Reagents for Interconverting Carboxylic Acids and Acid Derivatives

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  • Topic: Ether, Nucleophile, Nucleophilic substitution
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ETHERS Classification of Ethers:   Symmetrical ethers – two groups attached to O are identical Ex. CH3CH2OCH2CH3 – diethyl ether  Unsymmetrical ethers – two groups attached to O are not identical Ex. CH3CH2OCH3 – ethyl methyl ether Physical Properties of Ethers:   Ethers have much lower boiling points compared to alcohols of comparable MWs.  BPs of ethers increases with increasing MW.  BPs of isomeric ethers increase with increasing alkyl chain length.  BPs of ethers are about the same as those of alkanes of comparable MWs.  Solubility of ethers in water is comparable to that of alcohols.  As chain length of ethers increases, solubility in water decreases.  Branching of the alkyl chain results in greater solubility since London forces are weaker.  O 111o

 H3C

 CH3

  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'X

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

RO Na
-

-

+

+

H2

RO Na

+

+ 1/2 H2
+

ArO Na

-

+ H2O

 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 

+

R'X

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

-

OH

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

Example:
CH3 C C H CH3 H Cl2 H2O H OH H3C C C CH3 Cl H
-

OH

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

H X

X X C OH C

-

+ C

OH C

2.

Ring-opening of Epoxides Using Nucleophiles
O C C Nu H
+ -

O C Nu
-

C

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.

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