Halogenation of alkanes (2023)

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    Substitutional Methane Chlorination

    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 the complex transformations of combustion, the halogenation of an alkane appears straightforward.substitution reactionin which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane shown below is a simple example of this reaction.

    CH4+Kl2+ Energy → CH3Cl + HCl

    Since only two covalent bonds (C-H and Cl-Cl) are broken and two covalent bonds are formed (C-Cl and H-Cl), this reaction seems an ideal case for mechanistic studies and speculations. However, one complication is that all of the hydrogen atoms in an alkane can undergo substitution, resulting in a mixture of products as shown below.unbalanced equation. The relative amounts of the various products depend on the ratio of the two reagents used. In the case of methane, a large excess of the hydrocarbon favors the formation of methyl chloride as the main product; while an excess of chlorine favors the formation of chloroform and carbon tetrachloride.

    CH4+Kl2+ Energy → CH3Cl + CH2Kl2+ CHCl3+ CCl4+ HCl

    (Video) Halogenation of alkanes

    empirical considerations

    The following facts must be explained by any reasonable mechanism for the halogenation reaction.

    1. The reactivity of the halogens decreases in the following order: F2>Kl2> brother2> yo2.
    2. We will limit our attention to chlorine and bromine since fluorine is so explosively reactive that it is difficult to control and iodine is generally unreactive.
    3. Chlorination and bromination are usually exothermic.
    4. Energy input in the form of heat or light is required to initiate these halogenations.
    5. When light is used to initiate halogenation, thousands of molecules react to each photon of light absorbed.
    6. Halogenation reactions can be carried out in the gas or liquid phase.
    7. In the case of gas phase chlorination, the presence of oxygen (radical scavenger) inhibits the reaction.
    8. In the case of halogenation in the liquid phase, radical initiators such as peroxides facilitate the reaction.

    The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. The weakest covalent bond in the reagents is the halogen-halogen bond (Cl-Cl = 58 kcal/mol; Br-Br = 46 kcal/mol), so the initial step is the homolytic cleavage of this bond by heat or light and chlorine. Bromine both absorb visible light (are colored). A chain reaction mechanism has been described for the chlorination of methane. Bromination of alkanes occurs by a similar mechanism but is slower and more selective because a bromine atom is a less reactive hydrogen extractant than a chlorine atom, as reflected in the higher binding energy of H-Cl than H-Br.


    When alkanes larger than ethane are halogenated, isomeric products are formed. Thus, chlorination of propane gives both 1-chloropropane and 2-chloropropane as monochlorinated products. Four constitutionally isomeric dichlorinated products are possible, andfive constitutional isomersexist for trichlorinated propanes. Can you write structural formulas for the four dichlorinated isomers?

    \[CH_3CH_2CH_3 + 2Cl_2 \rightarrow \text{Vier} \; C_3H_6Cl_2\; \text{Isómero} + 2 HCl\]

    (Video) Halogenation of Alkenes & Halohydrin Formation Reaction Mechanism

    The halogenation of propane reveals an interesting feature of these reactions.All hydrogen atoms in a complex alkane do not have the same reactivity.. For example, propane has eight hydrogen atoms, six of which are structurally equivalent.primary, and the other two aresecondary. If all of these hydrogens were equally reactive, halogenation should produce a 3:1 ratio of 1-halopropane to 2-halopropane monohalogen products, reflecting the primary/secondary numbers. We don't observe that. Light-induced gas phase chlorination at 25°C gives 45% 1-chloropropane and 55% 2-chloropropane.

    CH3-CH2-CH3+Kl2→ 45 % CH3-CH2-CH2Kl+ 55 % CH3-CHKl-CH3

    The bromination results (induced light at 25 °C) are even more surprising, since 2-bromopropane represents 97% of the monobrominated product.

    CH3-CH2-CH3+brother2→ 3% CH3-CH2-CH2brother+ 97 % CH3-CHbrother-CH3

    These results strongly suggest that 2nd hydrogens are inherently more reactive than 1st hydrogens by a factor of approximately 3:1. Other experiments showed that hydrogens of the third degree are even more reactive with halogen atoms. Thus, light-induced chlorination of 2-methylpropane gave predominantly (65%) 2-chloro-2-methylpropane, the substitution product of the single 3° hydrogen, despite the presence of nine 1° hydrogens in the molecule.

    (CH3)3CH+Kl2→ 65 % (CH3)3CKl+ 35% (CH3)2CHCH2Kl

    (Video) Radical Halogenation of Alkanes

    From an overview of the two steps that make up the free radical chain reaction for halogenation, it should be clear that the first step (hydrogen abstraction) is thatproduct determining step. Once a carbon radical is formed, subsequent attachment to a halogen atom (in the second step) can only occur at the radical site. Consequently, from the analysis of this first step, an understanding of the preference for substitution at the 2nd and 3rd carbons should emerge.

    First step: R3CH+X·→ R3C·+ H-X

    Second step: R3C·+X2→ R3CX+X·

    Since the H-X product is common to all possible reactions, differences in reactivity can only be attributed to differences in C-H bond dissociation energies. In our earlier discussion of bond energies, we assumed average values ​​for all bonds of a given type, but now we see that strictly speaking this is not the case. In the case of carbon-hydrogen bonds, there are significant differences and specific dissociation energies (energy required to homolytically break a bond) have been measured for different types of C-H bonds. These values ​​are given in the following table.

    R (an R–H) Methyl Äthyl i-Propil t-butyl Phenyl selfish he is Vinyl
    bond dissociation energy
    103 98 95 93 110 85 88 112

    The difference in reported C–H bond dissociation energies for the primary (1st), secondary (2nd), and tertiary (3rd) sites is consistent with previously reported halogenation observations in which we would expect weaker bonds to break more easily than strong links. Based on this consideration, we would expect the benzyl and allyl sites to be exceptionally reactive towards radical halogenation, as experiment has shown. The methyl group of toluene, C6H5CH3, is readily chlorinated or brominated in the presence of free radical initiators (usually peroxides), and ethylbenzene is similarly chlorinated exclusively at the benzylic site. The hydrogens attached to the aromatic ring (referred to above as phenyl hydrogens) have relatively high bond dissociation energies and are unsubstituted.

    C6H5CH2CH3+Kl2→ C6H5CHKlCH3+HKl

    (Video) Halogenation of Alkanes

    Since carbon-carbon double bonds in liquid phase solutions rapidly add chlorine and bromine, radical substitution reactions of alkenes for these halogens must be carried out in the gas phase or by other halogenating reagents. One of these reagents is N-bromosuccinimide (NBS), shown in the second equation below. By using NBS as a brominating agent, allylic brominations are easily achieved in the liquid phase.

    Halogenation of alkanes (2)

    The homolyses of covalent bonds that define the bond dissociation energies listed above can be described by the general equation:

    R3C-H + Energy → R3C·+H·

    The dissociation energies of C–H bonds are commonly interpreted in terms of radical stability.However, this interpretation was disputed by Gronert.More importantly, when it comes to the selectivity of free radical reactions, it is the bond energies that matter, not why they are the way they are.


    (Video) The Halogenation of Alkanes


    What is halogenation of alkanes with example? ›

    The reaction of a halogen with an alkane in the presence of ultraviolet (UV) light or heat leads to the formation of a haloalkane (alkyl halide). An example is the chlorination of methane.

    What is the process of halogenation? ›

    Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance wherein the halogen atom ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

    What is halogenation of alkenes? ›

    Halogenation of alkenes means the addition of a halogen like bromine, or chlorine on the double bond of alkenes. Halogen dissociates into an ion consisting of opposite charges and attacks on the double bond to form dihalide.

    Why is halogenation of alkanes an important reaction? ›

    One of these reactions is halogenation, or the substitution of a single hydrogen on the alkane for a single halogen to form a haloalkane. This reaction is very important in organic chemistry because it opens a gateway to further chemical reactions.

    What happens in a halogenation reaction? ›

    A Halogenation reaction occurs when one or more fluorine, chlorine, bromine or iodine atoms replace hydrogen atoms in organic compound. The order of reactivity is fluorine > chlorine > bromine > iodine. Fluorine is especially aggressive and can react violently with organic materials.

    What does a halogenation reaction produce? ›

    Halogenation is a reaction that occurs when one or more halogens are added to a substance. Halogens comprise the seventh column in the periodic table and include fluorine, chlorine, bromine, iodine, and astatine. The resulting product of a halogenation reaction is known as a halogenated compound.

    What are the steps of halogenation of alkanes? ›

    Hint: Halogenation of alkanes means the substitution of a halogen atom(s) by the removal of one or more hydrogen atoms in the alkane. The mechanism of halogenations occurs in three steps: chain initiation, chain propagation, and chain termination.

    What is the first step of the halogenation of alkanes? ›

    The first step is the halogen radical abstracting the hydrogen from our alkane. We form our side product (HBr in this case) in this step. Next, we are going to have a reaction with another molecule of the halogen. This step generates the halogenated product and regenerates the halogen radical.

    What is the order of reactivity of halogenation of alkanes? ›

    The order of reactivity of different halogens towards halogenation of alkanes is: A. F2>Cl2>Br2>I2.

    What conditions are needed for halogenation? ›

    Energy input in the form of heat or light is necessary to initiate these halogenations. If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed. Halogenation reactions may be conducted in either the gaseous or liquid phase.

    What is an example of halogenation reaction? ›

    Halogenation is the addition of halogen atoms to a π‐bond system. For example, the addition of bromine to ethene produces the substituted alkane 1,2‐dibromoethane.

    What does halogenation of alkenes produce? ›

    The halides add to neighboring carbons from opposite faces of the molecule. The resulting product is a vicinal (neighboring) dihalide.

    What bonds are made in alkane halogenation reactions? ›

    Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed.

    What type of reaction occurs in the halogenation experiment? ›

    Generally, halogenation is the reaction of a halogen with an alkane in which the introduction of halogen atoms occurs into the organic molecule by an addition reaction or by a substitution reaction.

    What is the purpose of halogenation? ›

    Chlorine halogenation reactions go on to enable the production of myriad other compounds that are integral to commercial products, medical components, industrial goods, and more. The estimated global sales of products benefiting from chlorine halogenation reactions is $38 billion per year.

    What are 4 examples of alkanes? ›

    Methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10) are the first four alkanes.

    Which of the following is an example of a halogenation reaction? ›

    Example: Reaction of methane (CH4) with bromine (Br2)

    Here, one of the hydrogen atoms from the methane is replaced by a bromine atom.

    What happens when alkanes react with halogens? ›

    In the presence of light, or at high temperatures, alkanes react with halogens to form alkyl halides. Reaction with chlorine gives an alkyl chloride. Reaction with bromine gives an alkyl bromide. Unsaturated hydrocarbons such as alkenes and alkynes are much more reactive than the parent alkanes.

    What are the different types of halogenation? ›

    There are two basic types of halogenation reactions: (1) substitution reactions in which the halogen replaces another atom in the molecule, for example the chlorination of ethane and (2) addition reactions in which the halogen reacts with an unsaturated molecule, for example the reaction of chlorine or bromine with ...

    What is the most common reaction of alkanes? ›

    Combustion is the most important reaction of alkanes. Alkanes burn in oxygen to form carbon dioxide and water vapor.

    How do you remember the first 10 alkanes? ›

    For example the first 10 alkanes in order are , Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane and Decane. These can be memorised with “Many elephants prefer blue pinapples.

    What is the major product of halogenation of alkanes? ›

    Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms. Alkanes are notoriously unreactive compounds because they are non-polar and lack functional groups at which reactions can take place.

    What is the product produced from the halogenation of alkenes? ›

    The resulting product is a vicinal (neighboring) dihalide.

    What are the three steps for the reaction between alkanes and halogens? ›

    15.1: Free Radical Halogenation of Alkanes
    • Step 1: Initiation.
    • Step 2: Propagation.
    • Step 3: Termination.
    Jun 5, 2019

    What is the mechanism of halogenation in alkanes? ›

    Hint: Halogenation of alkanes means the substitution of a halogen atom(s) by the removal of one or more hydrogen atoms in the alkane. The mechanism of halogenations occurs in three steps: chain initiation, chain propagation, and chain termination.

    Does halogenation need a catalyst? ›

    Halogenation is an example of electrophillic aromatic substitution. In electrophilic aromatic substitutions, a benzene is attacked by an electrophile which results in substition of hydrogens. However, halogens are not electrophillic enough to break the aromaticity of benzenes, which require a catalyst to activate.


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