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Look in the table of pKas.
Yes @Preetha is right. You can find them online or in your textbook, but if you need or want specific ones, i can give you a table once I make it of the common pKa's that I used in my organic class that helped me in class and on the ACS exam for organic chemistry.
I have to compare..
These are some values of some of the pKa's that I found to be most important and useful during my organic series. I highlighted the acids to help you notice which acids are strong and which ones are weak. I HIGHLY suggest that you memorize the pKa of water as it is commonly used in o-chem often and just knowing it's pKa will help you determine where the reaction lies, and whether it would be appropriate to do a reaction and so forth.
I also highlighted a carboxilic acid. I think you might get into that one in the last part of organic, but it helps to know it as you end ochem 1 with alcohols, gringards, and oxidative reactions. Best of luck!
As far as your question, the way I look at is is resonance. Secondly, you want to see if there are more carbons. If you notice on the pKa table that I attached, carbons tend to have more basic pKa values. Secondly, I would notice the resonance. Benzenes are pretty darn stable molecules. It has complete resonance around it's whole structure. This makes the structure more stable, loweing its potential energy. Less likely to want to want to react due to it's stability.
So what is the Pka of benzene?
more no.'s of carbon= less Pka?
Since benzene is merely all carbons and hydrogens with high stability, I would assume its w/ in the same range as other carbons. If I remember correctly, benzene pKa = 43.
Yes. The more carbons, the more basic your molecule is.
more no.'s of carbon= less Pka? can you go a little more detail in this perhaps? little confused..
No. Remember, the lower the pKa the more acidic it is. The higher the pKa the more basic it is. Compounds like NH2 and OH- are highly basic.
You can base it off water ~ 15.7 (pH of about 7)
For instance: |dw:1359709156170:dw| Seeing as how NH2 is REEEEALLY high due to it's instability, the reaction, by just looking @ pKa's you know will proceed towards the product side. You have a 25 and a 38.
Usually it goes this way: If you have to compare the relative strength of acids, you need to compare the stability of their conjugate bases. Form the conjugate base of each of your acid, then look out for resonances. If their are resonances, the base is stable ie, easy to form, so the acid will lose easily its proton: stronger acid. If there are no resonances in the base, the acid is weaker.
Hehe! Privilege of old age and experience ;-)