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CBSE Class 12th Chemistry Notes: Alcohols, Phenols and Ethers (Part - III)

Oct 24, 2016 14:30 IST

    CBSE Class 12 Chemistry Notes on Chapter - 11: Alcohols, Phenols and Ethers are available in this article.

    This article is a continuation of the revision notes on CBSE Class 12 Chemistry, Chapter- Alcohols, Phenols and Ethers, Part-I and Part-II.

    In previous parts, you got acquainted with the alcohols and phenols along with their classification, preparation, properties, etc.

    In Part-III, you will get to know about the Ethers, their preparation, properties and uses. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.

    The main topics covered in this part are:

    o    Ethers

          •    Classification

          •    Nomenclature

          •    Structure

          •    Preparation

          •    Physical properties

          •    Chemical properties

          •    Uses of Ethers

    The key notes of the chapter are as follows:

    Ethers

    Ethers are the organic compounds in which an oxygen atom is connected to two alkyl or aryl groups. These are represented by the general formula R-O-R' where R and R' may be alkyl or aryl groups.

    For example:

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    Classification of Ethers

    On the basis of similarity or dissimilarity of alkyl or aryl groups attached to the central oxygen atom, ethers are classified into the two following categories:

    Simple or Symmetrical ethers:

    If R = R', then the ether is called simple or symmetrical ether. For example:

    Mixed or Unsymmetrical ethers:

    If R ≠ R’, then ether is called mixed or unsymmetrical ether. For example:

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    Nomenclature of Ethers

    Common naming system:

    First identify the alkyl or aryl groups attached to the central oxygen atom.

    Then arrange the alkyl or aryl groups alphabetically and write a suffix ether

    IUPAC naming system:

    In the JUPAC system, ethers are regarded as ‘alkoxy alkanes’ in which the ethereal oxygen (to which the two alkyl or aryl groups are attached) is taken along with smaller alkyl group while the bigger alkyl group is regarded as a part of the alkane.

    The oxygen atom is sp3 hybridised and the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The R−O−R’ bond angle is more than the typical tetrahedral angle of 109°. This is due to the interaction between the two bulky (–R) groups. The C–O bond length (141 pm) is almost the same as in alcohols.

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    Preparation of Ethers

    General methods of preparation of ethers are described below:

    (a) By dehydration of 1° alcohols (SN2 reaction):

    This method can be used to prepare ethers having primary alkyl groups only.

    2° and 3° alcohols on dehydration gives alkenes but not ethers.

    (b) By Williamson's synthesis (SN2 reaction):

    For example:

    Ethers containing substituted alkyl groups (2° or 3°) may also be prepared by this method.

    On the other hand alkyl halides (RX) yield only alkenes as main product.

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    Physical properties of Ethers

    •    Ethers are polar in nature and have a net di-pole moment even if the two R and R' groups are similar. This is due to their bent structure.

    •    Ethers have a lower boiling point than alcohols which is due to their inability to associate via intermolecular hydrogen bonding.

    •    Lower ethers (containing up to three carbon atoms) are completely miscible in water. The solubility of ethers decreases as the number of carbon atoms increases.

    •     Due to the presence of electron donating R groups and the lone pair on oxygen atoms, ethers act as Bronsted bases as well as Lewis bases.

    Chemical properties of Ethers

    (a) Reactions of ethers involving C−O bond cleavage

    Reaction with hydrogen halides (HX):

    For symmetrical ethers:

    R ‒ O ‒ R + HX → 2RX + H2O

    For asymmetrical ethers:

    R ‒ O ‒ R’ + HX → RX + R’‒OH

    For a given ether, the reactivity of hydrogen halides follows the order:

    HCl < HBr < HI

    For aromatic ethers:

    In case of aromatic ether, reaction with HI results in the cleavage of R−O bond due to the stability of aryl-oxygen bond.

    When one of the alkyl group is a tertiary group, the halide formed is a tertiary halide. This is due to the formation of more stable 3° carbonium ion.

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    (b) Electrophilic substitution

    The alkoxy group (-OR) on benzene ring is o- and p-directing and activates the aromatic ring towards electrophilic substitution.

    Some of the important electrophilic substitution reactions of aromatic ethers are

    •    Halogenation:

    •    Nitration:

    •    Friedel-Crafts reaction:

    Uses of Ethers

    •    Dimethyl ether is used as refrigerant and as a solvent at low temperature.

    •    Diethyl ether is used as an anaesthesia in surgery.

    •    Diethyl ether is also used as a solvent for oils, gums, resins etc.

    •    Phenyl ether is used as a heat transfer medium because of its high boiling point.

    CBSE Class 12th Chemistry Notes: Alcohols, Phenols and Ethers (Part - I)

    CBSE Class 12th Chemistry Notes: Alcohols, Phenols and Ethers (Part - II)

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