The benzyl or benzyl is a common substituent group in organic chemistry whose formula is C6H5CHtwo- or Bn-. Structurally it consists simply of the union of a methylene group, CHtwo, with a phenyl group, C6H5; that is, a carbon sp3 directly bonded 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 C6H5CHtwo-, being easily recognized in any compound; especially when attached to an oxygen or nitrogen atom, O-Bn or NBntwo, respectively.
This group is also found implicitly in a number of widely known compounds. For example, to benzoic acid, C6H5COOH, could be considered as a benzyl whose carbon sp3 it has undergone extensive oxidation; or benzaldehyde, C6H5CHO, from a partial oxidation; and benzyl alcohol, C6H5CHtwoOH, even less rusty.
Another somewhat obvious example of this group can be found in toluene, C6H5CH3, 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 groups.two of reactions that undesirably modify the product to be synthesized.
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In the first image the general representation of a compound with a benzyl group was shown: C6H5CHtwo-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 set.
Benzyl alcohol, for example, is derived from substituting OH for R: C6H5CHtwo-OH. If instead of OH it is the NH grouptwo, then the benzylamine compound arises: C6H5CHtwo-NHtwo.
If Br is the atom that replaces R, the resulting compound is benzyl bromide: C6H5CHtwo-Br; R for COtwoCl gives rise to an ester, benzyl chlorocarbonate (or carbobenzoxyl chloride); and OCH3 gives rise to benzyl methyl ether, C6H5CHtwo-OCH3.
Even (though not quite correctly), R can be assumed by a lone electron: the benzyl radical, C6H5CHtwo·, Product of the liberation of the radical R ·. Another example, although not included in the image, is phenylacetonitrile or benzyl cyanide, C6H5CHtwo-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..
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 carbon3.
Such hydrogens can be represented as: Bn-CH3, Bn-CHtwoR or Bn-CHRtwo. The compound Bn-CR3 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 a sp carbon3.
For example, consider methane, CH4, which can also be written as CH3-H. For the CH bond to break3-H in a heterolytic breakdown (radical formation), a certain amount of energy must be supplied (104kJ / mol).
However, the energy for the same C bond break6H5CHtwo-H is lower compared to that of methane (85 kJ / mol). Since this energy is less, it implies that the radical C6H5CHtwoIt is more stable than CH3·. 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.
The radical C was already considered6H5CHtwo·, Lacking the benzyl carbocation: C6H5CHtwo+. In the first there is an unpaired and solitary electron, and in the second there is an electronic deficiency. The two species are very reactive, and represent transient compounds from which the end products of the reaction originate..
Carbon sp3, after losing one or two electrons to form the radical or carbocation, respectively, it can adopt sp hybridizationtwo (trigonal plane), in such a way that there is the least possible repulsion between its electronic groups. But, if it happens to be sptwo, Like the aromatic ring carbons, can conjugation occur? The answer is yes.
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:
Note that where one of the benzylic hydrogens was, an orbital remained p with an unpaired electron (radical, 1e-), or void (carbocation, +). As can be seen, this orbital p is parallel to the aromatic system (the gray and light blue circles), with the double arrow indicating the beginning of the 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 do not lie in any orbital p aromatic ring; only in those belonging to the carbons in ortho and para positions with respect to CHtwo.
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..
It should be mentioned that this conjugation or resonance cannot occur in sp carbons3 most distant from the aromatic ring.
For example, the radical C6H5CHtwoCHtwoIt is much more unstable because the unpaired electron cannot conjugate with the ring when the CH group intervenestwo in between and with sp hybridization3. The same goes for C6H5CHtwoCHtwo+.
In summary: benzylic hydrogens are prone to react, either generating a radical or a carbocation, which in turn ends up originating the final product of the reaction. Therefore, they react via an SN mechanism.1.
An example is the bromination of toluene under ultraviolet radiation:
C6H5CH3 + 1 / 2Brtwo => C6H5CHtwoBr
C6H5CHtwoBr + 1 / 2Brtwo => C6H5CHBrtwo
C6H5CHBrtwo + 1 / 2Brtwo => C6H5CBr3
In fact, in this reaction Br radicals are produced..
On the other hand, the benzyl group itself reacts to protect the OH or NH groups.two 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..
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