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Efficiently Studying Organic Chemistry for students of chemistry, biochemistry, biology, pharmacy, and medicine von Breitmaier, Eberhard (eBook)

  • Erscheinungsdatum: 24.02.2016
  • Verlag: Georg Thieme Verlag KG
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Efficiently Studying Organic Chemistry

Study the essentials of organic chemistry efficiently! This e-book for bachelor and master students facilitates effective learning and is renowned for the quality of its content: 85 short chapters present each topic concisely, including questions for self-examination. Based on the author's long teaching experience, this book has been developed from lecture scripts of courses held in the USA and in Germany. It comprises the molecular orbital model to explain covalent bonding in organic molecules, the classes of organic compounds including natural products, polymers and biopolymers, basic concepts (orbital hybridization, resonance, aromaticity), types and mechanisms of organic reactions, and essential aspects of molecular structure such as atom connectivities, skeletal isomerism, conformation, configuration and chirality. The updated 2nd edition includes 4 new chapters on Selectivity and Specificity of Organic Reactions, Planning Organic Syntheses, Carbon-13 NMR, and two-dimensional NMR.


    Format: PDF
    Kopierschutz: AdobeDRM
    Seitenzahl: 256
    Erscheinungsdatum: 24.02.2016
    Sprache: Englisch
    ISBN: 9783132402652
    Verlag: Georg Thieme Verlag KG
    Größe: 5792 kBytes
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Efficiently Studying Organic Chemistry

16 Additions to Alkenes

Additions of molecules such as HX or X2 to the CC double bond are typical reactions of alkenes and the reverse of eliminations ( Chapters 11.1 , 11.2 , 15.1 ).
16.1 Addition of Hydrogen (Catalytic Hydrogenation)

Catalytic hydrogenation of alkenes to provide alkanes involves the addition of hydrogen to a CC double bond in the presence of a group 10 metal (Ni, Pd, Pt) as catalyst. As illustrated in Fig. 12.1 , the catalyst decreases the activation barrier of hydrogenation and thus the reaction proceeds more rapidly. The hydrogen molecule, activated at the surface of the catalyst, collides as a whole with the CC double bond, forming a cyclic transition state to induce a four-center reaction . Thus, after having added from one side of the CC double bond, both hydrogen atoms are attached at the same side of the CC double bond meaning that they will adopt a syn arrangement ( syn- or cis- addition). This stereospecificity, however, is undetectable in open-chain alkanes because of free rotation about the CC single bond arising from the hydrogenation.
16.2 Addition of Bromine (Bromination)

The addition of brownish-red bromine to an alkene giving an ,beta-dibromoalkane decolorizes the solution and thereby indicates the presence of a CC double bond. The mechanism of bromination starts with a polarization of the bromine molecule upon collision with the alkene, resulting in a transition state which is converted into an intermediate three-membered cyclic bromonium ion with loss of a bromide anion. This bromide anion adds from the backside of the bulky bromonium ion ( anti- or trans- addition), opening the strained three-membered ring to yield an ,beta-dibromoalkane.

The stereospecificity of this reaction is not detectable for open-chain alkenes because of free rotation about the newly formed CC single bond, so that 2,3-dibromobutane is obtained by bromination of both configurational isomers of 2-butene:

Chlorination of alkenes also follows the mechanism of bromination, involving an intermediate three-membered halonium ion. Fluorine is too reactive for a controlled reaction: all bonds are attacked and thus mixtures of compounds are produced. Iodinations proceed too slowly to be of synthetic significance.
16.3 Electrophilic Addition of Hydrogen Halide (Hydrohalogenation)

Hydrogen halides (HCl, HBr, HI) add to alkenes yielding alkyl halides. This reaction, called hydrohalogenation of alkenes, follows a stepwise ionic mechanism involving the proton of the hydrogen halide as an electrophile : In the first step, the electrophilic proton adds to the CC double bond ( electrophilic addition ), thus generating an intermediate carbenium ion which reacts with the halide anion in a fast reaction of the ions to produce the haloalkane (alkyl halide).

Ethene (H2C=CH2) is readily hydrochlorinated to give chloroethane (H3C-CH2-Cl). Hydrohalogenation of an unsymmetrically substituted alkene, such as propene in the simplest case, may produce two structural isomers of the resulting alkyl halides, with different positions of the halogens, called regioisomers . Thus, hydrobromination of propene may produce the regioisomers 2-bromopropane and 1-bomopropane:

The regioselectivity of hydrohalogenation, favoring one and discriminating against the other product, is predicted by the MARKOVNIKOV rule: Electrophilic addition preferentially involves the most stable carbenium ion. Since the relative stability of radicals ( Chapter 13.2 ) as well as of carbenium ions increases with increasing alkylation, the 2-propyl cation is more stable than th

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