Ethers are compounds with the formula R−O−R. They have traditionally been grouped with the alcohol. Meaning the H of alcohol is being replaced by an alkyl radical. This makes sense as long us you remember that the physical and chemical properties of H is very different from that of R. First, H is the smallest atom possible and second it can dissociate from the compound to give it acidic property. Both of these are not possible with ether.
Other than these the oxygen in ether can be attacked as in alcohol resulting in reactions frequently seen for alcohol.
NOMENCLATURE
The common usage would be to state the two alkyl groups and let it be know that it is an ether. Traditionally the alkyl are named in alphabetical order. Of course we can still identify the ether if you do not follow this rule.
However it is easier to name it as an alkoxy substituent if you have other functional groups like chloro, hydroxyl, etc, attached to the compound.
Tutorial 9.1
Name the compound using either the common name or the IUPAC nomenclature, which ever is simplier. Answer
PHYSICAL PROPERTIES
Like all hydrocarbons the melting and boiling point of an ether increases with increasing molecular size. However the oxygen introduces slight polarity to the molecule. So for the same hydrocarbon size the ether will have a much higher melting or boiling point, but the increase is very much lower than for alcohol.
| LENGTH | ALKANE | ETHER | ALCOHOL |
| 3 units 4 units 5 units 6 units |
Propane = −42.1 Butane = −0.5 Pentane = 36.1 Hexane = 68.7 |
Methoxymethane = −23.0 Methoxyethane = 10.8 Ethoxyethane = 34.5 Ethoxypropene = 63.6 |
Ethanol = 78.5 Propanol = 97.4 Butanol = 117.3 Pentanol = 138.0 |
The influence of the oxygen in ether is negligible above 4 units.
In terms of solubility it can fool around with water but will not get serious with it; not like alcohol. So it is often used to remove organic compounds from an aqueous medium, for example in preparative chemistry where water is the medium of reaction. The organic compound is generally extracted from the reaction mixture using diethyl ether. It can actually get close enough to the water molecule to get to the organic compound, but when left standing it will separate as an ether layer from the water. Diethyl ether has a density of 0.71 and so it is generally the upper layer. This application is also help by the fact that ether is not attack by acid or base at room temperature.
However, it is very important to remember that diethyl ether (and most ether used for this purpose) is very volatile and burn very easily. So no naked flame should be present when using ether. Also when removing the ether to obtain the product it is important not to evaporate to dryness. Ether can be easily oxidised to peroxides which are thermally unstable. It can decompose vigorously resulting in an explosion.
PREPARATION of ETHER
Ether can be prepared from alcohol and bromide by a SN2 reaction.
ROH + R'Br � R−O−R' + HBr
A simple alcohol is not aggressive enough to force the bromide to leave so it has first to be converted to a basic alkoxide.
ROH + K � R−O‾K+ + � H2
Now the alkoxide is too basic and will also execute an E2 Reaction. So the bromide used has to be a primary bromide, otherwise several by-reactions will occur; leading to low yield and difficulty in isolating the wanted product.
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Beside bromide the other good leaving group would be the sufonate ester, especially benzenesulfonate
Tutorial 9.2
If you want to prepare isopropyl methyl ether, which reaction would you choose:
- Sodium isopropoxide with methyl benzenesulfonate; or
- Sodium methoxide with isopropyl benzenesulfonate?
Explain your choice. Answer
These SN2 reactions to prepare ether is commonly known as the Williamson ether synthesis.
Symmetrical ether
Symmetrical ether is much easier to prepare. An alcohol is refluxed with excess acid, usually sulphuric acid.
| 2 ROH | H2SO4, 140�C � | R−O−R + H2O |
Higher temperature will promote Elimination Reaction. The Elimination Reaction is also the main reaction for secondary and tertiary alcohols.
Tutorial 9.3
Tert-butyl ethyl ether is prepared in more than 80% yield by heating tert-butanol and ethanol at about 100�C. Explain how such a high yield of the ether is possible from this reaction. Answer
REACTIONS
Let us continue from where we left off in our introduction: the physical and chemical properties of H is very different from that of R. We are aware that the ether does not have acidic property. We can heat ether with acid, BUT we must be aware that it can undergo SN2 reaction with HBr.
Unlike alcohols, ethers are not oxidised to aldehydes and ketones, BUT we must be aware that it can undergo free radical oxidation reaction.
| [O2] + | RCH2−O−R' | � |
� RCH −O−R' + HOO� |
| HOO� + | � RCH−O−R' |
� |
OOH │ RCH−O−R' Hydroperoxide |
| [O2] + | � RCH −O−R' |
� |
OO� │ RCH −O−R' |
| OO� │ RCH−O−R' + |
� RCH−O−R' | � |
OR' OR' │ │ RCH−OO−CHR Peroxide |
We refer to such reaction as autooxidation. So ethers left in the laboratory for some time must assumed to have a small amount of hydroperoxide and peroxide. When distilling ethers like isopropyl ether, diethyl ether, tetrahydrofuran, etc, the ether distills over leaving behind the less volatile peroxides. In high concentration such residue can cause a very nasty explosion. So never distill ether solvents to dryness. Alternatively you can shake the ether with aqueous ferrous sulphate to destroy the peroxides before performing the distillation.
A convenient test for the presence of the peroxides is to take a small portion of the ether (it is colourless) and shake it with a little aqueous potassium iodide. The peroxide will oxidise the iodide to iodine which will give a dark tint to the solution.
On the whole ether are considered "unreactive" and so are not useful as reagents. Its importance is mainly as solvents. The oxygen introduces slight polarity to the molecule and so it is mid-way between a non-polar and a highly polar solvent.
CYCLIC ETHERS
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O2 / Ag � Heat |
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Unlike halides and alcohols, ether can form cyclic compounds, since the oxygen is divalent. The simplest member would be ethylene oxide. It can be prepared by the oxidation of ethylene with air in the presence of a silver catalyst. Ethylene oxide used widely to sterise medical equipments, and as a reagent to prepare ethylene glycol which is used as antifreeze radiators.
Three-member cyclic ethers are commonly known as epoxide. Its IUPAC equivalent is oxirane.
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However the general method of preparing cyclic ether is by executing an intramolecular SN2 reaction between two adjacent active groups. The most widely used is the β−halo alcohol, which can easily be prepared by reacting an alkene with hypohalous acid. The β−halo alcohol is then heated with an alkaline, like potassium hydroxide, which removes the proton to give the β−halo alkoxide. The alkoxide then undertake a SN2 reaction upon the halo substituent to give about 70% of the cyclic ether.
Tutorial 9.4
If we refluxed the 3-chloro-2-butanol shown with KOH, which geometrical 2,3-dimethyloxirane isomer would we get? Explain your choice. (Read: All vertical groups are directed away from you.) Answer
REACTIONS of CYCLIC ETHERS
1. SN2
Unlike the straight chain ethers, the cyclic ethers are reactive as the oxirane ring easily opens up because of the steric strain. So cyclic ethers would react readily via SN2 Reactions.
- Hydrolysis
It hydrolyses easily in aqueous solution of diluted acid to give a glycol (or a β−diol).
- Hydrolysis in the presence of alcohol
Heated with an alcohol in the presence of an acid it will give a β−hydroxyl ether.
- With Alkoxide
It reacts with alkoxides, like potassium alkoxide, to give a β−hydroxyl ether.
- With a base
It reacts with a base to give oligomeric ethers known as glycol. Example:
+
+
�
The chain reaction can be terminated by a reaction between the alkoxide and any protonated molecule present; like water.
H2O
�
You should now recognised that this is actually a chain polymerization process. The propagating species here is an anionic alkoxide instead of a free radical. "i" can vary from 1 to a few hundred. For large value of "i" it will be more appropriate to refer to the product as polyol.
The terminal hydroxyl groups can also be replaced by an alkoxide group. The product R'O−[CH2CH(R)]i−OR'is then know as a glyme if "i" is small and a polyether when "i" is large.
- With terminal alkynes
The proton of terminal alkyne is acidic enough to give a "strong" conjugate base which can attack the oxirane.
NaNH2 / NH3
�
- With terminal alkynes
2. With Grignard reagents
Tutorial 9.5
Show how you can prepare CH3(CH2)i+2Br from CH3(CH2)iBr. Answer
This is an easy path for extending the length of an alkyl by two units.
LARGER CYCLIC ETHERS
 Oxetane |
      Tetrahydrofuran |
    Tetrahydropyran |
Since any compound of the type R-O-R' is known as an ether, cyclic ethers need not be exclusively three-member ring. We can have four-, five-, and six-member rings. Rings larger than six are much more difficult to prepare.
Since the reactivity of the cyclic ether is dependent on the steric strain on the ring, the tetrahydrofuran and the tetrahydropyran are just as stable as the straight chain ethers and are often used as solvents (meaning not very reactive).
If you like to known more about the nomenclature of cyclic ether click here.
Crown ethers are cyclic polyethers, (OCH2 CH2)i.CROWN ETHERS
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Aq. KOH � SN2 reaction |
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It certainly looks like a crown. This particular cyclic polyether is known as 18-crown-6. There are 18 atoms to the ring and six of them are oxygen. (Note: The molecule is not planer).
Crown ethers have a very special solvating property. The battery of oxygen atoms is very effective in holding onto cations. For example potassium permanganate can be dissolved in benzene with the help of 18-crown-6. The potassium is embraced by the lone-pairs of the oxygen atoms in the "cavity" of cyclic ether molecule. Since the ether is soluble in benzene it pulls the potassium ion everywhere it goes, with the permanganate ion tailing it.
The strength of solvation will depend very much on the size of the "cavity" of the oxygen trap. The cation must fit comfortably in the cavity so that the lone-pairs of all the oxygen can interact with it. So lithium cation is solvated by 12-crown-4 and sodium by 15-crown-5. This property has important commercial applications.
