Free Radical Halogenation Of Alkanes

There are just a few reactions that alkanes can go through. Halogenation, or the replacement of a single hydrogen on an alkane for a single halogen to generate a haloalkane, is one of these processes. This reaction is crucial in organic chemistry since it serves as a springboard for subsequent chemical reactions.

While there are few reactions that can be done with alkanes, there are many that can be done with haloalkanes. The chlorination of methane will be extensively examined in order to better understand the mechanism (a detailed look at the step-by-step method by which a reaction occurs). When methane (CH4) & chlorine (Cl2) are mixed at room temperature in the lack of light, nothing happens. However, if the circumstances are adjusted so that the reaction occurs at high temperatures (denoted by) or under ultraviolet irradiation, a product, chloromethane, is generated (CH3Cl).

ENERGETICS:

Examining the change in enthalpy (H) that occurs during the reaction is one technique to answer these questions.

H = (Energy invested in reaction) – (Energy released from reaction) 

When more energy is invested into a reaction than is released, the H is positive, the process is endothermic, and it is not energy-efficient. If the reaction gives off more energy than it takes in, the H is negative, the process is exothermic, and it is regarded as favorable. The difference between exothermic and endothermic reactions is depicted in the diagram below.

RADICAL CHAIN MECHANISM:

The radical chain mechanism is used to carry out the reaction. The reaction requires an energy input to start, but after that it is self-sustaining. The very first propagation step consumes one of the initiation products, while the next propagation step generates another, allowing the cycle to continue indefinitely.

STEP 1 – INITIATION:

The link between the chlorine molecules is broken during initiation (Cl2). This phase necessitates the expenditure of energy, and therefore is not energetically advantageous. After this stage, the reaction can continue indefinitely (as long as the reactants supply energy) without the need for additional energy. It’s worth noting that this section of the system can’t work without some external energy, such as light or heat.

STEP 2 – PROPAGATION:

The mechanism’s next two steps are known as propagation steps. A Cl radical reacts with a hydrogen on the methane in the first propagation step. This produces methyl radical and hydrochloric acid (HCl, the inorganic result of this reaction). More chlorine starting material (Cl2) is utilized in the second propagation phase, one of the Cl atoms becomes a radical, and the other joins with the methyl radical.

The initial propagation phase is endothermic, which means it absorbs heat (2 kcal/mol) that is not energetically advantageous. The second propagation phase, on the other hand, is exothermic, generating 27 kcal/mol. The second propagation phase is incredibly fast since it is so exothermic.

The second propagation step consumes a product from the first propagation phase (the methyl radical), and according to Le Chatelier’s principle, when the first step’s product is removed, the equilibrium shifts to its products. This concept regulates the occurrence of the unfavorable first propagation phase.

TERMINATION:

In the termination steps, all of the leftover radicals unite to form more product (CH3Cl), more reactant (Cl2), and even combinations of the two methyl radicals to form an ethane side product (in all possible ways) (CH3CH3).

CHLORINATION OF METHANE CONSTRAINTS:

After one chlorination, methane chlorination often does not come to an end. Monosubstituted chloromethane may be quite difficult to get. Di, tri, and sometimes even tetra-chloromethanes are generated instead. Using a considerably higher quantity of methane than chloride is one technique to avoid this problem.

 This minimises the possibility of a Cl radical colliding with a chloromethane and restarting the reaction to generate dichloromethane. One can have a relative degree of relation to the product using this way of managing product ratios.

CONCLUSION:

Experiments have revealed that the reaction does not occur when the alkane and halogen reactants are not exposed to UV light or heat. Once a reaction has begun, however, the heat or light source can be turned off but the reaction will proceed. 

Mechanism of halogenation the carbon hydrogen links in the methane molecule are low polarity covalent connections. A nonpolar covalent link exists between the halogen molecule and the oxygen molecule. UV radiation has enough energy to destroy the weaker nonpolar chlorine chloride link (58 kcal/mole), but not enough to break the stronger carbon hydrogen bond (104 kcal/mole). 

The creation of two extremely reactive chlorine free radicals results from the fracture of the chlorine molecule (chlorine atoms). An atom or a group of atoms is referred to as a free radical.

To summarize, this free radical chain reaction starts with a small number of free radicals and a large number of reactant molecules. The number of free radicals increases as the reaction progresses, whereas the number of reactant molecules drops. 

There are considerably more free radicals than reactant molecules near the end of the reaction. Termination steps become the major reactions at this stage of the overall reaction. All of the halogenation mechanisms are extremely fast, and the products are formed in microseconds.