Ether

Ether

Ether [Organic Chemistry]- BSC 1st Year

 

Syllabus       

  • Introduction
  • Nomenclature
  • Preparation: by Williamson’s synthesis,
  • Industrial synthesis-Dehydration of alcohol
  • laboratory preparation of Ether
  • Physical properties
  • Reactions of Ether with PCl5 and HX
  • Role of Solvent
  • Solubility
  • Phase transfer Catalysis.
  • Introduction:-  
  • Ethers are a class of compounds in which two alkyl group are linked to
  •  A divalent oxygen atom. They may be represented as 

                        R—O—R’

  • There are two types of ethers-
  • i) Symmetrical ether :-

  If both alkyl group in ether are same then called symmetrical ether.

            CH3—O—CH3 (diethyl ether)                                      

             CH3CH2—O—CH2CH3(diethyl ether)

  • ii) Unsymmetrical or mixed ether :-

    If both alkyl group in ether are different then called unsymmetrical ether.

            CH3—O—CH2CH3 (ethyl methyl ether),

             CH3—O—C3H7 (methyl propyl ether)

  • Nomenclature:-

 There are two types of nomenclatures

  • Common name :-

The name of alkyl group or aryl groups separately in alphabetical order followed by the word ether.  

      CH3—O—CH2CH3 (ethyl methyl ether), 

     CH3—O—C3H7 (methyl propyl ether)

  • IUPAC name :-

Ethers are named as alkoxy derivative of hydrocarbon, the larger of the two alkyl derivatives is regarded as the parent hydrocarbon.

CH3—O—CH3(methoxy methane),

CH3—O—CH2CH3(methoxy ethane)

Isomerism:

    Functional isomerism: The alcohol having two or more carbon atom can show functional isomer.

  H3 C ̶  CH2 ̶  OH                                    H3C ̶  0 ̶  CH3

 Ethanol or ethyl ether                  Methoxy methane or diethyl ether  

i) Williamson's Ether synthesis:

 Whe alkyl halide is heated with sodium alkoxide then ether is formed. This reaction is known as williamson's synthesis. Both symmetrical and unsymmetrical ether can be prepared by this method.

         R- OH     +     X- R'           R - O- R' +  NaX

     Sod.alkoxide  alkyl halide      ether

CH3ONa    +  Cl - CH2CH3 CH3 - O - CH2CH3  +  NaCl

Sod. methoxide                     ethyl methyl ether

The mechanism of williamson's synthesis follows substitution of alkoxide ion on primary alkyl halide.

            R - O-  + R - X    R - O - R  + X-

The reaction involves SN2 attack of an alkoxide ion primary alkyl halide.

In this reaction alkyl halide is always 10 (Primary) but alkoxide may be primary, secondary or tertiary. Because SN2 is highly favourable in case of primary alkyl halide.

In the preparation of t- butyl ethyl ether by williamson's synthesis sodium tert. Butoxide is used but not sodium ethoxide. Explain?

 In williamson's synthesis for preparation of t- butyl ethyl ether following two way can be used.

 way-I   

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  way-II

 

The way-ii is not feasible because alkoxide ions are not only nucleophile, but also strong base and tend to react with alkyl halide by elimination to give alkene.

 

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The 20 and 30 akyl halide readily undergo elimination with strongly basic alkoxide to form alkene.Hence, to prepare unsymmetrical ethers, such as t- butyl ethyl ether, 10 alkyl group should come from alkyl halide part(RX) and 20 and 30 alkyl groups should come from the alkoxide part.

ii) Dehydration of alcohols:  

    When excess of alcohol is heated with conc. H2SO4 at 14000c, two moles of alcohol lose one molecule of water to form symmetrical ether.

 

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For example:

 

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Mechanism - this mechanism involves nucleophilic substitution with protonated alcohil as the substrate and a second molecule of alcohol as nucleophile.

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