Chapter 18: Ethers Naming of Ethers Physical Properties Preparation of Ethers General Structure R O R' R S R' H S R' Ether Thiol Sulfide R or R' can be alkyl, aryl or vinyl and the R groups may be attached in a ring shape Simple ethers are named by naming the two substituent groups followed by the word ether. O ethyl methyl ether O diphenyl ether O tert-butyl isopropyl ether O cyclohexyl propyl ether If other functional groups are present then the group is considered to be an alkoxy substituent. O 3-propoxy-cyclohexene O OH 2-Methoxy-ethanol O 1-Propoxy-but-2-yne O O Dimethoxy-methane H 3 C O CH 3 112 ° Boiling points of ethers are elevated relative to similar alkanes. This is due to a molecular dipole caused by the R-O-R bond. Simple, symmetric ethers are prepared industrially by the dehydration of alcohols : OH diethyl ether synthesis: 2 H 2 SO 4 O OH OH 2 This reaction is an easy way to make symmetric ethers, yet is impractical for most laboratory applications.
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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.
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.
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.
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