Chapter 14 - Ethers, Epoxides, And Thiethers

Question 1

What is an ether?

Answer 1

R-O-R
Ethers are related to water, with alkyl groups replacing the hydrogen atoms. In an alcohol one hydrogen atom of water is replaced by an alkyl group. In an ether both hydrogen atoms are replaced by alkyl groups.

Question 2

Ether Properties

Answer 2

Relatively unreactive, stable with many types of reagents making them a good solvent for reactions and extractions, volatile

Question 3

Structure and Polarity of Ethers

Answer 3

Bent structure
Sp3 hybrid oxygen atom giving a nearly tetrahedral bond angle (110 degrees)
Strongly Polar

Question 4

Hydrogen Bonding of Ethers

Answer 4

Pure ethers cannot engage in hydrogen bonding because they have no O-H groups
Large dipole moments
Dipole-Dipole attractions
Can hydrogen bond with other compounds containing O-H or N-H bonds (hydrogen donors) with the lone pair from the oxygen being the hydrogen acceptor

Question 5

Ethers as Polar Solvents

Answer 5

Dissolve polar and non polar substances
Evaporate from product because of low boiling point
Non polar substances tend to be more soluble in ethers than alcohols because ethers have no hydrogen bonding networks to be broken up by the non polar solute
The non bonding pairs in ethers effectively solvated cations (do not solvated anions as well as alcohols)
Ionic substances are often insoluble in ether except for iodides, acetates, and other organic anions with large, diffused anions
No hydroxyl group
Normally unreactive towards strong bases
Frequently used as solvents for very strong polar bases
DME, THF, and dioxane are miscible in water
Diethyl ether is sparingly soluble in water

Question 6

Stable Complexes of Ethers with Reagents

Answer 6

Sharing of electrons stabilizes the reagent and helps keep it in solution
An Esther’s non bonding electrons also stabilizes borane, BH3
BF3 forms a stable complex with ethers
Crown ethers are large cyclic poly ethers that specifically solvated metal cations by completing the metal in the center of the ring (helps polar inorganic salts to dissolve in non polar organic solvents, allowing them to be used under aprotic conditions where uncomplexed anions may show greatly enhanced reactivity)
The crown ether complexes only the cation, leaving the anion bare and highly reactive

Question 7

Naming Ethers

Answer 7

Common - name the two alkyl groups on oxygen and adding the word ether (ex. Ethyl methyl ether CH3-O-CH2CH3)
IUPAC - use the more complex alkyl group as the root name and the rest of the ether as an alkoxy group (ex. Methoxyethane)

Question 8

Epoxides (Oxiranes)

Answer 8

Three membered cyclic ethers, usually formed by proxy acid oxidation of the corresponding alkenes.
Common name - add oxide to the name of the alkene that is oxidized (ex. Cyclohexene oxide)
Systematic Method 1- name the rest of the molecule and use the term epoxy as the substituent, giving the numbers of the two carbon atoms bonded to the epoxide oxygen (ex. Trans-1,2-epoxy-4-methylcyclohexane)
Systematic Method 2- names the epoxide a as derivatives of the parent compound, ethylene oxide, using oxirane as the name for ethylene oxide numbering the ring starting with the heteroatom (oxygen) and going in the direction to give the lowest substituents numbers (ex. 3,4-epoxy-4-ethyl-2-methylhexane)

Question 9

Oxetanes

Answer 9

Four membered cyclic ethers
Because of strain, more reactive than larger cyclic compounds and open-chain ethers
Not as reactive as highly strained epoxides
Ex. Oxetane C3H6O

Question 10

Furans (Oxolanes)

Answer 10

5 membered cyclic ethers
2 Saturated carbons
Ex. 3-methoxyfuran

Question 11

Pyrans (Oxanes)

Answer 11

6 membered cyclic ethers
2 saturated carbons
Ex. 4-methlypyran

Question 12

Dioxanes

Answer 12

Heterocyclic ethers with two oxygen atoms in a six membered ring
Ex. 1,4-dioxane

Question 13

Infrared Spectroscopy of Ethers

Answer 13

C-O stretch around 1000-1200 cm-1
IR only useful to show absence of carbonyl (C=O) groups and hydroxyl (O-H) groups
If the molecular formula contains an oxygen atom, the lack of carbonyl or hydroxyl absorptions in the IR suggests an ether

Question 14

Mass Spectrometry of Ethers

Answer 14

The most common fragmentation of ethers is cleavage next to one of the carbon atoms bonded to oxygen resulting in an Oxonium ion (oxygen with three bonds and a positive charge)
Another common cleavage is the loss of either of the two alkyl groups to give another Oxonium in or an alkyl cation
The four most abundant ions correspond to the molecular ion, loss of an ethyl group, alpha cleavage (cleavage at one of the alkyl groups), and loss of ethylene molecule combined with alpha cleavage

Question 15

NMR Spectroscopy of Ethers

Answer 15

C NMR: C-O around 65-90
H NMR: C-O around 3.5-4
If a compound containing C, H, and O has resonances in the correct range, and if there is no O-H stretch or C=O stretch in the IR, an ether is most likely the functional group.

Question 16

Williamson Ether Synthesis

Answer 16

Most reliable and versatile ether synthesis
SN2 attack of an alkoxide ion on an unhindered primary alkyl halide or tosylate
The alkoxide is commonly made by adding Na, K, or NaH to the alcohol

Question 17

Synthesis of Phenol Ethers

Answer 17

A phenol (aromatic alcohol) can be used as the alkoxide fragment, but not the halide fragment, for the Williamson Ether Synthesis

Question 18

Converting Two Alcohols to an Ether

Answer 18

Convert the more hindered alcohol to its alkoxide
Convert the less hindered alcohol to its tosylate or an alkyl halide (make sure the tosylate or halide is a good SN2 substrate)

Question 19

Alkoxymercuation-Demurcuration

COME BACK TO AT PAGE 637

Answer 19

Process adds a molecule of an alcohol across the double bond of an alkene. The product is an ether.