Diols

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of diols are discussed. Diols are a subset of carbohydrate compounds. They have a variety of commercial uses and are also useful in synthesis reactions as starting materials and protecting groups.

The Nature of the Diols

The term "diol" indicates that a molecule possesses two hydroxyl groups (−OH) as a primary identifying feature, while "polyol" typically refers to those with three or more. The hydroxyl groups in a diol may be positioned on any of the carbon atoms in the molecule, but they have special relevance when they are on adjacent carbon atoms, as in a vicinal diol (also known as a "glycol"), or on the same carbon atom, as in a geminal diol. The presence of two hydroxyl groups in close proximity brings two electron-rich oxygen atoms near to each other. Because this places strain on the molecule, geminal diols tend to be very unstable and usually decompose to a stable ketone structure by eliminating one of the hydroxyl groups, which bonds with a hydrogen cation (H⁺) to form a molecule of water. (A ketone is an organic compound in which the carbon atom of a carbonyl group, C=O, is bonded to two other carbon atoms.) Vicinal diols, on the other hand, tend to be very stable due to the formation of intramolecular hydrogen bonds between the two hydroxyl groups in a five-membered ring formation. Compounds in which the two hydroxyl groups are separated by one carbon atom are called "1,3 diols" and are even more stable, due to both the additional distance between the two oxygen atoms and the formation of intramolecular hydrogen bonds in a six-membered ring formation.

Due to their two hydroxyl groups and their ability to form hydrogen bonds, the smaller diols tend to be soluble in water and other polar solvents. The intramolecular hydrogen bonding presents little or no hindrance to dissolution by water molecules. Solubility does decrease as the size of the alkyl group (C_nH_2n+1) increases, however, due to the hydrophobic nature of hydrocarbons.

Diols typically have low melting points. The simplest diol, called ethylene glycol (C₂H₆O₂), is commonly used to change the freezing and boiling points of coolant solutions. A mixture of about equal parts of water and ethylene glycol is the standard liquid coolant for most internal combustion engines in cold climates, since the freezing point of such a mixture is about −36 degrees° Celsius (−32.8 degrees Fahrenheit) and its boiling point is significantly higher than the boiling point of water alone. Propylene glycol (C₃H₈O₂) is also often used in low-temperature applications.

Diols are a sort of intermediate between hydrophilic and hydrophobic compounds. They are often blended into different formulations to improve the solubility of certain materials in aqueous solutions.

Nomenclature of Diols

Diols are named according to the parent hydrocarbon from which they are formed. According to the International Union of Pure and Applied Chemistry (IUPAC) conventions for systematically naming organic compounds, the basic compound is first identified by the longest carbon chain in the molecular structure. Typically this is an alkane, or saturated hydrocarbon, but there is no stricture against the parent structure being identified as an alkene (unsaturated hydrocarbon with one or more carbon-carbon double bonds, C=C) or an alkyne (unsaturated hydrocarbon with one or more carbon-carbon triple bonds, C≡C). In the case of an alkene or alkyne, the location of the double or triple bond or bonds is indicated after the prefix describing the number of carbon atoms in the chain. For diols, the parent chain must include both alcohol functional groups. Next, the relevant functional groups and their positions in the carbon chain are identified, such that an alcohol group is given the lowest number possible, and the highest-priority functional group determines the final suffix. All other functional groups are listed alphabetically in the formal name of the compound, although certain groups, such as alkyls, are only indicated by prefixes and thus must either come first or follow another prefix. Following these rules, a five-carbon chain with two hydroxyl groups on the first and second carbon atoms, a methyl group (−CH₃) on the fourth carbon atom, and a C=C bond between the first and second carbon atoms would be named 4-methylpent-4-en-1,2-diol. The 4-methyl- prefix describes the location of the methyl group, pent- indicates that there are five carbon atoms in the chain, 4-en(e) describes the location of the C=C bond, and 1,2-diol describes the locations of the hydroxyl groups.

Some discretion must be exercised in naming diols so as not to confuse naming conventions. The term "glycol" is used exclusively in reference to vicinal diols and always accompanies an alkene name, as in ethylene glycol. Thus, the name hept-3-ene glycol applies to the compound properly named heptan-3,4-diol and not to a seven-carbon alkene with a C=C bond between the third and fourth carbon atoms and two hydroxyl groups elsewhere in the molecular structure. The best practice, at least until one is entirely familiar with naming conventions, is to name diols according to the IUPAC systematic conventions and avoid using the glycol naming method.

Formation of Diols

Ethylene glycol can prepared commercially from ethanol. Dehydration of the ethanol yields ethylene (or ethene) gas, which can undergo a catalyzed reaction with oxygen gas to produce the simple epoxide compound ethylene oxide. (An epoxide is a compound containing a three-membered ring structure made of one oxygen and two carbon atoms.) Hydration of the ethylene oxide then produces ethylene glycol. This is a standard method of producing diols in synthesis.

A compound with a C=C bond can be converted to the corresponding epoxide via reaction with a reagent such as perbenzoic acid or performic acid. These are acids with an extra oxygen atom between the carbonyl group (C=O) and the hydroxyl group. Subsequent hydrolysis of the epoxide produces the corresponding diol. Vicinal diols can similarly be produced by mild oxidation of an alkene C=C bond with potassium permanganate (KMnO₄) in alkaline solution. A C=C bond activated by an aryl group, or functional group derived from an aromatic ring, as in the compound styrene (or phenylethylene), can be converted into a vicinal diol by reaction with hydrogen peroxide (H₂O₂) and formic acid (CH₂O₂, also written as HCO₂H). All three mechanisms are shown below.

Geminal diols, which dehydrate very readily, are formed by hydration of the carbonyl group in a ketone or aldehyde. They are essentially never isolated because they dehydrate so readily and revert back to the carbonyl group.

Diols other than vicinal and geminal diols are generally prepared by reducing ketones or aldehydes or by hydrolyzing or substituting appropriate functional groups already present in the molecular structure.

Reactions of Diols

Diols are essentially a subset of the carbohydrate class of compounds, and reactions involving diols are generally applicable to the study of carbohydrates. In reactions, diols generally behave either as functional groups to be protected against other reactions or as protecting groups themselves. In a complex synthesis, it is often necessary to protect a carbonyl group against a reaction under conditions designed to alter another part of the molecule. Reducing agents being used to convert a nitrile (organic compound containing a carbon-nitrogen triple bond, or C≡N) into a primary amine, for example, will also reduce the carbonyl group to an alcohol. If the carbonyl group of a ketone or aldehyde reacts with ethylene glycol to form a ketal or acetal first, as shown below, a stable structure is produced that does not react with the reducing agent. After the desired reaction has been carried out, the carbonyl group can be recovered relatively easily by hydrolysis of the ketal or acetal.

This type of reaction can also be used to produce various substituted heterocyclic ring structures in larger molecules, as well as 1,3-dioxanes. A dioxane is a six-membered ring with two oxygen atoms in its ring structure.

Carbohydrates and closely related compounds contain numerous diol structures, and reaction conditions that affect one hydroxyl group in a carbohydrate molecule will affect all of the hydroxyl groups in the molecule. Thus, in order to alter one part of a carbohydrate molecule without affecting another, it is necessary to protect the hydroxyl groups that are to remain throughout the reaction process. This requires the use of specialized reagents to act as protective groups under the reaction conditions. One such reagent is the dimethyl acetal of 4-methoxybenzaldehyde, which effectively uses the diol to replace the two methyl groups in the acetal. A number of related aryl aldehydes can be used in the same way. Diols and other polyols are also useful in organometallic chemistry as ligands.

PRINCIPAL TERMS

• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• geminal: describes the relationship between two functional groups, usually similar or identical, bonded to the same (typically carbon) atom in the same molecule.
• hydroxyl group: a primary functional group consisting of an oxygen atom covalently bonded to a single hydrogen atom.
• polyol: an organic compound containing multiple hydroxyl groups.
• vicinal: describes the relationship between two functional groups bonded to adjacent (typically carbon) atoms in the same molecule.

Bibliography

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Morrison, Robert Thornton, and Robert Neilson Boyd. Organic Chemistry. 6th ed. Englewood Cliffs: Prentice, 1992. Print.

Wuts, Peter G. M., and Theodora W. Greene. Greene’s Protective Groups in Organic Synthesis. 4th ed. Hoboken: Wiley, 2006. Print.