The C-O bonds in ethers are polar, with O being - and each of the C atoms bearing a +. The C-O-C bond angle is 111o, slightly higher than the H-O-H bond angle in water because of the stronger electron repulsion between the bulky alkyl groups.
Preparation of Ethers: 1. Preparation of symmetrical ethers from alcohols H2SO4 R O R + H2O 2ROH 2. Williamson Ether Synthesis
R O Na
R O R' + NaX
Ar O Na + RX Ar O R + NaX Involves a nucleophilic substitution reaction. Halide ion is displaced by the alkoxide or phenoxide ion. Gives best results when 1o alkyl halides are used. Mechanism is SN2.
Alkoxides may be prepared by reacting an alcohol with NaH or by reacting an alcohol with Na metal. Phenoxides may be prepared by reaction of phenols with NaOH
ROH + NaH ROH + Na ArOH + NaOH
+ 1/2 H2
When 3o alkyl halides are reacted with alkoxide or phenoxide ions, elimination is the major reaction, not substitution When 2o alkyl halides are used, yield of ether is lower since elimination is a competing reaction. Use 1o alkyl halide in Williamson ether synthesis to get good yield of ether. Reactions of Ethers: 1. Cleavage of Ethers by Acids – usually carried out using concentrated acids (HI or HBr) and high temperatures
ROH R O R' + HX
RX + R'OH
Nucleophilic substitution reaction First step is protonation of ether. The alcohol that is formed can react with another HX to give an alkyl halide NET RESULT: Since excess HX is used: R-O-R’ + 2HX --> R-X + R’-X Ar-O-R + HX --> ArOH + RX Preparation of Epoxides: 1. 2. Oxidation of alkenes by peroxy acids (MCPBA) Cyclization of halohydrins O C C X halohydrin (where X = Br or Cl) epoxide H
O C C
+ H2O + X
Preparation of Halohydrins: Electrophilic addition of HO-Br or HO-Cl (Br2 or Cl2 + H2O) to alkenes OH C C + HO Cl C C Cl OH C C + HO Br C C Br
CH3 C C H CH3 H Cl2 H2O H OH H3C C C CH3 Cl H
O H3C C H C CH3 H
Reactions of Epoxides: 1. Acid-catalyzed ring-opening reactions of epoxides Preferred reaction for ring-opening of an epoxide is SN1. O C C +O H C C
X X C OH C
Ring-opening of Epoxides Using Nucleophiles
O C C Nu H
O C Nu
OH C Nu C
Epoxides can undergo nucleophilic attack that leads to ring-opening. Nucleophiles: -OH, -SH, RO-, RS-, Grignard reagents. Ring-opening goes by way of a SN2 reaction. Nucleophile attacks less substituted C of the epoxide. Attack of nucleophile on less substituted C occurs simultaneously with C-O bond breakage.