Allylic Alcohols

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of allylic alcohols are discussed. Allylic alcohols are very useful compounds in organic synthesis reactions, as they allow for the relatively easy formation of complex molecular structures from simple starting materials.

Hybrid Orbitals in Allylic Alcohols

Like other alcohols, allylic alcohols are characterized by the presence of a hydroxyl (−OH) functional group bonded to a saturated carbon atom—that is, a carbon atom that is attached to the maximum possible number of hydrogen atoms via single bonds only. The term "allylic" indicates that the hydroxyl group is bonded to a carbon atom that is adjacent to a carbon atom that is part of a carbon-carbon double bond (C=C). When this is the case, the carbon atom bonded to the hydroxyl group is said to be in the allylic position and can be referred to as an allylic carbon atom, while the hydroxyl group itself can also be called an allylic alcohol group.

An atom is made up of a central nucleus, consisting of positively charged protons and neutral neutrons, surrounded by negatively charged electrons. Electrons are often described as falling within electron shells and inhabiting specific regions about the nucleus known as orbitals. A carbon atom has four electrons in its valence, or outermost, electron shell: two electrons in the 2s orbital and two electrons in the 2p orbitals. Combining the 2s orbital with two of the three 2p orbitals results in the formation of three equivalent hybrid sp² orbitals. These three orbitals are arranged in a plane, to which the carbon atom’s remaining 2p orbital is perpendicular. When two adjacent carbon atoms are hybridized in this way, a C=C bond can form between them. The two bonds that make up this double bond are known as the sigma (σ) bond and the pi (π) bond. The sigma bond is formed by the end-to-end overlap of an sp² orbital from each atom, while the pi bond is formed by the side-by-side overlap of the non-hybridized 2p orbitals. Meanwhile, the allylic carbon atom hybridizes the 2s and all three of the 2p orbitals to form four equivalent sp³ hybrid orbitals, meaning that it cannot form a double bond with another carbon atom, unless one of its hybrid orbitals regresses.

Nomenclature of Allylic Alcohols

Allylic alcohols are not assigned any specific nomenclature under the International Union of Pure and Applied Chemistry (IUPAC) naming conventions. The compounds are named as normal alkenes (organic compounds that include two carbon atoms connected by a double bond) containing a hydroxyl group, the location of which is specified in the name.

It is important to note that the term "allylic alcohol" is not a synonym for the similar-sounding "allyl alcohol." The latter term refers to a specific compound—the simplest of the allylic alcohols, with the molecular formula C₃H₆O—that falls into the broader category of the former, which are compounds characterized by the presence of a hydroxyl functional group bonded to an allylic carbon atom.

Reactions of Allylic Alcohols

Allylic alcohols undergo all of the reactions typical of an alcohol, though they are more active in some ways than others. Substitution of the allylic hydroxyl group with a halide, or halogen anion, can be achieved by a reaction with hydrogen chloride (HCl) or hydrogen bromide (HBr). The halide compound dissociates into a halogen anion and a hydrogen cation (H⁺), and the cation, which is effectively a proton, is added to the −OH group, a process known as protonation. The loss of the resulting H₂O group from the alcohol molecule produces the allylic carbonium ion, which then bonds to the unattached halide ion.

Halogenated compounds, or organic compounds containing a halogen, are also effective in converting an allylic alcohol to the corresponding allylic halide. Allylic halides can be used in a variety of reactions, such as alkylation to form new carbon-carbon single bonds (C−C), allylic amines, and esters. Allyl chloride, for example, can be reacted with ammonia to produce allyl amine. Allyl alcohol is sufficiently acidic that it can be deprotonated—that is, lose a hydrogen cation—by a sufficiently strong base to form the corresponding allylic oxide salt of the metal, which can then be used in a number of different reactions. Deprotonation produces the alkoxide anion, which can then be used as a reagent in substitution reactions for the formation of allyl ethers.

Such reactions can be used to protect an allylic alcohol group in a molecule from undergoing an undesired side reaction in conditions meant to alter another part of a molecule. The allylic alcohol group can be first deprotonated to form the alkoxide anion and then reacted with a compound such as trimethysilyl chloride to produce the corresponding mixed-ether compound. This renders the allylic alcohol group safe from reaction when the other parts of the molecule are altered. The group can then be easily regenerated by a simple hydrolysis reaction that removes the protecting group, leaving the original hydroxyl group behind.

Production of Allylic Alcohols

Allylic alcohols are produced commercially by a variety of methods. Allyl alcohol can be produced by the dehydration of propylene glycol or, alternatively, hydrolysis of allyl chloride with a mild base. Similar processes are used to form an allylic alcohol functional group in another molecular structure. Manganese dioxide (MnO₂) can be used to oxidize a suitable C=C bond to the corresponding diol (a compound containing two hydroxyl groups). Carefully controlled dehydration of the diol product can subsequently produce an allylic alcohol. Further dehydration can produce a mixture of allylic alcohols.

An allylic position can be formed in a molecule in many ways, but allylic alcohols are rather more difficult to achieve directly. The addition of a vinyl Grignard reagent to a ketone—a strictly structured compound consisting of a carbonyl (C=O) group as well as other carbon-containing groups—can result in an allylic alcohols. However, the structure of the ketone must preclude the possibility of dehydration after the allylic alcohol has been formed, as this would alter the substance further.

In industry, allylic alcohols are used primarily to produce substances such as resins and plasticizers, which are in turn involved in the development of safety glass, coatings, and other products. Allylic alcohols are extremely toxic, and their production and use are regulated by various governmental bodies.

PRINCIPAL TERMS

• alcohol: an organic compound in which a hydroxyl is the primary functional group and is bonded to a saturated carbon atom.
• double bond: a type of chemical bond in which two adjacent atoms are connected by four bonding electrons rather than two.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydrolysis: the cleavage of a chemical bond caused by the presence of water.

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