26Aug 2020 by Vincent Summers No Comments

[12]Annulene and Two Simple Derivatives – Aromaticity?

Chemistry
Complex chemicals possess names almost equally complex. Occasionally, a simpler nomenclature can be employed. For basic aromatic compounds, composed of a ring of alternating single and double bonded carbon atoms in a ring, the name [n]annulene has been is commonly used. Examples, including [12]annulene are given below. Aromaticity We won't go into the theory behind aromaticity. What we will do is lay out the basic factors that students use to determine if a compound is aromatic. 1. The ring is composed of conjugated single and double bonds (...−C=C−C=...). 2. The molecule is relatively flat. 3. The number of available π-electrons equals 4n + 2 (a Hückel number), where n is generally a small positive integer.¹ 4. Crowding does not severely limit or prevent aromaticity. 5. Ring size affects aromaticity, but…
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13Feb 2018 by Vincent Summers No Comments

Is Cyclopropenethione Aromatic, As Is Its Oxygen Analog?

Chemistry, Logic
As small as it is, and even with a heteroatom in its make up, cyclopropenethione is aromatic, in the same way cyclopropenone is aromatic. Its aromaticity is not due to a theoretically electrically neutral structure, as in Figure 1, but to its "alternative" zwitterionic structure, shown in Figure 2. Aromatic Characteristics Hückel descriptors fall short of aromaticity if cyclopropenethione exhibited only the structure drawn in Figure 1. In addition to a closed and flat ring, aromaticity requires a 4n+2 number of π-electrons (pi), where n is usually a small integer. In the un-ionized form of Figure 1, each carbon atom has a π-electron, for a total of 3 electrons. This is because every double bond consists of one π-electron plus one σ-electron (sigma) per constituent atom. Widening Perspectives: Cyclopropenethione The…
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07Nov 2016 by Vincent Summers 4 Comments

Aromatic Resonance – What It Is and What It Is Not

Chemistry
[caption id="attachment_19938" align="alignright" width="440"] Prominent resonance structures.[/caption] Sometimes a chemistry or physics concept is misunderstood even by the reader of above-average intelligence and education. Such is the case with the chemist’s use of the term aromatic resonance. There are a small number of recognized rules for aromatic behavior. These include: cyclic geometery planarity 4n + 2 contributing, de-localized, conjugated (π) double bonds An Example of What Aromatic Resonance Is Not The best and oldest known example is benzene, C₆H₆. Benzene is cyclic (one ring), planar (flat), and has 6 delocalized, conjugated double bonds. Frequently, the organic chemist simply renders benzene’s structure… There is a carbon atom at each vertex of the hexagonal ring. Attached to each carbon and pointing outward from it is one hydrogen atom. Now each carbon atom…
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24May 2016 by Vincent Summers No Comments

Why Cyclodecapentaene is Not an Aromatic Hydrocarbon

Chemistry
[caption id="attachment_16073" align="alignright" width="440"] Fig 1. Cyclodecapentaene - all cis form.[/caption] There are aromatic hydrocarbons and there are non-aromatic hydrocarbons. For basic mono-ring structures that have alternating carbon-carbon double bonds, there is a rule called Hückel’s rule that defines whether a given hydrocarbon can be aromatic or not. It might seem, at first that cyclodecapentaene (C10H10 or [10]-annulene) should be aromatic. Yet, cyclodecapentaene is not an aromatic hydrocarbon. Let’s find out why not. A Few Simple Rules for Aromatic Behavior  The double bonds in the ring must be conjugated (alternating). This allows for the electrons to be delocalized [in conceptualization be flipped in either a clockwise or counterclockwise direction]. This allows for the presence of what is called a ring current.  The ring must be flat.  The mono-ring must feature…
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16May 2016 by Vincent Summers No Comments

Cyclobutadiene Antiaromatic or Other Destabilization Factors?

Chemistry
Clearly cyclobutadiene is not aromatic. Is cyclobutadiene antiaromatic? Or is its lack of stability due to other factors? Cyclobutadiene is a four-carbon hydrocarbon. The –diene part of its name indicates it has two double bonds. The cyclo– part indicates it is not a chain structure, but a cyclic (ring) structure. Now double bonds hold atoms tighter than single ones do. So they are shorter than single bonds. This causes cyclobutadiene to assume a rectangular shape. If it were square instead, it would indicate the four carbon to carbon bonds were identical, and aromatic. Cyclobutadiene's four hydrogen atoms point away from the ring. Analyzing the Structure Cyclobutadiene is flat. The two double bonds of cyclobutadiene are conjugated. That is, their ring structure alternates double bond, single bond, double bond, single bond.…
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13May 2016 by Vincent Summers 4 Comments

Is Cyclopropenone Aromatic or Not?

Chemistry
Is cyclopropenone aromatic? Before reading on, try to anticipate the answer to that question. When most students think of aromatic, they think of simple hydrocarbons—compounds composed of hydrogen and carbon atoms only. The best known example of an aromatic hydrocarbon is the 6-sided cyclic compound benzene—C₆H₆. Benzene, if drawn according to classical structures, has alternating single and double bonds between carbon atoms. Basic Qualifications for Aromaticity A double bond consists of one sigma (σ) and one pi (π) bond. If the ring is single rather than multiple (as, for instance, for naphthalene, which is comprised of two fused rings), and if it has 4n+2 completely conjugated π electrons in a closed loop or ring, is neutral, and is flat, the structure obeys Hückel’s rule and will be aromatic. There are…
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21Mar 2014 by Vincent Summers 2 Comments

The Aromatic Cyclopentadienyl Anion

Chemistry, Education
[caption id="attachment_16480" align="alignright" width="440"] Cyclopentadienyl Anion[/caption] Is it possible that for an ion to be aromatic? Yes. Consider the aromatic cyclopentadienyl anion. Hückel’s Rule dictates a flat ring with 4n + 2 π (pi) conjugated electrons. The smallest neutral ring with these qualifications has n = 1. It is benzene (C₆H₆). But Hückel’s law does not require an electronically neutral structure. The smallest aromatic ion is the cyclopropenyl cation¹ (C₃H₃⁺). Is there a negative ion that has 6 π electrons and is aromatic? The answer is yes. The aromatic cyclopentadienyl anion (C₅H₅⁻). Neutral Cyclopentadiene Neutral cyclopentadiene is flat. It is pentagon shaped. Three of the carbon-carbon bonds are single bonds. The other two are double bonds. At each of the four ends of the two double bonds there is one…
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14Dec 2012 by Vincent Summers No Comments

Aromatic Tropylium Ion

Chemistry
[caption id="attachment_20212" align="alignright" width="440"] Aromatic tropylium cation[/caption] Aromatics are separate category of organic compounds. Often aromatic compounds are electrically neutral. However, a few do carry a charge. They are ions.¹ In fact many aromatic ions would not be aromatic if they lost their charge. We will consider the aromatic tropylium cation in this article. Three Carbon Atoms The smallest aromatic ion is the cyclopropenyl cation. The neutral cyclopropene molecule possesses 4 π-electrons. That is not an aromatic number. By making it a cation, it loses 2 of them, meeting the qualification for aromatic compounds.¹ [caption id="attachment_16191" align="alignright" width="240"] Spacefill Model.[/caption] Four Carbon Atoms On the basis of π-electron count alone, one might suppose a cyclobutadienyl di-anion or a cyclobutadienyl di-cation, would display aromaticity. That is not the case. There are…
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