Question

Suggest a reason for the fact that ultraviolet light causes \(\mathrm{Cl}_{2}\) (gas) to cleave homolytically, to produce two chlorine radicals, rather than heterolytically, to produce \(\mathrm{Cl}^{+}\) and \(\mathrm{Cl}^{-}\).

Short answer

Ultraviolet light provides sufficient energy to break the weak bond between the two chlorine atoms in the Cl2 molecule. Homolytic cleavage is favored over heterolytic cleavage because the energy cost of separating charges in the gas phase is comparatively high, and chlorine atoms are more stable in their radical form than in their ionic form. Hence, ultraviolet light causes Cl2 gas to cleave homolytically, producing two chlorine radicals.

Step-by-step solution

Understanding homolytic and heterolytic cleavage

Homolytic cleavage is when a chemical bond breaks, and each of the two resulting fragments retains one electron from the bond. This produces free radicals, which have unpaired electrons and are highly reactive. In contrast, heterolytic cleavage occurs when one fragment retains both electrons from the bond, creating a negatively charged ion, and the other fragment becomes a positively charged ion.

Considering the chlorine molecule

Chlorine is a diatomic molecule, consisting of two chlorine atoms covalently bonded together by sharing an electron pair. The bond between the two chlorine atoms is relatively weak, making it more susceptible to cleavage when exposed to energy.

Understanding the energy of ultraviolet light

Ultraviolet light has a higher energy than visible light. When a molecule absorbs this high-energy light, it gains enough energy to break its chemical bonds. In the case of Cl2, the absorbed energy enables the bond between the two chlorine atoms to break.

Comparing the stability of the products

Upon bond cleavage, there are two possible outcomes: homolytic cleavage, producing two chlorine radicals, or heterolytic cleavage, producing Cl+ and Cl-. The latter requires separating the charges, creating a positive and a negative ion. This process is less favorable due to the energy cost of separating charges in gas phase and due to the fact that the single unpaired electron in chlorine atom is less stable in an ionic form than in a radical form.

The reason for homolytic cleavage in Cl2

The energy provided by ultraviolet light is sufficient to break the relatively weak bond between the two chlorine atoms in the Cl2 molecule. When the bond breaks, homolytic cleavage is favored over heterolytic cleavage because the energy cost of separating charges in the gas phase is comparatively high. In addition, chlorine atoms in their radical form are more stable than in their ionic form. Therefore, ultraviolet light causes Cl2 gas to cleave homolytically and produce two chlorine radicals.

Key concepts

Homolytic Cleavage

Homolytic cleavage is a fascinating chemical process that occurs when a bond breaks and each of the two atoms involved in the bond takes away one of the shared electrons. This results in the formation of radicals, which are atoms or molecules that have an unpaired electron. Radicals are an important topic in chemistry because they are highly reactive due to their unpaired electrons.

• In homolytic cleavage, the bond breaking is symmetrical.
• Each of the resulting radicals keeps one electron from the bond.
• Radicals play significant roles in various chemical reactions, especially in organic chemistry.

The process of homolytic cleavage is particularly relevant when discussing ultraviolet (UV) light. UV light has enough energy to break weak covalent bonds, often resulting in the generation of radicals. When chlorine molecules (\( \mathrm{Cl}_2 \)) absorb UV light, they undergo homolytic cleavage, forming two chlorine radicals. These radicals are reactive intermediates in many chemical reactions.
Understanding why UV light favors homolytic over heterolytic cleavage in chlorine molecules can be attributed to the greater stability of chlorine radicals compared to ionic forms.

Heterolytic Cleavage

Heterolytic cleavage is another possibility when it comes to bond breakage, but it involves a different outcome compared to homolytic cleavage. In heterolytic cleavage, one atom takes both electrons from the bond, leading to the formation of two charged species — one positively charged, and the other negatively charged. This process results in the formation of ions.

• Heterolytic cleavage is asymmetrical.
• This cleavage creates a positive ion and a negative ion.
• It often occurs in polar molecules where significant differences in electronegativity exist.

Chlorine molecules, however, do not favor heterolytic cleavage when exposed to UV light. The reason for this is that separating the electrons and forming ions in the non-polar chlorine gas phase is energetically costly. Moreover, forming ions like \( \mathrm{Cl}^+ \) and \( \mathrm{Cl}^- \) requires more energy compared to forming neutral radicals. This is why we'll typically observe chlorine undergoing homolytic cleavage rather than heterolytic cleavage under the influence of UV light.

Chlorine Radicals

Chlorine radicals are formed when a chlorine molecule undergoes homolytic cleavage. This type of radical is particularly reactive due to its unpaired electron, making it a pivotal player in a variety of chemical reactions.

• Chlorine radicals possess one unpaired electron, leading to high reactivity.
• They are often involved in chain reactions and are key intermediates in numerous chemical processes.
• In the context of UV light, chlorine radicals are preferred products over ionic forms due to lesser energy demands.

Because chlorine (\( \mathrm{Cl}_2 \)) has a relatively weak covalent bond between the two atoms, it can easily absorb energy from UV light, breaking this bond and forming radicals. These chlorine radicals can further react with other molecules, often initiating chain reactions that lead to the formation of various products. This reactivity is exploited in industrial and laboratory settings, especially in processes like chlorination. Understanding chlorine radicals and their behavior under different forms of energy like UV light is crucial for manipulating reactions involving chlorine.