Chemistry
Hydrolysis of Halogenoalkanes
The hydrolysis of halogenoalkanes involves the reaction of halogenoalkanes with water to form alcohols and hydrogen halides. This reaction is typically carried out in the presence of a strong nucleophile, such as hydroxide ions, which attack the carbon atom bonded to the halogen, leading to the substitution of the halogen with a hydroxyl group. The hydrolysis of halogenoalkanes is an important process in organic chemistry.
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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 - 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 - ePubOrganic Chemistry Study Guide
Key Concepts, Problems, and Solutions
- Robert J. Ouellette, J. David Rawn(Authors)
- 2014(Publication Date)
- Elsevier(Publisher)
Also, electronegative substituents near the carbon atom bearing the hydroxyl group increase its acidity. Halogen substituents inductively withdraw electron density from the oxygen atom and weaken the O—H bond. The halogens also stabilize the negative charge of the conjugate base—an alkoxide ion.Alcohols, like water, can be protonated. The product is a conjugate acid known as an alkyloxonium ion.9.12 Substitution Reactions of Alcohols
The hydroxyl group of alcohols react with hydrogen halides such as HBr to give haloalkanes. The order of reactivity is tertiary > secondary > primary. Hydrogen bromide suffices to form bromoalkanes, but zinc chloride is required as a catalyst for the reaction with hydrogen chloride. The substitution reactions of alcohols parallel that of haloalkanes. However, hydroxide ion is not the leaving group. Protonation of the hydroxyl group must occur to allow water to become the leaving group. In general, a weaker base is a better leaving group than a stronger base. Since hydroxide ion is a stronger base than water, it is a poor leaving group in both SN 1 and SN 2 reactions.The order of reactivity of alcohols in SN 1 reactions is tertiary > secondary > primary. This order parallels the stability of the carbocation intermediates that form in the reaction. This order of reactivity is reversed for SN 2 reactions; however, tertiary alcohols do not react by an SN 2 mechanism.9.13 Alternate Methods for the Synthesis of Alkyl Halides
Since undesirable rearrangements occur in acid-catalyzed reactions of alcohols, other methods have been developed to synthesize alcohols that do not require acid. In this section, we discussed two additional reagents that convert alcohols to haloalkanes. They are used for secondary and primary alcohols which react slowly with hydrogen halides.Thionyl chloride is used to convert alcohols to chloroalkanes. The by-products are sulfur dioxide and hydrogen chloride, both of which escape from the solution as gases. Phosphorus tribromide is used to convert alcohols to bromoalkanes. The by-product, phosphorous acid, is soluble in water.
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 - ePub- Robert J. Ouellette, J. David Rawn(Authors)
- 2015(Publication Date)
- Elsevier(Publisher)
7Nucleophilic Substitution and Elimination Reactions
7.1 Reaction Mechanisms and Haloalkanes
We introduced the concept of functional groups and their role in the organization of the structures of organic molecules in Section 1.9 . We described the importance of reaction mechanisms as an organizational device to classify chemical reactions in Section 2.9 . The details of the electrophilic addition reactions of alkenes (Section 4.9 ) and electrophilic substitution reactions of aromatic compounds (Section 5.5 ) are examples of two important reaction mechanisms. In this chapter we examine two more types of reactions mechanisms—nucleophilic substitution and elimination reactions. These mechanisms often occur in competition with one another and describe the reactions of several classes of compounds, such as haloalkanes (also called alkyl halides) and alcohols. In this chapter we focus on the substitution and elimination reactions of haloalkanes. These reactions illustrate the role of structure in determining the degree to which a given reaction mechanisms occurs.Reactivity of Haloalkanes
Haloalkanes have a halogen atom bonded to an sp3 -hybridized carbon atom. As a result of the greater electronegativity of the halogens, the carbon atom of the carbon-halogen bond bears a partial positive charge and the halogen atom has a partial negative charge.where X = F, Cl, Br, ISince a carbon-halogen bond is polar, a haloalkane has two sites of reactivity. One is at the carbon atom bonded to the halogen atom. This carbon atom is electropositive and reacts with nucleophiles. The second site of reactivity in a haloalkane is the hydrogen atom bonded to the carbon atom adjacent to the carbon atom bonded to the halogen atom. This hydrogen atom is more acidic than the hydrogen atoms in alkanes because the halogen atom on the adjacent carbon atom withdraws electron density by an inductive effect.
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- W. Brock Neely(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
172 158 Environmental Exposure from Chemicals I. INTRODUCTION Hydrolysis along with biodegradation (Chapter 6) and photodegradation (Chapters 8 and 9) is one of the significant environmental fate processes that act on many types of organic chemicals. The simple reaction shown in Equation I defines R -OH + x-+ H+ (1) the process. In this scheme a chemical transformation has occurred in which an organic molecule, R -X, reacts with water, forming a new carbon-oxygen bond and cleaving a carbon-X bond in the original molecule. The net effect is a direct displacement of X by OH. The importance of the scheme from an environmental point of view is that the resulting product is usually more easily degraded, metabolized and less toxic than the initial starting material. As knowledge of the fate and transport processes of chemicals increases, it is logical that the emphasis will change from field observation to prediction. Field experiments, where the chemical is added to an ecosystem and the disappearance is followed with time are very site specific. Information gained in this situation is very difficult to extrapolate to a new system. The predictive mode, on the other hand, requires a detailed knowledge both of the mechanism and the factors that perturb the mechanism. This entire book is devoted to the subject of prediction and the state of the art in understanding the various forces that are operating on a chemical. The present chapter will be confined to hydrolysis. Before attempting to assess the role of hydrolysis it is well to recognize that certain organic structures are resistant to attack. Table 1 illustrates the types of chemicals that are unreactive' while Table 2 lists functional groups that are potentially susceptible to hydrolysis. 1 • 2 Once a decision is made that the molecule can hydrolyze, it becomes important to have as much information as possible concerning the mechanism and laws governing the rate of reaction.
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
For example, the elements of hydrogen halide, H–X, may be eliminated from an alkyl halide, 2 . The functional group of an alkyl halide is a carbon-halogen, single bond, C – X , and the process by which the carbon-halogen bond and an adjacent carbon-hydrogen bond are converted into a carbon-carbon p -bond via dehydrohalogenation is an example of a functional group transformation . A carbon-carbon p -bond may also be formed by removing the elements of water from an alcohol, 3 , in which a C–OH single bond is the functional group; this reac-tion is called dehydration . Although other aspects of the chemistry of alkyl halides and alcohols will be presented in Sections 14.1–14.5 and 16.2, a brief introduction to these families is essential to understanding how they may be used as starting mate-rials for the synthesis of alkenes. 10 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. 332 Experimental Organic Chemistry ■ Gilbert and Martin C CHR 3 R 2 R 1 1 An alkene CH CH 2 X = Cl, Br, I An alkyl halide R 1 X R 3 R 2 CH CH 3 An alcohol R 1 OH R 3 R 2 + + – – 10.2 D E H Y D R O H A L O G E N A T I O N O F A L K Y L H A L I D E S The electronegative halogen atom of an alkyl halide polarizes the carbon-halogen bond so the carbon atom bears a partial positive charge, d + , and the halogen atom a partial negative charge, d − . This polarization may be transmitted through the s -bond network, a phenomenon referred to as an inductive effect , to enhance the acidity of hydrogen atoms on the b -carbon atom.
eBook - PDF- William H. Brown, Thomas Poon(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
190 YOU MAY HAVE HEARD of the term chlorofluorocarbons and their well‐documented harm to the environment. Chlorofluorocarbons belong to a larger class of compounds named haloalkanes or, in the common system of nomenclature, alkyl halides, compounds containing at least one halogen atom covalently bonded to an sp 3 hybridized carbon atom. The general symbol for an alkyl halide is R X, where X may be F, Cl, Br, or I: R X A haloalkane (An alkyl halide) In this chapter, we study two characteristic reactions of haloalkanes: nucleophilic sub- stitution and β‐elimination. We will see that haloalkanes can be quite useful molecules because they can be converted to alcohols, ethers, thiols, amines, and alkenes and are Haloalkane (alkyl halide) A compound containing a halogen atom covalently bonded to an sp 3 hybridized carbon atom; given the symbol RX. K E Y Q U E S T I O N S 7.1 How Are Haloalkanes Named? 7.2 What Are the Characteristic Reactions of Haloalkanes? 7.3 What Are the Products of Nucleophilic Aliphatic Substitution Reactions? 7.4 What Are the S N 2 and S N 1 Mechanisms for Nucleophilic Substitution? 7.5 What Determines Whether S N 1 or S N 2 Predominates? 7.6 How Can S N 1 and S N 2 Be Predicted Based on Experimental Conditions? 7.7 What Are the Products of β‐Elimination? 7.8 What Are the E1 and E2 Mechanisms for β‐Elimination? 7.9 When Do Nucleophilic Substitution and β‐Elimination Compete? H O W TO 7.1 How to Name Cyclic Haloalkanes 7.2 How to Recognize Substitution and β‐Elimination Reactions 7.3 How to Complete a Substitution Reaction 7.4 How to Predict the Type of Substitution Reaction a Haloalkane Will Undergo 7.5 How to Complete an Elimination Reaction 7.6 How to Draw Mechanisms 7.7 How to Predict the Type of β‐Elimination Reaction a Haloalkane Will Undergo C H E M I C A L C O N N E C T I O N S 7A The Environmental Impact of Chlorofluorocarbons 7B The Effect of Chlorofluorocarbon Legislation on Asthma Sufferers Carolyn A.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Nucleophilic substitution is any reaction in which Pdf_Folio:921 CHAPTER 18 Haloalkanes 921 one nucleophile is substituted for another. In the following general equation, Nu: − is the nucleophile, X is the leaving group, and substitution takes place on an sp 3 hybridised carbon atom. + Nu – nucleophile C leaving group X C X – + Nu nucleophilic substitution Halide ions, with a filled outer electron shell equivalent to the noble gases, make excellent leaving groups, which explains the value of haloalkanes for the synthesis of other molecules. The process of nucleophilic substitution is evident from the name; an atom or group is replaced (substituted) by another. This is not so clear with -elimination, where atoms or groups are removed from two adjacent carbon atoms. For example, H and X could be removed from a haloalkane, or H and OH from an alcohol, to give a carbon–carbon double bond in both cases. Because all nucleophiles are also bases, nucleophilic substitution and base-catalysed -elimination are competing reactions. The degree to which each occurs is governed by subtle variations in structure and reaction conditions that we will discuss later. The ethoxide ion, CH 3 CH 2 O − , for example, is both a nucleophile and a base. With bromocyclohexane, it can react as a nucleophile (pathway shown in red) to give ethoxycyclohexane (cyclohexyl ethyl ether) or as a base (pathway shown in blue) to give cyclohexene and ethanol. OCH 2 CH 3 nucleophilic substitution ethanol – OCH 2 CH 3 As a base, ethoxide ion attacks this hydrogen atom. As a nucleophile, ethoxide ion attacks this carbon atom. CH 3 CH 2 O – ethanol β-elimination + Br CH 3 CH 2 OH H Na + + Br – + Na + + Br – + In this chapter, we study both of these organic reactions. Using them, we can convert haloalkanes to compounds with other functional groups including alcohols, ethers, thiols, sulfides, amines, nitriles, alkenes and alkynes.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
C H A P T E R 240 6 PROPERTIES AND SUBSTITUTION REACTIONS OF ALKYL HALIDES Nucleophilic Reactions Not all substitutions are a good thing; for instance, we would not want to accidentally use salt in place of the needed amount of sugar in a batch of chocolate chip cookies. But with some substitutions, we get something even better. In organic chemistry that is often the case, since nucleophilic substitution reactions (which we will learn about in this chapter) allow the conversion of functional groups within a given molecule into entirely different functional groups, leading to new compounds with distinct properties. Moreover, nature utilizes a number of specific substitution reactions that are required for life. IN THIS CHAPTER WE WILL CONSIDER: • what groups can be replaced (i.e., substituted) or eliminated • the various mechanisms by which such processes occur • the conditions that can promote such reactions [ WHY DO THESE TOPICS MATTER? ] At the end of the chapter, we will show an example where just a few substitution reactions can convert table sugar into a sweetener that has no calories—a sugar substitute that is not salty, but is in fact 600 times sweeter than sugar itself! See for additional examples, videos, and practice. photo credit: (sugar bowl) Sylvie Shirazi Photography/Getty Images (salt pouring) Tom Grill/Getty Images (sugar pouring) Tom Grill/Getty Images 6.1 ALKYL HALIDES 241 6.1 ALKYL HALIDES • An alkyl halide has a halogen atom bonded to an sp 3 -hybridized (tetrahedral) carbon atom. • The carbon–halogen bond in an alkyl halide is polarized because the halogen is more electronegative than carbon. Therefore, the carbon atom has a partial positive charge (δ+) and the halogen has a partial negative charge (δ−). X C δ+ δ– • Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) accord- ing to the number of carbon groups (R) directly bonded to the carbon bearing the halogen atom (Section 2.5).
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