Quick Orgo Question

[Dentman1]

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What is more basic CH3NH2 or C6H5NH2?

What is more acidic CH3OH or C6H5OH?

On other words, how does chain lenght affect acidity or basicity?

 

[Philippines03j]

Dont quote me on anything but these are my thoughts.

First look at what is attached. In this case we have a CH3 group (sp3 hybridization) and C6H5 (Sp2 hydbridized). We then look to see which group has the best electron donating group. The stronger the donating group the more basic it will be. The stronger the withdrawing group the more acidic it will be.

In this case we have an aryl group attached which is a weaker donating group than the alkyl group. I believe the ch3Nh2 should be more basic than the c6H5Nh2.

The c6h5oh(Pka=10) should be more acidic than the ch3oh (Pka about 16).


let me know if that makes sence.

 

[loveoforganic]

This doesn't have as much to do as chain length as it does resonance. The lone pair of the nitrogen in aniline can play into the conjugation, delocalizing it, and thus decreasing the basicity relative to methylamine.

Same thing for phenol - upon deprotonation, the anionic lone pair can play into the conjugation of the ring, delocalizing the charge, and thus increasing the stability of the conjugate base and thus increasing the acidity of phenol relative to methanol.

 

[DrSpeed]

What is more basic CH3NH2 or C6H5NH2?

What is more acidic CH3OH or C6H5OH?

On other words, how does chain lenght affect acidity or basicity?

first, when I think these through I put everything in terms of acidity and then flip the answer if is a base question.

Try reasoning it out this way: look at the carbon attached to the acid; the more electronegative it is the more the hydrogen is attracted to it and therefore the more difficulty that hydrogen has in dissociating. the harder time the hydrogen has leaving, the fewer there will be dissociated at equilibrium therefore the weaker the acid and visa versa.

Now look at whats attached to that carbon. Is it an electron donating group or an electron withdrawing group (think back to benzene activation and deactivation. most of these questions will have either an activator or deactivator being compared)? If its electron donating, the carbon is more electronegative, the hydrogen has a harder time dissociating, and its a weaker acid (flip side makes it a stronger base).

Now apply that to the above...

 

[DrSpeed]

I had to go look it up... from McMurry:

"Phenols are more acidic than alcohols because the phenoxide anion is resonance stabilized. Delocalization of the negative charge over the ortho and para postions of the aromatic ring (ie, it withdraws electrons from the carbon that the acid is attached to) results in increased stability of the phenoxide anion relative to undissociated phenol. Substituted phenols can be either more or less acidic that phenol itself. Phenols with an electron withdrawing group are generally more acidic because these substituants delocalize the negative charge where those with donating groups are less acidic because the concentrate the charge."

 

[Dentman1]

Ok so what if it was comparing CH3NH2 and C2H5NH2, which is more basic?
and CH3OH and C2H5OH, which is more acidic?

 

[DrSpeed]

Ok so what if it was comparing CH3NH2 and C2H5NH2, which is more basic?
and CH3OH and C2H5OH, which is more acidic?

same reasoning...
look at the carbon in methanol. it only has hydrogen attached to it and hydrogen is neither donating or withdrawing (again refer to benzene. the H's attached to the ring have zero effect in considering OMP activation).

Now look at the carbon next to the alcohol in ethanol. it has an electron donating group attached to it, therefore carries a slightly higher negative charge compared to the corresponding carbon in methanol. this higher negativity means it holds the hydrogen tighter, therefor fewer H's dissociate at equilibrium, therefor a weaker acid.

If you check the pKa's methanol = 15.5 and ethanol = 15.9.

Now apply that logic to CH3NH2 and C2H5NH2 and flip it you can see that ethylamine is the stronger base. Again, if you check pKa's: methylamine = 10.64 and ethylamine = 10.70

http://www.zirchrom.com/organic.htm is a great source if you want to check pKa's