Halogenation of alkanes (2023)

Metanecloration by substitution

Halogenation is the replacement of one or more hydrogen atoms in an organic compound with a halogen (fluorine, chlorine, bromine or iodine). In contrast to complex burns, the halogenation of an alkeanicReplacement reactionwhere a C-H connection is broken and a new C-X connection is formed. The chlorination of methane shown below provides a simple example of this reaction.

Pez₄+ CL₂+ Energia → CH₃CL + HCL

Since only two covalent bonds (C-H and CL-CL) are interrupted and two covalent bonds (C-CL and H-CL) are formed, this reaction seems to be an ideal case for mechanistic exams and speculation.Hydrogen atoms of an alkay replacement, which leads to a mixture of products, as shown belowUnbalanced equationThe relative amounts of different products depend on the proportion of the two reagents used. In the case of methane, a large excess of hydrocarbons favors the formation of methyl chloride as a main product; a surplus in chlorine promotes the formation of chloroform and carbontetrachlor

Pez₄+ CL₂+ Energia → CH₃CL + CH₂CL₂+ CHCl₃+ CCL₄+ HCL

Empirical considerations

The following facts must be accommodated by an appropriate mechanism for halogenation reaction.

1. The reactivity of halogens decreases in the following order: F.₂> CL₂> BR₂> Yo₂.
2. We will limit our attention to chlorine and bromine, since fluoride is so explosively reactive that it is difficult to control, and iodine is generally not reactive.
3. Clorinies and brominations are generally exothermic.
4. Energy entry in heat or light is required to begin these halogenations.
5. When the light is used to begin halogenation, thousands of molecules react to each photon absorbed by the light.
6. Haloging reactions can be performed in the gas or liquid phase.
7. In the chlorinations of the gas phase, the presence of oxygen (a radical trap) inhibits the reaction.
8. In the liquid phase, radical initiators, such as peroxides, facilitate reaction.

The most plausible mechanism for halogenation is a chain reaction in which neutral intermediate products, such as free radicals or atoms.The initial step is, therefore, the homolitic division of this heat or light connection, keep in mind that the castle and bromo they take the visible light (they are colorful). A chain reaction mechanism has been described to chlorinMethane. However, it is slower and more selective, since a bromatoma is a less reactive hydrogen absent than a chlorine atom, which is reflected in the greatest H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-CS.

selectivity

If the greatest alkanes that are inhabited, isomeric products will be formed.Five constitutional isomersIt exists for trichloric propagation. Can you write structural formulas for the four soméros densely?

\ [CH_3CH_2CH_3 + 2CL_2 \ REJETROW \ Text {Quatro} \;C_3H_6CL_2 \;\ text {isomere} + 2 hcl \]

Propana halogenation shows an interesting feature of these reactions.All hydrogens in a complex Alcón do not have the same reactivityFor example, propane has eight hydrogen, six of which are structurally equivalentPrimaryAnd the other two aresecondary. ° C results in 45% 1 chlorine propane and 55% 2 propane chlorine.

Pez₃-Ch₂-Ch₃+ +CL₂→ 45% CH₃-Ch₂-Ch₂CL+ 55% CH₃-ChCL-Ch₃

The results of the bromation (induced light at 25 ° C) are even more surprising and represent 97% of the mono bromine product.

Pez₃-Ch₂-Ch₃+ +BR₂→ 3% CH₃-Ch₂-Ch₂BR+ 97% CH₃-ChBR-Ch₃

These results suggest that the second hydrogen is more reactive than the 1st-hydogenic for a factor of approximately 3: 1. The experiments than the Hydogenic 3rd are even more reactive for halogen atoms. Methylpropane predominantly (65%) 2-clore-2-Methylpropane, the product of the replacement of the only 3rd hydrogen, despite the existence of nine 1 1 hydrogen in the molecule.

(CH₃)₃CH +CL₂→ 65% (CH₃)₃CCL+ 35% (CH₃)₂Iglesia₂CL

A review of the two steps of which the chain reaction of the free radical chain consists in halogenation must be clear that the first step (hydrogen abstraction) isProduct determination stageIf a carbon radical is formed, the subsequent connection to a halogen atom (in the second step) can only occur in the radical area.

First step: Riñonal₃CH +X·→ R₃C·+ H-X

Second step: Riñonal₃C·+ +X₂→ R₃CX+ +X·

Since the H-X product is common to all possible reactions, the differences in reactivity can only be attributed to the differences in the dissociation energies of the C-H connections. In our previous discussion on the energy of the title, we accept average values for all thecertain types, but now we see that this is not rigorous. In the case of carbon hydrogen bonds, there are significant differences and specific dissociation energies (energy required to break a link) for different types of C-H connections.

The difference in the dissociation energy of the C-H connection to primary (1st), secondary (2nd) and tertiary (3 °) agrees with the halogenation observations given above, since we hope that the weakest titles will breakMore easily than these strong ties. After this reasoning, we would expect that the points of Benzil and Alilas in free radicalization are exceptionally reactive, as the experiments showed. The Metilo de Toluol group, C.₆H₅Pez₃It is slightly chlorinated or joined in the presence of initiators of free radicals (usually peroxides), and ethylbenzene are similarly chlorinated in the benzyl location. Hydrogen (called phenyl -hydrogen above) is relatively high in the aromatic ring) and notThey replace.

C₆H₅Pez₂Pez₃+ +CL₂→ C₆H₅PezCLPez₃+ HCL

Because double carbon bonds in liquid phase solutions add chlorine and bromine quickAlilicals

The homolisms of covalent union that define the dissociation energies of the connection listed above can be described by the general equation:

Riñonal₃C-H + Energie → R₃C·+ H·

The dissociation energies of the C-H connection are generally interpreted in terms of radical stability.However, this interpretation was questioned by Gronrt.Especially with respect to the selectivity of free radicals, it is the energies of the titles that are important and not why they are.