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Could you please give me a reasonable cause why benzene ring is found in every organic compound in food, pharmaceuticals, dyes . In addition, benzene is a known canceriogen, why shouldn't it be toxic when found in food items for example. I hope that I've formulated my question very clearly and if someone could reformulate my question in a better fashion, it would be highly appreciated. Thanks in advance for all your answers

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    $\begingroup$ Benzene ring is not benzene. You would be dead without benzene ring. $\endgroup$
    – Poutnik
    Mar 20, 2022 at 20:54
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    $\begingroup$ Nature does not tell us why the benzene cycle is so frequent in natural products. Maybe it is due to the remarkable stability of this ring. But in fact we must admit that we do not know everything in chemistry. Nature is stronger than chemists. $\endgroup$
    – Maurice
    Mar 20, 2022 at 21:33
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    $\begingroup$ Benzene is a compound, which is a liquid and toxic. Phenyl gruop is the cyclic group (what you called "benzene ring") resulting from substituting one or more hydrogen in benzene with any other group. Most of those compounds with phenyl group are not terribly toxic (e,g., tylenol and aspirin). $\endgroup$ Mar 20, 2022 at 21:55
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    $\begingroup$ Benzene rings are common but not ubiquitous in organic compounds. For example, of the amino acids that make up all proteins in living things (so most of you) only 3 out of 20 have benzene rings in them. $\endgroup$
    – matt_black
    Mar 20, 2022 at 22:03
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    $\begingroup$ Related: Why six C atoms are usually seen in cyclic compounds? $\endgroup$
    – ron
    Mar 20, 2022 at 22:18

2 Answers 2

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By "benzene ring", I assume that you mean the aryl functional group:

$\hspace{8 cm}$ aryl

Molecules with the above motif - called arenes - are common throughout nature and are present in a multitude of chemical reactions. The specific group above is called a "phenyl" group since there is only one substitution on the ring, but more substitutions are possible.

This type of structure is common in molecules for a multitude of reasons. The primary one is that it is very stable; certain cyclic conjugated groups, like the one above, have a property known as aromaticity (see Huckel's rules for more). Aromatic compounds are stable because the electron density inside the ring is completely delocalized. In fact, these types of rings usually only undergo substitution mechanisms, and rarely addition mechanisms unless forcing conditions (high temperature, high pressure, etc.) are utilized. You can read more about electrophilic aromatic reactions here, but the important thing to note is that when these groups react, they rarely do so by completely breaking this delocalization (as would otherwise occur in an addition reaction).

Additionally, although benzene is carcinogenic, the molecules that are derived from it can be either carcinogenic or not carcinogenic. Most simple derivatives of benzene (with only one substitution on the ring) fall roughly equally into either category: toluene and aniline are likely not carcinogenic, while phenol is a known carcinogen.

Going further to more complicated rings, we can see that what determines a molecule's carcinogenicity is less concrete than you might think. Back in the 1960s and 1970s, there was a huge environmental controversy when a class of compounds known as polychlorinated biphenyls (PCBs), which have two phenyl groups (see example below), were found to be carcinogenic to humans and nature:

$\hspace{5 cm}$ PCB

These kinds of molecules are part of a larger class of compounds called polycyclic aromatic hydrocarbons (PAHs) - almost all of which have some adverse human effect, usually carcinogenic.

However, not all aryl-containing substances are carcinogenic. You mentioned food in your question - most aryl rings in foods are found in aromatic amino acids, such as the tyrosine molecule below:

$\hspace{5 cm}$ tyrosine

Heterocylic aromatic groups can also be found in nucleic acids, such as cytosine (don't be fooled by the nitrogens replacing the carbons in the ring - it's still an aromatic compound!):

$\hspace{7 cm}$ cytosine

In plants, you can find aromatic compounds in most plant hormones, such as auxins (the molecule below is indole-3-acetic acid, the most common auxin):

$\hspace{6 cm}$ IAA

Aromatic rings are also a common motif in many pharmaceutical compounds of interest.

Ultimately, if you view this from a chemical evolution perspective, delocalized aromatic rings are more "favored" in nature because of their stability and nonreactivity. Whether a molecule is carcinogenic or not can be influenced by the presence of an aromatic ring, but there are a plethora of other factors at play.

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To restate the question: what is being referred to as "benzene ring" is a six-membered aromatic carbon ring. The question asks why this structure is ubiquitous, despite the simplest form of this structure being a carcinogen.

The simplest answer is that properties (chemical, physical, toxilogical) are features of the entire molecule, not of a fragment of the molecule. There's a popular meme that makes this point using NaCl: sodium metal reacts spectacularly with water; chlorine is a toxic gas. Salt is neither toxic nor water-reactive. Likewise, structures with aromatic six-membered rings are not necessarily carcinogenic.

In addition to being a very stable structure, as pointed out by the previous answer, the six membered ring has geometric advantages - it is easier to form. The aromatic six-membered ring is flat and has electronic properties that can be used in the assembly of host-guest molecular interactions. Shape plays a huge part in biologically relevant chemical systems.

Benzene is carcinogenic, but the very similar molecule toluene is not.

enter image description here

It turns out that even this seemingly minor modification of the structure changes the shape/electronic properties enough to alter the key interactions that are necessary to change DNA. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1469723/

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