Is the cyclopentadienyl anion planar

The Hückel-Rule

When is an aromatic an aromatic?

Not only benzene and the direct benzene derivatives such as toluene, phenol etc. belong to the aromatics group, but also many other compounds, some of which only look vaguely like benzene.

According to the rule established by Erich Hückel (1896-1980) around 1930 (Hückel rule), a compound is aromatic if the following conditions are met:

  1. The molecule is ring-shaped.
    It doesn't matter whether the ring consists of 3, 4, 5, 6, 7 or more atoms. It also doesn't matter whether the atoms are carbon or other atoms (e.g. N, S or O).

  2. All atoms of the ring are sp2-hybridized.
    The decisive factor here is that every ring atom has a pz-Orbital available, in which pi-electrons can stay. In this system from pz-Orbitals must not be a gap. A sp3-hybridized atom would create such a gap because there is no pz-Orbital provides.

  3. The ring is planar.
    That means that all atoms of the ring lie in one plane. In this way the pz-Orbitals of the ring atoms overlap.

  4. The number of π electrons in the ring system obeys the rule (4n + 2).
    For benzene, n has the value 1, for naphthalene the value 2. A ring of 8 carbon atoms with 4 double bonds would have 8 π electrons, the number 8 does not meet the condition (4n + 2), so this compound would not be aromatic .

  5. The pi electrons are all delocalized across the ring.
    That means that every π-electron can be in every pz- Hold the orbital as long as the number of 2 electrons per orbital is not exceeded.

Aromatic compounds that comply with Hückel's rule are very stable and try to maintain this energetically favorable state even in chemical reactions.

Example biphenyl

Let's take a look at the compound biphenyl or the molecules of this compound. Biphenyl molecules can be described as two benzene rings that are linked by a C-C bond.

The biphenyl molecule with its pz orbitals

As you can see in this picture, biphenyl consists of two benzene rings. Each ring is aromatic in itself, so biphenyl is also an aromatic. The Hückel rule always applies to a ring system, not to the entire molecule. Each ring has 6 electrons and therefore fulfills the Hückel rule with n = 1.

Example cyclooctatetrene

Cyclooctatetraene

The cyclooctatetraene molecule does not meet the Hückel rules!

The molecule is ring-shaped, so rule 1 is fulfilled, and all C atoms are sp2-hybridized, which also fulfills the second rule. However, the ring is not planar, as can be seen in the illustration, so the third rule is not fulfilled. The pz orbitals can only overlap in pairs, but no system can be formed from all 8 pz orbitals. The fourth rule is also not fulfilled, because there is no n for which 4n + 2 = 8 holds. Because of the incomplete overlap of the pz orbitals, the 8 pi electrons are not delocalized over the entire ring, but remain localized in the respective double bond.

Example cyclopentadiene

The cyclopentadiene molecule and its anion

The cyclopentadiene molecule does not meet the Hückel rules!

The molecule is ring-shaped, so rule 1 is fulfilled, but not all carbon atoms are sp2-hybridized, so rule 2 is not fulfilled. The "upper" carbon atom in the picture on the left is namely sp3-hybridized. The ring is largely planar (rule 3), but there is no n with 4n + 2 = 4, which means that the fourth rule is not fulfilled. Because of the loophole in sp2-Ring is also not met rule 5.

In the presence of a strong base, the cyclopentadiene can release a proton relatively easily, so it acts like a weak acid. The pKS.-Value of cyclopentadiene is 16[2]. So for a hydrocarbon, cyclopentadiene is a very strong acid! Why is that?

In the cyclopentadienyl anion, which is created by releasing the proton, the "upper" carbon atom is no longer sp3-hybridized, but sp2-hybridized, like all carbo-anions. The gap in the pz orbitals is thus filled, which means that rule 2 and rule 5 are fulfilled at the same time. The number of pi electrons also increases from 4 to 6, which then also fulfills rule 4.

"It is a regular pentagonal, planar, cyclic ion; as well, it has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity."[1]

Example anthracene

The anthracene molecule

The anthracene molecule fulfills the Hückel rules!

The entire molecule is to be seen as a large ring of three sections (rule 1 fulfilled), all ring atoms are sp2-hybridized, so rule 2 is fulfilled. Rule 3 is met because the entire molecule is planar. With n = 3, 4n + 2 = 14 applies, which corresponds exactly to the number of pi electrons, and rule 5 is also fulfilled, there is no gap or interruption in the ring.