- Examples of compounds with benzyl group
- Benzyl hydrogens
- Carbocations and benzyl radicals
- Resonance in the benzyl group
- Other radicals
- Reactions
- References
The benzyl or benzyl is a substituent group in common organic chemical whose formula is C 6 H 5 CH 2 - or Bn. Structurally, it consists simply of the union of a methylene group, CH 2, with a phenyl group, C 6 H 5; that is, an sp 3 carbon linked directly to a benzene ring.
Therefore, the benzyl group can be seen as an aromatic ring attached to a small chain. In some texts, the use of the abbreviation Bn is preferred instead of C 6 H 5 CH 2 -, being easily recognized in any compound; especially when it is attached to an oxygen or nitrogen atom, O-Bn or NBn 2, respectively.
Benzyl group. Source: IngerAlHaosului
This group is also found implicitly in a number of widely known compounds. For example, benzoic acid, C 6 H 5 COOH, could be considered a benzyl whose sp 3 carbon has undergone exhaustive oxidation; or benzaldehyde, C 6 H 5 CHO, from partial oxidation; and benzyl alcohol, C 6 H 5 CH 2 OH, even less oxidized.
Another somewhat obvious example of this group can be found in toluene, C 6 H 5 CH 3, which can undergo a certain number of reactions as a result of the unusual stability resulting from benzyl radicals or carbocations. However, the benzyl group serves to protect the OH or NH 2 groups from reactions that undesirably modify the product to be synthesized.
Examples of compounds with benzyl group
Benzyl group compounds. Source: Jü
In the first image the general representation of a compound with a benzyl group was shown: C 6 H 5 CH 2 -R, where R can be any other molecular fragment or atom. Thus, by varying R a high number of examples can be obtained; some simple, others just for a specific region of a larger structure or assembly.
Benzyl alcohol, for example, is derived from substituting OH for R: C 6 H 5 CH 2 -OH. If instead of OH it is the NH 2 group, then the benzylamine compound arises: C 6 H 5 CH 2 -NH 2.
If Br is the atom that replaces R, the resulting compound is benzyl bromide: C 6 H 5 CH 2 -Br; R for CO 2 Cl gives rise to an ester, benzyl chlorocarbonate (or carbobenzoxyl chloride); and OCH 3 gives rise to the benzyl methyl ether, C 6 H 5 CH 2 -OCH 3.
Inclusive (although not completely correctly), R can be assumed by a single electron: the benzyl radical, C 6 H 5 CH 2 ·, product of the liberation of the radical R ·. Another example, although not included in the picture, is phenylacetonitrile or benzyl cyanide, C 6 H 5 CH 2 -CN.
There are compounds where the benzyl group hardly represents a specific region. When this is the case, the abbreviation Bn is often used to simplify the structure and its illustrations.
Benzyl hydrogens
The above compounds have in common not only the aromatic or phenyl ring, but also benzylic hydrogens; these are the ones that belong to the sp 3 carbon.
Such hydrogens can be represented as: Bn-CH 3, Bn-CH 2 R or Bn-CHR 2. The Bn-CR 3 compound lacks benzyl hydrogen, and therefore its reactivity is less than that of the others.
These hydrogens are different from those that are usually attached to an sp 3 carbon.
For example, consider methane, CH 4, which can also be written as CH 3 -H. In order for the CH 3 -H bond to be broken in a heterolytic cleavage (radical formation), a certain amount of energy must be supplied (104kJ / mol).
However, the energy for the same breaking of the C 6 H 5 CH 2 -H bond is lower compared to that of methane (85 kJ / mol). As this energy is lower, it implies that the radical C 6 H 5 CH 2 · is more stable than CH 3 ·. The same happens to a greater or lesser degree with other benzylic hydrogens.
Consequently, benzylic hydrogens are more reactive in generating more stable radicals or carbocations than those caused by other hydrogens. Why? The question is answered in the next section.
Carbocations and benzyl radicals
The radical C 6 H 5 CH 2 · was already considered, missing the benzyl carbocation: C 6 H 5 CH 2 +. In the first there is an unpaired and solitary electron, and in the second there is an electronic deficiency. The two species are highly reactive, and represent transient compounds from which the end products of the reaction originate.
The sp 3 carbon, after losing one or two electrons to form the radical or carbocation, respectively, can adopt sp 2 hybridization (trigonal plane), in such a way that there is the least possible repulsion between its electronic groups. But if it happens to be sp 2, just like the aromatic ring carbons, can a conjugation occur? The answer is yes.
Resonance in the benzyl group
This conjugation or resonance is the key factor to explain the stability of these benzyl or benzyl-derived species. The following image illustrates such a phenomenon:
Conjugation or resonance in the benzyl group. The other hydrogens were omitted to simplify the picture. Source: Gabriel Bolívar.
Note that where one of the benzylic hydrogens was, there was a p orbital with an unpaired electron (radical, 1e -), or empty (carbocation, +). As can be seen, this p orbital is parallel to the aromatic system (the gray and light blue circles), with the double arrow indicating the start of conjugation.
Thus, both the unpaired electron and the positive charge can be transferred or dispersed through the aromatic ring, since the parallelism of their orbitals favors it geometrically. However, these are not located in any p orbital of the aromatic ring; only in those belonging to the carbons in ortho and para positions with respect to CH 2.
That is why the light blue circles stand out above the gray ones: in them the negative or positive density of the radical or carbocation, respectively, is concentrated.
Other radicals
It should be mentioned that this conjugation or resonance cannot occur at sp 3 carbons more distant from the aromatic ring.
For example, the radical C 6 H 5 CH 2 CH 2 · is much more unstable because the unpaired electron cannot conjugate with the ring due to the intervening CH 2 group and sp 3 hybridization. The same is true for C 6 H 5 CH 2 CH 2 +.
Reactions
In summary: benzylic hydrogens are prone to react, either generating a radical or a carbocation, which in turn ends up causing the final product of the reaction. Therefore, they react through an SN 1 mechanism.
An example is the bromination of toluene under ultraviolet radiation:
C 6 H 5 CH 3 + 1 / 2Br 2 => C 6 H 5 CH 2 Br
C 6 H 5 CH 2 Br + 1 / 2Br 2 => C 6 H 5 CHBr 2
C 6 H 5 CHBr 2 + 1 / 2Br 2 => C 6 H 5 CBr 3
In fact, in this reaction Br · radicals are produced.
On the other hand, the benzyl group itself reacts to protect the OH or NH 2 groups in a simple substitution reaction. Thus, an ROH alcohol can be 'benzylated' using benzyl bromide and other reagents (KOH or NaH):
ROH + BnBr => ROBn + HBr
ROBn is a benzyl ether, to which its initial OH group can be returned if it is subjected to a reductive medium. This ether should remain unchanged while other reactions are carried out on the compound.
References
- Morrison, RT and Boyd, RN (1987). Organic Chemistry. (5th Edition). Addison-Wesley Iberoamericana.
- Carey, FA (2008). Organic Chemistry. (6th Edition). McGraw-Hill, Interamerica, Editores SA
- Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
- Wikipedia. (2019). Benzyl group. Recovered from: en.wikipedia.org
- Dr. Donald L. Robertson. (December 5, 2010). Phenyl or Benzyl? Recovered from: home.miracosta.edu
- Gamini Gunawardena. (2015, October 12). Benzylic Carbocation. Chemistry LibreTexts. Recovered from: chem.libretexts.org