Reactivity with Halogens: Interhalogens
Master the formation, reactivity, and VSEPR geometries of Interhalogen Compounds.
When halogens react with each other, they form a fascinating class of compounds called Interhalogen Compounds. Because halogens have varying electronegativities and sizes, the less electronegative atom acts as the central atom, forcing the more electronegative atom(s) to surround it. These compounds are extremely reactive and serve as powerful halogenating agents.
1. Formula and Types ($XX'_n$)
Interhalogens are represented by the general formula $XX'_n$, where $n = 1, 3, 5, \text{ or } 7$.
- $X$ (The Central Atom): Must be the halogen with the larger size and lower electronegativity.
- $X'$ (The Terminal Atom): Must be the halogen with the smaller size and higher electronegativity (very often Fluorine).
- As the ratio $\frac{\text{Radius of } X}{\text{Radius of } X'}$ increases, the value of $n$ increases. Because Iodine is the largest and Fluorine is the smallest, the maximum number of atoms can only fit around Iodine. Hence, $IF_7$ is the only interhalogen with $n=7$.
| Type | Examples | Oxidation State of Central Atom ($X$) |
|---|---|---|
| $XX'$ | $ClF, BrF, IF, BrCl, ICl, IBr$ | +1 |
| $XX'_3$ | $ClF_3, BrF_3, IF_3, ICl_3$ | +3 |
| $XX'_5$ | $ClF_5, BrF_5, IF_5$ | +5 |
| $XX'_7$ | $IF_7$ (Only example) | +7 |
2. Preparation & Reactivity Trend
A. Preparation
They are prepared by direct combination of the halogens under specific conditions. The product formed depends heavily on the ratio of the reactants.
(Equal volume)
(Excess F₂)
B. The Reactivity Anomaly
Interhalogen compounds are MORE reactive than their constituent halogens (except Fluorine).
Reason: The $X-X'$ bond in interhalogens is polar (due to electronegativity difference) and dissimilar in orbital overlap, making it weaker than the non-polar $X-X$ bond in pure halogens. The only exception is the $F-F$ bond, which is exceptionally weak due to intense lone-pair repulsion.
3. Hydrolysis (A Classic Exam Trap)
When interhalogens undergo hydrolysis, they break down into a halide ion and an oxoacid. The trick is knowing which halogen forms which product.
- The smaller, more electronegative halogen ($X'$) forms the Halide ion ($X'^-$).
- The larger, less electronegative halogen ($X$) forms the Oxoacid ($HOX, HOXO, \text{etc.}$ ).
$ICl + H_2O \rightarrow HCl + HOI$
(Cl is smaller $\rightarrow$ HCl; I is larger $\rightarrow$ Hypoiodous acid)
4. Highly Tested VSEPR Geometries
Because the central atom possesses lone pairs of electrons along with bond pairs, the shapes of these molecules are distorted from perfectly symmetrical geometries.
Figure 1: The presence of lone pairs distorts the geometries in $ClF_3$ and $BrF_5$. $IF_7$ has no lone pairs on the central iodine atom.
Interhalogen compounds (especially $ClF_3$ and $BrF_3$) are exceptionally useful fluorinating agents. $ClF_3$ is famously used in the enrichment of Uranium for nuclear power, as it reacts with solid Uranium to produce volatile $UF_6$ gas.
$U(s) + 3ClF_3(l) \rightarrow UF_6(g) + 3ClF(g)$
Mastery Check: Interhalogens
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