Why is 1 3 cyclopentadiene acidic

There are several examples of cationic and anionic compounds with unexpected stabilities that suggest that they are aromatic. It is important to understand how charged carbons in these compounds will affect the determination of aromaticity. There are two main situations which need to be considered: The conjugation of a carbocation and the conjugation of a carbanion.

A carbocation carbon is surrounded by three electron groups giving it sp 2 hybridization. The remaining unhybridized p orbital holds the carbocation's positive charge and is vacant of pi electrons.

Although a carbocation is capable of extending conjugation it does not add to the compound's pi electron count. A carbanion carbon is surrounded by four electron groups and would normally be sp 3 hybridized. However, to obtain the stabilizing effects of conjugation, carbanion carbons can becomes sp 2 hybridized putting the set of lone pair electrons into the unhybridized p orbital. Because the carbanion's p orbital contains two electrons in the form of a set of lone pair electrons, it increases a compound's pi electron count by 2.

One of the most well know examples of an aromatic ion is the 1,3-cyclopentadiene ion. However, if a proton is removed form the CH 2 group to form the cyclopentadienyl anion, the carbon atom becomes sp 2 hybridized and the two electrons of the resulting lone pair occupy the newly produced p orbital.

Moreover, this new p orbital overlaps with the p orbitals already present allowing for cyclic delocalization of pi electrons about the entire ring. The resonance hybrid can be shown by drawing a series of five resonance form. As discussed in Section The 6 pi electrons gained by forming an anion is enough to completely fill the bonding MO's in the diagram giving the cyclopentadienyl anion aromaticity.

One of the effects of the aromaticity of the cyclopentadienyl anion is that the acidity of 1,3-cyclopentadiene is unusually strong. As previously discussed, stabilizing the conjugate base increases the acidity of the corresponding acid. In this case, the conjugate base of 1,3-cyclopentadiene, the cyclopentadienyl anion, is stabilized through aromaticity. This makes 1,3-cyclopentadiene one of the most acidic hydrocarbons known with a pK a of This is almost 10 30 times more acidic than cyclopentane.

Because of its acidity, cyclopentadiene can be deprotonated by moderately strong bases such as NaOH. Other solutions. Name the following compounds:. View Full Material. Draw the structure of 3-quinuclidinone. What is the approximat Draw the product of each of the following reactions:.

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Active Oldest Votes. Well put! Another example is cyclooctatetraene. Edit: Huckel's Rule: Aromaticity - Antiaromaticity If we look at the energetic positioning of the molecular orbitals MO's in a cyclic, conjugated polyene, we can quickly understand the basis for Huckel's rule. Improve this answer. It comes from the molecular orbital diagrams. See these earlier SE Chem questions, ref1 , ref2. Sign up or log in Sign up using Google. Sign up using Facebook.

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