Zwitterion

3 Key excerpts on "Zwitterion"

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  eBook - ePub

  Conducting Polymers for Advanced Energy Applications

  • Ram K. Gupta(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  Laschewsky and Rosenhahn 2019 ).

  Besides single molecular Zwitterions, there are also polymeric Zwitterions (PZ). Further in the text, ZI will correspond to single molecular Zwitterions and PZ to polymeric Zwitterions. According to the terminology defined by Laschewsky, polyZwitterions are polymers, which “bear, within their structural repeating unit, the same amount of cationic and anionic groups” (Laschewsky 2014 ). In other words, the structural units of PZ are Zwitterions and the overall electric charge is zero. Some authors distinguish strong PZ, where every unit has two oppositely charged ionic groups, and weak PZ, where only a fraction of units bear two ionic groups. (Kharlampieva, Izumrudov, and Sukhishvili 2007 ) Standard cationic groups in PZ are ammoniums and imidazoliums, while anionic groups are sulfanate, carboxylate, and phosphate groups. The most abundant class of synthetic PZ are poly(methacrylates) such as poly(carboxybetaine methacrylate) (PCBMA), poly(sulfobetaine methacrylate) (PSBMA), or poly(phosphoryl choline methacrylate) (PMPC) derivatives. Monomers corresponding to those polymers are shown in Figure 20.2 .

  Figure 20.2 Three canonical monomers used for synthesis of polyZwitterions: (a) CBMA, (b) SBMA, (c) MPC. Value of n lies typically between 1 and 3 methylene groups.

  Monomers might differ by the n number, which is the number of methylene groups that separate the cationic and anionic moieties (Figure 20.2), i.e., a spacer. The length of a spacer is crucial and determines the strength of intermolecular interactions of polyZwitterions (Li et al. 2019 ), which as a consequence determine their physiochemical properties and application potential. A noble theoretical chemist Mark Ratner in his memorable seminar in 2004, said, “The chemistry of XX-th century was about intramolecular interactions; the chemistry of XXI century will be about intermolecular interactions.” Therefore, Section 20.2

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  eBook - ePub

  Isocyanide Chemistry

  Applications in Synthesis and Material Science

  • V. Nenajdenko(Author)
  • 2012(Publication Date)
  • Wiley-VCH

    (Publisher)

  8 Zwitterions and Zwitterion-Trapping Agents in Isocyanide Chemistry Ahmad Shaabani, Afshin Sarvary, and Ali Maleki

  8.1 Introduction

  The combinatorial chemistry, sequential transformations, and one-pot multicomponent reactions (MCRs), in which three or more starting materials react to form a product, are always resource-effective and environmentally acceptable, and thus “greener” as compared to multistep reactions. Moreover, MCRs offer significant advantages over conventional linear-step syntheses, by reducing time and also saving money, energy, and raw materials. Consequently, both economical and environmental benefits are achieved. Yet, at the same time, diversity can be achieved for building up libraries, simply by varying each component. Because of the unique reactivity of the isocyanide functional group, which undergoes facile additions with nucleophiles and electrophiles, isocyanide-based MCRs (IMCRs) are among the most versatile, in terms of the number and variety of compounds with potential biological and medicinal activities which can be generated. As a result of these benefits, the chemistry of the isocyanides and the use of IMCRs have been the subject of many reviews [1–10].

  The term “Zwitterion” is derived from the German word “zwitter” (hybrid); hence, a Zwitterion is a neutral molecule with a positive and a negative electrical charge at different locations within that molecule; occasionally, Zwitterions may be referred to as “inner salts.”

  Usually, dipolar compounds are not classified as Zwitterions. For example, amine oxides are not Zwitterions in true terms of the definition, which specifies that there must be unit electrical charges on the atoms. The distinction lies in the fact that the plus and minus signs on the amine oxide signify formal charges, but not electrical charges. Other compounds which sometimes are referred to as Zwitterions (mistakenly according to the definition above) include nitrones and 1,2- or 1,3-dipolar compounds [11].

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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)

  hybrid, as it is a dipolar ion. However, overall, the Zwitterion is considered an electrically neutral species even though it carries formal charges within the molecule itself. The quantity of the formal charge is greater than the partial electric dipole on, let’s say, a polar H–Cl molecule. Hence, we would expect the electrostatic interaction between the formal charges to be stronger than that between the partial dipoles. Due to this dipolar nature of Zwitterions, amino acids exhibit physical properties rather unusual for covalent molecules but similar to those of ionic compounds.

  Q:

  Taking the example of aspartic acid, can an intramolecular acid–base reaction occur between the amine group and the side chain carboxyl group instead?

  A:

  Not likely. Both the –COOH groups on the α-carbon and the R-group have different acidic strength. We would expect the –COOH group on the α-carbon to be more acidic than the one on the R-group due to the presence of an electron-withdrawing amine group that is also bonded to the α-carbon. This electron-withdrawing group would enhance the acidity of the –COOH group by stabilizing the conjugate base that forms (refer to Chapter 8 ). Thus, being more acidic, the −COOH group on the α-carbon would be more likely to dissociate and hence suppress the dissociation of the less acidic –COOH on the R-group. In addition, the proximity of both groups on the α-carbon makes the protonation more favorable as the close proximity of the two oppositely charged groups that form, the –NH+ 3 and –COO− groups, allows intramolecular ionic interaction to take place.

  12.3 Physical Properties of Zwitterions

  12.3.1 Melting and Boiling Points

  One evidential fact that amino acid exists as Zwitterions is their high melting points, and this, for the simplest amino acid glycine, is 233°C. Under normal conditions, you will find them to be colorless crystalline solids.

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