Question

Which of the following is the most acidic compound?

• A. $\stackrel{+}{N}{H}_3-C{H}_2-OH$
• B. $N{O}_2-C{H}_2-OH$
• C. $HC\equiv C-C{H}_2-OH$
• D. $I-C{H}_2-OH$

Answer 1

The correct option is A $\stackrel{+}{N}{H}_3-C{H}_2-OH$

$\text{Acidic strength}\propto -\text{Inductive effect}$
Higher the inductive effect, higher will be the acid strength of the compound.
-I effect : The permanent shift in electron density away from the carbon chain towards the electron withdrawing group is called -I effect. It operates through sigma $\left(\sigma \right)$ bonds only i.e. no displacement of $\pi$ electrons.
The order of -I effect is,
$\stackrel{+}{-N{R}_3}>\stackrel{+}{-S{R}_2}>\stackrel{+}{-N{H}_3}>-N{O}_2>-S{O}_{3}H>-CN>-CHO>-COR>-COOH>F>Cl>Br>I>-OR>-OH>-N{R}_2>-NHR>-N{H}_2>-C\equiv CH>-C_6{H}_5>-C=C{H}_2>-H$
Thus, $\stackrel{+}{-N{H}_3}$ has the highest -I effect on the given compound. Hence, it is the most acidic compound.