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18.1: Names and Properties of Ethers

  • Page ID
    36365
  • Objectives

    After completing this section, you should be able to

    1. write two acceptable names for a simple dialkyl ether, given its Kekulé, shorthand or condensed structure.
    2. name a complicated ether by the IUPAC system, given its Kekulé, shorthand or condensed structure.
    3. draw the Kekulé, condensed or shorthand structure of an ether, given an acceptable name.
    4. explain why the boiling point of an ether is generally higher than that of an alkane of similar molecular mass.

     

    Structure of EthersEdit section

    Ethers are a class of organic compounds that contain an sp3 hybridized oxygen between two alkyl groups and have the formula R-O-R'. these compounds are used in dyes, perfumes, oils, waxes and other industrial uses.  Aliphatic ethers have no aryl groups directly attached to the ether oxygen. 

    Aliphatic Ethers.svg

    Examples of Aliphatic Ethers

    Aromatic ethers have at least one aryl ring directly attached to the ether oxygen.  In aryl ethers, the lone pair elections on oxygen are conjugated with the aromatic ring which significantly changes the properties of the ether. 

    Aryl Ethers.svg

    Example of Aromatic Ethers

    The sp3 hybridization of oxygen gives ethers  roughly the same geometry as alcohols and water.  The R-O-R' bond angle is close to what is expected in a tetrahedral geometry.  The bond angle of dimethyl ether is 112o which is larger than the H-O-H bond angle in water (104.5o) due to the steric repulsion of the methyl groups.

    Ether Bond Angle.svg

    The presence of an electronegative oxygen atom gives ethers a small dipole moment.

    Dipole.png

    Comparisons of Physical Properties of Alcohols and Ethers

    Ethers, unlike alcohols,  have no hydrogen atom on the oxygen atom (that is, no OH group). Therefore, there is no intermolecular hydrogen bonding between ether molecules, which makes their boiling points much lower than an alcohol with similar mass. Despite the presence of a small dipole moment, ethers have boiling points that about the same alkanes of comparable molar mass. (Table 18.1.2).

    Table 18.1.2 Comparison of Boiling Points of Alkanes, Alcohols, and Ethers
    Condensed Structural Formula Name Molar Mass Boiling Point (°C) Intermolecular Hydrogen Bonding in Pure Liquid?
    CH3CH2CH3 propane 44 –42 no
    CH3OCH3 dime thyl ether 46 –25 no
    CH3CH2OH ethyl alcohol 46 78 yes
    CH3CH2CH2CH2CH3 pentane 72 36 no
    CH3CH2OCH2CH3 diethyl ether 74 35 no
    CH3CH2CH2CH2OH butyl alcohol 74 117 yes

    Ether molecules do have an oxygen atom, however, and engage in hydrogen bonding with water molecules. Consequently, an ether has about the same solubility in water as the alcohol that is isomeric with it. For example, dimethyl ether and ethanol (both having the molecular formula C2H6O) are completely soluble in water, whereas diethyl ether and 1-butanol (both C4H10O) are barely soluble in water (8 g/100 mL of water).

     

    Peroxide Formation

    Many ethers can react with oxygen to form explosive peroxide compounds n a free radical process called autoxidation.  For this reason ethers should not be stored for long periods of time and should not be stored in glass bottles.  The danger is particularly acute when ether solutions are distilled to near dryness.  The hydroperoxides can become more concentrated during a distillation because they tend to have a slightly higher boiling point than the corresponding ether.  Before performing an ether distillation great care should be taken to test for the presence of peroxides. 

    Autooxidation.svg

    Naming Ethers

    When no other functional group is present, simple ethers are often given common functional class names.  Both alkyl groups attached to the oxygen atom are named as substituents (in alphabetical order) and then the word ether is added.  The common names for alkyl substituents discussed in Section 3.3 are often used.

    Example \(\PageIndex{1}\)

    Example 1.png

    IUPAC nomenclature for ethers should be used for complicated ethers, compounds with more than one ether linkage, and compounds where other functional groups are present with an ether.  In these cases, an RO group of the ether is named as an alkoxy substituent.

    Common alkoxy substituents are given names derived from their alkyl component.  The suffix -yl is replaced with -oxy. (Table 18.1.1):

    Table 18.1.1: Common Alkyl and Alkoxy Groups
    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

    Example \(\PageIndex{2}\)

    Example 2.png

     

    Cyclic Ethers

    In cyclic ethers are a type of heterocycle with one or more oxygens located in the ring.  Many cyclic ethers have common names and are often used as solvents due to their inert nature.  These ring structures are often found in biological molecules such as sugars and DNA.  The rings are numbered so that an oxygen gets position 1. 

    Ether Ring Numbering.svg

    Example \(\PageIndex{3}\)

    Example 3.png

    Exercise \(\PageIndex{1}\)

    Please name the following ethers:

    a)

    b)

    c)

    d)

    e)

    f)

    g)

    Answers

    a) 3-Isopropoxypentane

    b) 1,3-Dimethoxybenzene

    c) 2-Methyltetrahydropyran

    d) Cyclopentyl ethyl ether

    e) 4-Bromo-1-ethoxybenzene

    f) Dicyclohexyl ether

    g) 4-Butoxycyclohexene

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