Which of the following has least acidic hydrogen???
Answers
HELLO THERE!
How to determine the acidity of Hydrogen?
Note: An acid is one which donates a proton. After a proton is donated, the structure formed is called as its conjugate base. More stable is the conjugate base, more will be the tendency for the acid to donate it's H⁺, and hence, more acidic will be the acid.
Now, when it comes to determining the acidity of Hydrogen, you must see to what atom the hydrogen is attached. If the atom is more electronegative, it attracts the shared pair of electron towards itself, and hence H⁺ goes out easily, making the acid more acidic.
In the given compound, the Hydrogen atom in all options are attached to carbon, which is attached to S and O.
We know, that electronegativity of O > S.
So, O pulls the shared pair of electrons from the carbon with more power than S. In turn, there is a development of positive charge on the carbon atom, which thus attracts the electron cloud from the hydrogen atom.
Consider Option (1): Two O atoms are attached to the C atom. Hence, there will be a greater pull on the electron cloud of C atom in this compound, hence H⁺ will be donated easily. Thus, in option (1) , the hydrogen is the most acidic.
Consider Option (2): Here, we have one O and one S atom surrounding the Carbon. Since Electronegativity of S < O, in this case, the pull on electron cloud of Carbon will be much lesser than that in Option (1). Thus, the Hydrogen in this case is less acidic than Option (1). In fact, electronegativity values of carbon and sulphur are same (2.5), so there will be no effective pull on electron cloud.
Consider Option (3): Here, we have two S atoms surrounding the carbon atom. Again, electronegativity of S < O, so in this case, the pull on electron cloud of Carbon by S will be far less than that in Option (1) and (2). Thus, the pull on electron cloud of H atom by C atom will also be less, due to which the H⁺ ion can't escape easily. Hence, Hydrogen atom in this case is lesser acidic than Options (1) and (2).
The last option, Option (4): Here, two S atoms and a methoxy group is attached to the carbon atom. Methoxy group has negative inductive effect, i.e., it pulls the electron cloud of Carbon towards itself, and the two S atoms are also attached. Since an additional electronegative group is also attached, the electron cloud from carbon will be attracted with more power than in Option (3), so the Hydrogen of Option (4) will be more acidic than that of Option (3).
Hence, we conclude that the hydrogen in Option (3) is the least acidic.
That was my concept. Correct me if I'm wrong :)