Haloalkane

9 Key excerpts on "Haloalkane"

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  eBook - ePub

  Understanding 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  A halogenolkane is obtained when one or more hydrogen atoms of an alkane molecule have been replaced by halogen atoms via the free radical substitution reaction.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

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  The carbon–halogen bond can be very strong, especially for fluorine, and the properties that halogen atoms impart on the molecule can be very use- ful. The strongly bound, non-reactive chemistry of the halogens in haloalka- nes means these substances are ideal as flame retardants, in fire extin- guishers (figure 18.4) and as stable refrigerants and solvents. While these applications arise from the generally low reactivity of Haloalkanes, chemists also understand that, under the right reaction conditions, Haloalkanes can be controllably converted to other functional groups including alcohols, ethers, thiols, amines and alkenes. This makes them very versatile synthetic building blocks that are often used to generate important compounds used in medicine, materials and agriculture. Understanding how Haloalkanes react allows great control over the transformations of simple molecules into more complex and valuable products. In this chapter, we study two characteristic reactions of Haloalkanes: nucleophilic substitution and  -elimination. 18.1 Haloalkanes LEARNING OBJECTIVE 18.1 Describe the chemical properties of Haloalkanes using correct chemical terminology. Haloalkanes are compounds containing a halogen atom covalently bonded to an sp 3 hybridised carbon atom. The general symbol for a Haloalkane is RX, where X may be F, Cl, Br or I. a Haloalkane (an alkyl halide) R X Of all the Haloalkanes, the chlorofluorocarbons (CFCs) manufactured under the trade name Freon ® are the most widely known. CFCs are nontoxic, nonflammable, noncorrosive and odourless. Originally, they seemed to be ideal replacements for the hazardous compounds, such as ammonia and sulfur dioxide, formerly used in refrigeration systems. Among the CFCs most widely used for this purpose were trichlorofluoromethane (CCl 3 F, Freon-11) and dichlorodifluoromethane (CCl 2 F 2 , Freon-12). The CFCs also found wide use as industrial cleaning solvents.

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  Introduction to Organic Chemistry

  • 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.

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  Brown's Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  190 Carolyn A. McKeone/Photo Researchers, Inc. Asthma patient using a metered‐dose inhaler to deliver the drug albuterol. The drug is propelled by Haloalkanes such as 1,1,1,2‐ tetrafluoroethane. Inset: A molecule of 1,1,1,2‐tetrafluoroethane (HFA‐134a). 7 Haloalkanes 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.

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  Keynotes in Organic Chemistry

  • Andrew F. Parsons(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  5 Halogenoalkanes

  Key point . Halogenoalkanes (RX) are composed of an alkyl group bonded to a halogen atom (X = F, Cl, Br or I). As halogen atoms are more electronegative than carbon, the C–X bond is polar and nucleophiles can attack the slightly positive carbon atom. This leads to the halogen atom being replaced by the nucleophile in a nucleophilic substitution reaction and this can occur by either an S

  N

  1 (two-step ) or S

  N

  2 (concerted or one-step) mechanism . In competition with substitution is elimination , which results in the loss of HX from halogenoalkanes to form alkenes. This can occur by either an E1 (two-step ) or E2 (concerted ) mechanism . The mechanism of the substitution or elimination reaction depends on the structure of the halogenoalkane, the solvent and the nucleophile/base.

  5.1 Structure

  Halogenoalkanes, R–X, have an alkyl group (R) joined to a halogen atom by a single bond. The larger the size of the halogen atom (X = I > Br > Cl > F), the weaker the C–X bond (Appendix 1). With the exception of C–I, the C–X bond is polar; the carbon atom bears a slight positive charge and the electronegative halogen atom bears a slight negative charge.

  The use of hashed and wedged line notation is discussed in Section 3.3.2 Hybridisation is introduced in Section 1.5 Naming halogenoalkanes is discussed in Section 2.4

  Halogenoalkanes have an sp3 carbon atom and so have a tetrahedral shape.

  • An aliphatic halogenoalkane has the carbon atoms in a chain and not a closed ring, e.g. 1-bromohexane, CH3 (CH2 )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

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  Experimental Organic Chemistry

  A Miniscale & Microscale Approach

  • John Gilbert, Stephen Martin(Authors)
  • 2015(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  331 C H A P T E R Alkenes As discussed in the preceding chapter, alkanes are saturated hydrocarbons. You encounter such compounds in your daily life, especially in the form of fuels to power automobiles, trains, planes, and many power plants that generate electric-ity. These fuels also contain compounds that are known as alkenes, which are unsaturated hydrocarbons. In this chapter, you will discover that alkenes are much more interesting organic compounds because they undergo a variety of different chemical reactions, not just the radical reactions that are typical of alkanes. Thus, although you may have considered the chemistry of alkanes “boring,” you will find that the chemistry of alkenes is rich and characterized by a number of fascinating transformations of the carbon-carbon double bond, the functional group found in alkenes. 10.1 I N T R O D U C T I O N A functional group is an atom or group of atoms that governs the chemical and physical properties of a family of compounds. The introduction and manipulation of these functional groups are major objectives in modern organic chemistry. In this chapter, we will explore the chemistry of alkenes, 1 , which are organic compounds possessing a polarizable carbon-carbon double bond, a p -bond, as the functional group. Methods for introducing a carbon-carbon double bond into a molecule from alkyl halides and alcohols are presented first, and then some of the addition reac-tions that characterize this functional group are examined. Elimination reactions are among the most common ways to produce a carbon-carbon p -bond. 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 .

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  Keynotes in Organic Chemistry

  • Andy Parsons(Author)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  5. ALKYL HALIDES Key point. Alkyl halides are composed of an alkyl group bonded to a halogen atom (X = F, Cl, Br, I). As halogen atoms are more elec- tronegative than carbon, the CX bond is polar and nucleophiles can attack the slightly positive carbon atom. This leads to the halogen atom being replaced by the nucleophile in a nucleophilic substitution reaction, and this can occur by either an S N 1 (two-step) mechanism or an S N 2 (concerted or one-step) mechanism. In competition with substitution is elimination, which results in the loss of HX from alkyl halides to form alkenes. This can occur by either an E1 (two-step) mechanism or an E2 (concerted ) mechanism. The mechanism of the substitution or elimination reaction depends on the alkyl halide, the solvent and the nucleophile/base. 5.1 Structure Alkyl halides have an alkyl group joined to a halogen atom by a single bond. The larger the size of the halogen atom (X = I > Br > Cl), the weaker the CX bond (Appendix 1). The CX bond is polar, and the carbon atom bears a slight positive charge and the electronegative halogen atom bears a slight negative charge. Alkyl halides have an sp 3 carbon atom and hence have a tetrahedral shape. • An aliphatic alkyl halide has the carbon atoms in a chain and not a closed ring, e.g. 1-bromohexane, CH 3 (CH 2 ) 5 Br. • An alicyclic alkyl halide has the carbon atoms in a closed ring, but the ring is not aromatic, e.g. bromocyclohexane, C 6 H 11 Br. • An aromatic alkyl halide has the carbon atoms in a closed ring, and the ring is aromatic, e.g. bromobenzene, C 6 H 5 Br. 5.2 Preparation 5.2.1 Halogenation of alkanes Alkyl chlorides or bromides can be obtained from alkanes by reaction with chlorine or bromine gas, respectively, in the presence of UV light. The reaction involves a radical chain mechanism. R 3 C R C R R X X = F, Cl, Br, I δ+ δ– R = alkyl or H sp 3 hybridisation (tetrahedral) X

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  Organic Chemistry

  • 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.

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  General Introduction: Hydrocarbons, Halogen Derivatives

  A Modern Comprehensive Treatise

  • S. Coffey(Author)
  • 2016(Publication Date)
  • Elsevier

    (Publisher)

  Some of their properties will be dealt with in discussing individual compounds and others in describing the preparation of halogenated alkenes and alkynes. In general, as the atomic weight of the halogen increases the stability of the polyhalogeno-alkane decreases because of the decrease in strength of the carbon to halogen bond and also because of the steric effect caused by packing the larger halogen atoms adjacent to one another as is well illustrated in the series C F 4 , CC1 4 , C B r 4 and C I 4 . The perfluoroalkanes are particularly stable and are not attacked by con-centrated nitric, fuming sulphuric or mixed acids, permanganate or chromic acid. They begin to break down on heating alone at ~ 7 0 0 0 and with sodium or potassium at 6oo°-8oo°. Because of the great stability of the fluorocarbon ske-leton, they give rise to homologous series such as C n F 2 n + 1 I a n d C n F 2 n + 1 C 0 2 H containing functional groups similar to those arising from the hydrocarbons i.e. C n H 2 n + 1 I , etc. In many of their reactions the perfluoroalkyl halides differ from the correspond-ing hydrocarbon compounds because of the different electronic forces in the molecules, fluorine being the most electronegative element, having lone pairs I P O LYHALOGENOPARAFFIN S 507 of electrons and, when attached to carbon, being able to exhibit a hyper-conjugative effect in the opposite sense to that exhibited by hydrogen. The reactions of these polyhalogeno-compounds are mainly elimination reactions when there is hydrogen in the molecule, and dehalogenation reactions. In the former the order of elimination is HI > HBr > HC1 > H F and in the latter, 1 2 > Br 2 > Cl 2 . As yet there are no cases in the aliphatic series in which fluorine alone has been eliminated but in the alicyclic series perfluorocyclohexane can be converted to hexafluorobenzene by passing the vapour over heated iron gauze (/. C. Tatlow et al., Tetrahedron, i960, 9, 240).

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