Ortho-fluoro aniline and pH reactions

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Dhamnekar Winod
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Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

Consider ortho-fluoro aniline (o-FC6H4NH2).

a.Give an electron-dot formula for this compound and indicate with an arrow which atoms can accept a proton. Circle the atom that actually does accept a proton from water.

b.Determine whether o-FC6H4NH2 is a stronger or weaker base than aniline itself (C6H5NH2) and explain why this is the case.

c.Calculate the pH of a 0.1 M solution of o-FC6H4NH2.

d.Draw the structural formula of the conjugate acid of o-FC6H4NH2.

e.Calculate the pH of a 0.010 M solution of the conjugate acid of o-FC6H4NH2.

What are answers to all these questions? How to answer all these questions? Let me answer all these questions one by one.
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

Answer to a.
Chemical structure of ortho-fluoro aniline is as follows:

Image Nitrogen atom can accept proton from water.

Answer to b.

o-FC6H4NH2 is stronger base than C6H5NH2 because F can act as base (electron pairs donor)
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

Both are wrong . What you draw is para fluor aniline not the ortho one.
And anilin pka 4,6 is stronger base as the fluoraniline pka 3,2. Reason strong - I effect of flourine, M - effect is miner.
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

Are Yoy talking about mesomeric effect and inductive effect of Fluorine?

Image Is this correct chemical structure drawing for ortho-fluoro aniline?
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

Now it is correct. Yes i talk about inductive and mesomeric effect.
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

Fluorine atom will accept proton from water in ortho-fluoro aniline.

You said pKa of the o-fluoro aniline is 3.2, so its pH is 3.2 as well.

Now, how to answer part d and e?
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

Wrong. The only atom which accept proton will be nitrogen to form the ammonium.
The calculation is similar the first exercise with the trimethylamin.
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

Let me answer part e.
Conjugate acid of o-FC6H4NH2 is o-FC6H4NH3
Reaction equation is FC6H4NH2 + H2O → FC6H4NH3⁺ + OH⁻

So, 1.6E-11=x*x/0.01M → x= 4.0e-7, Hence pOH=-log(4.0e-7)= 6.4. so, pH=14.0-6.4=7.6 Is this computaion correct?
Last edited by Dhamnekar Winod on Mon Mar 22, 2021 4:53 am, edited 1 time in total.
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

Where does the 0,00625 coming from. pH should be alkaline because its a amin
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

I edited the answer. Please check it out.
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

I dont know what you doing, where this numbers come from. pKa is 3.2 already
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

If pKa is 3.204, then ka= 0.000625, this means 0.000625* 0.01= x * x , → x= 0.0025 Now is this hydroxide ion concentration? If it is so, then poH=-log(0.0025)=2.602, this means pH= 14.000 -2.602=11.398 Is this computation correct?
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

Looks better now
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Re: Ortho-fluoro aniline and pH reactions

Post by Dhamnekar Winod »

How to use pH solver available on webqc.org to answer this question?
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Re: Ortho-fluoro aniline and pH reactions

Post by ChenBeier »

The given pKs is 3.2 for FC6H4NH3+ , for the solver has to be used the corresponding pKb value of 10.8.
pH about 8.1
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