Question

Out of $2 -$chloroethanol and ethanol which is more acidic and why?
A. $2 -$Chloroethanol due to +I effect of $Cl$
B. Ethanol due to +I effect of $C{H_3}$
C. $2 -$Chloroethanol due to –I effect of $Cl$
D. Ethanol due to –I effect of $C{H_3}$

Answer 1

The electronegativity of an element plays a key role in deciding the acidity of a molecule. The more electronegative the atom the more it will attract the bonded pair of electrons towards itself.

Complete step by step answer:
A compound is more acidic if it readily donates a hydrogen ion bonded to the compound. This is explained by the following equation:
$R - H \to {R^ - } + {H^ + }$
The acidity is shown as the ability to release ${H^ + }$ ion. The acidity is dependent on the formation of the ${R^ - }$ ion. The more facile is the formation of ${R^ - }$ ion the more readily the molecule loses H+ ion and the more acidic the molecule is.
The strength of the acidity is dependent on various factors. The factors which affect the acidity of a molecule are as follows:
a) Electronegativity: The more electronegative the bonded atom the more is its electron attracting power. Thus the electron density of bonded electrons move towards the more electronegative atom and thus loss of ${H^ + }$ ion becomes easy. In general, the acidity increases from left to right along the periodic table. As halogens the most electronegative so $HF$, $HCl$ , $HBr$ , $HI$ are strong acids.
b) Anion size: The size of the anions also governs the acidity of a molecule. As more shells are added down the group in the periodic table the acidity increases from top to bottom in the periodic table.
c) Resonance: Resonance plays an important role in deciding the acidity of a molecule. For example the phenol molecule is acidic because of the formation of phenoxide anion. In phenoxide ion the negative charge on oxygen atom is in resonance with the benzene ring which accounts for its acidity.
4) Hybridization: The hybridization of an atom also affects the acidity of a molecule. For example the acetylene is more acidic than ethene which is more acidic than ethane. The more the s-character of a molecule the more acidic the molecule becomes which follows the trend $sp > s{p^2} > s{p^3}\;$.
5) Inductive effect: The electron withdrawing power of the substituent increases the acidity of a compound. The electron donating power of the substituent decreases the acidity. This is indicated as _I for electron withdrawing groups and +I for electron donating groups.
The given compounds are $2 -$ chloroethanol and ethanol. In $2 -$ chloroethanol as chlorine is more electronegative and has –I inductive effect it will attract the bonded pair of electrons towards itself. In ethanol the ethyl group has +I inductive effect and thus lower the acidity of ethanol.
Hence,option C is the correct answer.

Note: The acidity of the molecule is expressed by the pKa values which measure the dissociation constant of the acid molecule. The lower the pKa value the more acidic the molecule is.