Alcohols, Phenols and Ethers__Sania Matte PPT.pptx

Ethers preparation

1. Alcohols, Phenols and Ethers Topic: Ethers Presented by: Sania S. Matte

2. Ethers Nomenclature 1. CH3-O-CH3 Methoxy methane 2. C2H5-O-C2H5 3. CH3-CH2-O-CH3 Methoxy ethane 4. CH3-CH-O-CH3 Methoxy propane CH3 CH3 5. CH3-O-C-CH3 CH3

3. Preparation of Ethers 1. By dehydration of alcohols Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of reaction product, alkene or ether depends on the reaction conditions. For example: ethanol is dehydrated to ethene in the presence of sulphuric acid at 443K. At 413K ethoxyethane is the main product.

4. The formation of ether is nucleophilic bimolecular reaction (SN2 )

5. The method is suitable for preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows SN1 pathway when the alcohol is secondary or tertiary. The dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over susbstitution and as a consequence, alkenes are easily formed.

6. 2. Williamson synthesis It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide R-X + R`O Na R-O-R` + NaX The reaction involves SN2 attack of an alkoide ion on primary alkyl halide. Better results are obtained if the alkyl halide is primary.

7. Better results are obtained if the alkyl halide is primary.

8. In case of secondary and tertiary alkyl halides, elimination competes over substitution. If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CH3ONa with (CH3)3C-Br gives 2-methylpropene. It is because alkoxides are not only nucleophiles but strong bases as well. They react with alkyl halides leading to elimination reaction.

9. Phenols are also converted to ether by this method. In this, phenol is used as the phenoxide moiety. OH O Na O- R + NaOH R-X Phenol Phenoxide Ether

10. Physical Properties The C-O bonds in the ethers are polar and thus, ethers have a net dipole moment. FORMULA CH3(CH2)3CH3 C2H5-O-C2H5 CH3(CH2)3-OH n-Pentane Ethoxyethane Butan-1-ol B.P/K 309.1 307.6 390 The large difference in boiling points of alcohols and ethers is due to the presence of hydrogen bonding in alcohols

11. Chemical Properties Cleavage of C-O bonds in ethers Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions Excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules. R- O -R + HX RX +R- OH (Excess) R- OH + HX R-X + H2O

12. Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide. O- R OH + H -X + R-X Ethers with two different alkyl groups are also cleaved in the same manner. R- O -R` + HX R-X + R`- OH

13. The order of reactivity of hydrogen halides is as follows: HI>HBr> HCl. When one of the alkyl group is tertiary group, the halide formed is tertiary halide. CH3 CH3 CH3 C O CH3 + HI CH3 OH + CH3 C I CH3 CH3

14. CH3 O HI O H + CH3 Methylphenyl oxonium ion O HI OH + CH3I Anisole Phenol Methyl iodide

15. II . Electrophilic substitution The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.

16. Halogenation: Phenylalkyl ethers undergo usual halogenation in the benzene ring e.g., anisole undergoes halogenation with bromine in ethanoic acid even in the absence of iron(III) bromide catalyst

17. b) Friedael -Crafts reaction: Anisole undergoes Friedel-Crafts reaction i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride as catalyst

18. c) Nitration: Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole

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