Question

Which of the following hydrogen bonds is likely to be the weakest?

• A. C$-$H$\cdots$O
• B. N$-$H$\cdots$O
• C. O$-$H$\cdots$O
• D. O$-$H$\cdots$F

Hint:

Using a simple frame or just bolding for the box Key Points: H-bond strength depends on the polarity of the X$-$H bond. Higher electronegativity of X leads to a more polar X$-$H bond and a stronger H-bond. Electronegativity order: F>O>N >> C. C$-$H bonds are generally considered nonpolar or weakly polar, leading to very weak H-bonds (if any).

Answer 1

The Correct Option is A

Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (typically F, O, or N) and is attracted to another nearby highly electronegative atom. The strength of the hydrogen bond depends significantly on the polarity of the bond involving the hydrogen atom.

• Electronegativity Order: F>O>N>C.
• Bond Polarity: The polarity of the X−H bond increases as the electronegativity of X increases. Therefore, the partial positive charge (\(\delta^+\)) on the hydrogen atom follows the order: H in F−H>H in O−H>H in N−H>H in C−H.
• Hydrogen Bond Strength: A larger partial positive charge on the hydrogen atom leads to a stronger electrostatic attraction with the lone pair of the acceptor atom (O or F in these examples), resulting in a stronger hydrogen bond.

Comparing the options:

• (D) O−H…F: Involves H bonded to O (highly electronegative) and attracted to F (most electronegative). Strong H-bond.
• (C) O−H…O: Involves H bonded to O and attracted to O. Strong H-bond.
• (B) N−H…O: Involves H bonded to N (less electronegative than O) and attracted to O. Moderate H-bond, weaker than O−H…O.
• (A) C−H…O: Involves H bonded to C. Carbon has significantly lower electronegativity compared to N, O, and F. The C−H bond is almost nonpolar, meaning the hydrogen atom carries a very small partial positive charge. Consequently, the electrostatic attraction to the oxygen atom is very weak.

Therefore, the C−H…O interaction represents the weakest hydrogen bond among the given options (sometimes it's considered too weak to be a "true" hydrogen bond, but relative to the others, it is the weakest).