Symmetrical Vs Asymmetrical Aromatics: How To Distinguish?

Aromaticity is a property of unusually stable organic molecules such as benzene. Aromatic molecules have an extremely high resonance energy, undergo substitution rather than addition reactions, and have delocalized pi-electrons.

To determine whether a molecule is aromatic or not, there are four key rules to follow:

Condition #1 for Aromaticity: The Molecule Must Be Cyclic

The first rule for aromaticity is that the molecule must be cyclic, meaning it has a ring structure. If there is no ring, the molecule cannot be aromatic.

Condition #2 for Aromaticity: Every Atom in the Ring Must Be Conjugated

The second rule states that every atom in the ring must be conjugated. In other words, there must be a continuous ring of p-orbitals around the ring that build up into a larger cyclic pi system.

Condition #3 for Aromaticity: The Molecule Must Have [4n+2] Pi Electrons

The third rule is that the molecule must have the correct number of pi electrons, following Hückel's rule. The cyclic, conjugated molecule must have 2, 6, 10, 14, 18, or 22 pi electrons to be aromatic.

Condition #4 for Aromaticity: The Molecule Must Be Flat

The fourth and final condition for aromaticity is that the molecule must be flat or planar.

Now, let's apply these rules to determine whether a molecule is symmetrical or unsymmetrical. Symmetry in molecules refers to the arrangement of atoms and their spatial distribution. A symmetrical molecule has an axis of symmetry, meaning that if you rotate the molecule around this axis, it will look the same as the original molecule. On the other hand, an unsymmetrical molecule does not have an axis of symmetry, and you can tell if it has been rotated.

To identify whether a molecule is symmetrical or unsymmetrical, you can apply the four conditions for aromaticity and also consider the types of ligands attached to the doubly bonded carbon atoms. If the ligands attached to the carbon atoms are the same, the molecule is likely to be symmetrical. In contrast, if the ligands are different, the molecule is likely to be unsymmetrical.

Symmetrical compounds have the same ligands attached to the doubly bonded carbon atoms

For example, consider the following image of two symmetrical alkene compounds:

! [Two Symmetrical Alkene Compounds](https://i.imgur.com/m3r5v8N.png)

Here, the ligands attached to the doubly bonded carbon atoms are a methyl group and a hydrogen atom. These compounds are symmetrical because they have the same ligands attached to each double-bonded carbon atom.

On the other hand, consider the following example of an unsymmetrical aliphatic alkene:

! [An Unsymmetrical Aliphatic Alkene](https://i.imgur.com/h7b5j0t.png)

In this compound, a methyl group and a hydrogen atom are attached to one carbon atom, while two hydrogen atoms are attached to the other. This compound is unsymmetrical because there are different ligands attached to each carbon atom.

Another example of an unsymmetrical alkene is shown below:

! [An Unsymmetrical Aromatic Alkene](https://i.imgur.com/y5h4h4r.png)

Here, one carbon atom is attached to a hydrogen atom and an adjacent carbon atom of the ring structure, while the other carbon atom is attached to a methyl group and an adjacent carbon atom of the ring structure.

The key difference between symmetrical and unsymmetrical alkenes is the ligands attached to the doubly bonded carbon atoms. Symmetrical alkenes have the same ligands, while unsymmetrical alkenes have different ligands attached to these carbon atoms.

Unsymmetrical compounds have different ligands attached to the doubly bonded carbon atoms

For example, in the case of an unsymmetrical aliphatic alkene, there is a methyl group and a hydrogen atom attached to the right-side carbon atom and two hydrogen atoms attached to the left side carbon atom. The chemical compound becomes unsymmetrical because there are different ligands attached to each carbon atom.

Another example of an unsymmetrical alkene is an aromatic compound. It has two doubly bonded carbon atoms attached to different ligands; one carbon atom is attached to a hydrogen atom and an adjacent carbon atom of the ring structure, whereas the other carbon atom is attached to a methyl group and an adjacent carbon atom of the ring structure.

When an unsymmetrical reagent is added to an unsymmetrical alkene, the negative component of the reagent attaches itself to the unsaturated carbon atom that has a lower number of hydrogen atoms than the positive part of the reagent.

Symmetrical compounds are often called symmetric for short

Symmetry is a fundamental concept in chemistry. It can be used to predict or explain many of a molecule's chemical properties, such as whether or not it has a dipole moment, as well as its allowed spectroscopic transitions.

The symmetry of a molecule is determined by the existence of symmetry operations performed with respect to symmetry elements. A symmetry element is a line, a plane, or a point in or through an object, about which a rotation or reflection leaves the object in an orientation indistinguishable from the original.

A symmetry operation is a movement of the atoms which leaves the molecule indistinguishable from the original. There are four symmetry operations: rotation (Cn), reflection (σ), inversion (i) and improper rotation (Sn). Each symmetry operation is performed with respect to a symmetry element, which is either an axis (rotation), a plane (reflection), a point (inversion), or a combination of axis and plane perpendicular to this axis (an improper rotation).

The axis of highest order is called the main, or principal, axis and has the highest value of n among the Cn axes present. A molecule can have more than one symmetry axis.

A molecule with no symmetry elements other than the identity operation, E, is called asymmetric. Such an object is necessarily chiral.

Unsymmetrical compounds are often called asymmetric for short

In chemistry, a molecule is considered unsymmetrical when its appearance changes if you turn it about an axis of symmetry. In other words, the original and rotated states of an unsymmetrical molecule are distinguishable from one another.

Unsymmetrical molecules have an arrangement of atoms that lacks symmetry. For example, a molecule with a carbon atom bonded to four different atoms or groups would be considered unsymmetrical.

Unsymmetrical molecules are important because their symmetry or asymmetry affects how they respond to light waves, form bonds, and operate biologically. For instance, carbon dioxide is a symmetrical molecule where a carbon atom is set in the middle with oxygen atoms at each end. This symmetry allows CO2 in the atmosphere to let incoming sunlight pass through while blocking infrared rays from the warm land and oceans, contributing to the greenhouse effect and global warming.

In the field of organic chemistry, an unsymmetrical alkene is an alkene in which the pair of ligands on one doubly bonded carbon is different from the other.

Examples of symmetrical and unsymmetrical compounds include dithioacetals and alkenes, respectively

Symmetry in compounds refers to the arrangement of atoms or functional groups around a central atom or axis. In the context of aromatics, symmetry refers to the arrangement of substituents around the aromatic ring. Symmetrical compounds have identical substituents on each carbon atom of the double bond, resulting in a balanced and identical structure. On the other hand, unsymmetrical compounds have different substituents on each carbon atom of the double bond, leading to an asymmetrical structure.

Dithioacetals are symmetrical compounds that are frequently used in synthetic organic chemistry. They have been found to be effective as structural motifs in antiviral agents against plant pathogens. However, most reports focus on symmetrical dithioacetals, and there is a lack of general methods for synthesising unsymmetrical dithioacetals. Alkenes, on the other hand, can be symmetrical or unsymmetrical. Symmetrical alkenes, such as ethene (C2H4), have the same substituents on both sides of the double bond, while unsymmetrical alkenes, like but-1-ene (CH3=CH-CH2-CH3), have different substituents on each side.

Frequently asked questions