Ranking acid problem

[tdkyun]

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Hey guys, I think my brain is fried😴 so I need your help. I have DAT in 2 days and I guess I was worried, I just couldn't fall asleep (finally fell asleep around 5:30am).

1) CH3OH
2) CH3OH2+
3) CH3NH2
4) CH3NH3+

I need someone to rank these species in the order from the strongest acid to the weakest acid. And please give brief explanation to support your conclusion. Thanks.

 

[pbure9]

I'm pretty sure the order would be 2>4>1>3.

First of all, anything with a positive charge is a stronger acid, because its conjugate base is a neutral/stable base. Also, its helpful to know acidity increases from left to right and down a group. And finally, CH3OH is more acidic than CH3NH2, because the conj bases have the negative charge on the O and the N respectively. O is more electronegative, therefore prefers the negative charge > nitrogen making CH3O- the more stable conjugate base and thus, CH3OH the stronger acid.

Hope thats right and helpful.

 

[tdkyun]

Thank you.
Here is another one.

Can someone rank them too?
Thanks.

 

[nze82]

Thank you.
Here is another one.

Can someone rank them too?
Thanks.

Electron withdrawing groups (in this case NO2) that are attached to the ring increase the acidity of the compound (decrease basicity). I think you can rank them as follows:

B>A>C>D

The reason why B is the most basic is that, when it picks up a proton, the + charge will be delocalized on the ring via resonance, stabilizing the compound. A comes next, because it doesn't have any electron withdrawing groups attached to the ring. However, A is less basic than B, because the + charge cannot be delocalized on the ring, due to the presence of the intervening -CH2-. Finally, C is more basic than D, because it only has one electron withdrawing group attached to the ring.

 

[192LT192]

Electron withdrawing groups (in this case NO2) that are attached to the ring increase the acidity of the compound (decrease basicity). I think you can rank them as follows:

B>A>C>D

The reason why B is the most basic is that, when it picks up a proton, the + charge will be delocalized on the ring via resonance, stabilizing the compound. A comes next, because it doesn't have any electron withdrawing groups attached to the ring. However, A is less basic than B, because the + charge cannot be delocalized on the ring, due to the presence of the intervening -CH2-. Finally, C is more basic than D, because it only has one electron withdrawing group attached to the ring.

i think its A>B>C>D bc when you make resonance around the ring in B, it actually is ruinining the aromaticity of the ring. so A is the best base bc aromaticity is not affected. Resonance works with ACIDITY bc it leaves a negative charge (aka 2 lone electrons) on one C in the ring which still gives 6 pi electrons which therefore maintains aromaticity. with basicity, aromaticity is ruined with resonance so its weaker. and then adding elecron withdrawing grps actually makes it worse so those are even weaker.

 

[loveoforganic]

ABCD is right, but not really for the reason you gave. The lone pair on an aromatic amine can resonate into the benzene, decreasing the localization of the lone pair, and thus the basicity. Thus, the alkyl amine is more basic than the aryl amine.

 

[pbure9]

i think its A>B>C>D bc when you make resonance around the ring in B, it actually is ruinining the aromaticity of the ring. so A is the best base bc aromaticity is not affected. Resonance works with ACIDITY bc it leaves a negative charge (aka 2 lone electrons) on one C in the ring which still gives 6 pi electrons which therefore maintains aromaticity. with basicity, aromaticity is ruined with resonance so its weaker. and then adding elecron withdrawing grps actually makes it worse so those are even weaker.

I'd have to agree here. First of all aromaticity increases acidity. Also, if you protonate the aniline and benzylamine, you see that you actually have more resonance forms for A, whereas in the protonated aniline, the Nitrogen already has 4 bonds and you can't resonate outside the ring. So in A, your conj acid is weaker, making it the stronger base. Have to agree with 192LT192

 

[ttran9229]

I'd have to agree here. First of all aromaticity increases acidity. Also, if you protonate the aniline and benzylamine, you see that you actually have more resonance forms for A, whereas in the protonated aniline, the Nitrogen already has 4 bonds and you can't resonate outside the ring. So in A, your conj acid is weaker, making it the stronger base. Have to agree with 192LT192

I agree with the order, but not 100% with the explanation. Unlike acid strength comparisons, base strength comparison is based on the stability of the base itself. If the base is stable, then it will be less reactive and therefore is a weaker base. The problem clearly tests one's knowledge of resonance although other factors affect basicity. In order of their effects, these include resonance, induction, strain, sterics, electronegativity, hybridization, conjugation, aromaticity, and environment.