24.11 Preparation of Aryl Ethers. Typical Preparation is by Williamson Synthesis ONa + RXRXRXRX OROROROR NaX + SN2SN2SN2SN2.

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Presentation transcript:

24.11 Preparation of Aryl Ethers

Typical Preparation is by Williamson Synthesis ONa + RXRXRXRX OROROROR NaX + SN2SN2SN2SN2

Typical Preparation is by Williamson Synthesis ONa + RXRXRXRX OROROROR NaX + SN2SN2SN2SN2 but the other combination X + RONa fails because aryl halides are normally unreactive toward nucleophilic substitution

acetone heat Example ONa + CH 3 I OCH 3 (95%)

acetone, heat Example OHOHOHOH + (86%) H2CH2CH2CH2C CHCH 2 Br K 2 CO 3 OCH 2 CH CH 2

Aryl Ethers from Aryl Halides F NO 2 + KOCH 3 CH 3 OH 25°C OCH 3 NO 2 + KFKFKFKF (93%) nucleophilic aromatic substitution is effective with nitro-substituted (ortho and/or para) aryl halides

24.12 Cleavage of Aryl Ethers by Hydrogen Halides

Cleavage of Alkyl Aryl Ethers OArR H + O Ar R H Br Br –+ +

Cleavage of Alkyl Aryl Ethers OArR H + O Ar R H Br Br –+ + R Br + OAr H An alkyl halide is formed; never an aryl halide!

+ Example OCH 3 OH OHOH (85-87%) CH 3 Br (57-72%) HBr heat

24.13 Claisen Rearrangement of Allyl Aryl Ethers

Allyl Aryl Ethers Rearrange on Heating OH CH 2 CH CH 2 OCH 2 CH CH 2 200°C (73%) allyl group migrates to ortho position

rewrite as Mechanism OCH 2 CH CH 2 O O H keto-to-enol isomerization OH

Claisen rearrangement is an example of a sigmatropic rearrangement. A  bond migrates from one end of a conjugated  electron system to the other. this  bond breaks this  bond forms “conjugated  electron system” is the allyl group Sigmatropic Rearrangement O O H

24.14 Oxidation of Phenols: Quinones

The most common examples of phenol oxidations are the oxidations of 1,2- and 1,4-benzenediols to give quinones. (76-81%) QuinonesOHOHOO Na 2 Cr 2 O 7, H 2 SO 4 H2OH2OH2OH2O

The most common examples of phenol oxidations are the oxidations of 1,2- and 1,4-benzenediols to give quinones. (68%) Quinones O O Ag 2 O diethyl ether OHOH CH 3

Alizarin (red pigment) Some quinones are dyes OO OH OH

Ubiquinone (Coenzyme Q) n = 6-10 involved in biological electron transport Some quinones are important biomolecules CH 3 O OO CH 3 n

Vitamin K (blood-clotting factor) Some quinones are important biomolecules O CH 3 O

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