aromaticity Archives - Quirky Science

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…

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…

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…

12Aug 2015 by Vincent Summers 1 Comment

Are Additional Varieties of Aromaticity Achievable?

Chemistry

[caption id="attachment_7679" align="alignright" width="380"] A stick model of the aromatic molecule, benzene.[/caption] I am a retired chemist. I have no laboratory, and I have no close affiliations, but I do retain a brain. One of my favorite topics is aromaticity. Organic chemistry, essentially the study of carbon chemistry, features aliphatic compounds and aromatic compounds. Aromatic compounds, even being constructed from alternating (conjugated) double bonds, possess resistance against undergoing addition reactions, tending toward substitution reactions instead. An Idea Begins to Take Shape Realizing this, I began to wonder about a kind of aromaticity that might utilize triple bonds, rather than double bonds. However carbon-carbon triple bonds force other atoms to take positions along the same axis as the triple bond. A ring created from alternating single and triple carbon-carbon bonds would…

23Mar 2014 by Vincent Summers No Comments

The Quintessential Aromatic Hydrocarbon Benzene (Pt.1)

Chemistry

[caption id="attachment_14959" align="alignright" width="440"] Cyclohexane - Chair Form[/caption] Benzene? What's that? Living creatures universally share an important characteristic: they all are constructed of carbon-containing compounds. For that reason, chemists call such compounds organic. Now the term organic has taken on additional meaning. Compounds that are similar, but not found in nature, are also called organic compounds. One group of organic compounds, whether found in nature or not, possess special properties—chemical and physical—that put them into a category apart. Once it was thought such compounds were distinguishable by smell or aroma. Each was labeled an aromatic, and—the property itself—aromaticity. Simple Hydrocarbons The simplest hydrocarbon1 is methane (CH4)—a gas. It possesses a tetrahedral structure, with a carbon atom at its center and four hydrogen atoms at the corners (See Figure 1). Since…

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…

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…