Question

The correct order of acidic strength is/are (A) (B) (C) \(\mathrm{CH}_{3}-\mathrm{OH}>\mathrm{H}_{2} \mathrm{O}>\mathrm{EtOH}>\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}>\mathrm{R}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}>\mathrm{NH}_{3}\) (D) \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}>\mathrm{H}_{2} \mathrm{O}>\mathrm{NH}_{3}>\mathrm{CH}_{4}\)

Short answer

The correct order of acidic strength is: \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}>\mathrm{H}_{2}\mathrm{O}>\mathrm{NH}_{3}>\mathrm{CH}_{4}\).

Step-by-step solution

Understanding Acidic Strength

Acidic strength refers to the ability of a compound to donate a proton (H+) in a chemical reaction. In general, compounds with more electronegative atoms bonded to the hydrogen atom will be more acidic, as the electronegative atom will be more likely to attract and stabilize the charge of the released proton. Bond strength also plays a role, with weaker bonds being more likely to break and donate the proton.

Analyzing Compound C

In compound C, we have the following order: \(\mathrm{CH}_{3}-\mathrm{OH}>\mathrm{H}_{2}\mathrm{O}>\mathrm{EtOH}>\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}>\mathrm{R}-\mathrm{C}\equiv \mathrm{C}-\mathrm{H}>\mathrm{NH}_{3}\). In terms of acidity, the order is \(\mathrm{CH}_{3}-\mathrm{OH}> \mathrm{H}_{2}\mathrm{O} >\mathrm{EtOH}> \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) since all of them have the hydrogen atom bonded to an oxygen atom. The difference in their acidity arises due to different electron-donating groups present in those compounds. The order \(\mathrm{R}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}>\mathrm{NH}_{3}\) might be considered incorrect as the electronegativity of the nitrogen atom in \(\mathrm{NH}_{3}\) is higher than carbon, which should make \(\mathrm{NH}_{3}\) more acidic.

Analyzing Compound D

In compound D, we have the following order: \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}>\mathrm{H}_{2}\mathrm{O}>\mathrm{NH}_{3}>\mathrm{CH}_{4}\) This order is established based on the strength of the bond between hydrogen and the corresponding atom. The order of bond strength is \(\mathrm{HF} > \mathrm{HCl} > \mathrm{HBr} > \mathrm{HI}\). Since weaker bonds are more likely to break and donate the proton, the order of acidic strength should be \(\mathrm{HI} > \mathrm{HBr} > \mathrm{HCl} > \mathrm{HF}\). Between \(\mathrm{H}_{2}\mathrm{O}\), \(\mathrm{NH}_{3}\), and \(\mathrm{CH}_{4}\), the order is correct as \(\mathrm{H}_{2}\mathrm{O}>\mathrm{NH}_{3}>\mathrm{CH}_{4}\) based on the electronegativity of oxygen, nitrogen, and carbon, respectively. Therefore, compound D seems to have the correct ordering of acidic strength.

Conclusion

Comparing compounds C and D, it appears that compound D has the correct order of acidic strength. This is due to the consideration of bond strength and electronegativity of the atoms within the compounds. The correct order of acidic strength is: \(\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}>\mathrm{HF}>\mathrm{H}_{2}\mathrm{O}>\mathrm{NH}_{3}>\mathrm{CH}_{4}\).

Key concepts

Electronegativity

Electronegativity is like a magnet that atoms have for electrons. It's a way to measure how strongly an atom attracts the electrons in a bond. When it comes to acidity, electronegativity is a key player! Atoms with higher electronegativity are able to hold onto electrons more tightly. This is important because the more electronegative an atom bonded to hydrogen is, the more it encourages the hydrogen to leave as a proton, or \(H^+\). This process of hydrogen leaving as a proton defines a substance’s acidity.In the context of acidic strength, an electronegative atom bonded to hydrogen will make the molecule more acidic. A great example is the halogen group: fluorine, chlorine, bromine, and iodine. They all are highly electronegative, which helps them attract and stabilize the negative charge left when they donate a proton. With acidic strength, the more electronegative the atom attached to hydrogen, the stronger the acid tends to be, but that's also affected by bond strength, which we will touch on later. For example:

• Oxygen is more electronegative than nitrogen and carbon. Thus, water (\(H_2O\)) is more acidic than ammonia (\(NH_3\)) and methane (\(CH_4\)).
• Iodine, though less electronegative than fluorine, forms a strong acid with hydrogen due to other factors like bond strength.

Proton Donation

Proton donation is central to understanding how substances behave as acids. Simply put, an acid is something that donates a proton, which is a positive hydrogen ion \(H^+\). The easier it is for a molecule to lose or donate that proton, the stronger the acid is. But not all molecules are equally willing to give them up!There are a few things at play when it comes to how readily a molecule will donate a proton:

• **Electronegativity.** As mentioned earlier, the presence of an electronegative atom can stabilize the negative charge left behind, facilitating proton donation.
• **Stability of the conjugate base.** Once the acid donates a proton, the rest of the molecule is a conjugate base. The stability of this conjugate base is crucial; the more stable it is, the more easily a proton is donated.
• **Molecular Structure.** The presence of electron-withdrawing groups in the molecule can also enhance proton donation by stabilizing the conjugate base further.

Think of proton donation somewhat like someone giving away money. If they know their bank account remains stable afterwards, they are more likely to give it away! Similarly, a stable conjugate base means a molecule will donate a proton more readily.

Bond Strength

Bond strength between atoms is an influential factor in determining acidic strength. It refers to how much energy is needed to break the bond between two atoms. In the case of acids, we specifically look at the bond between hydrogen and the atom it's attached to.For acids, weaker bonds mean stronger acids. This might seem a bit backward, but here's why: if the bond between the hydrogen and its attached atom is weak, it can break more easily, allowing the hydrogen to be donated as a proton \(H^+\).Consider the halogen acids again:

• **Hydrogen iodide (HI).** This has the weakest bond with hydrogen in the halogen group, which makes \(HI\) a very strong acid.
• **Hydrogen fluoride (HF).** Although fluorine is highly electronegative, the bond it forms with hydrogen is quite strong, making \(HF\) a relatively weaker acid compared to \(HI\), \(HBr\), and \(HCl\).

The concept of bond strength also explains why certain compounds, despite having highly electronegative atoms, may not be as acidic. For example, in water \(H_2O\), the strong bond between oxygen and hydrogen needs considerable energy to break, which affects its acidity.Remember:- **Weaker bonds = easier proton donation.**- **Stronger bonds = more difficult to break, less acidic.**