Question

Arrange the following in increasing order of acidic strength: I. $H_2SO_4$ II. $(CH_3)_3CH(SH)$ III. $CH_3CH_2^+OH_2$ IV. $CH_3CH_2CH_3$

• A. IV < II < I < III
• B. IV < II < III < I
• C. IV < I < II < III
• D. IV < III < II < I

Answer 1

Answer: (B)

IV < II < III < I

Answer 2

The acidic strength of a compound is determined by its ability to donate a proton ($H^+$). A stronger acid readily donates a proton, resulting in a stable conjugate base. We can compare the acidic strengths by considering the stability of their conjugate bases or by comparing their pKa values (lower pKa indicates stronger acid).

Let's analyze each compound:

I. $H_2SO_4$ (Sulfuric acid): This is a strong mineral acid. It dissociates to form $HSO_4^-$. The conjugate base $HSO_4^-$ is highly stabilized by resonance, distributing the negative charge over multiple oxygen atoms. The first dissociation of $H_2SO_4$ is very strong, with a pKa around -3.

II. $(CH_3)_3CSH$ (tert-Butyl thiol): This is a thiol. Thiols are more acidic than alcohols because the S-H bond is weaker than the O-H bond, and the thiolate anion ($(CH_3)_3CS^-$) is more stable than the corresponding alkoxide anion ($(CH_3)_3CO^-$) due to the larger size of the sulfur atom, which allows better dispersal of the negative charge. The pKa of simple thiols is around 10-11.

III. $CH_3CH_2^+OH_2$ (Protonated ethanol): This is a protonated alcohol, an oxonium ion. Positively charged species are generally more acidic than their neutral counterparts. This species acts as an acid by donating a proton to form neutral ethanol ($CH_3CH_2OH$). The conjugate base is ethanol, which is a relatively stable neutral molecule. The acidity of protonated alcohols is similar to that of the hydronium ion ($H_3O^+$), which has a pKa of -1.74. The pKa of protonated ethanol is around -2.4.

IV. $CH_3CH_2CH_3$ (Propane): This is an alkane. Alkanes are extremely weak acids. They can theoretically donate a proton from a C-H bond to form a carbanion ($CH_3CH_2CH_2^-$ or $(CH_3)_2CH^-$). Carbanions are very unstable because the negative charge is localized on a carbon atom with low electronegativity and lacks significant stabilization. The pKa of alkanes is very high, typically around 50.

Now let's arrange them in increasing order of acidic strength, which corresponds to decreasing order of pKa:

• IV (Propane): pKa ~ 50 (weakest acid)
• II (tert-Butyl thiol): pKa ~ 10
• III (Protonated ethanol): pKa ~ -2.4
• I (Sulfuric acid): pKa ~ -3 (strongest acid)

Comparing the pKa values: 50 > 10 > -2.4 > -3.

The order of increasing acidity is the order of decreasing pKa: IV < II < III < I