Halogenation is defined as the substitution of a halogen for one or more hydrogen atoms in an organic molecule (fluorine, chlorine, bromine or iodine). The halogenation of an alkane appears to be a straightforward substitution reaction in which a C-H bond is broken and a new C-X bond is generated, unlike the complicated changes of combustion. A basic example of this reaction is the chlorination of methane, as shown below.
CH4+ Cl2+energy →CH3Cl+HCl
This reaction appears to be an ideal scenario for mechanistic inquiry and hypothesis because only two covalent bonds are broken (C-H & Cl-Cl) and two covalent bonds are generated (C-Cl & H-Cl). One issue is that all of an alkane’s hydrogen atoms can be substituted, resulting in a mixture of products, as indicated in the unbalanced equation below. The proportions of the two reactants utilized determine the proportional amounts of the various products. A considerable excess of methane favors the creation of methyl chloride as the primary product, whereas an excess of chlorine favors the formation of chloroform and carbon tetrachloride.
CH4+ Cl2+energy → CH3Cl+CH2Cl2+CHCl3+CCl4+HCl
Overview of Propane
Isomeric products are generated when alkanes bigger than ethane are halogenated. As a result of the chlorination of propane, mono-chlorinated compounds include 1-chloropropane and 2-chloropropane. Four constitutionally isomeric dichlorinated products are feasible, and tri chlorinated propanes has five constitutional isomers.
Can you write the four dichlorinated isomers’ structural formulas?
CH3CH2CH3+2Cl2 →Four C3H6Cl2 isomers+2HCl —1
A fascinating aspect of these reactions is shown by the halogenation of propane. The reactivity of all hydrogens in a complex alkane is not equivalent. Propane, for example, includes eight hydrogens, six of which are structurally equivalent main hydrogens and two of which are secondary hydrogens. If all of these hydrogen atoms were equally reactive, halogenation should result in a 3:1 ratio of mono-halogenated 1-chloropropane to 2-chloropropane mono-halogenated products, according to the primary/secondary numbers. This does not appear to be the case. At 25 degrees Celsius, light-induced gas phase chlorination yields 45 % 1-chloropropane and 55 percent 2-chloropropane.
CH3–CH2–CH3+ Cl2→45% CH3–CH2–CH2Cl+55% CH3-CHCl-CH3
These findings strongly suggest that 2o-hydrogens are more reactive intrinsically than 1°-hydrogens by a factor of roughly 3:1. Experiments revealed that 3o-hydrogens are even more reactive toward halogen atoms than 2°-hydrogens. Despite the existence of nine 1°-hydrogens in the molecule, light-induced chlorination of 2-methylpropane yielded primarily (65%) 2-chloro-2-methylpropane, the substitution product of the sole 3°-hydrogen.
CH33CH+ Cl2 → 65% CH33CCl+35% CH32CHCH2Cl
An examination of the two steps that make up the halogenation free radical chain reaction should reveal that the first (hydrogen abstraction) is the product determining step.
Following the formation of a carbon radical, further bonding with a halogen atom (in the second phase) can only take place at the radical site. As a result, an examination of this first step is required to comprehend the predilection for replacement at the 2° and 3° carbon atoms.
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R3CH+X∙ → R3C∙ +H-X
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R3C∙+X2 → R3CX +X∙
Because the H-X product is common to all conceivable reactions, changes in reactivity can only be attributed to variances in C-H bond dissociation energy. In our previous discussion of bond energy, we assumed average values for all bonds of a given class, but we now see that this is not strictly true. There are considerable variances in carbon-hydrogen bonds, and specific dissociation energies (energy required to break a bond homolytically) for several types of C-H bonds have been measured.
Propane and Halogen
Propane: C3H8 is a highly flammable gaseous alkane found in crude petroleum and natural gas that is utilized primarily as a fuel and in chemical synthesis.
At normal temperature and pressure, it is a gas, but it may be compressed into a liquid and transported. As a byproduct of natural gas processing and petroleum refining, it is commonly used as a fuel in domestic and industrial applications, as well as in low-emissions public transportation.
Halogen: any of the five elements fluorine, chlorine, bromine, iodine, and astatine, which belong to the periodic table’s group VIIA and are generally found in the free state as diatomic molecules.
The term “halogen” refers to a substance that produces salt. When halogens react with metals, a variety of salts are produced, including calcium fluoride, sodium chloride (table salt), silver bromide, and potassium iodide.
The halogens are the only periodic table group that comprises elements in all three primary states of matter at ordinary temperatures and pressures. When halogens are combined with hydrogen, acids are produced.
Conclusion
Alkanes can be halogenated by adding a halogen gas and energy, with the halogens’ reactivity proceeding in the following order:
Cl2>Br2>I2
UV radiation or heat triggers a chain reaction in this reaction, which cleaves the covalent bond between the two atoms of a diatomic halogen. The halogen radicals can then take protons from the alkanes, allowing them to mix or react to produce additional radicals. Alkanes can be halogenated in a variety of ways, and the end product is usually a combination of halogenated compounds.
Propane monobromination Propane is brominated with diatomic bromine in this process. The stability of the intermediate radicals plays a role in the product distribution in this reaction, which is outside the scope of this atom.
The majority of halogens are made from minerals or salts.