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 bonds form when a hydrogen atom, already bonded to a highly electronegative atom (like F, O, or N), is attracted to another nearby electronegative atom. The hydrogen bond's strength directly relates to the polarity of the bond involving the hydrogen atom.\n\n

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

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• (D) O−H…F: H bonded to O (highly electronegative) and attracted to F (most electronegative). Strong H-bond.
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• (C) O−H…O: H bonded to O and attracted to O. Strong H-bond.
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• (B) N−H…O: H bonded to N (less electronegative than O) and attracted to O. Moderate H-bond, weaker than O−H…O.
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• (A) C−H…O: H bonded to C. Carbon's electronegativity is significantly lower than N, O, and F. The C−H bond is nearly nonpolar, resulting in a very small partial positive charge on the hydrogen atom. Consequently, the electrostatic attraction to the oxygen atom is very weak.
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\n Therefore, the C−H…O interaction represents the weakest hydrogen bond among the given options (sometimes considered too weak to be a "true" hydrogen bond, but is the weakest relative to the others).\n\n