Halogenoalkanes

8 Key excerpts on "Halogenoalkanes"

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  A halogenolkane is obtained when one or more hydrogen atoms of an alkane molecule have been replaced by halogen atoms via the free radical substitution reaction.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

  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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  eBook - PDF

  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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  eBook - ePub

  Fundamental Aliphatic Chemistry

  Organic Chemistry for General Degree Students

  • P. W. G. Smith, A. R. Tatchell(Authors)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  IV

  Halogen Derivatives of Aliphatic Hydrocarbons*

  Publisher Summary

  The monosubstituted compounds are known as alkyl halides and can be represented by the general formula Cn H2n+1 X, where X denotes any halogen atom. The complete series of alkyl halides can be derived by replacing any hydrogen atom in paraffin by the appropriate halogen atom. The simplest members of the alkyl halide series are gases or volatile liquids. The boiling points throughout the series progressively increase with increasing molecular weight. Alkyl halides are covalent compounds being almost insoluble in water though readily soluble in organic solvents. Their inertness and insolubihty in cold concentrated sulfuric acid is exploited as their method of purification because this reagent removes alcohols and olefins that may be present as contaminants. The monohalogen derivatives of the alkanes are prepared from paraffins, olefins, alcohols, or carboxylic acids and by methods involving halogen exchange. The alkyl halides undergo a number of important reactions in which halogen is replaced by another monovalent group. These substitution reactions are also accompanied by elimination reactions to give olefins. Alkyl halides are also used in the laboratory to form organometallic compounds, which find wide use as synthetic intermediates.

  THE ALKYL HALIDES

  As has been noted previously the main reaction of the paraffins is that of substitution by the halogens, to yield halogenated hydrocarbons. The monosubstituted compounds are known as the alkyl halides, and can be represented by the general formula Cn H

  2n +1

  X where X denotes any halogen atom. The complete series of alkyl halides can therefore be derived by replacing any hydrogen atom in a paraffin by the appropriate halogen atom. The common and also the systematic method of naming these compounds is illustrated with reference to the following alkyl chlorides which may be derived from the simple paraffins. For example, methane CH4 and ethane CH3 ·CH3 , lead to methyl chloride CH3 Cl (systematically chloromethane) and ethyl chloride CH3 ·CH2

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  eBook - PDF

  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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  eBook - PDF

  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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  Ether & Hydrocarbons (Important Class of Organic Compounds)

  • (Author)
  • 2014(Publication Date)
  • Academic Studio

    (Publisher)

  ________________________ WORLD TECHNOLOGIES ________________________ Chapter 10 Alkane Chemical structure of methane, the simplest alkane Alkanes (also known as paraffins or saturated hydrocarbons ) are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e., hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are satura-ted compounds). Alkanes belong to a homologous series of organic compounds in which the members differ by a constant relative molecular mass of 14. Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C 2 -alkane). An alkyl group, generally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms, for example a methyl or ethyl group. The simplest possible alkane (the parent molecule) is methane, CH 4 . There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated, and is a hydrocarbon. Saturated oils and waxes are ________________________ WORLD TECHNOLOGIES ________________________ examples of larger alkanes where the number of carbons in the carbon backbone tends to be greater than 10. Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular tree upon which can be hung the interesting biologically active/reactive portions (functional groups) of the molecule.

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  eBook - PDF

  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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