Dehydrohalogenation of Alkyl Halides

5 Key excerpts on "Dehydrohalogenation of Alkyl Halides"

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  Experimental 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.

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  Chemistry of Dehydrogenation Reactions and Its Applications

  • Syed Shahabuddin, Rama Gaur, Nandini Mukherjee(Authors)
  • 2024(Publication Date)
  • CRC Press

    (Publisher)

  1 Introduction to Dehydrogenation Reactions of Organic Compounds

  Syed Shahabuddin, Nandini Mukherjee, and Rama Gaur

  DOI: 10.1201/9781003321934-1

  1.1 INTRODUCTION

  Dehydrogenation reactions include a wide variety of reactions where hydrogen is completely or partially eliminated from an organic compound to form a new compound (e.g., conversion of saturated into unsaturated compounds) [1 ]. Dehydrogenation reactions have applications in the production of hydrogen (H2 ), alkenes, cycloalkanes, aromatic compounds, imines, and oxygenates such as carbonyl compounds [2 ]. Alkenes, imines, and oxygenates are crucial chemical intermediates for producing solvents, polymers, rubbers, detergents, insecticides, and pharmaceuticals [3 ].

  Dehydrogenation is most important in the petrochemical industry especially in the refinery cracking process, where olefins are synthesized from saturated hydrocarbons [4 ]. Suitable hydrocarbon feed-stocks (e.g., naphtha) are subjected to cracking in order to produce industrially important olefins such as ethylene, propylene, butene derivatives, and butadiene derivatives in large volumes. Traditionally, steam cracking and fluid catalytic cracking (FCC) of C2+ hydrocarbons (ethane, propane, etc.) have been extensively used to produce alkenes. However, as there is an increasing demand for a specific alkene to be produced, building new specific steam crackers or FCC units for each individual alkene is not a frugal choice. Dehydrogenation reaction, in contrast, provides a comparatively flexible, cost-effective method for producing single alkenes. The dehydrogenation of ethylbenzene to produce an important monomer ‘styrene’ has become a preferred commercial route in the polymer industry.

  The benefits of building dehydrogenation reactors are the production not only of specific alkenes but also of aldehydes and ketones. Although the carbonyl compounds can be easily prepared via the oxidation of alkenes or alcohols, dehydrogenation routes may offer advantages such as a higher selectivity for products or the availability of a wider variety of feedstock [5 ]. Dehydrogenation reaction is also extremely useful in synthesizing α,β-unsaturated carbonyls as common organic building blocks in natural product and pharmaceutical synthesis. The introduction of a C=C group adjacent to an electron withdrawing group (EWG) provides greater synthetic opportunity that utilizes the resulting polarized double bond, since EWG is susceptible to regioselective functionalization by several methods [6 , 7 ]. Acceptorless dehydrogenation of alcohols, especially biomass-derived carbohydrates, provides an atom-economical method to synthesize carbonyl derivatives and a low-temperature route for selective H2 production [8 –14 ]. It also has synthetic applications in tandem coupling reactions involving C-N and C-C bond formation for the synthesis of imines and amides and the β-functionalization of alcohols [15 –24

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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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  Pyrolytic Methods in Organic Chemistry

  Application of Flow and Flash Vacuum Pyrolytic Techniques

  • Roger Brown(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  It will be evident from this that alkyl halides pyrolyzed in an unconditioned flow apparatus are likely to decompose by a number of competing mechanisms. 96 4. Elimination Reactions Unimolecular ß-elimination of hydrogen halides in the gas phase was originally considered to be a 4-center cis-elimination (42) but Maccoll and co-workers 9 2 ' 9 6 favor a highly polar transition state approaching a tight carbonium ion-halide ion pair (43). Smith and Kelly 5 0 give a critical dis-cussion of the advantages and difficulties associated with such a model for the transition state, and discuss the polar semi-ion pair transition state of Benson and Bose. 9 7 Equations (4.53-(4.56) show four examples of jS-eliminations of hydrogen halides under flow and static conditions. VA H* Hal H' Hal 42 43 ^ CHs ci I C H 3 CH 3 CH 3 CH 3 ÇH 3 r J Α Λ r R r 9 8 1 / ^ -^ / --> HCl + + [ R e f -9 8 1 / / clean Pyrex Π I l ^ J r^ flow reactor, y CH 3 Cl Pyrex packing I C H T C H 3 C H 3 C H 3 29 : 71 (4.53) r ^ V * 3 0 0 --3 5 0 7 2 0 0 -4 0 0 m m ) | ^ | + ^ ^ ^ ^ I J seasoned static l Ï) reactor ( C H 3 ) 2 C H I JLÜEDLR ' C H 2 = C H C H 3 + HI [Refs. 95, 100] (4.55) reactor FÇH—ÇC1F ? 5 2 _ > F — C H = C < C I + Q F 6 [Ref. 101] I ι qjj.nzt .be F J (4.56) C* u <-T 5 sec 7 C l H 62% 3.5% IV. ß-Eliminations and y-Eliminations 97 Alkyl halides may also undergo thermal rearrangements in the gas phase very similar to those which occur in solution via ionic or ion pair inter-mediates. Fields et al. 102 have reported the isomerization of a number of chlorocyclopropanes to 3-chloropropenes at 500°-650° and have proposed concerted migration of chlorine [Eqs. (4.57) and (4.58)]. DePuy et α/. 1 03 have found similar isomerizations of cyclopropyl acetates [e.g. Eq. (4.59)].

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  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2016(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.

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