Name: why ring nitrogen changes amine basicity
Uploaded: 2023-04-30T10:39:04.000Z
Duration: 1 min 25 s
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La basicité des amines hétérocycliques est influencée à la fois par les effets de résonance et d’hybridation.
Pour comprendre l’effet de résonance, considérons deux amines hétérocycliques aromatiques : la pyridine et le pyrrole. Les électrons non liés de l’azote pyridine résident dans une orbitale hybride sp2. Comme ils ne contribuent pas au sextuor aromatique, ils sont disponibles pour la protonation.
Dans le pyrrole, les électrons non liants sont présents dans une orbitale p et font partie du système π aromatique. Ici, la protonation est défavorable car l’aromaticité est perdue dans l’acide conjugué.
Comme le pKa du pyrrole protoné est inférieur au pKa de la pyridine protonée, la pyridine est plus basique que le pyrrole.
Pour l’effet d’hybridation, considérons l’hybridation de l’azote amine. Au fur et à mesure que le caractère du pourcentage augmente, les électrons non liés sont maintenus plus étroitement et sont moins disponibles pour la protonation.
Avec moins de caractère s de l’azote pipéridine hybridé sp3, il est facilement protoné avec une valeur pKa plus élevée de l’acide conjugué. Ainsi, la pipéridine est plus basique que la pyridine.

basicité des amines aromatiques hétérocycliques

Basicity of Heterocyclic Aromatic Amines

Les amines hétérocycliques, où l'atome d'azote fait partie d'un système alicyclique, ont une basicité similaire aux alkylamines. Il est intéressant de noter que l’amine hétérocyclique ayant un atome d’azote faisant partie d’un cycle aromatique a beaucoup moins de basicité que son homologue alicyclique correspondante. Pour cette raison, comme le montre la figure 1, la pipéridine (pKb = 2.8) est significativement plus basique que la pyridine (pKb = 8.8).

<img alt="Figure1" src="/files/ftp_upload/12527/12527-PT_Figure_1.svg" style="text-align: center; height: 110px;">

Figure 1. Comparaison de la basicité de la pipéridine et de la pyridine.

Cette différence de basicité peut être attribuée à l'état d'hybridation des orbitales contenant une paire isolée d'électrons sur l'atome N, comme le montre la figure 2. Dans le cas de la pipéridine, la paire isolée réside dans une orbitale hybride sp3 ayant un s inférieur. caractère, rendant la paire isolée plus disponible pour présenter une basicité envers l'acide. D'autre part, dans la pipéridine, la paire isolée réside dans une orbitale hybride sp2 contenant un caractère s beaucoup plus élevé. Par conséquent, le doublet isolé est plus étroitement lié au cycle aryle et moins disponible pour présenter une basicité envers l’acide.

<img alt="Figure2" src="/files/ftp_upload/12527/12527-PT_Figure_2.svg" style="text-align: center; height: 60px;">

Figure 2. L'effet des orbitales hybridées sur la basicité.

Le pyrrole est beaucoup moins basique que la pyridine, ayant une valeur pKb de 15. La paire libre d'électrons sur l'atome N illustrée à la figure 3 réside dans une orbitale p pure et est parfaitement alignée avec les orbitales p des atomes C, qui participent à la l'aromaticité de la bague. Par conséquent, le doublet libre d’électrons sur l’atome N du pyrrole est délocalisé par résonance dans tout l’anneau. À l’inverse, le doublet libre sur l’atome N de la pyridine étant dans une orbitale hybridée sp2 est aligné perpendiculairement aux autres orbitales sp2 des atomes C participant à la résonance. Ainsi, la paire d’électrons libres sur l’atome N de la pyridine est beaucoup plus disponible que celle du pyrrole, ce qui entraîne une basicité plus élevée de la pyridine.

<img alt="Figure3" src="/files/ftp_upload/12527/12527-PT_Figure_3.svg" style="text-align: center; height: 90px;">

Figure 3. La structure de Lewis du pyrrole.

L'imidazole contient deux atomes de N dans un cycle à cinq chaînons et constitue un hétérocycle vital présent dans de nombreuses protéines. Cette amine hétérocyclique avec un pKb de 7 est plus basique que la pyridine d'un facteur 100. L'atome de N ressemblant à celui du pyrrole n'est pas basique. L'autre atome de N est basique et extrait H de l'acide, donnant naissance à son acide conjugué, qui est stabilisé par la résonance, comme illustré sur la figure 4. Cette stabilisation par résonance de la base conjuguée entraîne une basicité accrue de l'imidazole par rapport à la pyridine.

<img alt="Figure4" src="/files/ftp_upload/12527/12527-PT_Figure_4.svg" style="text-align: center; height: 125px;">

Figure 4. La stabilisation de la résonance dans les ions imidazolium.

La basicité des amines constitue un outil pratique précieux pour séparer les amines d’un mélange contenant d’autres composés neutres. Ceci est réalisé en dissolvant le mélange impur dans de l'éther et en l'agitant avec de l'eau dans une ampoule à décanter. Une fois les deux couches séparées, le drainage de la couche d’eau élimine le sel et la plupart des impuretés inorganiques. Lorsqu'un acide aqueux dilué est ajouté à la couche organique, les amines sont sélectivement protonées dans l'acide correspondant et se dissolvent dans la couche aqueuse. L'égouttage de la couche organique élimine l'impureté organique neutre. Une basification lente de la couche aqueuse régénère à nouveau l'amine libre, qui pourrait être extraite à l'aide d'un nouveau volume d'éther. L'évaporation ultérieure de l'éther donne une amine pure.

Basicité des amines aromatiques hétérocycliques

Les amines hétérocycliques, où l’atome N fait partie d’un système alicyclique, ont une basicité similaire à celle des alkylamines. Il est intéressant de noter que l’amine hétérocyclique ...

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why ring nitrogen changes amine basicity

Heterocyclic amines can show very different basicity depending on where the nitrogen atom sits in the ring. When nitrogen is part of an alicyclic ring, the amine behaves much like an alkylamine. When nitrogen is part of an aromatic ring, the compound is usually much less basic than its alicyclic counterpart.

Piperidine and pyridine show this difference clearly. Piperidine is significantly more basic than pyridine, with pKb values of 2.8 and 8.8, respectively. The reason is the orbital that holds the lone pair on nitrogen. In piperidine, the lone pair is in an sp3-hybridized orbital with lower s character, so it is easier to use in reactions with acid. In pyridine, the lone pair is in an sp2-hybridized orbital with higher s character, so it is held more tightly and is less available.

Pyrrole is even less basic than pyridine. Its pKb value is 15. The lone pair on nitrogen is in a pure p orbital, and it lines up with the p orbitals of the carbon atoms in the ring. That lone pair helps the ring stay aromatic, so it is delocalized by resonance. In pyridine, by contrast, the lone pair is in an sp2 orbital that points away from the ring system, so it is much more available for basic behavior.

Imidazole has two nitrogen atoms in a five-membered ring and is found in many proteins. It is more basic than pyridine by a factor of 100, with a pKb of 7. The nitrogen that acts like pyrrole is not basic. The other nitrogen is basic and can accept H from acid. Its conjugate acid is stabilized by resonance, and that stabilization helps explain the higher basicity of imidazole.

The basicity of amines is also useful in the lab for separating amines from neutral compounds. A common method uses ether and water in a separatory funnel. The water layer is drained first to remove the salt and most inorganic impurities. Then dilute aqueous acid is added to the organic layer. The amines become protonated and move into the aqueous layer, while the neutral organic impurity stays behind. The aqueous layer is then basified slowly to regenerate the free amine, which can be extracted with fresh ether and recovered after the ether is evaporated.

Why Ring Nitrogen Changes Amine Basicity

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Amines: Types and Classifications

Amines are organic derivatives of ammonia. They form when one or more ammonia protons are replaced by alkyl or aryl groups. Chemists sort amines by the number of organyl groups attached to the nitrogen atom. Primary amines have one organyl group on nitrogen. Secondary amines have two, and tertiary amines have three. If a fourth organyl group is present, the nitrogen carries a formal positive charge. These compounds are called quaternary ammonium salts. Amines are also grouped as aliphatic or...

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Primary amines are named by following a few clear organic chemistry rules. These rules help identify the amino group, written as –NH2, and show how it connects to the carbon chain. Common names for primary amines use the alkyl group name plus the suffix -amine. In IUPAC naming, the final -e of the parent alkane is changed to -amine, which gives an alkanamine name. The longest carbon chain attached to the amine nitrogen is chosen, and the carbon bonded to nitrogen gets the lowest locant. Any...

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Naming Secondary, Tertiary, and Quaternary Amines

Secondary and tertiary amines are ammonia derivatives with two or three hydrogen atoms replaced by alkyl groups. They may be symmetrical, with the same alkyl groups attached to nitrogen, or unsymmetrical, with more than one kind of alkyl group present. These compounds are often named as alkylamines in standard naming. For symmetrical secondary and tertiary amines, the prefixes di- and tri- are used. For unsymmetrical amines, each substituent is named and listed in alphabetical order. IUPAC...

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Naming Aniline and Ring Amine Derivatives

Naming aniline and ring amine derivatives follows clear rules for aromatic and heterocyclic amines. The simplest aromatic amine is phenylamine, which has an –NH2 group directly attached to an aromatic ring. The common name for this skeleton is aniline. Functionalized anilines often use the prefixes ortho-, meta-, and para- to show where a substituent is placed on the ring. Many of these compounds also have trivial, or common, names. For systematic naming, the ring is numbered from the carbon...

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Amines have a nitrogen atom with a lone pair of electrons. That lone pair affects the shape of the molecule, the bond angles, and the bond lengths around nitrogen. In most amines, nitrogen is hybridized and attached to three substituents. The bond angle is about 108°, which is a little less than the 109.5° tetrahedral angle. The C–N–H bond angle is slightly larger at 112°, and the carbon-nitrogen bond length is 147 pm. This bond is longer than the carbon-oxygen bond in alcohols, which is 143...

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Aliphatic amines act as bases because the nitrogen atom can accept a proton from an acid. They can also behave as Lewis bases, since the nitrogen lone pair can form a covalent bond with an electrophile. These properties make amines useful examples of basic behavior in organic chemistry. Amine basicity is commonly described in two ways. One method uses the basicity constant, K b, for the reaction in which the amine removes a proton from water. A lower K b means a stronger base. By this measure,...

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Why Aromatic Amines Are Less Basic

Aromatic amines are less basic than aliphatic amines because the nitrogen lone pair can share electrons with the aryl ring through resonance. When the lone pair is delocalized, it is less available to accept a proton, so the amine shows weaker basicity. Substituents on the aryl ring can change this effect. Electron-donating groups increase electron density on the ring and raise the basicity of the amine. Electron-withdrawing groups have the opposite effect and can lower basicity sharply. A...

basicity of heterocyclic aromatic amines

Heterocyclic amines can show very different basicity depending on where the nitrogen atom sits in the ring. When nitrogen is part of an alicyclic ring, the amine behaves much like an alkylamine. When nitrogen is part of an aromatic ring, the compound is usually much less basic than its alicyclic counterpart. Piperidine and pyridine show this difference clearly. Piperidine is significantly more basic than pyridine, with pKb values of 2.8 and 8.8, respectively. The reason is the orbital that...

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Alkylation of Ammonia to Make Amines

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Azide Route to Primary Amines

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Gabriel Synthesis for Primary Amines

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Routes to Amines from Oximes and Nitro Groups

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Making Amines from Nitriles and Amides

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Making Amines with Reductive Amination

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Carbon-Loss Routes to Primary Amines

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Primary Amine Synthesis by Rearrangement

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Using Acylation to Modify Amines

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Hofmann Elimination and Alkene Formation

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Syn Elimination in Cope Reaction

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Nitrous Acid Reactions with Amines

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1&deg; amines to diazonium or aryldiazonium salts: diazotization with nano2 mechanism

Diazotizing Primary Amines with Nitrous Acid

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2&deg; amines to n-nitrosamines: reaction with nano2

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Nitrosamine Formation from Secondary Amines

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Making Aryl Halides from Diazonium Salts

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Arenediazonium Salt Reactions: Phenols and H

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Diazo Coupling and Azo Dye Formation

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key terms and concepts

Compare piperidine, pyridine, pyrrole, and imidazole to see how nitrogen placement changes basicity and how amines are separated in the lab.

Why Ring Nitrogen Changes Amine Basicity | JoVE K12

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Why Ring Nitrogen Changes Amine Basicity

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