Difference Between Allyl and Vinyl

Main Difference – Allyl vs Vinyl

Allyl and vinyl are two different organic functional groups. Both have C-C double bonds but in different positions. In vinyl group, C=C is directly attached to the rest of the chain.  In contrast to vinyl, allyl group is attached to the rest of the molecule through –CH₂ group. This is the main difference between Allyl and Vinyl groups.

What is Vinyl

Vinyl indicates the –CH=CH₂ functional group, which can be formed by removing hydrogen from ethylene molecule. Therefore, the general molecular formula of vinyl compounds is R- CH=CH₂, where R is any other group of atoms. Vinylic groups have sp² hybridized carbon atoms. These compounds are very reactive, and readily polymerize to form vinyl polymers. Polyvinyl chloride, polyvinyl fluoride, and polyvinyl acetate are some examples of vinyl polymers. The Figure 1. Illustrates the mechanism of polyvinyl chloride synthesis.

Figure 1. Free radical vinyl polymerization mechanism

Vinylic compounds are generally used for plastic manufacturing since it is durable, cheaper, etc. In addition, vinyl has many other uses, because of its ability to be combined with various additives and modifiers.

What is Allyl

Allyl indicates a functional group with structural formula H₂C=CH-CH₂-R, where R is the rest of the molecule. It consists of methylene bridge (-CH₂-) in between the vinyl group (-CH=CH₂) and the rest of the molecule. Therefore, allyl group contains sp² hybridized vinyl carbon atoms and sp³ hybridized allyl carbon atom. The allylic carbon atom is more reactive than normal alkanes, and it can easily form a stable carbocation because it is adjacent to vinyl carbon that can delocalize electrons to stabilize the positive charge. The Figure 2. Illustrates the resonance stabilization of allylic carbocation.

Due to the stability of the carbocation, allyl compounds radially form intermediates during the reaction. For example, S_N1 reaction.

Difference Between Allyl and Vinyl

General Molecular Formula

Allyl: The general molecular formula is RCH₂CH=CH₂. The Vinyl group is attached to rest of the molecule through –CH₂– group.

Vinyl: The general molecular formula is RCH=CH₂. The Vinyl group is directly attached to rest of the molecule.

Hybridization of Carbon Skeleton

Allyl: This contains sp³ hybridized allyl carbon atom, which is adjacent to sp² hybridized carbon atom.

Vinyl: This contains sp² hybridized carbon atoms.

Uses

Allyl: Allylic compounds are precursors to many natural products, including natural rubber and biosynthesis f terpenes.

Vinyl: Vinylic compounds can produce vinylic polymers such as PVC, PVF, PVAc, etc.

Stability of Carbocation

Allyl: Allylic carbon atom can form stable carbocations due to electron delocalization.

Vinyl: Vinylic carbocations are very unstable due to lack of p-character.