Dehydrohalogenation of Alkyl Halides

8 Key excerpts on "Dehydrohalogenation of Alkyl Halides"

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  eBook - ePub

  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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  eBook - ePub

  Keynotes in Organic Chemistry

  • 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 7.2.1

5.2 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 4.6.2
This 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 synthesis
Primary, secondary and tertiary halogenoalkanes are defined in Section 2.1
The 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 4.11
The mechanism of these reactions depends on whether a primary (RCH2 OH), secondary (R2 CHOH) or tertiary alcohol (R3