In this section, we will learn about the nature of Carbon-Halogen Bond and the trends observed in Electronegativity, Bond Polarity, Bond Length, Bond Enthalpy and the Dipole Moment.
Nature of Carbon-Halogen Bond
Halogens on forming a covalent bond with Carbon pulls the electrons in the bond towards itself. This property is known as the electronegativity. The unequal sharing of the electron cloud results in electron-deficient Carbon to get a partial positive charge denoted as (delta)+ and the electron-rich halogen atom to get a partial negative charge shown as (delta)-. This charge separation results in making Carbon-halogen bond, a polar covalent bond.
Electronegativity value changes with the atomic size and the atomic number. It increases if we go from left to right and from bottom to top of the periodic table making Fluorine the most electronegative element. Linus Pauling assigned it a value of 4 and calculated the electronegativity values of other elements relative to Fluorine. This scale is called Pauling scale of electronegativity.
In general, a bond of Carbon with more a electronegative element from group 15-17 is considered polar with Carbon carrying (delta)+ charge. However, when carbon forms a bond with metal atoms, its polarity is reversed as Carbon is now more electronegative and thereby carries a (delta)- charge.
Electronegativity difference to predict nature of bond
Electronegativity difference is a great tool to predict the nature of bond whether it is polar, non-polar or ionic. If it is less than 0.5, then it is a non-polar covalent bond. Example, Carbon-Hydrogen or Hydrogen-Hydrogen. If electronegativity difference is in the range of 0.5-2, the bond is a polar covalent bond. A carbon-halogen bond is a perfect example so is the hydrogen-fluorine bond. For values above 2, the bond is an ionic bond. For example, Silver Fluoride, Potassium Fluoride, Sodium Bromide and Sodium Chloride. Therefore, due to decreasing electronegativity of the halogens, as we go down the group 17 of the periodic table, a Carbon-Fluorine Bond is found to be the most polar followed by Carbon-Chlorine, Carbon-Bromine and least is for Carbon-Iodine.
Nature of Carbon-Iodine bond
If you take a look at the electronegativity values of Carbon and Iodine; for both, it is 2.5. That means, there is not much difference in the electronegativity, and hence, it would be treated as a non-polar bond. However, polarity not only depends on the electronegativity difference but also on the polarizability of the atoms involved in a covalent bond.
'Polarizability is the ability of the electrons in the atom to distort or shift due to external influences.' It could be an external electric field or presence of polar molecules nearby. Iodine due to its position in the periodic table has large size due to the involvement of more shells and therefore, the higher number of electrons. The large size makes it difficult for the attractive force of the nucleus to pull the outside electrons towards itself thereby decreasing the nuclear charge. These outside electrons are therefore available for distortion or dispersion depending on the nature of the surrounding medium.
Let us take the example of Methyl Iodide which is a Carbon-Iodine bond. Even though there is no electronegativity difference between them, it has a polarizable atom, Iodine. So in a chemical reaction, due to the presence of the other polar molecules in the reaction medium, the polarity is induced in Carbon-Iodine bond. Iodine being a polarizable atom would pull the electron clouds towards itself carrying a partial negative charge and carbon, partial positive charge thereby making Carbon-Iodine a polar bond.
Two things we learned here, that electronegativity and polarizability of halogen influence the polarity of carbon-halogen bond and also, electronegativity and bond polarity decrease if we go down in a group from Fluorine to Chlorine, Bromine, and Iodine.
Another change that happens is in the bond length. As new shells are added to the group, and size of the halogen increases; and on forming a bond with Carbon, its bond length would also increase. As Fluorine is the smallest element and Iodine, the largest, Carbon-Fluorine Bond would be smallest and Carbon-Iodine bond the longest.
Bond Enthalpy of Carbon-Halogen Bond
Due to the increasing bond length between the Carbon-Halogen, the energy required to break such a bond would be least. This energy is called bond enthalpy and it minimum for Carbon-Iodine bond and maximum for, smallest Carbon-Fluorine Bond. As bond enthalpy is high for Fluoroalkanes, it is highly unreactive, and Iodoalkanes are very reactive.
Another physical property affected by the bond polarity is the dipole moment. Dipole moment calculates the total molecular polarity by taking into consideration the electronegativity difference leading to separation of charges and the distance between the two charges (µ= q xd). As electronegativity and the bond polarity decreases from Fluorine to Iodine, the dipole moment is expected to decrease. However, the dipole moment of Chloromethane is greater than that of Fluoromethane because the bond length of the Carbon-Fluorine is shortest even though it is the most polar.
Attempt this quiz to check your understanding of this concept.
For complete videos and tutorials, subscribe at curlyarrows.com/subscribe