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Allyl: allyl unit, carbocation, radical, examples

The term ‘allyl’ is due to the botanical name used to refer to garlic: Allium sativum , from whose oils the diallyl disulfide compound, H 2 C = CHCH 2 SSCH 2 CH = CH 2 , was isolated in 1892 , responsible in part for its characteristic odors. In fact, many allylic compounds, that is, those that have the allyl group, are found in garlic and vegetables.

Allyl group

The above image shows the skeletal formula of the allyl group. Beyond the sinuosities on the right we have the rest of the molecule; if it is alkyl, it will be represented by the symbol R. 

Allyl is easy to recognize because it closely resembles the vinyl group. However, allyl can also be cyclic, going unnoticed in compounds like cyclopentene.

Allylic unit and its parts

More important than the allyl group, is the allylic unit which is the same for all allyl compounds. This is: C = CC. The C = C end corresponds to the vinyl carbons. All atoms bonded to these two carbons will also be called vinyl substituents. For example, if they are hydrogens, CH 2 = CH, we are talking about vinyl hydrogens.

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While, on the other hand, the -C end corresponds to allylic carbon. All atoms or groups attached to this carbon will be called allylic substituents. Therefore, allylic compounds are precisely all those that have a functional group (OH, S, F, COOH, etc.) attached to the allylic carbon.

Vinyl carbons are sp 2 hybridized , so they are more electronegative than allyl carbon, sp 3 hybridized . This difference in electronegativity increases the acidity of the allylic hydrogens, the formation of the allylic carbanion being probable. But more profitable in terms of organic synthesis is the allyl carbocation, which will be explained below.

Carbocation

Resonance structures

Resonance structures of the allylic carbocation. Source: Dissolution / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The above image shows the allylic carbocation. Note that the positive charge, (+), appears first on the allylic carbon. However, the electrons in the double bond will immediately be attracted to this charge, so they will move in the direction of the allyl carbon atom.

Consequently, we have two resonance structures (left of the image). Now, the positive charge is placed on one of the vinyl carbons. Again, the electrons of the double bond on the allylic side will be attracted to the positive charge again, and will return to their initial position. This is repeated over and over again, at unimaginable speeds.

The result: the positive charge, +1, is delocalized or dispersed between the three atoms of the allylic unit; but concentrating only on the two extreme carbons. Thus, one of the vinyl carbons retains a 1/2 + charge, while the allyl carbon remains with the other half of the charge, adding +1.

A more appropriate way to represent the allyl carbocation is by its resonance hybrid (right of image). Thus, it is observed that the positive charge is distributed throughout the allylic unit.

Stability and distribution of positive charge

The delocalization of the positive charge gives stability to the allylic carbocation. It is so much so, that it equates to a secondary carbocation in terms of stability.

In the image, since it is only the allylic unit, it is assumed that the distribution of the positive charge is equivalent for both carbons (+1/2 for each one). But this is not true for all allylic compounds. Thus, there will be more or less positive carbocations; which means, they will be more or less reactive.

Consider for example the allyl cation:

2 C = CH-CH + ↔ H 2 C + -CH = CH 2

The two resonance structures are equivalent by distributing the positive charge. The same does not happen, however, with the 1,1-dimethylallyl cation:

2 C = CH-C + (CH 3 ) 2 ↔ H 2 C + -CH = C (CH 3 ) 2

In the structure on the left, the positive charge is more stabilized by the presence of the two methyl groups, which donate part of their negative densities to allylic carbon.

Meanwhile, vinyl hydrogens do not in themselves add anything to the positive charge on vinyl carbon. Therefore, the structure on the left will contribute more to the resonance hybrid of this allyl cation.

It is crucial to keep in mind that it is the resonance hybrids that most closely approximate the true state of these carbocations, and not their separate structures.

Radical

Resonance structures of the allyl radical. Source: Orgonaute / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The upper image now shows the two resonance structures of the allylic radical in their respective allylic unit. Note that the nature of its development is the same as for the carbocation: the unpaired electron (·) is delocalized between the two end carbons. Therefore, each one will have a “half electron” (1/2 ·).

What has been explained for the carbocation and the allylic radical also applies to their respective carbanion, where each of the two mentioned carbon atoms will have a half negative charge (-1/2).

Examples of allylic compounds

Several examples of allylic compounds will be mentioned to finalize. In each of them the allylic unit will be present:

-Alyl chloride, H 2 C = CH-CH 2 -Cl

-Allyl methyl sulfide, H 2 C = CH-CH 2 -S-CH 3 (another of the compounds responsible for the smell of garlic)

-Alycine, H 2 C = CH-CH 2 -S (O) -S-CH 2 -CH = CH 2

-Crotyl alcohol, CH 3 CH = CH-CH 2 OH (note that one of the vinyl hydrogens is substituted by a methyl)

-Allyl acetate, H 2 C = CH-CH 2 -OC (O) CH 3

-Alyl bromide, H 2 C = CH-CH 2 -Br

-Allylamine, H 2 C = CH-CH 2 -NH 2 (basic unit for more complex allylamines that are used as antifungal agents)

-Dimethylallyl pyrophosphate, (CH 3 ) 2 C = CH-CH 2 -OPO 2 -O-PO 3

In the following pair of allylamines, flunarizine and naftifine, both with pharmacological effects, we can see the allyl group:

Structural formulas of two allylamines. Source: Hbf878 / CC0.

In both it is on the right: that double bond interposed between the piperazine rings (the hexagonal one with two nitrogen atoms) and the benzene one. Note that to identify the allyl it is essential to remember the allylic unit: C = CC, since it is in open chains, or in closed structures.

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