and N will hold the proton

thanks so much! this is the kind of answer i was looking for! much appreciated!!!

 

[morrisol]

no problem buddy

can i ask you one more question.
when do you use an ICE table? i can never seem to know when to use one and when i dont need to use one

 

[sapientnarwhal]

thanks. if you dont mind i have a few questions about how best to approach certain things that i would love opinions on. there are certain concpts that i tend to struggle the most on.

1) how do i know when im dealing with an acid/base or redox reaction separately
2) how do i know if a reaction is multiple steps or not
3) do salts only form when you have a strong acid and strong base reacting? and if not how do i know when a salt forms and how do i identify a salt?
4) also how do i know if something will hydrolyze?
5) if a compound is a weak acid or base then how can it completley dissociate in water?

i know some of this stuff i will learn better when doing orgo but if you had any suggestions that would be great.

These are topics you should learn on your own time or with a tutor. Like I said, review this content and do practice problems. If you still have questions we are happy to answer.

I can send you excellent content review materials if you PM me.