The acetoxy group, often abbreviated as OAc, is a functional group in organic chemistry that plays an important role in chemical synthesis. Its presence within a larger molecule can influence reactivity and enable specific transformations.
Defining the Acetoxy Group
The acetoxy group has the chemical formula −OCOCH3 and a structure of −O−C(=O)−CH3. It consists of a carbonyl group (a carbon atom double-bonded to an oxygen atom) single-bonded to another oxygen atom, which is then bonded to a methyl group (CH3). The name “acetoxy” is a shortened form of “acetyl-oxy.”
The acetoxy group differs from the acetyl group (−C(=O)−CH3). While the acetyl group only has a methyl group directly attached to a carbonyl, the acetoxy group inserts an oxygen atom between the carbonyl and the rest of the molecule. This structural difference leads to distinct chemical properties and applications.
Protecting Alcohols with OAC
In organic synthesis, a “protecting group” is temporarily attached to a reactive functional group to prevent it from reacting during other chemical transformations. This is necessary because some reagents are very strong and would react with multiple parts of a molecule. For instance, if a molecule contains both an ester and an alcohol, and the goal is to reduce only the ester, a strong reducing agent like lithium aluminum hydride would also react with the alcohol unless it is protected.
The acetoxy group serves as a common protecting group for alcohol functionalities. Converting them into acetates (by adding an acetoxy group) temporarily masks their reactivity. This acetylation process typically involves reacting the alcohol with an acetyl halide, such as acetyl chloride, in the presence of a base like triethylamine, or with acetic anhydride alongside a base and a catalyst such as pyridine with DMAP.
Once the desired reactions are complete, the acetoxy group can be removed to regenerate the original alcohol. This deprotection step can be achieved using various methods, including treatment with an aqueous base (at a pH greater than 9), an aqueous acid (at a pH less than 2), or an anhydrous base like sodium methoxide in methanol. The choice of deprotection method depends on the other functional groups present in the molecule.
OAC in Oxymercuration
The acetoxy group also plays a role in the oxymercuration-demercuration process, a reaction used to add water across a carbon-carbon double bond (alkene) to form an alcohol. This reaction typically utilizes mercuric acetate, which contains two acetoxy groups attached to a mercury atom, represented as AcO−Hg−OAc. The primary purpose of using mercuric acetate in this reaction is to facilitate the addition of water without forming highly reactive carbocation intermediates, which could lead to unwanted rearrangements in the molecule.
During the oxymercuration step, the alkene’s double bond acts as a nucleophile, attacking the mercury ion from mercuric acetate. This interaction causes one of the acetoxy groups to be ejected from the mercury atom. Subsequently, the mercury atom forms a three-membered ring with the two carbon atoms of the original double bond, creating a cyclic intermediate called a mercurinium ion. This mercurinium ion is positively charged, and its formation is a key step that prevents carbocation rearrangements.
Following the formation of the mercurinium ion, a water molecule attacks the more substituted carbon atom of the ring, leading to the addition of a hydroxyl (−OH) group. The acetoxy group that was initially ejected then removes a hydrogen atom from the newly attached water molecule, completing the oxymercuration part of the reaction and yielding an organomercurial alcohol. In the subsequent demercuration step, this mercury-containing group is replaced by a hydrogen atom using a reducing agent like sodium borohydride (NaBH4), ultimately yielding the desired alcohol product.