Amines Basicity

7 Key excerpts on "Amines Basicity"

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  eBook - ePub

  Organic Chemistry Study Guide

  Key Concepts, Problems, and Solutions

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2014(Publication Date)
  • Elsevier

    (Publisher)

  Heterocyclic aromatic amines have rings that are numbered using a selected nitrogen atom as the number one atom. The Chemical Abstract System (CAS) of heterocyclic ring nomenclature has been accepted by the IUPAC. We use CAS names in this text.

  23.4 Physical Properties of Amines

  Amines may be gases, liquids, or solids depending on their molecular weight and structure. The boiling points of primary and secondary amines are higher than those for alkanes of similar molecular weight because these amines form intermolecular hydrogen bonds. Tertiary amines have lower boiling points than isomeric primary and secondary amines because tertiary amines do not have an N—H bond to form intermolecular hydrogen bonds. The lower molecular weight amines are soluble in water as a result of hydrogen bonding to water molecules.

  23.5 Basicity of Amines

  The basicity of amines of different classes do not follow a simple pattern because the number of groups bonded to nitrogen affects the electron density at the nitrogen atom. And, the stability of the conjugate acid in the solvent has a major affect on basicity. Thus, the basicity of amines can be explained only for amines with similar structures at the nitrogen atoms.

  The basicity of an amine is increased by electron-donating groups and decreased by electron-withdrawing groups. Aryl amines are less basic than alkyl-substituted amines because some electron density provided by the nitrogen atom is distributed throughout the aromatic ring. Basicity is expressed using K b values measured from the reaction of the amine with water. An alternate indicator of basicity is pK b , which is −log K b . A strong base has a large K b and a small pK b . The basicity of amines is also expressed by the acidity of their conjugate acids. A strong base has a weak conjugate acid, as given by a small value of K a and a large pK a .

  The basicity of heterocyclic amines depends on the location of the electron pair of the nitrogen atom, its hybridization, and whether or not resonance stabilization is possible. In pyrrole, the electron pair is part of the aromatic system. As a result, pyrrole is a very weak base. Pyridine is a weaker base than saturated amines of similar structure because its electron pair is in an sp2

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  eBook - ePub

  Pharmaceutical Chemistry E-Book

  • David G. Watson(Author)
  • 2011(Publication Date)
  • Churchill Livingstone

    (Publisher)

  Chapter 3 Amines David G. Watson Chapter contents Introduction 35 Factors affecting the p Ka value of a base 36 Alkyl groups 36 Electron withdrawing groups 39 Salt formation 48 Reactions of amines in relation to storage and formulation 48 Oxidation 48 Acylation 48 Schiff’s base formation 49 Quaternary amines 52 Introduction The amine group can be considered as the single most important group for conferring pharmacological activity. This is because it is the principle group within organic molecules that bears a positive charge. When it is present in a drug molecule it can exert biological effects through electrostatic interactions with negatively charged groups within proteins and also compete with positively charged cations such as sodium, potassium and calcium ions in biological processes. Amines are derived from ammonia and have the structures shown in Figure 3.1 for mono-, di- and trisubstituted nitrogen. Figure 3.1 Amines. The structures may look planar but, in fact, the structures of amines are more or less tetrahedral, like tetrasubstituted carbon, as shown for trimethylamine in Figure 3.2. In the case of nitrogen, the position that would be occupied by a fourth bond in tetrasubstituted carbon is occupied by a lone pair of electrons. Since nitrogen is less electronegative than oxygen the lone pair within an amine structure is more available than the lone pairs on an oxygen atom to interact with a proton. An approximate measure of the availability of lone pairs of electrons is given by the energy of the highest occupied molecular orbital (HOMO). The deeper the ‘energy well’ that the molecular orbital occupies the less available the electrons are for bond formation with a proton. Thus water has a HOMO of −12.5 eV while ammonia has a HOMO of −10.5 eV, and thus its electrons are more available and it protonates more readily than water

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  eBook - ePub

  Organic Chemistry

  An Acid-Base Approach, Third Edition

  • Michael B. Smith(Author)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Nitrogen is the blue atom. It is clear that less of the nitrogen atom is exposed in trimethylamine due to the presence of the three methyl groups. For an acid-base reaction, anything that makes it more difficult for nitrogen to approach another atom diminishes the base strength of the amine and based on accessibility to the nitrogen. Therefore, the tertiary amine is less basic. This observation contrasts with the greater amount of nitrogen that is exposed in the secondary amine or the primary amine, consistent with greater basicity. It is important to note that this effect is described for amines that are in solution, not in the gas phase. However, all the reactions of amines in this book will be done in a solvent. Based on inductive and substitution effects at nitrogen in amines, it is possible to categorize the base strength of amines and the trend in solution is NH 3 < R 3 N < RNH 2 < R 2 NH, where secondary amines are the strongest bases and ammonia is the weakest base. The basicity of an amine can be compared to that of ammonia. A putative acid-base reaction of ammonia with water gives the ammonium ion and the hydroxide ion. Similarly, the reaction of methanamine with water gives methylammonium hydroxide and the hydroxide ion. The ammonium salt products are weak acid. The p K a of the ammonium ion (from ammonia) is 9.2 whereas the p K a for the methylammonium ion (from methylamine) is 10.64. 3 With this knowledge, the equilibrium reaction that converts the ammonium salts to ammonia or amine can be used to measure the basicity of ammonia or the amine. The ammonium ion has a lower p K a, so it is a stronger acid than the methylammonium ion. It is known that a stronger acid will generate a weaker conjugate base and a weaker acid will generate a stronger conjugate base. Methanamine generates the weaker conjugate acid so it must be more basic than ammonia

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  eBook - PDF

  Chemistry for Today

  General, Organic, and Biochemistry

  • Spencer Seager, Michael Slabaugh, Maren Hansen, , Spencer Seager, Spencer Seager, Michael Slabaugh, Maren Hansen(Authors)
  • 2021(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  H N H H O H R O H H 18 amine H N H R9 O H R O H H 28 amine R0 N H R9 R O H 38 amine .... .... .... .... .... FIGURE 16.1 Hydrogen bonding between amines in water. Denis Val/Shutterstock.com Copyright 2022 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. 516 Chapter 16 16.4A Basicity and Amine Salts The single most distinguishing feature of amines is their behavior as weak bases. They are the most common organic bases. We know from inorganic chemistry that ammonia behaves as a Brønsted base by accepting protons, and in the process becomes a conjugate acid. The reaction of NH 3 with HCl gas illustrates this point (Reaction 16.1): H N: 1 H H H ammonia Cl H N H H H 1 Cl – 1 ammonium ion chloride ion (16.1) We also remember from inorganic chemistry that Brønsted bases such as ammonia react with water to liberate OH 2 ions (Reaction 16.2): H N: 1 H H H ammonia OH H N H H H 1 OH – 1 ammonium ion hydroxide ion water (16.2) Because the amines are derivatives of ammonia, they react in similar ways. In water, they produce OH – ions: General reaction: RiNH 2 1 H 2 O } RiNH 3 1 1 OH 2 (16.3) Specific reaction: CH 3 iNH 2 1 H 2 O } CH 3 iNH 3 1 1 OH 2 (16.4) methylamine methylammonium ion Example 16.6 Completing Reactions Complete the following reaction: CH 3 iNHiCH 3 1 H 2 O } Solution Primary, secondary, and tertiary amines are all basic and exhibit the same reaction in water. CH 3 iNHiCH 3 1 H 2 O } CH 3 iN 1 H 2 iCH 3 1 OH 2 ✔ LEARNING CHECK 16.6 Complete the following reactions: CH 2 CH 3 CH 3 N CH 3 1 H 2 O a.

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  eBook - PDF

  Organic and Biological Chemistry

  • H. Stephen Stoker(Author)
  • 2015(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  However, in acid–base consid-erations, the presence of this lone pair of electrons is of vital importance. ▶ Amines, like ammonia, have a pair of unshared electrons on the nitrogen atom present. These unshared electrons can accept a hydrogen ion from water. Thus both amines and ammonia pro-duce basic aqueous solutions. ▶ Substituted ammonium ions always contain one more hydro-gen atom than their “parent” amine. They also always carry a 1 + charge, whereas the “parent” amine is a neutral molecule. Determining Names for Substituted Ammonium and Substituted Anilinium Ions Name the following substituted ammonium or substituted anilinium ions. CH 3 CH 3 A NH O O 1 CH 3 CH 3 CH 3 A NH O O 1 CH 3 CH 3 CH 2 O A O NH 3 O CH 1 CH 3 CH 2 O CH 3 O CH 2 O NH 2 O 1 a. c. b. d. E X A M PL E 6 -3 (continued) 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. 210 CHAPTER 6 Amines and Amides Amines are stronger proton acceptors than oxygen-containing organic compounds such as alcohols and ethers; that is, they are stronger bases than these compounds. A 0.1 M aqueous solution of methylamine has a pH of 11.8, and a 0.1 M aqueous solution of aniline has a pH of 8.6. These solutions are sufficiently basic to turn red litmus paper blue. Carboxylic acid salts (Section 5-8) are the only other type of organic compound sufficiently basic to turn red litmus paper blue. ◀ Solution a. The parent amine is diethylamine. Replacing the word amine in the parent name with ammonium ion generates the name of the ion, diethylammonium ion.

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  Acidity and basicity in chemistry

  • Saeed Farrokhpay(Author)
  • 2023(Publication Date)
  • Arcler Press

    (Publisher)

  Amines are a very abundant class of organic alkalies. They are composed of the –NH 2 group coupled to a carbon atom. In water, amines form weak alkaline solutions: CH 3 NH 2 + H 2 O – → CH 2NH 3 + OH – (aq) (9) Amines are the byproducts of microorganisms degrading nitrogenous organic compounds like proteins. They frequently smell strong like “rotten fish.” This is not coincidental, since seafood includes a high concentration of nitrogen-containing chemicals that degrade rapidly. Methylamine, CH 3 NH 2 , is particularly suitable to make itself known to humans since it is a gas at room temperature. When lemon juice or another acidic material is added to fish, the methylamine is converted to the methylammonium ion CH 3 NH + 3 . Since ions are non-volatile, they contain no fragrance (Greenberg, 1966). Acidity and Basicity in Chemistry 26 1.9.7. Oxides as Acids and Bases The separation of oxygen compounds into acidic and basic oxygen compounds is broadly analogous to the divide of –OH compounds. The compounds of oxygen of the Groups 1–2 highly electropositive metals have the O 2– ion. Such ion is another example of a proton acceptor which is strong as compared to OH – and hence can’t reside in water. As a result, ionic oxides tend to produce extremely basic solutions: O 2 – + H 2 O – → 2OH – (aq) (10) In other circumstances, like MgO, the solid is sufficiently insoluble that it causes a minimal change in pH when added to water. Furthermore, CaO, often referred to as quicklime, is enough soluble to create a highly alkaline solution when adequate heat is generated; the result is the somewhat dissolvable slaked lime, Ca(OH) 2 (Pearson and Songstad, 1967). Transition metal-oxygen compounds are often insoluble solids with relatively complicated comprehensive lattices. While certain would soluble in acids, they are not acidic when dissolved in H 2 O.

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  eBook - ePub

  Essentials of Organic Chemistry

  For Students of Pharmacy, Medicinal Chemistry and Biological Chemistry

  • Paul M. Dewick(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  a values. In other words, they are very strong acids and will lose a proton readily. Conversely, the non-protonated compounds are weak bases.

  4.5.3 Hybridization effects

  We have seen above that acidity is influenced by the hybridization of the atom to which the acidic hydrogen is attached. The acidity of a C–H bond was found to increase as the s character of the bond increased. The more s character in the orbital, the closer the electrons are held to the nucleus. Similar reasoning may be applied to basicity. If the lone pair is in an sp 2 or sp orbital, it is held closer to the nucleus and is more difficult to protonate than if it is in an sp 3 orbital.

  Accordingly, we find that a nitrile nitrogen (lone pair in an sp orbital) is not at all basic (pK a about –10), though ethylamine (lone pair in an sp 3 orbital) has pK a 10.7. Imines (lone pair in an sp 2 orbital) are less basic than amines. Cyclohexanimine (the imine of cyclohexanone) has pK a 9.2, and is less basic than cyclohexylamine (pK a 10.6).

  Similarly, alcohols (sp 3 hybridization), although they are themselves rather weak bases, are going to be more basic than aldehydes and ketones (sp 2 hybridization)

  4.5.4 Resonance/delocalization effects

  Delocalization of charge in the conjugate base anion contributes to stabilization of the anion, and thus ionization of the acid is enhanced. Delocalization effects in bases are more likely to stabilize the base rather than the conjugate acid, and thus tend to reduce the basicity. Refer again to Table 4.8 for a summary of various groups that may contribute resonance effects.

  Pre-eminent amongst examples is the case of amides, which do not show the typical basicity of amines. Acetamide, for example, has pK a − 1.4, compared with a pK a 10.7 in the case of ethylamine. This reluctance to protonate on nitrogen is caused by delocalization in the neutral amide, in which the nitrogen lone pair is able to overlap into the π system. This type of resonance stabilization would not be possible with nitrogen protonated, since the lone pair is already involved in the protonation process. Indeed, if amides do act as bases, then protonation occurs on oxygen, not on nitrogen. Resonance stabilization is still possible in the O -protonated amide, whereas it is not possible in the N -protonated amide. Note that resonance stabilization makes the O -protonated amide somewhat less acidic than the hydronium ion (pK a

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