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Haloalkanes and Haloarenes - Nature of C-X Bond

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The C−XC-X bond (where X=F,Cl,Br,IX = F, Cl, Br, I) is a polar covalent bond. Since halogens are more electronegative than carbon, the carbon atom carries a partial positive charge (delta+\\delta^+) and the halogen atom carries a partial negative charge (delta−\\delta^-).

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As the size of the halogen atom increases down the group (F<Cl<Br<IF < Cl < Br < I), the C−XC-X bond length increases. Consequently, the bond strength or bond dissociation enthalpy decreases from C−FC-F to C−IC-I.

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The dipole moment (mu\\mu) of haloalkanes generally decreases as the electronegativity of the halogen decreases. However, an anomaly exists: CH3ClCH_3Cl has a higher dipole moment than CH3FCH_3F because the significantly longer bond length of C−ClC-Cl outweighs the higher electronegativity of Fluorine.

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Bond enthalpy order: C−F>C−Cl>C−Br>C−IC-F > C-Cl > C-Br > C-I. This is because as the size of the orbital of the halogen increases, the overlap with the sp3sp^3 orbital of carbon becomes less effective.

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Bond length order: C−F<C−Cl<C−Br<C−IC-F < C-Cl < C-Br < C-I due to the increase in atomic radii of the halogens down the group.

📐Formulae

mu=qtimesd\\mu = q \\times d

textBondLengthOrder:C−F<C−Cl<C−Br<C−I\\text{Bond Length Order: } C-F < C-Cl < C-Br < C-I

textBondEnthalpyOrder:C−F>C−Cl>C−Br>C−I\\text{Bond Enthalpy Order: } C-F > C-Cl > C-Br > C-I

textDipoleMomentOrder:CH3Cl>CH3F>CH3Br>CH3I\\text{Dipole Moment Order: } CH_3Cl > CH_3F > CH_3Br > CH_3I

💡Examples

Problem 1:

Explain why the dipole moment of CH3ClCH_3Cl (1.860D1.860 D) is greater than that of CH3FCH_3F (1.847D1.847 D).

Solution:

muCH3Cl>muCH3F\\mu_{CH_3Cl} > \\mu_{CH_3F}

Explanation:

Dipole moment is the product of charge (qq) and bond length (dd). Although FF is more electronegative than ClCl, the C−ClC-Cl bond length is significantly larger than the C−FC-F bond length. This increase in distance (dd) more than compensates for the smaller charge (qq) on ClCl compared to FF.

Problem 2:

Arrange the following bonds in decreasing order of their bond dissociation enthalpy: C−F,C−Cl,C−Br,C−IC-F, C-Cl, C-Br, C-I.

Solution:

C−F>C−Cl>C−Br>C−IC-F > C-Cl > C-Br > C-I

Explanation:

As the size of the halogen atom increases, the C−XC-X bond length increases. Larger bonds are weaker and require less energy to break, thus bond dissociation enthalpy decreases as we go from Fluorine to Iodine.

Nature of C-X Bond - Revision Notes & Key Formulas | CBSE Class 12 Chemistry