Phenol ether

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The general structure of a phenol ether. Phenolether.png
The general structure of a phenol ether.

In chemistry, a phenol ether (or aromatic ether) is an organic compound derived from phenol (C6H5OH), where the hydroxyl (-OH) group is substituted with an alkoxy (-OR) group. Usually phenol ethers are synthesized through the condensation of phenol and an organic alcohol; however, other known reactions regarding the synthesis of ethers can be applied to phenol ethers as well. Anisole (C6H5OCH3) is the simplest phenol ether, and is a versatile precursor for perfumes and pharmaceuticals. [1] Vanillin and ethylvanillin are phenol ether derivatives commonly utilized in vanilla flavorings and fragrances, while diphenyl ether is commonly used as a synthetic geranium fragrance. [2] [1] Phenol ethers are part of the chemical structure of a variety of medications, including quinine, an antimalarial drug, and dextromethorphan, an over-the-counter cough suppressant.

Contents

Nomenclature

Phenol ethers follow the same nomenclature of regular ethers; ethers have the structure R-O-R’, but phenol ethers require that one of the substituents to be a phenyl group (abbreviated Ph), signifying a simple general structure of Ph-O-R’.  As a result, the IUPAC nomenclature of phenol ethers will often take the form of “alkoxybenzene” or “phenoxyalkane,” where the alkane is some sort of hydrocarbon substituent.

The preference of the benzene ring in nomenclature relies on whether the alkane has more or less carbons than the benzene ring itself. Anisole is formally known as methoxybenzene, and is formed through the condensation of methanol (CH3OH) and phenol; due to the methyl group attached to the ethereal oxygen being smaller than the aromatic benzene ring, the benzene takes priority when naming the molecule. However, 1-phenoxyoctane has an octane substituent, which has a greater number of carbons than a benzene ring.

Examples of phenol ethers and their nomenclature. 5-phenoxynonane follows the same naming scheme as 1-phenoxyoctane, due to nonane having more carbons than a benzene ring. Phenoletherexamples.png
Examples of phenol ethers and their nomenclature. 5-phenoxynonane follows the same naming scheme as 1-phenoxyoctane, due to nonane having more carbons than a benzene ring.

When substituents on aromatic rings are present, standard IUPAC nomenclature should be followed when naming aromatic compounds.

Structure and properties

Phenol ethers, similarly to regular ethers, are less hydrophilic than its precursors, phenols and alcohols, both of which can donate and accept hydrogen bonds. Phenol ethers, however, are still able to accept hydrogen bonds through the ethereal oxygen, allowing for its slight solubility in polar solvents. However, the presence of the aromatic ring reduces its solubility in polar solvents such as water and ethanol. Diethyl ether has higher water solubility of 8 g per 100 mL, versus diphenyl ether, with a solubility of 0.002 g per 100 mL. [3] [4]

The presence of the aromatic ring also draws electrons away from the ethereal oxygen, making the hydrolysis of a phenol ether significantly more difficult than that of an alkyl ether. [5] The ethereal oxygen must be significantly nucleophilic in order for the ether to undergo acid-catalyzed hydrolysis.

Preparation

Phenol ethers can be synthesized through an acid-catalyzed condensation of phenols and an alcohol. Phenols include phenol itself, benzenediols, polyphenols, and other phenol-derived molecules.

An acid catalyzed condensation between phenol and ethanol, forming ethoxybenzene. Phenolethercondens.png
An acid catalyzed condensation between phenol and ethanol, forming ethoxybenzene.

However, this synthesis risks the self-condensation of alcohol itself (e.g. ethanol self-condenses to form diethyl ether). A more common and higher-yielding reaction is the Williamson ether synthesis, where a phenol is converted by a strong base to the phenoxide ion, which can subsequently be reacted with an alkyl halide via nucleophilic substitution to form the desired phenol ether. Primary alkyl halides work best, as secondary and tertiary alkyl halides prefer the E2 elimination product. [6] This ether synthesis removes the risk of self-condensation, and yields can be as high as 95% in the laboratory.

A Williamson ether synthesis between p-ethylphenolate and bromoethane to form 4-ethyl-1-ethoxybenzene. Williamsonether.png
A Williamson ether synthesis between p-ethylphenolate and bromoethane to form 4-ethyl-1-ethoxybenzene.

Bis-aryl ethers (such as diphenyl ether) cannot be synthesized through the Williamson ether synthesis, however, as aryl halides cannot undergo nucleophilic substitution. As such, an Ullmann condensation can be employed: an aryl halide is able to react with phenol (or its derivatives) to form a bis-aryl ether in the presence of a copper-based catalyst, such as copper(II) oxide. [7]

An Ullmann condensation between p-methylphenolate and bromobenzene in the presence of a copper catalyst to form 4-methyl-1-phenoxybenzene. Ullmanncondensation.png
An Ullmann condensation between p-methylphenolate and bromobenzene in the presence of a copper catalyst to form 4-methyl-1-phenoxybenzene.

Applications and occurrence

Omeprazole and elemicin are examples of useful molecules containing phenol ether substituents. Phenoletherirl.png
Omeprazole and elemicin are examples of useful molecules containing phenol ether substituents.

Phenol ethers are often utilized in pharmaceutical design as a substituent that acts as an hydrogen-bond acceptor but not a hydrogen-bond donor; this allows many oral medications to follow Lipinski’s rule of five. [8] By replacing the acidic hydrogen on phenol with that of an alkyl group, the toxicity of phenols is also reduced; the LD50 of phenol in rats when administered orally is 317 mg/kg, compared to 3500-4000 mg/kg for anisole, the methyl ether. [9] [10] Furthermore, ethers are significantly more hydrophobic than phenols and can be more easily absorbed by the digestive system than the phenol substituent itself, and allows for oral intake of such medicines. [11] For instance, omeprazole, an oral medication that treats acid reflux, contains two phenol ether substituents.

Due to the increased hydrophobicity of phenol ethers compared to traditional phenols, phenol ethers are often present in the essential oils of plants. [12] Anethole, a simpler compound containing only one phenol ether substituent, is the main component in the oil of anise fruits. Elemicin, a naturally-occurring organic compound containing three phenol ether substituents, is a major component in the oils of nutmeg and mace. [13]

Related Research Articles

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

<span class="mw-page-title-main">Ether</span> Organic compounds made of alkyl/aryl groups bound to oxygen (R–O–R)

In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula R−O−R′, where R and R′ represent the alkyl or aryl groups. Ethers can again be classified into two varieties: if the alkyl or aryl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether". Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

<span class="mw-page-title-main">Haloalkane</span> Group of chemical compounds derived from alkanes containing one or more halogens

The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

The following outline is provided as an overview of and topical guide to organic chemistry:

<span class="mw-page-title-main">Aryl group</span> Molecular groups or substituents derived from an aromatic ring

In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams, analogous to “R” used for any organic substituent. “Ar” is not to be confused with the elemental symbol for argon.

<span class="mw-page-title-main">Williamson ether synthesis</span> Method for preparing ethers

The Williamson ether synthesis is an organic reaction, forming an ether from an organohalide and a deprotonated alcohol (alkoxide). This reaction was developed by Alexander Williamson in 1850. Typically it involves the reaction of an alkoxide ion with a primary alkyl halide via an SN2 reaction. This reaction is important in the history of organic chemistry because it helped prove the structure of ethers.

<span class="mw-page-title-main">Alkylation</span> Transfer of an alkyl group from one molecule to another

Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene. Alkylating agents are reagents for effecting alkylation. Alkyl groups can also be removed in a process known as dealkylation. Alkylating agents are often classified according to their nucleophilic or electrophilic character. In oil refining contexts, alkylation refers to a particular alkylation of isobutane with olefins. For upgrading of petroleum, alkylation produces a premium blending stock for gasoline. In medicine, alkylation of DNA is used in chemotherapy to damage the DNA of cancer cells. Alkylation is accomplished with the class of drugs called alkylating antineoplastic agents.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

In organic chemistry, an aryl halide is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. The haloarene are different from haloalkanes because they exhibit many differences in methods of preparation and properties. The most important members are the aryl chlorides, but the class of compounds is so broad that there are many derivatives and applications.

<span class="mw-page-title-main">Methoxy group</span> Chemical group (–OCH3)

In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula R−O−CH3.

<span class="mw-page-title-main">Anisole</span> Organic compound (CH₃OC₆H₅) also named methoxybenzene

Anisole, or methoxybenzene, is an organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound is mainly made synthetically and is a precursor to other synthetic compounds. Structurally, it is an ether with a methyl and phenyl group attached. Anisole is a standard reagent of both practical and pedagogical value.

<span class="mw-page-title-main">Sulfonic acid</span> Organic compounds with the structure R−S(=O)2−OH

In organic chemistry, sulfonic acid refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2(OH) group a sulfonyl hydroxide. As a substituent, it is known as a sulfo group. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound is the parent sulfonic acid, HS(=O)2(OH), a tautomer of sulfurous acid, S(=O)(OH)2. Salts or esters of sulfonic acids are called sulfonates.

<span class="mw-page-title-main">Triphenylphosphine</span> Chemical compound

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether.

Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfur is vital for life. Of the 20 common amino acids, two are organosulfur compounds, and the antibiotics penicillin and sulfa drugs both contain sulfur. While sulfur-containing antibiotics save many lives, sulfur mustard is a deadly chemical warfare agent. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus of oil refineries.

In organic chemistry, Madelung synthesis is a chemical reaction that produces indoles by the intramolecular cyclization of N-phenylamides using strong base at high temperature. The Madelung synthesis was reported in 1912 by Walter Madelung, when he observed that 2-phenylindole was synthesized using N-benzoyl-o-toluidine and two equivalents of sodium ethoxide in a heated, airless reaction. Common reaction conditions include use of sodium or potassium alkoxide as base in hexane or tetrahydrofuran solvents, at temperatures ranging between 200–400 °C. A hydrolysis step is also required in the synthesis. The Madelung synthesis is important because it is one of few known reactions that produce indoles from a base-catalyzed thermal cyclization of N-acyl-o-toluidines.

<span class="mw-page-title-main">Carbonate ester</span> Chemical group (R–O–C(=O)–O–R)

In organic chemistry, a carbonate ester is an ester of carbonic acid. This functional group consists of a carbonyl group flanked by two alkoxy groups. The general structure of these carbonates is R−O−C(=O)−O−R' and they are related to esters, ethers and also to the inorganic carbonates.

<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

Organocopper chemistry is the study of the physical properties, reactions, and synthesis of organocopper compounds, which are organometallic compounds containing a carbon to copper chemical bond. They are reagents in organic chemistry.

Heteroatom-promoted lateral lithiation is the site-selective replacement of a benzylic hydrogen atom for lithium for the purpose of further functionalization. Heteroatom-containing substituents may direct metalation to the benzylic site closest to the heteroatom or increase the acidity of the ring carbons via an inductive effect.

Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and acylation Friedel–Crafts reaction.

<span class="mw-page-title-main">Phenylsodium</span> Chemical compound

Phenylsodium C6H5Na is an organosodium compound. Solid phenylsodium was first isolated by Nef in 1903. Although the behavior of phenylsodium and phenyl magnesium bromide are similar, the organosodium compound is very rarely used.

References

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