Chemistry
Halogenation of Alkanes
Halogenation of alkanes is a chemical reaction in which a halogen, such as chlorine or bromine, replaces a hydrogen atom in an alkane molecule. This reaction is a type of substitution reaction and is typically carried out in the presence of ultraviolet light or heat. The halogenation of alkanes is an important process for the synthesis of various organic compounds.
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10 Key excerpts on "Halogenation of Alkanes"
eBook - ePubOrganic Chemistry
Concepts and Applications
- Allan D. Headley(Author)
- 2019(Publication Date)
- Wiley(Publisher)
Alkanes are fairly inert compounds; some alkanes are used as solvents to provide inert media for different reactions that we will see in later chapters. In addition to combustion, alkanes undergo another type of reaction that is very important to organic chemists, and that is the reaction with halogens, specifically chlorine or bromine in the presence of energy in the form of heat or light. As the name suggests, this reaction involves the reaction of alkanes and bromine or chlorine and is called bromination or chlorination of alkanes, respectively. These reactions are performed in the presence of light or heat and are described as substitution reactions since a hydrogen or more than one hydrogen atoms of an alkane reactant are substituted for a halogen or more than one halogen in the product. Most of the reactions that will be encountered in this chapter involve the substitution of one hydrogen in the alkane for a halogen in the product.The products produced upon the chlorination of alkanes are alkyl halides, most are important industrial raw material for the synthesis of numerous other chemicals. For example, chloromethane was used once as a refrigerant, but discontinued owing to its flammability and toxicity. Today, it is widely used as a chemical intermediate for the production of different compounds, including polymers. Other chloroalkanes are widely used as a solvent in the research labs and in the production of rubber and in the petroleum refining industry. The chlorination of methane is shown in Reaction (14‐5) .(14‐5)To be able to predict the products of these and other similar reactions, a thorough understanding of how the reaction occurs is essential. Before we examine the reaction mechanism for the chlorination of alkanes, let us examine the process of bond reorganization for the reaction shown in Reaction (14‐5) . It should be obvious that this reaction is a substitution reaction in which a Cl─Cl bond has to be broken and the chlorine atoms form two new bonds, a new H─Cl bond and a new C─Cl bond in the CH3
eBook - ePub- Andrew F. Parsons(Author)
- 2013(Publication Date)
- Wiley(Publisher)
5 Br.- An alicyclic halogenoalkane has the carbon atoms in a closed ring, but the ring is not aromatic, e.g. bromocyclohexane, C6 H11 Br.
- An aromatic halogenoalkane has the carbon atoms in a closed ring and the ring is aromatic, e.g. bromobenzene, C6 H5 Br.
The formation of bromobenzene from benzene is discussed in Section 7.2.15.2 Preparation
5.2.1 Halogenation of Alkanes
Chloroalkanes (RCl) or bromoalkanes (RBr) can be obtained from alkanes (RH) by reaction with chlorine or bromine gas, respectively, in the presence of UV radiation. The reaction involves a radical chain mechanism.Radicals are introduced in Section 4.1 Radical reactions are discussed in Section 4.6.2This is a substitution reaction as a hydrogen atom on the carbon is substituted for a Cl or Br atom. A mixture of halogenated products is usually obtained if further substitution reactions can take place.Chloroalkanes, such as CH2 Cl2 (dichloromethane), are common solvents in organic synthesisPrimary, secondary and tertiary halogenoalkanes are defined in Section 2.1The ease of halogenation depends on whether the hydrogen atom is bonded to a primary, secondary or tertiary carbon atom. A tertiary hydrogen atom is more reactive because reaction with a halogen atom (X• ) produces an intermediate tertiary radical, which is more stable (and therefore more readily formed) than a secondary or primary radical (Section 4.3).5.2.2 Halogenation of Alcohols
Alcohols (ROH) are converted into halogenoalkanes using a number of methods. All methods involve ‘activating’ the OH group to make this into a better leaving group (Section 5.3.1.4). Reaction mechanisms are introduced in Section 4.11The mechanism of these reactions depends on whether a primary (RCH2 OH), secondary (R2 CHOH) or tertiary alcohol (R3
eBook - PDF- H. Stephen Stoker(Author)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
A halogenated alkane is an alkane derivative in which one or more halogen atoms are pres-ent. Similarly, a halogenated cycloalkane is a cycloalkane derivative in which one or more halogen atoms are present. Produced by halogenation reactions (Section 1-17), these two types of compounds represent the first class of hydrocarbon derivatives (Section 1-3) formally considered in this text. Alkanes have the general molecular formula C n H 2 n + 2 (Section 1-4). Halogenated alkanes containing one halogen atom have the general molecular formula C n H 2 n + 1 X; a halogen atom has replaced a hydrogen atom. If two halogen atoms are present in a halogenated alkane, the general molecular formula is C n H 2 n X 2 . Since cycloalkanes have the general molecular formula C n H 2 n (Section 1-12), a halogenated cycloalkane with one halogen atom present will have a general molecular formula of C n H 2 n 2 1 X. Nomenclature of Halogenated Alkanes The IUPAC rules for naming halogenated alkanes are similar to those for naming branched alkanes, with the following modifications: 1. Halogen atoms, treated as substituents on a carbon chain, are called fluoro -, chloro -, bromo -, and iodo -. 2. When a carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered of equal rank in determining the numbering sys-tem for the chain. The chain is numbered from the end closer to a substituent, whether it be a halo group or an alkyl group. 3. Alphabetical priority determines the order in which all substituents present are listed. 1. The most important chemical use for alkanes involves their reaction with a. oxygen b. chlorine c. bromine d. no correct response 2. The chemical products formed when an alkane undergoes complete combustion are always a. C and H 2 b. C and H 2 O c. CO 2 and H 2 O d. no correct response 3. Which of the following is not a product when an alkane molecule (R — H) reacts with a halogen molecule (X 2 )? a.
eBook - ePubUnderstanding Advanced Organic and Analytical Chemistry
The Learner's ApproachRevised Edition
- Kim Seng Chan, Jeanne Tan;;;(Authors)
- 2016(Publication Date)
- WS EDUCATION(Publisher)
CHAPTER 7
Halogen Derivatives
7.1 Introduction
Halogenoalkanes, also known as alkyl halides, are saturated organic compounds that contain the −C−X functional group (X = F, Cl, Br or I). They are important derivatives of alkanes and have the general formula Cn H2n+1 X. An example is bromoethane:Halogenoalkanes do not occur naturally. In fact, they are the by-products of the reaction of alkanes or alkenes with halogen, as these hydrocarbons are commonly found in petroleum. Halogenoalkanes are generally known as the “workhorse” in organic chemistry as they are very useful intermediates to be converted to other more important specialty chemicals of greater economic value. Some halogenoalkanes, such as chlorofluorocarbon, can also be harmful to the environment.Halogenoarenes (or aryl halides) are aromatic compounds with a halogen atom directly attached to the benzene ring. Similar to halogenoalkanes, halogenoarenes do not occur naturally and are in fact synthesized by reacting aromatic compounds isolated from petroleum with halogens.7.2 Nomenclature
A halogenolkane is obtained when one or more hydrogen atoms of an alkane molecule have been replaced by halogen atoms via the free radical substitution reaction. Other than this, halogenoalkanes can also be obtained when hydrogen halide (HX) or the diatomic halogen molecules add across an alkene double bond through the electrophilic addition mechanism. Thus, one can simply perceive halogenoalkanes as substituted alkanes. Therefore, halogenoalkanes are named in a similar manner to alkanes — the suffix ends in — ane
eBook - PDF- John M. McIntosh(Author)
- 2018(Publication Date)
- De Gruyter(Publisher)
This will greatly simplify review. It is also important to realize that the reactions must be learned frontwards and backwards. That is – we will see a reac-tion where A gives B under certain conditions. You should remember this in terms of how A reacts and also how to prepare B. https://doi.org/10.1515/9783110565140-005 62 | 5 Reactions of Alkanes, Alkenes, and Alkynes 5.2.2 Halogenation The replacement of hydrogen atoms by halogen (usually chlorine) is another common reaction of alkanes. The products are called alkyl halides . (Alkyl is the term used to describe a general structure of the type C n H 2 n + 1 ). The reaction is used frequently in industrial processes CH 3 CH 3 + Cl 2 → CH 3 CH 2 Cl + HCl The products, particularly if they are polyhalogenated (i.e., they contain several halo-gen atoms), are useful as flame retardants, insecticides, herbicides, and solvents. When alkanes of more complex structures are used, it is found that tertiary hydro-gens are replaced at a faster rate than secondary which, in turn, are replaced faster than primary hydrogens. It is frequently difficult to get clean replacement of one type to the complete exclusion of others and as a result, mixtures of products are com-monly obtained. If these mixtures can be used directly, this poses no problem, but frequently very undesirable properties are associated with the impurities. An exam-ple of this can be found in the chlorination of an organic molecule called phenol. The desired product – 2,4,6-trichlorophenol is contaminated with another product called dioxin, which has the reputation, perhaps undeserved, of being one of the most toxic compounds known. 5.3 Electrophilic Addition to Alkenes: Our First Mechanism E + E X X E Fig. 5.1 Alkenes (olefins) are electron-rich molecules; that is they contain more electrons than are required to hold the atoms together in the molecule. Therefore, they can be con-sidered to be nucleophilic compounds.
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
For example, alkanes are converted into alkyl chlorides or bromides, R–X (X 5 CI or Br, respectively), by a free-radical process in which a mixture of the alkane and halogen is heated at 200–400 8 C or is irradiated with ultraviolet light (Eq. 9.1). Under these conditions, the s -bond of molecular chlorine or bromine undergoes homolytic cleavage (Eq. 9.2) to generate chlo-rine and bromine atoms. These conditions provide sufficient energy to promote homo-lytic cleavage of the s -bond of molecular chlorine or bromine to generate chlorine and bromine atoms, respectively, which are free radicals (Eq. 9.2); the amount of energy required to effect this reaction is called the bond dissociation energy . Generating chlo-rine and bromine atoms is essential to initiating the reaction between an alkane and molecular chlorine or bromine to form alkyl halides. These may then be transformed into a variety of other functional groups. An environmentally damaging reaction associ-ated with the formation of chlorine atoms by photochemical degradation of chlorocar-bons is discussed in the Historical Highlight Keeping it Cool , which is available online. R H + X 2 R X + H X heat or h ν Alkyl halides (mixture) X = Cl or Br (9.1) X • + X • X X heat or h ν (9.2) 9 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 310 Experimental Organic Chemistry ■ Gilbert and Martin The procedures described in Sections 9.2 and 9.3 illustrate methods for trans-forming alkanes to alkyl chlorides and bromides by free-radical substitution reactions.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
We say that this addition occurs with anti selectivity. Alternatively, we say that the addition of halogens is stereospecific involving anti addition of the halogen atoms. Step 1: Reaction of the electrons of the carbon–carbon double bond with bromine forms a bridged bromonium ion intermediate in which bromine bears a positive formal charge. C C C C Br + + + C C – a bridged bromonium ion intermediate Br Br Br + – Br Br Step 2: A bromide ion (a Lewis base) attacks a carbon atom of the three-membered ring (a Lewis acid) from the side opposite the bridged bromonium ion, opening the three-membered ring. C + C C C Br Br anti orientation of added bromine atoms a Newman projection of the product – Br Br Br Br As we can see from the Newman projection above, these bromine atoms are trans to each other but, in open-chain alkanes, this relative position is rapidly scrambled by normal bond rotation around the carbon– carbon bonds. On the other hand, such rotation is not possible in a cycloalkene so the bromine atoms remain on opposite sides of the ring. H H + Br 2 trans-1,2-dibromocyclopentane H H Br Br Reduction of alkenes: formation of alkanes Most alkenes react quantitatively with molecular hydrogen, H 2 , in the presence of a transition metal catalyst to give alkanes. Commonly used transition metal catalysts include platinum, palladium, ruthenium and nickel. Yields are usually quantitative or nearly so. Because the conversion of an alkene to an alkane 840 Chemistry involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or catalytic hydrogenation. + H 2 25 °C, 3 × 10 5 Pa Pd cyclohexene cyclohexane The metal catalyst is used as a finely powdered solid, which may be supported on some inert material such as powdered charcoal or alumina.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
For example, consider the products that are formed when cyclopentene is treated with molecular bromine (Br 2 ): Br Br Br Br + Br 2 Notice that addition occurs in a way that places the two halogen atoms on opposite sides of the π bond. This mode of addition is called an anti addition. For most simple alkenes, halogenation appears to proceed primarily via an anti addition. Any proposed mechanism must be consistent with this observation. WATCH OUT The products of this reaction are a pair of enantiomers. They are not the same compound. Students are often confused with this particular example. For a review of enantiomers, see Section 5.5. 8.10 Halogenation and Halohydrin Formation 389 A Mechanism for Halogenation Molecular bromine is a nonpolar compound, because the Br—Br bond is covalent. Nevertheless, the molecule is polarizable, and the proximity of a nucleophile can cause a temporary, induced dipole moment (Figure 8.10). This effect places a partial positive charge on one of the bromine atoms, rendering that position electrophilic. Many nucleophiles are known to react with molecular bromine: Nuc Br Br + δ– δ+ – We have seen that π bonds are nucleophilic, and therefore, it is reasonable to expect an alkene to attack molecular bromine as well: Br + + Br Br Br δ– δ+ + – Although this step seems plausible, there is a fatal flaw in this proposal. Specifically, the production of a free carbocation is inconsistent with the observed anti stereospecificity of halogenation. If a free carbocation were produced in the process, then both syn and anti addition would be expected to occur, because the carbocation could be attacked from either side: Br Br Br + + — Br Syn addition Anti addition δ– δ+ + – This mechanism does not account for the observed anti stereospecificity of halogenation.
- John A. Joule, Norbert Krause, Martin Oestreich, Jörg Rademann, Ernst Schaumann, Thomas Wirth, John A. Joule, Norbert Krause, Martin Oestreich, Jörg Rademann, Ernst Schaumann, Thomas Wirth(Authors)
- 2017(Publication Date)
- Thieme Chemistry(Publisher)
403 35.1.5.1.12 Synthesis of 1-Chloro-n-Heteroatom-Functionalized Alkanes (n ‡2) by Addition across C=C Bonds (Update 2017) T. Wirth and F. V. Singh General Introduction Organochlorine compounds are important chemical entities for synthetic organic chem- istry and have numerous applications in the area of pharmaceutical and agricultural chemistry. [1–4] Chlorine is found in various naturally occurring organic compounds, some of which have biological properties. [5] Addition of molecular chlorine to alkenes is one of the familiar routes to achieve organochlorine derivatives. [6–9] The first report on the addition of chlorine to alkenes was published in 1877, just a century after the discov- ery of molecular chlorine. [10] In the early days, molecular chlorine was used as source of chlorine to construct C-Cl bonds despite the challenges of handling this gaseous re- agent. [11,12] In the past few decades, other chlorine sources have been used for the con- struction of C(sp 3 )-Cl bonds. Various approaches used for the construction of C-Cl bonds by addition to alkenes published in the years up to 2006 are compiled in Section 35.1.5.1. This update covers the chlorination of alkenes using various chlorinating reagents, focus- ing mostly on results reported since 2006. 35.1.5.1.12.1 Method 1: Dichlorination of Alkenes Dichlorination of alkenes can be achieved simply by the addition of chlorine sources to al- kenes. Generally, vicinal anti-dichlorination of alkenes is more common and very few re- ports are available of syn-dichlorination. Most organic chemists depend on molecular chlorine for the dichlorination of alkenes. [11,12] In 2003, Iskra and co-workers developed a more convenient approach for vicinal dichlorination of alkenes by passive transport of molecular chlorine through a fluorous solvent. [13] Since 2003, molecular chlorine as the source of chlorine has rarely been used for dichlorination reactions.
eBook - ePubSurvival Guide to Organic Chemistry
Bridging the Gap from General Chemistry
- Patrick E. McMahon, Bohdan B. Khomtchouk, Claes Wahlestedt(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
electron donation .- Directly attached carbon groups (alkyl-substituent groups) are more efficient than substituted hydrogen atoms at donation of electron density to electrophilic carbon. This is due to the versatility (inductive and hyperconjugative effects) of the bonding electrons in the carbon group.
- The net result is that a positive charge or radical on electrophilic carbon is more stable (i.e., "“less unfavorable”) whenever the electrophilic carbon itself is bonded to the greatest number of alkyl groups (carbon groups ).
- Summary:
12.2 FREE RADICAL Halogenation of Alkanes/ALKYL GROUPS
12.2.1 REACTION CONCEPTS
- The overall reaction is a net substitution of a hydrogen for a halogen in an alkane to form an alkyl halide; the other product is a hydrogen halide. The carbon being substituted can be methyl, primary, secondary, or tertiary. The halogen is usually either diatomic Cl2 or Br2 . Light often is used to initiate the reaction.
- The rate of the reaction depends primarily on the alkyl-substitution pattern of the carbon which undergoes the halogen/hydrogen exchange.
- The rate-limiting step involves the breaking of the C — H bond to form a carbon radical. The ease of bond breaking follows the trends for radical stability.
- Resulting summary: (R = specifically an alkyl group )
- Hydrogens can be designated by the type of carbon they are bonded to: Tertiary hydrogen is a hydrogen bonded to a tertiary carbon; a secondary hydrogen is a hydrogen bonded to a secondary carbon; and a primary hydrogen is a hydrogen bonded to a primary carbon.
- The major product in a free radical reaction generally follows the relative rates of reaction for the corresponding hydrogen exchange . This is especially true for reaction with the halogen Br2 . Exchange of H for X will occur primarily at the tertiary C — H position if present in the molecule, followed by a secondary C — H followed by a primary C—H. Major product formation, based on potential available C—H
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