November 13, 2019, 04:18:48 PM
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Topic: Hydrolysis of dialkylaminoethyl ethers  (Read 432 times)

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Online rolnor

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Hydrolysis of dialkylaminoethyl ethers
« on: June 29, 2019, 08:38:24 AM »
I am trying to explain why it is so hard to hydrolyze a dialkylaminoethyl ether with proton-acids. The explanation I got from hollytara is that the first positive charge on nitrogen that forms repelles a second charge on the neighboring ether-oxygen. To visuallize this I have studied a related system, diaminoalkanes. In this system the first pKa is much higher then the second in ethylenediamine,  going to longer diaminoalkanes, propylene diamine, butylene diamine, pentane diamine and hexamethylene diamine this effect goes down. The explanation is the same, the first charge on nitrogen makes it harder for a second charge to form if the nitrogens are close. I have plotted 10exp (pKa1-pKa2) against the number of methylene groups in the amines, going from n=2 to n=6 and the correlation is very clear. I want you input in this, can you see any error in my reasoning?
 The scale on the y-axis is logarithmic.
« Last Edit: June 29, 2019, 10:13:00 AM by rolnor »

Offline hollytara

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #1 on: July 01, 2019, 05:19:33 PM »
That looks reasonable - an electric field weakens as the square of the distance through space, but there is also a through bond component that probably weakens more rapidly. 

Online rolnor

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #2 on: July 02, 2019, 08:47:45 AM »
Yes, the correlation is not simply x=y2 and an inductive effect is probably contributing. The molecules are also not straght chains but a mix of conformers, each contributing to the avarage distance between the twoo charged nitrogens. It would be very intersting to se the pKa2 for hydrazine that would be n=0, the effect is probably very big if my model is correct. Excel fails to make a good curve-fitting, the regression analysis must probably consist of a mix of logarithmic and exponential parts.

Online rolnor

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #3 on: July 02, 2019, 09:27:55 AM »
I found the valeus for hydrazine, pKa1 is 8,0 and pKa2 is -1,0. I put it in my model and it fits beautifully;


Offline pgk

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #4 on: July 02, 2019, 10:27:38 AM »
Indeed, alkyl length increases the acid strength (conjugate acids like protonated amines, included) and decreases the basicity of bases, due to the positive induction effect.
But in case of dialkylaminoethyl ethers cleavage, the problem is also kinetic because it is a consecutive reactions route that follows three steps.
1). Ionization of the hydracid but this is instant and doesn’t count in reaction kinetics.
2). Protonation of the oxygen atom, which is also fast but reversible.
3). Attack of the halogen anion to the carbon that is in α-position of the protonated oxygen.
Already, amine group consumes one molecule of the hydracid and decreases the protons concentration.
But this is not all. Hydracids are strong acids with negative pka and thus, Henderson-Hasselbach equation is not applicable. So, the protons concentration must be calculated by taking into account the acid dissociation, as well as the water auto-ionization and the protonated amine ionization; which means that the concentration of halogen anions that attack to the diprotonated dialkylaminoethyl ether, is quite lower than the protons concentration, whatever that means to the overall reaction kinetics.
In other words, dialkylaminoethyl ethers cleavage is quite slow and needs more heat than usual, in order to be adequately accelerated.
« Last Edit: July 02, 2019, 10:46:06 AM by pgk »

Online rolnor

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #5 on: July 02, 2019, 11:02:16 AM »
Here I am comparing pKa1 and pKa2 in diamines and the very large effect when n=2 (ethylenediamine) and n=0 (hydrazine) is comming from part Coloumb repulsion and inductive effect. If pKa in mono-amines were plotted against chainlength the effect you are mentioning would be revealed pgk. n=1 is not possible to meassure because the diaminomethane is not stable.

Offline pgk

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #6 on: July 02, 2019, 12:59:39 PM »
My reply was an answer to the question: “why it is so hard to hydrolyze a dialkylaminoethyl ether with proton-acids?”

Online rolnor

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #7 on: July 02, 2019, 04:15:57 PM »
OK pgk! Do you have any comment on the correlation on the diagram?

Offline pgk

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #8 on: July 03, 2019, 11:15:12 AM »
CORRECTION:
Alkyl length decreases the strength of carboxylic acids and increases the basicity of aliphatic amines, due to the positive induction effect of the methylene groups.
I apologize for possible confusions.

Offline pgk

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Re: Hydrolysis of dialkylaminoethyl ethers
« Reply #9 on: July 03, 2019, 11:15:52 AM »
As mentioned above, alkyl length increases the basicity of aliphatic amines, due to the (cumulative) positive induction effect of the electron donating, methylene groups. But amine group is also an electron donating group with positive inductive properties that additionally increases the basicity of both amines of aliphatic diamines, together with the length of alkyl chain; which is observed as a decreasing order of pka values difference, within alkyl chain length. 
Furthermore and especially for hydrazine, the α-effect of the one amine group might also play an important role to such a high pka difference.
Alpha effect
https://en.wikipedia.org/wiki/Alpha_effect
« Last Edit: July 03, 2019, 11:29:59 AM by pgk »

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