Chemistry
Hydrohalogenation of Alkenes
Hydrohalogenation of alkenes is a chemical reaction in which a hydrogen halide (such as HCl or HBr) adds across the carbon-carbon double bond of an alkene, resulting in the formation of a halogenated alkane. The reaction proceeds via a carbocation intermediate and follows Markovnikov's rule, where the hydrogen atom adds to the carbon atom with more hydrogen substituents.
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11 Key excerpts on "Hydrohalogenation of Alkenes"
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 - 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- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
This step is an S N 2 process and must therefore proceed via a 390 CHAPTER 8 Addition Reactions of Alkenes back-side attack (as seen in Section 7.4). The requirement for back-side attack explains the observed stereochemical requirement for anti addition. The stereochemical outcome for halogenation reactions is dependent on the configuration of the starting alkene. For example, cis-2-butene will yield different products than trans-2-butene: Br Br Br Br + Br 2 cis-2-Butene Br Br Br Br Meso Br 2 trans-2-Butene Anti addition across cis-2-butene leads to a pair of enantiomers, while anti addition across trans- 2-butene leads to a meso compound. These examples illustrate that the configuration of the starting alkene determines the configuration of the product for halogenation reactions. Halohydrin Formation When bromination occurs in a non-nucleophilic solvent, such as CHCl 3 , the result is the addition of Br 2 across the π bond (as seen in the previous sections). However, when the reaction is performed in the presence of water, the bromonium ion that is initially formed can be captured by a water mol- ecule, rather than bromide: Bromonium ion En + H H Br O H H Br O + + The intermediate bromonium ion is a high-energy intermediate and will react with any nucleophile that it encounters. When water is the solvent, it is more likely that the bromonium ion will be captured by a water molecule before having a chance to react with a bromide ion (although some dibromide product is likely to be formed as well). The resulting oxonium ion is then deprotonated to give the product (Mechanism 8.5).
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
The addition of the unsymmetrical reagent hydrogen bromide, H–Br, to an alkene is shown in Equation 10.16, and the acid-catalyzed addition of water, or hydration , of an alkene is depicted in Equation 10.17. In both reactions, the p -bond is protonated to form the more stable intermediate carboca-tion according to Markovnikov’s rule. This carbocation may then rearrange to a more stable carbocation (Sec. 10.3), or, as shown in the present case, may undergo direct nucleophilic attack from both faces of the planar carbocation to give the observed alkyl bromide or alcohol. These two examples of electrophilic additions to alkenes are regioselective processes because the two unsymmetrical reactants com-bine predominantly in one orientational sense to give one regioisomer preferentially. CH Br + R 1 C H R 3 R 2 H CH R 1 C R 3 R 2 + Br: – H Br CH R 1 C R 3 R 2 As mixture of stereoisomers (10.16) 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. 360 Experimental Organic Chemistry ■ Gilbert and Martin CH + R 1 C H + R 3 R 2 H 2 O H CH R 1 C R 3 R 2 + H 2 O: B: – H HO + CH R 1 C H R 3 R 2 As mixture of stereoisomers H HO CH H + B R 1 C R 3 R 2 (10.17) Since the ionic addition of hydrogen bromide and the acid-catalyzed addition of water to an alkene proceed via planar carbocations, the formation of products derived from rearranged carbocations is relatively common.
eBook - PDF- David R. Klein(Author)
- 2016(Publication Date)
- Wiley(Publisher)
Many steps 3 2 1 TDBMS = O CH 3 OTBDMS O CH 3 OTBDMS O CH 3 O O H 3 COCO H Si t-Bu CH 3 CH 3 Try Problems 8.46, 8.70 PRACTICE the skill APPLY the skill need more PRACTICE? 390 CHAPTER 8 Addition Reactions of Alkenes REVIEW OF REACTIONS 1. Hydrohalogenation (Markovnikov) 2. Hydrohalogenation (anti -Markovnikov) 3. Acid-catalyzed hydration and oxymercuration-demercuration 4. Hydroboration-oxidation 5. Hydrogenation 6. Bromination 7. Halohydrin formation 8. Anti dihydroxylation 9. Syn dihydroxylation 10. Ozonolysis O O H 1) OsO 4 2) NaHSO 3 H 2 O NaOH, cold KMnO 4 1) RCO 3 H 1) O 3 2) DMS Br 2 , H 2 O X HX Br 2 ROOR HBr, H 3 O + H 2 Pt Br 1) Hg(OAc) 2 , H 2 O 2) NaBH 4 OH 1) BH 3 ∙ THF 2) H 2 O 2 , NaOH 2) H 3 O + Br Br En + OH En + OH Br En + OH OH En + OH OH En + 1 2 3 4 5 6 7 8 9 10 REVIEW OF CONCEPTS AND VOCABULARY SECTION 8.1 • Addition reactions are characterized by the addition of two groups across a double bond. SECTION 8.2 • Alkenes are abundant in nature. • Ethylene and propylene, both formed from cracking petroleum, are used as starting materials for a wide variety of compounds. SECTION 8.3 • Addition reactions are thermodynamically favorable at low temperature and disfavored at high temperature. SECTION 8.4 • Hydrohalogenation reactions are characterized by the addi- tion of H and X across a π bond, where X is a halogen. • For unsymmetrical alkenes, the placement of the halogen represents an issue of regiochemistry. Hydrohalogenation reactions are regioselective, because the halogen is gen- erally installed at the more substituted position, called Markovnikov addition. • In the presence of peroxides, addition of HBr proceeds via an anti-Markovnikov addition. • The regioselectivity of an ionic addition reaction is deter- mined by the preference for the reaction to proceed through the more stable carbocation intermediate. • When one new chiral center is formed, a racemic mixture of enantiomers is obtained.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
The result is formation of a halohydrin as the major product. If the halogen is bromine, it is called a bromohydrin, and if chlorine, a chlorohydrin. + C C X OH X 2 + + H 2 O + HX Halohydrin (major) C C X X vic-Dihalide (minor) X = Cl or Br C C Halohydrin formation can be described by the following mechanism. Halohydrin Formation from an Alkene A MECHANISM FOR THE REACTION [ [ Step 1 C C X X C C X + Halonium ion + X − Halide ion This step is the same as for halogen addition to an alkene (see Section 8.11A). C C X + Halonium ion + O C C Protonated halohydrin Here, however, a water molecule acts as the nucleophile and attacks a carbon of the ring, causing the formation of a protonated halohydrin. H H O H H + O H H C C X X O H Halohydrin + H O + H H The protonated halohydrin loses a proton (it is transferred to a molecule of water). This step produces the halohydrin and hydronium ion. Steps 2 and 3 δ+ δ− The first step is the same as that for halogen addition. In the second step, however, the two mechanisms differ. In halohydrin formation, water acts as the nucleophile and attacks one carbon atom of the halonium ion. The three-membered ring opens, and a protonated halohydrin is produced. Loss of a proton then leads to the formation of the halohydrin itself. Write a mechanism to explain the following reaction. + (as a racemic mixture) Br OH Br OH Br 2 H 2 O PRACTICE PROBLEM 8.15 366 CHAPTER 8 ALKENES AND ALKYNES II: Addition Reactions • If the alkene is unsymmetrical, the halogen ends up on the carbon atom with the greater number of hydrogen atoms. Bonding in the intermediate bromonium ion is unsymmetrical. The more highly substi- tuted carbon atom bears the greater positive charge because it resembles the more stable carbocation. Consequently, water attacks this carbon atom preferentially.
eBook - ePubIntroductory Organic Chemistry and Hydrocarbons
A Physical Chemistry Approach
- Caio Lima Firme(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
the mechanism of Hydrohalogenation of Alkenes in polar, protic medium is also asynchronous concerted.POLAR ADDITION OF HYDROGEN HALIDE TO ALKENES: APOLAR SOLVENT
Second Order Kinetics
As mentioned above, there are three competing mechanisms for polar addition of HX to alkenes using apolar solvent. The second order kinetics has a much higher barrier than third and fourth order kinetics for polar HX addition in apolar solvent. Then, the polar addition of HX to alkenes in apolar solvent occurs mainly at low temperatures by fourth order kinetics and impurity-free media. However, when there are impurity traces, it was observed that second order kinetics is preferred, since the catalyst will be the impurity instead of one or two hydrogen halide besides the one adding to the alkene (Mayo and Katz 1947).Let us firstly show the mechanism of the second order kinetics of this reaction. We use hydrogen bromide and propene as an example of this mechanism in Fig. 17.5 . However, this is applicable to several other alkenes and also to hydrogen chloride replacing hydrogen bromide.Firstly, the reactants form a π complex, which is a minimum at PES where HBr is placed vertically towards the alkene’s π bond. Then, π bond electrons attack the hydrogen from HBr, promoting H-Br bond cleavage and σ electrons movement towards the bromine atom. During the transition state (which is associated with the rate determining step), alkene’s π bond and H-Br σ bond are partially broken to partially form a new bond (between one vinylic carbon and the hydrogen from HBr). Moreover, the opposite vinylic carbon holds a partial positive atomic charge (δ+ ) keeping its sp2 hybridization while the other vinylic carbon forming a new σ bond changes its hybridization to sp2.5 (in a pseudo-tetrahedral geometry) and halogen atom holds a partial negative atomic charge (δ +
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
• Both additions are regioselective and follow Markovnikov’s rule: C C C C H X H X X H Haloalkene gem-Dihalide HX HX C C 8.18 Addition of Hydrogen Halides to Alkynes PRACTICE PROBLEM 8.23 Alkenes are more reactive than alkynes toward addition of electrophilic reagents (i.e., Br 2 , Cl 2 , or HCl). Yet when alkynes are treated with one molar equivalent of these same electrophilic reagents, it is easy to stop the addition at the “alkene stage.” This appears to be a paradox and yet it is not. Explain. 8.19 Oxidative Cleavage of Alkynes 383 The hydrogen atom of the hydrogen halide becomes attached to the carbon atom that has the greater number of hydrogen atoms. 1-Hexyne, for example, reacts slowly with one molar equivalent of hydrogen bromide to yield 2-bromo-1-hexene and with two molar equivalents to yield 2,2-dibromohexane: 2,2-Dibromohexane 2-Bromo-1-hexene HBr HBr Br Br Br The addition of HBr to an alkyne can be facilitated by using acetyl bromide (CH 3 COBr) and alumina instead of aqueous HBr. Acetyl bromide acts as an HBr precursor by reacting with the alumina to generate HBr. For example, 1-heptyne can be converted to 2-bromo-1-heptene in good yield using this method: Br (82%) ‘‘HBr’’ CH 3 COBr/alumina CH 2 Cl 2 Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture. These reactions take place through a free-radical mecha- nism (Section 10.10): H Br HBr peroxides (74%) 8.19 Oxidative Cleavage of Alkynes Treating alkynes with ozone followed by acetic acid, or with basic potassium permanganate followed by acid, leads to cleavage at the carbon–carbon triple bond. The products are carbox- ylic acids: R—C C—R′ RCO 2 H + R′CO 2 H R—C C—R′ RCO 2 H + R′CO 2 H Three alkynes, X, Y, and Z, each have the formula C 6 H 10 . When allowed to react with excess hydrogen in the presence of a platinum catalyst each alkyne yields only hexane as a product.
- David R. Klein(Author)
- 2017(Publication Date)
- Wiley(Publisher)
1) BH 3 • THF 2) H 2 O 2 , NaOH Hydroboration- oxidation Treatment with an alkene gives an anti-Markovnikov addition of H and OH across the alkene. The reaction proceeds exclusively via a syn addition. H 2 , Pt Hydrogenation Treatment with an alkene gives a syn addition of H and H across the alkene. Br 2 Bromination Treatment with an alkene gives an anti addition of Br and Br across the alkene. Br 2 , H 2 O Halohydrin formation Treatment with an alkene gives an anti addition of Br and OH across the alkene, with the OH group being installed at the more substituted position. 1) RCO 3 H 2) H 3 O + Anti Dihydroxylation Treatment of an alkene with a peroxy acid (RCO 3 H) converts the alkene into an epoxide, which is then opened upon treatment with aqueous acid to give a trans-diol. KMnO 4 , NaOH, cold Syn Dihydroxylation Treatment with an alkene gives a syn addition of OH and OH across the alkene. 1) OsO 4 2) NaHSO 3 , H 2 O Syn Dihydroxylation Treatment with an alkene gives a syn addition of OH and OH across the alkene. 1) O 3 2) DMS Ozonolysis Ozonolysis of an alkene causes cleavage of the C=C bond, giving two compounds, each of which possesses a C=O bond. Solutions 8.1. (a) An alkene is treated with HBr (in the absence of peroxides), so we expect a Markovnikov addition of H and Br across the bond. That is, Br is installed at the more-substituted position: (b) An alkene is treated with HBr in the presence of peroxides, so we expect an anti-Markovnikov addition of H and Br across the bond. That is, Br is installed at the less-substituted position: CHAPTER 8 239 (c) An alkene is treated with HBr (in the absence of peroxides), so we expect a Markovnikov addition of H and Br across the bond. That is, Br is installed at the more-substituted position: (d) An alkene is treated with HCl, so we expect a Markovnikov addition of H and Cl across the bond.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
BINAP can be used as a chiral ligand to form a complex with ruthenium, producing a chiral catalyst capable of achieving very pronounced enantioselectivity: R OH 95% enantiomeric excess R OH Ru(BINAP)Cl 2 P P Ru Cl Cl Ph Ph Ph Ph H 2 9.9 HALOGENATION AND HALOHYDRIN FORMATION Experimental Observations Halogenation involves the addition of X 2 (either Br 2 or Cl 2 ) across an alkene. As an example, con- sider the chlorination of ethylene to produce dichloroethane: Cl Cl (97%) Cl 2 This reaction is a key step in the industrial preparation of polyvinylchloride (PVC): PVC Cl Cl Cl Cl Vinyl chloride Petroleum Cl Cl Cl 2 Halogenation of alkenes is only practical for the addition of chlorine or bromine. The reaction with fluorine is too violent, and the reaction with iodine often produces very low yields. The stereospecificity of halogenation reactions can be explored in a case where two new chiral centers are formed. 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. 420 CHAPTER 9 Addition Reactions of Alkenes 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 9.10). This effect places a partial positive charge on one of the bromine atoms, rendering that position electrophilic.
eBook - ePub- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
As σ-bonds are more stable than π-bonds, the most common reaction of C=C double bonds is the transformation of π-bonds into σ-bonds. When alkene and hydrogen gases are reacted in the presence of a catalyst, such as platinum, palladium, or nickel, two hydrogen atoms add to the C=C double bond to yield alkanes [94]. This reaction is called hydrogenation. The reaction is exothermic and the heat released is called the heat of hydrogenation and is about −20 to −30 kcal/mol (−80 to 125 kJ/mol), indicating that the product formed is more stable than the alkene. In this section, we will focus only on the reduction of carbon–carbon double bonds as a wide variety of functional groups can be reduced by catalytic hydrogenation. Depending on the type of the catalyst and reaction conditions, the reduction reactions are categorized into two groups: Heterogeneous catalytic reduction Homogeneous catalytic reduction 4.7.1 Heterogeneous Catalytic Reduction Heterogeneous catalysts have their advantages and disadvantages. After completion of the hydrogenation reaction, the heterogeneous catalysts are separated by filtration from the reaction medium because they do not dissolve in solvents. Hydrogenation reactions are generally carried out with heterogeneous catalysts. Alkenes do not react directly with hydrogen gas under normal conditions. A temperature of at least 500 °C is required for the reaction to take place. The activation energy required for adding hydrogen is quite high. The hydrogen–hydrogen bond must be weakened for reduction. Otherwise, it is also a challenging process. The activation energy of hydrogenation reactions decreases when a catalyst is used. The task of the catalyst is to lower the activation energy (Figure 4.2). Generally, the alkene is dissolved in alcohol, hydrocarbon, or acetic acid. A small portion of the catalyst is added to the reaction mixture. The reaction proceeds using hydrogen at atmospheric pressure
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