Nature of the C X Bond in Haloalkanes

The C in the C–X bond is carbon while the halogen is denoted as X. The high electronegativity of halogen makes the electron cloud attract more towards itself and therefore gains a slight negative charge, while the carbon attains a slight positive charge. As the halogens need only one electron to achieve their nearest inert gas configuration, that is, their octet state, so between one carbon and one halogen atom, only one sigma bond is formed. The C–X bond length in haloarenes increases due to the increase in atomic size from fluorine to astatine and the bond dissociation strength decreases.

Haloarenes Nature of C–X Bond

The chemical compounds containing arenes are known as haloarenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. It contains halogen atoms attached to sp2 hybridized carbon atom(s) of an aryl group. The C–X bond's nature depends on both the halogen of the compound and the nature of carbon in the benzene ring. The alphabet "X" generally denotes halogen. Halogens are group 17 elements that have very high electronegativity. Going down the group, the elements are namely, fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The highest electronegativity is of fluorine (F). The elements in this group need one more electron to complete the nearest noble gas configuration.

The carbon in haloarenes which is a 14th group element has lesser electronegativity as compared to that of the halogens which have higher electronegativity.

Salient Points on the Nature of C–X Bonds in Haloarenes

The salient points on the nature of C–X bonds in haloarenes are as follows: 

• We know that the halogens are more electronegative than carbon and due to this high electronegativity, it makes the electron cloud attract more towards itself and therefore gains a slight negative charge, while the carbon attains a slight positive charge. Thus, the C–X bond in haloarenes is polarised.

• Between one carbon atom and one halogen atom, only one sigma bond is formed because halogens need only one electron to reach the nearest inert gas configuration, that is, the octet state.

• The atomic size increases from fluorine to astatine; therefore, as a result, the C–X bond length in haloarenes also increases, and the bond dissociation strength decreases.

• Moving down the group, the electronegativity decreases, and we know that dipole moment depends on the difference in electronegativity of carbon and halogens. Therefore, the dipole moment also decreases down the group. Although, there is an exception of C–Cl and C–F dipole moments., Cl has less electronegativity than F, but the dipole moment of C–Cl bond is more than C–F. 

Solved Examples

1. Which C–X bond is strongest?

Answer: Carbon–fluoride bond.

Except for the carbon–iodine bond, the carbon–halogen bonds are polar, because the electron pair is pulled closer to the halogen atom than the carbon. This is because the halogens are more electronegative than carbon apart from iodine. Fluorine is the most electronegative that pulls the electron pair more strongly than the other halogens. Therefore, the carbon–fluorine bond is the strongest. 

2. Which of the following orders is correct regarding the bond enthalpy ε(C−X) in an alkyl halide (RX)?

(a) ε(C−I) < ε(C−Br) < ε(C−Cl)

(b) ε(C−I) < ε(C−Br) > ε(C−Cl)

Answer: Bond enthalpy C−X (where X is Cl, Br, I) decreases with increase in the atomic number of X. Hence, the correct order is option a.

3. What are the factors responsible for low reactivity of aryl halides towards nucleophilic substitution?

Answer: The following factors are responsible for the low reactivity of aryl halides towards nucleophilic substitution:

1. The bond length of C–X in haloarene is smaller than C–X bond length in alkyl halide.

2. By any nucleophile, it is difficult to displace halogen in haloarenes.

3. A nucleophile easily replaces halogen.

4. Attack of nucleophiles on aryl halides becomes an electron-rich molecule due to the presence of pi bonds.