In pubchem, i found the pKa of DMF is -0.3. However it is a strong base, which means its pKa value should be high (>16).
Hi
Yes it is correct. DMF is a derivative of formamide, the amide of formic acid.
Amides (known as acide amines) are not basic as amines, DMF is not a strong base, the pH of a 0.5 M DMF solution is 6.7
Best regards,
Thnaks Gilles.
What made me confused is, the pKa is indicative of concentration of disscoiated relative ti undisscociated species. So when DMF has pKa of -0.3 it means it dissociates giving rise protons and conjugate (DMF without H+). However as you mentioned the pH of 0.5 M solution of DMF is 6.7. could you clarify more to abolish my misunderstanding.
Hi Muhammad,
Surely what could be happening is that below pH = -0.30 the carbonyl group of DMF is protonated by hyperacidic media and thus this solvent is acting as a cationic acid. After pH = -0.30 this proton is released and it therefore, along pH values from 0.0 to 14.0 at 298.2 K DMF is normaly acting in neutral way.
Best regards
Dear Muhammad,
DMF is a neutral lewis base, in aqueous solution, DMF is protonated according to the relation:
CHO-N(CH3)2 + H+ -----> CHOH=N+(CH3)2
As pKa of (H2O/OH-) is greater than pKa of (CHO-N(CH3)2 / CHOH=N+(CH3)2) the equilibrium is displaced in the way CHOH=N+(CH3)2-----> CHO-N(CH3)2 + H+
DMF is a polar aprotic solvent, there are no H+ transfer from the base to the acid and so no dissociation giving rise protons...
I have another question related to it. Plaese clarify if possible.
If there is a solution of aniline in DMF and if we add HCl to it which will be protonated first DMF or aniline?
Kalpeshkumar,
Similar to in aqueous solvents, the most basic (least acidic) group will be protonated by HCl, provided the reaction is downhill.
HCl is a difficult acid to give you an answer for because the acidity of HCl in DMF will depend on the amount of water present.
Lets take triflic acid instead, which is a very strong acid and will be able to protonate both DMF and Aniline;
The pKa of protonated DMF in DMF is, by definition, 0, similar to the pKa of H3O+ in H2O being 0. This is how pKa scales are usually referenced, with the pKa scale being defined by the free energy change of the following reaction in DMF;
2 DMF -> [DMF-H]+ + [DMF]-
This reaction is much less favorable than the equivalent reaction for water which brackets the pKa scale to 14;
2 H2O -> [H3O]+ + [OH]- (Keq = 10^-14).
This is generally why nonaqueous solvents have such larger pKa scales. In DMF the pKa scale is >20, from what I recall.
Going back to your question, the protonation of a mixture containing excess aniline and DMF relative to HOTf will only protonate the aniline after equilibrium has been acheived. This is because the pKa of aniline is ~ 2-3 in DMF, whereas the pKa of protonated DMF is 0. Even if DMF is initially protonated, the reaction
[DMF-H]+ + Aniline -> DMF + [Anilinium]+ (Keq = 10^2-3)
Will be quite favorable.
For nonaqueous pKa scales and more information, I would recommend the works of Izutsu
ftp://nozdr.ru/biblio/kolxo3/Ch/Izutsu%20K.%20Electrochemistry%20in%20nonaqueous%20solutions%20(Wiley,%202002)(ISBN%203527305165)(O)(352s)_Ch_.pdf
Cheers
If pKa is negative it means the acid form of the conjugated pair (HDMF+, protonation at the nitrogen) is a strong acid, i.e. it does not exist in water. You woulds need to go to negative pH values [H+] larger than 1M to protonate DMF. In other words, the conjugated base, DMF, has a non-basic character in water.
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