Reactivity Towards Halogens

Reactivity Towards Halogens Chemistry Notes

Reactivity Towards Halogens :

→ The elements of group – 15 form two types of halide i.e, EX₃ and EX₃ But nitrogen cannot form penta halides due to the absence of vacant d-orbitals in its outer most shell.

→ Trihalides : In trihalides, element directly combines with halogen. Except NCl₃ NBr₃ and NI₃, all other trihalides are stable. These three halides are not stable due to large difference in atomic size of nitrogen and halogens (CI, Br, I).

→ Trihalides are covalent in nature and have pyramidal shape. In trihalides, the central atom is sp³-hybridised. Three sp³-hybridised orbitals having unpaired electron overlap with the orbitals of halogen atoms and form three sigma (σ) bonds. While the fourth sp³-hybridised orbital having lone pair of electron do not form bond with halogen atoms. Due to presence of this lone pair of electron the shape of trihalides become pyramidal.

→ Due to availability of lone pair of electrons, trihalides behave as Lewis bases, as they can release electron pair easily. However, NF₃ has a very little tendency to donate lone pair of electrons as the electronegativity of fluorine atom is very high. Hence, the order of Lewis basic strength is as follows

NF₃, < NCl₃ < NBr₃ < NI₃ (Lewis basic strength)

→ The acidic strength of trihalides of phosphorus is as follows
PCl₃ > PBr₃ > PI₃ (Acidic strength)

→ Some other important points related to trihalides are as follows

• Except BiF₃ other trihalides are covalent in nature. BiF, is ionic in nature.
• On moving down the group the ionic character of trihalides increases because the size of cation increases (Fajan’s rule).

NCl₃ < PCl₃ < AsCl₃ < SbCl₃ < BiCl₃ (Ionic character)

→ All the tribalides of nitrogen except NF₃ (which is inert and stable) are unstable and decomposed with explosion. The unstablity of NCl₃, NBr₃, NI₃ is due to the low polarity of N-X bond and large difference in atomic size of nitrogen and halogen.

→ Basic character of trihalides decreases down the group due to increase in size of central atom and decrease in availability of lone pair of electrons.
NE₃ > PF₃ > AsF₃ > SbF₃ > BiF₃
(Lewis basic strength)

Pentahalides :

→ Nitrogen does not form pentahalides because it does not have empty d-orbitals in its valence shell. While other elements of group – 15 can form pentahalides due to presence of empty d-orbitals, hence PCl₅ is known but NCl₅ is not known.

→ Pentahalides have trigonal bipyramidal structure in gaseous as well as in liquid state. In pentahalides the central atom is sp³ d-hybridised.

→ As pentahalides are comparatively less stable thus, trigonal bipyramidal structure is not a regular structure and is not very stable. In pentahalides three halogen atoms occupy equatorial positions while the other two occupy axial positions. The structure of pentahalide is not regular because all the bond angles are not same. The axial bonds are 90° while equatorial bonds are of 120° each. As axial position experience greater repulsion hence axial bonds are longer than equatorial bonds.

P – Cl_(a) > P – Cl_(a) (Bond length)

Hence on heating pentahalides decomposes into stable trihalides.

→ X-ray studies reveals that the solid PCl₅ is an ionic compound. It exist as [PCl ₄]⁺ [PCl₆]^–, PBr₅ and PI₅ also exist in ionic form in crystalline form.

→ Penta halides behaves as Lewis acids because of vacant d-orbitals.

PCl₅ + Cl^– → [PCl₆]^–
SbCl₅ + Cl^– → [SbCl₆]^–

→ The stability of pentahalildes decreases down the group. As the stability of + 5 oxidation state decreases down the group wihle the stability of + 3 oxidation state increases down the group. It is according to the inert pair effect.

Chemistry Notes