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10.5: Names and Physical Properties of Ethers

  • Page ID
    201182
  • Ethers are compounds having two alkyl or aryl groups bonded to an oxygen atom, as in the formula R1–O–R2. The ether functional group does not have a characteristic IUPAC nomenclature suffix, so it is necessary to designate it as a substituent. To do so the common alkoxy substituents are given names derived from their alkyl component (below):

    Alkyl Group Name Alkoxy Group Name
    CH3 Methyl CH3O– Methoxy
    CH3CH2 Ethyl CH3CH2O– Ethoxy
    (CH3)2CH– Isopropyl (CH3)2CHO– Isopropoxy
    (CH3)3C– tert-Butyl (CH3)3CO– tert-Butoxy
    C6H5 Phenyl C6H5O– Phenoxy

    Ethers can be named by naming each of the two carbon groups as a separate word followed by a space and the word ether. The -OR group can also be named as a substituent using the group name, alkox

    Example \(\PageIndex{1}\)

    CH3-CH2-O-CH3 is called ethyl methyl ether or methoxyethane.

    The smaller, shorter alkyl group becomes the alkoxy substituent. The larger, longer alkyl group side becomes the alkane base name. Each alkyl group on each side of the oxygen is numbered separately. The numbering priority is given to the carbon closest to the oxgen. The alkoxy side (shorter side) has an "-oxy" ending with its corresponding alkyl group. For example, CH3CH2CH2CH2CH2-O-CH2CH2CH3 is 1-propoxypentane. If there is cis or trans stereochemistry, the same rule still applies.

    Examples \(\PageIndex{2}\)

    • \(CH_3CH_2OCH_2CH_3\), diethyl ether (sometimes referred to as just ether)
    • \(CH_3OCH_2CH_2OCH_3\), ethylene glycol dimethyl ether (glyme).


    Exercises \(\PageIndex{2}\)

    Try to name the following compounds using these conventions:

    J

    J

    Try to draw structures for the following compounds:

    • 2-pentyl 1-propyl ether J
    • 1-(2-propoxy)cyclopentene J

    Common names

    Simple ethers are given common names in which the alkyl groups bonded to the oxygen are named in alphabetical order followed by the word "ether". The top left example shows the common name in blue under the IUPAC name. Many simple ethers are symmetrical, in that the two alkyl substituents are the same. These are named as "dialkyl ethers".

    • anisole (try naming anisole by the other two conventions. J )

    • oxirane

    1,2-epoxyethane, ethylene oxide, dimethylene oxide, oxacyclopropane,

    • furan (this compound is aromatic)

    tetrahydrofuran

    oxacyclopentane, 1,4-epoxybutane, tetramethylene oxide,

    • dioxane

    1,4-dioxacyclohexane

    Exercise \(\PageIndex{2}\)

    Try to draw structures for the following compounds-

    • 3-bromoanisole J
    • 2-methyloxirane J
    • 3-ethylfuran J

    Heterocycles

    In cyclic ethers (heterocycles), one or more carbons are replaced with oxygen. Often, it's called heteroatoms, when carbon is replaced by an oxygen or any atom other than carbon or hydrogen. In this case, the stem is called the oxacycloalkane, where the prefix "oxa-" is an indicator of the replacement of the carbon by an oxygen in the ring. These compounds are numbered starting at the oxygen and continues around the ring. For example,

    ex cred pic better.jpg

    If a substituent is an alcohol, the alcohol has higher priority. However, if a substituent is a halide, ether has higher priority. If there is both an alcohol group and a halide, alcohol has higher priority. The numbering begins with the end that is closest to the higher priority substituent. There are ethers that are contain multiple ether groups that are called cyclic polyethers or crown ethers. These are also named using the IUPAC system.

    Sulfides

    Sulfur analogs of ethers (R–S–R') are called sulfides, e.g., (CH3)3C–S–CH3 is tert-butyl methyl sulfide. Sulfides are chemically more reactive than ethers, reflecting the greater nucleophilicity of sulfur relative to oxygen.

    References

    1. Schore, Neil E. and Vollhardt, K. Peter C. Organic Chemistry: Structure and Function. New York: Bleyer, Brennan, 2007.
    2. Winter, Arthur. Organic Chemistry for Dummies. Hoboken, New Jersey: Wiley, 2005.
    3. Pellegrini, Frank. Cliffs QuickReview Organic Chemistry II. Foster City, CA: Wiley, 2000

    Problems

    Name the following ethers:

    ETHER PROBS.bmp

    (Answers to problems above: 1. diethyl ether; 2. 2-ethoxy-2-methyl-1-propane; 3. cis-1-ethoxy-2-methoxycyclopentane; 4. 1-ethoxy-1-methylcyclohexane; 5. oxacyclopropane; 6. 2,2-Dimethyloxacyclopropane)

    Objectives

    After completing this section, you should be able to

    1. write the normally accepted name for a crown ether, given its structure.
    2. draw the structure of a crown ether, given its normally accepted name.
    3. describe, briefly, the uses of crown ethers.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • crown ether

    Study Notes

    A “crown ether ” is a cyclic ether containing several (i.e., 4, 5, 6 or more) oxygen atoms. As we have indicated in the objectives above, a detailed knowledge of these compounds is not required in this course.

    It is possible to dissolve ionic compounds in organic solvents using crown ethers. Cyclic polyether with four or more oxygen atoms separated by two or three carbon atoms. All crown ethers have a central cavity that can accommodate a metal ion coordinated to the ring of oxygen atoms., cyclic compounds with the general formula (OCH2CH2)n. Crown ethers are named using both the total number of atoms in the ring and the number of oxygen atoms. Thus 18-crown-6 is an 18-membered ring with six oxygen atoms (part (a) in Figure 18.7.1 ). The cavity in the center of the crown ether molecule is lined with oxygen atoms and is large enough to be occupied by a cation, such as K+. The cation is stabilized by interacting with lone pairs of electrons on the surrounding oxygen atoms. Thus crown ethers solvate cations inside a hydrophilic cavity, whereas the outer shell, consisting of C–H bonds, is hydrophobic. Crown ethers are useful for dissolving ionic substances such as KMnO4 in organic solvents such as isopropanol [(CH3)2CHOH] (Figure 18.7.1). The availability of crown ethers with cavities of different sizes allows specific cations to be solvated with a high degree of selectivity.

    8ca69547ad5ca24de2594253a8d7b210.jpg

    Figure 18.7.1: Crown Ethers and Cryptands (a) The potassium complex of the crown ether 18-crown-6. Note how the cation is nestled within the central cavity of the molecule and interacts with lone pairs of electrons on the oxygen atoms. (b) The potassium complex of 2,2,2-cryptand, showing how the cation is almost hidden by the cryptand. Cryptands solvate cations via lone pairs of electrons on both oxygen and nitrogen atoms.

    Figure 18.7.2: Effect of a Crown Ether on the Solubility of KMnO4 in Benzene. Normally which is intensely purple, is completely insoluble in benzene which has a relatively low dielectric constant. In the presence of a small amount of crown ether, KMnO4 dissolves in benzene as shown by the reddish purple color caused by the permanganate ions in solution.

    Cryptands (from the Greek kryptós, meaning “hidden”) are compounds that can completely surround a cation with lone pairs of electrons on oxygen and nitrogen atoms (Figure 18.7.1b). The number in the name of the cryptand is the number of oxygen atoms in each strand of the molecule. Like crown ethers, cryptands can be used to prepare solutions of ionic compounds in solvents that are otherwise too nonpolar to dissolve them.

    3fd71b71df27cd639997451927c59a19.jpg

    Figure 18.7.3: Ion–Dipole Interactions in the Solvation of Li+ Ions by Acetone, a Polar Solvent

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