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Cleavage of Ethers

Williamson Ether Synthesis : SN2 reaction with alkoxide as nucleophile on a primary carbon with a good leaving group

R O Na X R'

R=any alkyl, phenyl or vinyl group

X=any good leaving group (Cl, Br, I or tosylate)

RO R'

Alkoxides can be genarated with sodium hydroxide in some cases or sodium hydride (NaH).

R OHNaH

R O Na H 2

Williamson ether synthesis reactions are usually only done with a primary electrophilc substrate sinceotherwise E2 elimination reactions would compete.

Alkoxymercuration of Alkenes

Hg(CF 3COO) 2

OH

OHgO 2CF 3

NaBH 4 O

Very simlar to alcohols synthesis using oxymercuration. This varierty uses the trifluoracetoxy ligand for Hginstead of acetate. An alcohol serves as the nucleophile instead of water.

A mercurinium bridge is formed followed by nucleophilc attack by the alcohol and subsequent reduction.The alcohol nucleophile will attack the more subsituted carbon due to greater partial positive charge at thissite.

Ethers may be cleaved to yield an alcohol and an alkyl halide. Strong conditions are usually necessary forether cleavage. Note: If two or more equivalents of acid are used further dehydration can occur on formed alcohols.

HI or HBr in refluxing water are need for cleavage of ether with primary or secondary substituents. Theseethers are cleaved via an S N2 mechansim.

With tertiary, allylic, or benzylic substituetns an S N1 mechanism is observed for cleavage which can bedone with strong acids such as H 2SO 4 or CF 3COOH.

OH2SO 4

OH

OH

H

OH

O OHI

H

I

OH

I

nucleophile attacks less substituted carbon

more stable carbocation formed



Claisen Rearrangement

OLook forthis group!!! ∆

Thermally catalyzed, 6 electron pericyclic

O O

H

OH

C yclic Ethers and E poxides

Cyclic ethers have essentially the same reactivity of their acyclic counterparts. Epoxides are anexception, owing their reactivity to a strained three membered ring.

O

O

O

O

tetrahydrofuran(THF) 1,4-dioxane

ethylene oxide(an epoxide)

Pre partion of Epoxides

Epoxides can be formed by oxidation of a doble bond with a peroxy acid (peracid). A common exampleof such a reagent is meta -chloroperoxybenzoic acid ( m-CPBA).

OO

H

O

Cl

m-CPBAO

m-CPBA

Mechanism:

O

O

H

O

R

O O

OH

R

Epoxide formation via an intramolecular Williamson ether synthesis of a halohydrin.

OH

Br

NaOH O

Br

O

groups anti to one another

Br2

H2O



Opening/Cleavage of Epoxides

Crown Ethers

Epoxides can be opened in a few different ways. Epoxides react at conditions milder then those needed tocleave non-strained ethers. Nucleophilic attack occurs from the backside of the epoxide oxygen. It is oftenpossible for mixtures of products to result.

O H Cl O H Cl

attack is S N2 like: on the less substituted carbon

OHCl

Acid Catalyzed: Primary and Secondary carbons

Acid Catalyzed: Tertiary carbons

OH Cl

O H

O H

Cl

OH

Cl

attack is SN1 like: more stable carbocation formed followed by attack major

product

majorproduct

Base-Catalyzed Epoxide Openigs-occur due to ring strain

OOH

O

HO

H2O

OH

HO

attack is S N2 like: on the less substituted carbon

Hydroxide acts as a nuclophile displacing the epoxide oxygen as a leaving group. Other nucleophiles,such as Grignard reagents, alkoxides, hydrides, can perform this same reaction.

O

O

O

OO

O

18-crown-6

K

Host molecules for cation guests of various sizes.

Named by specifying the number of atoms and the number of oxygensseparated by the word crown.

(total atom #)-crown-(oxygen #)



Thiols and Sulfides

Thiols (Mercaptans)- Sulfur analog of alcohols, named like alcohols except use thiol instead of - olSulfides- Sulfur analog of ethers, named like ethers except use sulfide instead of the word ether

SH Mercapto Group

Br

Preparation of thiolsNaSH

SN2 SH Note: prone to many side products

Br SHH2N NH2

S

NH2

NH2

SH2O

OH

Disulfide formation from thiols

SH2

oxidationBr 2 or I 2

SS

reductionZn, H

H2N

SH

OOH

cysteine

S

Sulfides

R X SR

SN2

S H3C I S

Trialkyl Sulfonium Ion

S

Oxidation of Sulfides to Sulfones

H2O2S

O

m-CPBAS

O

O

Preparation

DMSO(dimethylsulfoxide)

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