Введение
Свойство конъюгированных систем с 4n делокализованными электронами
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
Антиароматичность – химическое свойство циклической молекулы с π-электронной системой, которая обладает более высокой энергией, то есть менее стабильна из-за наличия в ней 4n делокализованных (π или неподеленных) электронов, в отличие от ароматичности. В отличие от ароматических соединений, которые подчиняются правилу Гюкеля ([4n+2] π-электронов) и являются высокостабильными, антиароматические соединения высоконестабильны и высокореактивны. Чтобы избежать нестабильности, связанной с антиароматичностью, молекулы могут изменять форму, становясь неплоскими и, следовательно, нарушая некоторые π-взаимодействия. В отличие от диамагнитного кольцевого тока, присутствующего в ароматических соединениях, антиароматические соединения обладают парамагнитным кольцевым током, который можно наблюдать с помощью спектроскопии ЯМР. Примерами антиароматических соединений являются пентален (A), бифенилен (B), циклопентадиенил-катион (C). Прототипический пример антиароматичности, циклобутадиен, является предметом дискуссий, при этом некоторые ученые утверждают, что антиароматичность не является основным фактором, способствующим его дестабилизации. Циклооктатетраен является примером молекулы, принимающей неплоскую геометрию, чтобы избежать дестабилизации, возникающей вследствие антиароматичности. Если бы он был плоским, он имел бы одну восьмиэлектронную π-систему вокруг кольца, но вместо этого принимает форму лодки с четырьмя отдельными π-связями. Поскольку антиароматические соединения часто недолговечны и с ними трудно работать экспериментально, энергия антиароматической дестабилизации часто моделируется с помощью расчетов, а не экспериментов. Критерии IUPAC для антиароматичности следующие:
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
Молекула должна быть циклической. Молекула должна быть плоской. Молекула должна иметь полную конъюгированную π-электронную систему внутри кольца. Молекула должна содержать 4n π-электронов, где n – любое целое число в конъюгированной π-системе. Это отличается от ароматичности только четвертым критерием: ароматические молекулы имеют 4n+2 π-электрона в конъюгированной π-системе и, следовательно, подчиняются правилу Гюкеля. Неароматические молекулы либо нециклические, либо неплоские, либо не имеют полной конъюгированной π-системы внутри кольца.
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
+ Сравнение ароматичности, антиароматичности и неароматичности
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
Ароматическая Антиароматическая Неароматическая
Циклическая? Да Да Не удовлетворяет хотя бы одному из критериев
Имеет полностью конъюгированную систему p-орбиталей в кольце молекулы? Да Да Нет
Плоская? Да Да Нет
Количество π-электронов в конъюгированной системе? 4n+2 (т.е. 2, 6, 10,…) 4n (4, 8, 12,…) N/A
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
Наличие плоской кольцевой системы необходимо для максимизации перекрывания p-орбиталей, составляющих конъюгированную π-систему. Это объясняет, почему плоская циклическая молекула является ключевой характеристикой как ароматических, так и антиароматических молекул. Однако на практике трудно определить, полностью ли конъюгирована молекула, просто взглянув на ее структуру: иногда молекулы могут искажаться, чтобы снять напряжение, и это искажение может нарушить конъюгацию. Таким образом, необходимо предпринять дополнительные усилия для определения того, действительно ли определенная молекула является антиароматической. Антиароматическое соединение может проявлять свою антиароматичность как кинетически, так и термодинамически. Как будет обсуждаться далее, антиароматические соединения обладают исключительно высокой химической реакционной способностью (высокая реакционная способность не является "показателем" антиароматического соединения, она лишь указывает на то, что соединение может быть антиароматическим). Антиароматическое соединение также может быть распознано термодинамически путем измерения энергии циклической конъюгированной π-электронной системы. В антиароматическом соединении количество энергии конъюгации в молекуле будет значительно выше, чем в соответствующем соединении-эталоне. На практике рекомендуется тщательно проанализировать структуру потенциально антиароматического соединения, прежде чем утверждать, что оно действительно является антиароматическим. Если экспериментально определенной структуры рассматриваемой молекулы не существует, необходимо провести вычислительный анализ. Потенциальная энергия молекулы должна быть исследована для различных геометрий, чтобы оценить любое отклонение от симметричной плоской конфигурации. Многие ароматические и антиароматические соединения (бензол и циклобутадиен) слишком малы, чтобы иметь протоны внутри кольца, где эффекты экранирования и дезэкранирования могут быть более диагностически полезными для определения того, является ли соединение ароматическим, антиароматическим или неароматическим. Анализ химического сдвига, не зависящего от ядра (NICS), является методом вычисления экранирования (или дезэкранирования) кольца в центре кольцевой системы для прогнозирования ароматичности или антиароматичности. Отрицательное значение NICS указывает на ароматичность, а положительное – на антиароматичность.
Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i. e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy. Examples of antiaromatic compounds are pentalene (A), biphenylene (B), cyclopentadienyl cation (C). The prototypical example of antiaromaticity, cyclobutadiene, is the subject of debate, with some scientists arguing that antiaromaticity is not a major factor contributing to its destabilization. Cyclooctatetraene is an example of a molecule adopting a non planar geometry to avoid the destabilization that results from antiaromaticity. If it were planar, it would have a single eight electron π system around the ring, but it instead adopts a boat like shape with four individual π bonds. Because antiaromatic compounds are often short lived and difficult to work with experimentally, antiaromatic destabilization energy is often modeled by simulation rather than by experimentation. The IUPAC criteria for antiaromaticity are as follows:
The molecule must be cyclic. The molecule must be planar. The molecule must have a complete conjugated π electron system within the ring. The molecule must have 4n π electrons where n is any integer within the conjugated π system. This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π electrons in the conjugated π system and therefore follow Hückel’s rule. Non aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. + Comparing aromaticity, antiaromaticity and non aromaticity AromaticAntiaromaticNon aromatic Cyclic? Yes Yes Will fail at least one of these Has completely conjugated system of p orbitals in ring of molecule? YesYes Planar? YesYes How many π electrons in the conjugated system? 4n+2 (i. e., 2, 6, 10, )4n (4, 8, 12, ) N/A
Having a planar ring system is essential for maximizing the overlap between the p orbitals which make up the conjugated π system. This explains why being a planar, cyclic molecule is a key characteristic of both aromatic and antiaromatic molecules. However, in reality, it is difficult to determine whether or not a molecule is completely conjugated simply by looking at its structure: sometimes molecules can distort in order to relieve strain and this distortion has the potential to disrupt the conjugation. Thus, additional efforts must be taken in order to determine whether or not a certain molecule is genuinely antiaromatic. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound. In reality, it is recommended that one analyze the structure of a potentially antiaromatic compound extensively before declaring that it is indeed antiaromatic. If an experimentally determined structure of the molecule in question does not exist, a computational analysis must be performed. The potential energy of the molecule should be probed for various geometries in order to assess any distortion from a symmetric planar conformation. Many aromatic and antiaromatic compounds (benzene and cyclobutadiene) are too small to have protons inside of the ring, where shielding and deshielding effects can be more diagnostically useful in determining if a compound is aromatic, antiaromatic, or nonaromatic. Nucleus Independent Chemical Shift (NICS) analysis is a method of computing the ring shielding (or deshielding) at the center of a ring system to predict aromaticity or antiaromaticity. A negative NICS value is indicative of aromaticity and a positive value is indicative of antiaromaticity.
Примеры
Хотя существует множество молекул, которые, судя по расчетам, кажутся антиароматическими, число молекул, действительно являющихся антиароматическими, значительно меньше. Это связано с тем, что обычно нельзя получить производные антиароматических молекул путем присоединения дополнительных антиароматических углеводородных колец и т.п., поскольку молекула, как правило, теряет либо свою плоскую структуру, либо свою сопряженную систему π-электронов и становится неароматической. В этом разделе представлены только неоспоримые примеры антиароматических соединений. Пентален – хорошо изученное как экспериментально, так и теоретически в течение десятилетий антиароматическое соединение. Он дициклический, плоский и содержит восемь π-электронов, что соответствует определению антиароматичности, данному IUPAC. Дианионные и дикационные состояния пенталена ароматичны, поскольку удовлетворяют правилу 4n + 2 π-электронов Гюкеля.
Гексадегидро-[12]аннулен
Как и его родственник [12]аннулен, гексадегидро[12]аннулен также является антиароматическим. Его структура была изучена вычислительным путем с использованием расчетов ab initio и теории функционала плотности и подтверждена как антиароматическая.
Циклобутадиен
Циклобутадиен – классический пример антиароматического соединения, приводимый в учебниках. Обычно его представляют как плоскую циклическую молекулу с 4 π-электронами (4n при n=1) в сопряженной системе. Однако давно существует вопрос, действительно ли циклобутадиен является антиароматическим, и недавние открытия позволяют предположить, что это может быть не так. Циклобутадиен особенно неустойчив, и изначально это объяснялось его антиароматичностью. Однако в двух параллельных связях циклобутадиена степень двойственности выше, чем в остальных, а π-электроны не делокализованы между этими двумя двойными связями, что придает молекуле прямоугольную, а не квадратную форму.
Циклооктатетраен
Циклооктатетраен – еще один пример молекулы, которая не является антиароматической, хотя на первый взгляд может показаться таковой. Циклооктатетраен принимает конформацию в виде ванны (то есть, напоминающую лодку). Поскольку он не плоский, даже имея 4n π-электронов, эти электроны не делокализованы и не конъюгированы. Следовательно, молекула не является ароматической. Хотя антиароматический характер циклобутадиена остается предметом дискуссий, снятие антиароматичности обычно рассматривается как движущая сила этой реакции. Антиароматичность также может существенно влиять на значение pKa. Линейное соединение пропен имеет pKa равный 44, что относительно кисло для sp3-углеродного центра, поскольку образующийся аллильный анион может быть стабилизирован резонансом. Аналогичная циклическая система, по-видимому, обладает еще большей стабилизацией за счет резонанса, поскольку отрицательный заряд может быть делокализован по трем атомам углерода вместо двух. Однако циклопропенильный анион содержит 4 π-электрона в циклической системе и на самом деле имеет значительно более высокое значение pKa, чем у пропена, поскольку он является антиароматическим и, следовательно, дестабилизирован. Избавление от антиароматичности иногда может быть движущей силой реакции. В следующей кето-энольной таутомеризации образующийся энол более стабилен, чем исходный кетон, хотя кетон содержит ароматическое бензольное кольцо (синего цвета). Однако присутствует также антиароматическое лактонное кольцо (зеленого цвета). Снятие антиароматической дестабилизации обеспечивает движущую силу, которая перевешивает даже потерю ароматичности бензола.