Indeed, we have seen in past chapters that amines react with electrophiles in several polar reactions see for example the nucleophilic addition of amines in the formation of imines and enamines in Section The ammonium ions of most simple aliphatic amines have a p K a of about 10 or However, these simple amines are all more basic i. Thus, the free energy difference between an alkylamine and an alkylammonium ion is less than the free energy difference between ammonia and an ammonium ion; consequently, an alkylamine is more easily protonated than ammonia, and therefore the former has a higher p K a than the latter.
In this section we consider the relative basicity of amines. When evaluating the basicity of a nitrogen-containing organic functional group, the central question we need to ask ourselves is: how reactive and thus how basic and nucleophilic is the lone pair on the nitrogen? In other words, how much does that lone pair want to break away from the nitrogen nucleus and form a new bond with a hydrogen. The lone pair electrons makes the nitrogen in amines electron dense, which is represents by a red color in the electrostatic potential map present below left.
Amine are basic and easily react with the hydrogen of acids which are electron poor as seen below. Amines are one of the only neutral functional groups which are considered basis which is a consequence of the presence of the lone pair electrons on the nitrogen. This gives the nitrogen in the resulting ammonium salt four single bonds and a positive charge. Amines react with water to establish an equilibrium where a proton is transferred to the amine to produce an ammonium salt and the hydroxide ion, as shown in the following general equation:.
The equilibrium constant for this reaction is the base ionization constant K b , also called the base dissociation constant:. However, Kb values are often not used to discuss relative basicity of amines. It is common to compare basicity's of amines by using the K a 's of their conjugate acids, which is the corresponding ammonium ion. Fortunately, the K a and K b values for amines are directly related. Adding these two chemical equations together yields the equation for the autoionization for water:.
Most simple alkyl amines have pK a 's in the range 9. Aromatic herterocyclic amines such as pyrimidine, pyridine, imidazole, pyrrole are significantly weaker bases as a consequence of three factors. The first of these is the hybridization of the nitrogen.
In each case the heterocyclic nitrogen is sp 2 hybridized. The increasing s-character brings it closer to the nitrogen nucleus, reducing its tendency to bond to a proton compared to sp 3 hybridized nitrogens. The very low basicity of pyrrole reflects the exceptional delocalization of the nitrogen electron pair associated with its incorporation in an aromatic ring.
Basicity of common amines pK a of the conjugate ammonium ions. Alkyl groups donate electrons to the more electronegative nitrogen.
The inductive effect makes the electron density on the alkylamine's nitrogen greater than the nitrogen of ammonia.
The small amount of extra negative charge built up on the nitrogen atom makes the lone pair even more attractive towards hydrogen ions. Correspondingly, primary, secondary, and tertiary alkyl amines are more basic than ammonia. The nitrogen atom is strongly basic when it is in an amine, but not significantly basic when it is part of an amide group. While the electron lone pair of an amine nitrogen is localized in one place, the lone pair on an amide nitrogen is delocalized by resonance.
The electron density — in the form of a lone pair — is stabilized by resonance delocalization, even though there is not a negative charge involved. The electrostatic potential map shows the effect of resonance on the basicity of an amide. The map shows that the electron density, shown in red, is almost completely shifted towards the oxygen. This greatly decreases the basicity of the lone pair electrons on the nitrogen in an amide. Extraction is often employed in organic chemistry to purify compounds.
Liquid-liquid extractions take advantage of the difference in solubility of a substance in two immiscible liquids e. However, it is possible to have four organic substituents on the nitrogen, making it an ammonium cation with a charged nitrogen center.
Amines are able to hydrogen bond. As a result, the boiling points of these compounds are higher than those of the corresponding phosphines, but lower than those of the corresponding alcohols, which hydrogen bond to a stronger extent. Amines also display some solubility in water. However, the solubility decreases with an increase in carbon atoms, due to the increased hydrophobicity of the compound as the chain length increases.
Aliphatic amines, which are amines connected to an alkyl chain, display solubility in organic polar solvents. Aromatic amines, which are amines that participate in a conjugated ring, donate their lone pair of electrons into the benzene ring, and thus their ability to engage in hydrogen bonding decreases. This results in a decrease in their solubility in water and high boiling points.
Amines are bases, and their basicity depends on the electronic properties of the substituents alkyl groups enhance the basicity; aryl groups diminish it , steric hindrance, and the degree of solvation of the protonated amine. In general, the effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity.
Thus, the basicity of an amine can be expected to increase with the number of alkyl groups on the amine. Additionally, the effect of the aromatic ring delocalizes the lone pair of electrons on nitrogen into the ring, resulting in decreased basicity. The solvation of protonated amines changes upon their conversion to ammonium compounds. Quaternary ammonium salts usually exhibit the lowest solubility of the series. Industrially, amines are prepared from ammonia by alkylation with alcohols.
They can also be prepared via reduction of nitriles to amines using hydrogen in the presence of a nickel catalyst. Amines are quite reactive due to their basicity as well as their nucleophilicity.
Most primary amines are good ligands and react with metal ions to yield coordination complexes. One of the most important reactions for amines is their formation of imines, or organic compounds where nitrogen participates in a double bond, upon reacting with ketones or aldehydes.
Amines are ubiquitous in biology. Many important molecules are amine-based, such as neurotransmitters and amino acids. Their applications in the world include being starting material for dyes and models for drug design. They are also used for gas treatment, such as removing CO 2 from combustion gases. Boundless vets and curates high-quality, openly licensed content from around the Internet.
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