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Acidity is defined as the proton releasing ability in a solution of a compound. The more is the ability of releasing H⁺, the more it shows acidic character. The main driving force of releasing H⁺ is the stabilisation of the conjugate base. We can also determine acidity by the length of the M-H bond. (M= metal or non- metal.) The longer is the bond, the less energy is required to break it and more acidity to show.

S-H hasa longerbond length than O-H bond for which it is easier to break the former than the later and as a matter of fact, the former releases H⁺ ion more as well as shows more acidity.

Acidity comparison CH₃CH₂SH > CH₃CH₂OH.

The more the stability of the conjugate base, the more will be the acidity of the compound. The conjugate bases of the above compounds are

Among the following groups, S has vacant d orbital which delocalises the negative charge resides on it easily. It is the most stabilised one among all. O being an electronegative element delocalised the electron through p orbital though the extent of delocalisation will be less compared to the former one as the p orbital is smaller than d and p is not vacant totally like d orbital. N-H bond having a negative charge on Nitrogen is very unstable as its p orbital gets half field stabilisation by completing its octet. So, it is least stable in its conjugate base form.

In CH₃CH₂CH₃, C-H bond breaking is not favorable due to its high covalency. So, it is less acidic than CH₃CH₂SH and CH₃CH₂OH but more acidic than CH₃CH₂NH₂.

The final order for decreasing acidity is :

CH₃CH₂SH > CH₃CH₂OH> CH₃CH₂CH₃> CH₃CH₂NH₂