Allyl
Table of Contents
The allyl or allylic group is an important organic substituent derived from an alkene and therefore contains a carbon–carbon double bond (C=C). Its structure is represented by the formula –CH2–CH=CH2. The general formula of an allyl compound is R–CH2–CH=CH2, where R may represent an alkyl group, an aryl group, or another substituent such as a halide or hydroxyl group. [1-4]
The allyl group is found in numerous natural and synthetic compounds. It plays a vital role in organic synthesis, particularly in the production of industrial chemicals, polymers, and pharmaceuticals. For instance:
- Allyl alcohol (CH2=CH–CH2OH) serves as a key intermediate in the manufacture of glycerol, plasticizers, and resins.
- Allyl chloride (CH2=CH–CH2Cl) is a major precursor to epichlorohydrin, used in the preparation of epoxy resins and synthetic rubbers.
Structure and Bonding
The allyl group is derived from propene (CH3–CH=CH2) by removing one hydrogen atom from the carbon adjacent to the double bond. Structurally, it can be viewed as a methylene bridge (–CH2–) attached to a vinyl group (–CH=CH2). Therefore, it exhibits characteristics of both single and double bonds. [4,5]
The group consists of a three-carbon chain:
- C1 and C2 are called vinylic carbons. They are sp2 hybridized, form σ-bonds and share a π-bond between them.
- C3 is called allylic carbon. It is sp³ hybridized and bonded to C2, two hydrogen atoms, and R.
The unhybridized p orbital on C3 can overlap with the π-bond of the C1=C2 double bond, allowing partial electron delocalization across all three carbons. This property allows the allyl group to exhibit resonance stabilization and participate in reactions involving allylic intermediates such as radicals, anions, and cations.
Nomenclature
The names of allyl compounds follow the conventions established by the International Union of Pure and Applied Chemistry (IUPAC). However, in practice, common names are more widely used because they are shorter, more recognizable, and well-established in both academic and industrial contexts. [2,3]
When the allyl group is attached to another atom or functional group, the compound is named by adding the prefix “allyl–” before the name of the main functional group.
| Molecular Formula | Description | Common Name | Preferred IUPAC Name |
|---|---|---|---|
| CH2=CH–CH2Cl | Contains a chlorine attached to the allylic carbon | Allyl chloride | 3-Chloroprop-1-ene |
| CH2=CH–CH2OH | Contains a hydroxyl group bonded to the allylic carbon | Allyl alcohol | Prop-2-en-1-ol |
| CH2=CH–CH2NH2 | Contains an amine group bonded to the allylic carbon | Allylamine | Prop-2-en-1-amine |
| CH2=CH–CH2OC(=O)CH3 | An ester formed from allyl alcohol and acetic acid | Allyl acetate | Prop-2-enyl acetate |
Some hydrocarbons also contain allylic positions even though they are not strictly allyl compounds. For example, dienes such as 1,4-pentadiene (CH2=CH–CH2–CH=CH2) and 1,5-hexadiene (CH2=CH–(CH2)2–CH=CH2) contain allylic carbons. Similarly, the cyclic compound cyclopenta-1,3-diene possesses an allylic carbon at the 5 position. These compounds illustrate how the allyl framework can appear within larger systems, extending beyond the simple allyl compounds.
Chemical Reactions [6]
1. Allylation Reactions
Allylation refers to the introduction of an allyl group into an organic molecule, typically through nucleophilic substitution at the allylic carbon. This process often involves allyl halides, such as allyl chloride (CH2=CH–CH2Cl) or allyl bromide (CH2=CH–CH2Br), which serve as sources of the allyl group.
General Reaction:
CH2=CH–CH2X + R–H + → R–CH2–CH=CH2 + HX (in presence of a base)
Example: Conversion of allyl chloride to allyl alcohol using sodium hydroxide.
CH2=CH–CH2Cl + NaOH → CH2=CH–CH2OH + NaCl
2. Oxidation Reactions
The allyl group can undergo oxidation reactions that convert it into more oxygenated compounds such as allyl alcohols, aldehydes, or carboxylic acids, depending on the oxidizing agent used and the reaction conditions.
(a) Mild Oxidation
Mild oxidizing agents like dilute potassium permanganate (KMnO4) or osmium tetroxide (OsO4) add oxygen across the double bond or oxidize the allylic position to produce allyl alcohols.
CH2=CH–CH3 → CH2=CH–CH2OH (in presence of dil. KMnO4)
(b) Strong Oxidation
Under stronger oxidative conditions, such as with acidic KMnO4 or chromic acid (H2CrO4), the allyl group can undergo further oxidation or cleavage at the double bond to yield aldehydes or carboxylic acids.
CH2=CH–CH2OH → CH2=CH–CHO (in presence of acidic H2CrO4)
Vinyl vs. Allyl Groups
The vinyl group (–CH=CH2) and the allyl group (–CH2–CH=CH2) are closely related organic groups that differ slightly in structure but significantly in their chemical behavior. The vinyl group contains two carbon atoms, while the allyl group contains three, with an additional methylene bridge (–CH2–) connecting to the double bond. [7]
In the vinyl group, the substituent R is directly attached to one of the sp2-hybridized carbons of the double bond. In contrast, in the allyl group, the point of attachment is an sp3-hybridized methylene carbon adjacent to the double bond. This structural difference gives rise to distinct bonding and reactivity characteristics.
