All You Need To Know About Control Of Halogenation

When one or even more fluorine, chlorine, bromine, or iodine atoms exchange hydrogen atoms in an organic molecule, a halogenation reaction occurs. Fluorine > chlorine > bromine > iodine is the sequence of reactivity. Fluorine is a particularly aggressive element, capable of severe reactions with organic compounds. It is also the most stable of the organohalogens, and removing a fluorine atom once added is difficult. Iodine, on the other hand, is more difficult to add to an organic molecule, but the iodine atom is easily removed after an iodo organic has formed. As a result, the halogen atom’s electronegativity is a driving force in halogenation processes. The reactions are also influenced by the type of the halogenated substrate molecule.

Saturated hydrocarbons are halogenated by a free radical process, unsaturated organics halogenate via an addition reaction, and aromatics halogenate via electrophilic substitution.

Importance of Halogenation Reaction

Halogenation reactions are significant in both bulk and fine chemical synthesis, and halogenation products and intermediates are widely used in medicines, polymers & plastics, refrigerants, fuel additives, fire retardants, and agro products.

In pharmaceuticals, adding fluorine or chlorine atoms to a molecule can boost its therapeutic action potential. Organobromine & organoiodines are very important as intermediate compounds since they allow you to add functional groups to substrate while also allowing you to make more complicated structures. C-Cl and C-Br bonds, for example, can be hydrolyzed to alcohols, which can then be oxidised to generate ketones, aldehydes, and acids. Double bonds can be produced through elimination reactions.

Bromination of organic molecules is a common step in the formation of a Grignard reagent, which provides a synthetic method for generating C-C bonds. The Friedel-Crafts reaction is widely used to alkylate aromatic rings, and alkyl halides are important reagents in this process.

Halogenation reactions produce some important commercial chemicals and products. Chloroform is fluorinated to produce chlorodifluoromethane, which then is transformed to fluoroethylene & polymerized to generate PTFE in a famous example. Another example is the addition of chlorine to ethylene to produce dichloroethane, which is then polymerized to produce PVC.

Is Catalyst Required For Halogenation Reaction?

To increase the electrophilicity of the halogen, halogenation processes may require a catalyst. Electrophilic substitution processes of aromatic compounds, for example, necessitate the use of a catalyst. Bromine & chlorine really aren’t electrophilic enough to cause hydrogen substitution by themselves, thus they need the existence of a Lewis acid. As an example, AlCl3 or AlBr3 are common catalysts for halogenation of aromatic rings. The halogen’s bonding becomes more polarised in the presence of the Lewis acid, as the enhanced positively charged halogen is a significantly stronger electrophile.

Since fluorine really is a strong electrophile and so this reaction can be quite energetic, fluorination of aromatics doesn’t really require a catalyst. The amount of fluorine substitution that happens is difficult to manage, and much more than one fluorine atom can halogenate the aromatic ring. Metal halides are ineffective catalysts for iodine substitution on an aromatic ring, but an oxidant like nitric acid can transform iodine to HIO3 and allow benzene to be iodized.

Chlorination of Methane in Presence Of Sunlight

Whenever a mixture of methane & chlorine is subjected to UV radiation, such as sunshine, a substitution reaction occurs, yielding chloromethane as the organic product.

                                                      CH₄ + Cl₂ 🡪 CH₃Cl + HCl

The reaction, however, does not end there; all of the hydrogens within methane could be exchanged by chlorine atoms. You could get chloromethane, dichloromethane, trichloromethane, or tetrachloromethane as a result.

                                                    CH₄ + Cl₂ 🡪 CH₃Cl + HCl

                                                   CH₃Cl + Cl₂ 🡪 CH₂Cl₂ + HCl

                                                   CH₂Cl _{2 +} Cl₂ 🡪 CHCl₃ + HCl

                                                    CHCl₃ + Cl₂ 🡪 CCl₄ + HCl

You may suppose that the amounts of methane & chlorine you used could determine the product you received, but it’s not that straightforward. You can get CCl₄ if you utilise enough chlorine, but any other proportions will always result in a combination of results.

Mechanism

The development of numerous substitution products such as di-, tri-, and tetrachloromethane can be described in the same way that the initial chloromethane was formed. All you have to do now is keep an eye on the potential collisions as the reaction progresses.

Synthesis of Dichloromethane

You’ll recall that the overall reaction equation for the first stage is 

                                            CH₄ + Cl₂ 🡪 CH₃Cl + HCl

Methane is used up as the reaction progresses, and chloromethane takes its place. That implies the debate about how a chlorine radical is most likely to hit shifts during the reaction. As time passes, the likelihood of it colliding with a chloromethane molecule instead of a methane molecule increases.

When this occurs, the chlorine radical may remove a hydrogen atom from chloromethane just like it can from methane. In this new situation:

                                                   CH₃Cl + Cl 🡪 CH₂Cl + HCl

In a new propagation step, the chloromethyl radical can combine with a chlorine molecule, forming dichloromethane and regenerating a chlorine radical.

                                                     CH₂Cl + Cl₂ 🡪 CH₂Cl₂ + Cl

These propagation steps are repeated until the chain is broken by two radicals colliding & merging.

Making Tetrachloromethane and Trichloromethane

Obviously, the possibility of the dichloromethane getting struck by a chlorine radical increase with time, resulting in these propagation steps yielding trichloromethane:

                                                CH₂Cl₂ + Cl 🡪 CHCl₂ + HCl

                                                CHCl₂ + Cl₂ 🡪 CHCl₃ + Cl

In a new propagation step, the chloromethyl radical can combine with a chlorine molecule, forming dichloromethane and regenerating a chlorine radical.

These propagation phases proceed until the chain is broken by any 2 radicals colliding, at which point the chain is broken.

How To Produce Only One Main Product?

Of course, if you wanted tetrachloromethane, you could use a lot of chlorine and ultimately all of the hydrogens would be replaced.

If you only wanted chloromethane, you could favour it by using a large amount of methane, so that the chances of a chlorine radical hitting a methane were constantly greater than anything else – but you’d still end up with a mixture of products.

There is no obvious technique to obtain dichloromethane or trichloromethane in large quantities.

Conclusion

Fluorine, chlorine, bromine, & iodine all have quite distinct interactions with methane. The most reactive element is fluorine. A combination of fluorine and methane will explode if no safeguards are taken. Methane and chlorine have an easy-to-control interaction, whereas bromine is much less reactive than chlorine. Methane does not react with iodine, on the other hand.

Because two molecules of gaseous products are generated from two compounds of gaseous starting products, the reaction entropy of methane halogenation is close to zero. As a result, the reaction enthalpy (H°) determines the thermodynamics of methane halogenation first and foremost. Because halogenation is a gas-phase reaction, the activation energy of methane halogenation is equal to the dissociation energy of the relevant halogen.