Amines Basicity
Amines are organic compounds that contain a basic nitrogen atom. Their basicity is due to the lone pair of electrons on the nitrogen, which can accept a proton to form a positively charged ammonium ion. The basicity of amines increases with the number of alkyl groups attached to the nitrogen atom, as these groups donate electron density, making the lone pair more available for protonation.
3 Key excerpts on "Amines Basicity"
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...Aromatic substituents affect basicity. Activating substituents increase electron density in the aromatic ring which helps to stabilize the ammonium ion and increase basic strength. Deactivating groups have the opposite effect. Substituents capable of interacting with the aromatic ring by resonance have a greater effect on basicity if they are at the ortho or para positions. Reactivity Amines react as nucleophiles or bases since they have a readily available lone pair of electrons which can participate in bonding. Primary and secondary amines can act as weak electrophiles or acids with a strong base, by losing an N–H proton to form an amide anion (R 2 N −). Spectroscopic analysis Evidence for primary and secondary amines include N–H stretching and bending absorptions in the IR spectrum as well as a D 2 O exchangeable proton in the 1 Hnmr spectrum. Related topics (A3) sp 3 Hybridization (A4) sp 2 Hybridization (C3) Intermolecular bonding (C4) Properties and reactions (E4) Organic structures (G3) Base strength (O3) Reactions of amines (P3) Infra-red spectroscopy (P4) Proton nuclear magnetic resonance spectroscopy (P5) 13 C nuclear magnetic resonance spectroscopy (P6) Mass spectrometry Structure Amines consist of an sp 3 hybridized nitrogen linked to three substituents by three σ bonds. The substituents can be hydrogen, alkyl, or aryl groups, but at least one of the substituents has to be an alkyl or aryl group. If only one such group is present, the amine is defined as primary. If two groups are present, the amine is secondary. If three groups are present, the amine is tertiary (Section C6). If the substituents are all alkyl groups, the amine is defined as being an alkylamine. If there is at least one aryl group directly attached to the nitrogen, then the amine is defined as an arylamine. The nitrogen atom has four sp 3 hybridized orbitals pointing to the corners of a tetrahedron in the same way as an sp 3 hybridized carbon atom...
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...If an alcohol reacts with a base that is much weaker than an alkoxide, however, the value of K a may be quite small or approach 1 (a 50:50 mixture). Previous sections have clearly established that the polarized O—H bonds of carboxylic acids and alcohols make those compounds acidic. The N—H bond of an amine is also polarized because nitrogen is more electronegative than carbon. In principle, the N—H unit of an amine can be classified as an acid. However, amines are considered to be weak acids (p K a, 36–40), requiring very strong bases for an acid-base reaction. The p K a of ammonia is reported to be 36, and N- ethylethan-1-amine is >36. In general, aliphatic amines have p K a values between 36 and 42. 2.7.2 Bases Just as there are “organic acids,” there are also “organic bases.” The most workable definition of any base is a species that contains an atom capable of donating two electrons to an electron deficient center. Using the Brønsted–Lowry definition, such a reaction will form a new covalent bond to a hydrogen atom. For the most part, organic bases contain a heteroatom such as N, O or S. Any atom that has unshared electrons pairs in a neutral molecule can potentially function as a base. Likewise, an anion, which has a negative charge localized on an atom, can function as a base. The nature of various organic bases will be explored. Although carbonate (CO 3 2−) and bicarbonate (HCO 3−) are often utilized, amine units in amino acid residues are perhaps the most common base in biochemical systems. If a typical organic amine (e.g., trimethylamine) reacts with a Brønsted–Lowry acid (e.g., HCl), the basic nitrogen atom donates two electrons to hydrogen to form a new N—H bond, and the product is trimethylammonium chloride. The ammonium salt, (CH 3) 3 NH +, is the conjugate acid of this reaction. The p K a of an ammonium salt derived from an amine is typically ~10–11, where the p K a of HCl is about −7...
eBook - ePubSustainable Carbon Capture
Technologies and Applications
- Humbul Suleman, Philip Loldrup Fosbøl, Rizwan Nasir, Mariam Ameen, Humbul Suleman, Philip Loldrup Fosbøl, Rizwan Nasir, Mariam Ameen(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
...There are several groups of amines: a series of primary amines with different patterns of substitutions marked by red circles; a series of cyclic, sterically unhindered secondary amines marked by black diamonds; and a few commercially relevant amines, monoethanolamine (MEA), diethanolamine (DEA), and ammonia (NH 3). There is a distinct linear relationship between the protonation constants and both k + and K for most amines; this relationship is indicated by the dashed black lines. Within the collection of amines represented in this graph, there are very few outliers; notably, they all feature reduced reactivities relative to the others with similar protonation constants. This is straightforwardly explained by steric hindrance. The proton, which defines the protonation constant, is much smaller than CO 2. Sec. butylamine (SBA) and 2-amino-1-propanol (2-AP) can be seen as MEA analogues with sterically bulky methyl groups next to the amine nitrogen, thus they are sterically hindered. DEA is a secondary amine, which is not restrained. The remaining ammonia, NH 3, is not sterically hindered, but its interactions with the solvent, H 2 O, are substantially different from the other amines; for this reason, it does not follow the general trend. Figure 2.3 The linear relationship between the Lewis basicity, as defined by the protonation constant log 6 and the reaction rate constant log k 7 (a) and the overall carbamic acid stability constant log K 7 (b) for sterically unhindered amines is visualized by the dashed line. Sterically hindered amines react slower and form a less stable reaction product with CO 2. Note that k 7 and K 7 are equivalent to k + and K, respectively, from Eq. 2.1. Source : Reprinted with permission from W. Conway, X. G. Wang, D. Fernandes, R. Burns, G. Lawrance, G. Puxty and M. Maeder...
