Question

The hypochlorite ion, \(\mathrm{ClO}^{-}\), acts as a weak base. (a) Is \(\mathrm{ClO}^{-}\)a stronger or weaker base than hydroxylamine? (b) When \(\mathrm{ClO}^{-}\) acts as a base, which atom, \(\mathrm{Cl}\) or \(\mathrm{O}\), acts as the proton acceptor? (c) Can you use formal charges to rationalize your answer to part (b)?

Short answer

In summary, (a) the hypochlorite ion (ClO⁻) is a weaker base than hydroxylamine, as its pKb value is higher. (b) The oxygen atom in ClO⁻ acts as the proton acceptor due to its higher electronegativity compared to chlorine. (c) Formal charges support this choice, as accepting a proton neutralizes the negative charge on the oxygen atom, resulting in a more stable molecule.

Step-by-step solution

(a) Comparing basic strength of ClO⁻ and hydroxylamine

To determine if ClO⁻ is a stronger or weaker base than hydroxylamine, we can look at the pKb values of each base. A base with a lower pKb value is considered a stronger base. The pKb value of hypochlorite ion (ClO⁻) is approximately \(10.24\). On the other hand, the pKb value of hydroxylamine is approximately \(8.09\). Since the pKb value of hydroxylamine is lower than that of the hypochlorite ion, hydroxylamine is a stronger base than ClO⁻.

(b) Identifying proton acceptor in ClO⁻

When a species acts as a base, it means that it accepts a proton (H⁺ ion) from another species, which is termed as the proton donor. In the case of the hypochlorite ion (\(\mathrm{ClO}^{-}\)), one of the atoms, either Cl or O, has a lone pair of electrons that can be used for this proton acceptance. Since oxygen is more electronegative than chlorine, it will more likely have the lone pair of electrons being available for proton acceptance. Therefore, the oxygen atom in the hypochlorite ion (\(\mathrm{ClO}^{-}\)) will act as the proton acceptor.

(c) Using formal charges to rationalize the proton acceptor

Formal charge is the charge assigned to an atom in a molecule, assuming that the electrons within a bond are shared equally between the atoms. An atom in a molecule with a formal charge is more likely to react or participate as a proton acceptor or donor if it can stabilize the overall charge of the molecule or ion. In the hypochlorite ion, the chlorine atom has a formal charge of \(+1\), while the oxygen atom has a formal charge of \(-1\). When the oxygen atom accepts a proton, the negative charge on the oxygen atom will be neutralized, leading to a more stable overall molecule with the formation of a new O-H bond. This is in accordance with our earlier prediction that the oxygen atom in the hypochlorite ion (\(\mathrm{ClO}^{-}\)) acts as the proton acceptor, and using formal charges has helped to rationalize this prediction.

Key concepts

Hypochlorite as a Weak Base

Understanding the behavior of the hypochlorite ion, \(\text{ClO}^-\), as a weak base is crucial in acid-base chemistry. A weak base is characterized by its partial ionization in water, meaning it does not fully dissociate to accept protons in a solution. When it comes to comparing the basic strength, such as between hypochlorite and hydroxylamine, a key value to look at is the pKb. A lower value indicates a stronger base, which in this case, shows hydroxylamine as a stronger base than hypochlorite with pKb values of approximately 8.09 and 10.24, respectively.

The ability of hypochlorite to act as a base, although weaker, is important in many biological and chemical processes. For instance, its presence in household bleaches and pool sanitizers utilizes its weak basic properties to maintain pH levels and react with various contaminants.

Despite its weaker base status, hypochlorite still plays a significant role in environmental and industrial applications, showcasing the versatility and importance of even feeble proton acceptors in chemistry.

Proton Acceptor in Bases

Bases are substances that can accept protons, typically from water or acids, thereby increasing the pH of a solution. The proton acceptor capability is a key feature of bases in acid-base reactions. In the case of the hypochlorite ion, \(\text{ClO}^-\), the decision of which atom acts as the proton acceptor is determined by electronegativity and the availability of a lone pair of electrons.

Oxygen, being more electronegative than chlorine, holds onto its electrons more tightly and therefore usually has a lone pair ready to accept a proton. This proton acceptance results in the formation of hydroxide, \(\text{OH}^-\), when hypochlorite ion, \(\text{ClO}^-\), functions as a base. This behavior aligns with the Bronsted-Lowry theory, which helps us understand how substances like hypochlorite function in various chemical reactions and environments.

Formal Charge and Reactivity

Formal charge is a tool chemists use to understand the distribution of electrons in a molecule or ion. It helps predict the reactivity, stability, and structure of the species in question. When an atom has a non-zero formal charge, it is often more reactive. For example, in the hypochlorite ion, \(\text{ClO}^-\), the chlorine atom possesses a formal charge of +1, whereas the oxygen atom has a formal charge of -1.

The oxygen's negative formal charge indicates an increased electron density, and when it accepts a proton, it neutralizes its charge, stabilizing the molecule in the process. This tendency to seek charge neutrality often drives the reactivity patterns observed in bases. Thus, the concepts of formal charge and reactivity are intertwined, providing insights into the behavior of ions like hypochlorite in chemical reactions and aiding in the prediction of proactive sites within molecules.