Introduction to Ether & Epoxides:
We know alcohols family with their rich chemical reactivity but ethers - compounds containing a R-O- R unit are less reactive and gives relatively few chemical reactions. This lack of reactivity of ether makes them valuable as inert solvents in a number of chemical reactions.
Unlike most ethers, epoxides – compounds in which the C – O – C unit forms a three-membered ring - are very reactive substances due to molecular strain.
Ethers are defined as symmetrical or unsymmetrical depending on whether the two groups bonded to oxygen are the same or different. Unsymmetrical ethers are also called mixed ethers. Dimethyl ether is a symmetrical while ethyl methyl ether is an unsymmetrical ether.
Cyclic ethers have their oxygen as part of a ring. They are heterocyclic compounds. Oxygen heterocycles of commonly encountered ring sizes have specific IUPAC nomenclature.
In each case the ring is numbered starting at the oxygen. The IUPAC rules also permit oxirane (without substituents) to be called ethylene oxide. Tetrahydrofuran and tetrahydropyran are acceptable synonyms for oxolane and oxane, respectively.
There are many compounds, often used as reaction solvents, are the
diethers
1,2,-dimethoxy ethane and 1,4-dioxane. Diglyme, also a commonly used solvent, is a
tri ether
The sulfur analogs (RS –) of alkoxy groups are called alkyl thio groups while RSH are known as alkane thiol. Sulfur hetero cycles have names analogous to their oxygen relatives, except that ox- is replaced by thi-
Thus the sulfur heterocycles containing three-, four-, five-, and six- membered rings are named
thiirane, thietane, thiolane,
& thiane respectively.
Structure and bondangle in ethers and epoxides Bonding in ethers is readily understood by comparing ethers with water and alcohols. Van der Waals strain involving alkyl groups causes the bond angle at oxygen to be larger in ether than alcohols, and larger in alcohols than in water. An extreme example is di-Isopropyl ether, where steric hindrance between the isopropyl groups is responsible for a dramatic increase in the C – O – C bond angle.  Carbon-oxygen bond distances are some what shorter than carbon-carbon bond distances. The C – O bond distances in dimethyl ether (141 pm) and methanol (142pm) are similar to one another, and both are shorter than the C – C bond distance in ethane (153 pm). Insertion of an oxygen atom into a three-membered ring requires its bond angle to be seriously distorted form the normal tetrahedral value. In ethylene oxide, for example, the bond angle at oxygen is 61.5.  Thus epoxides, like cyclopropanes, are strained. They tend to undergo reactions that open the three-membered ring by cleavage of one of the carbon-oxygen bonds. Physical properties Physical properties of ethers are very similar with alkanes and alcohols. With respect to boiling point, ethers resemble alkanes more than alcohols due to less polarity than alcohol while; with respect to solubility in water, ethers resemble alcohols more than alkanes due to H–bonding with water.  In general, the boiling point of alcohols are unusually high because of hydrogen bonding. Attractive forces are present in liquid phase of ethers and alkanes, but lack – OH groups so cannot take part in intermolecular hydrogen bonding, are much weaker, so their boiling points are less. If we think about structure of ether, the presence of an oxygen atom permits ethers to participate in hydrogen bonding to water molecules. These attractive forces are responsible for solubility of ethers in water to approximately the same extent as comparably constituted alcohols. Alkanes cannot take part in hydrogen bonding with water. Crown ethers : Cyclic compounds containing 4 or more ether linkages in a ring of 12 or more atoms. these compounds is known as crown ethers , because their molecular models are similar to crowns. Systematic nomenclature of crown ethers is actually a short hand description where by the word crown is followed by the total number of atoms in the ring and is followed by the number of oxygen atoms.  15-Crown-5 and 18-crown-6 are a cyclic pentamer and hexamer, respectively, of repeating -OCH2CH2- units; they are polyethers based on ethylene glycol (HOCH 2CH2OH) as the parent alcohol. The metal ion-complexing nature of crown ethers are explain by their effects on the solubility and reactivity of salts in nonpolar solvents. Potassium fluoride is ionic and insoluble in benzene (covalent), but 0.05 M solutions can be prepared when 18-crown-6 is present. This high solubility of potassium fluoride in benzene is explain by the formation of a stable complex, cage like structure, is stabilized by ion-dipole forces between K+ and the six oxygen atoms of the crownether. Potassium ion, with an ionic radius of 265 pm, adjust completely with in the 260- to 320-pm inside cavity of 18-crown-6. Nonpolar CH2 groups dominate the outer side of the complex, make its polar interior, and permit the complex to soluble in nonpolar solvents such as covalent benzene. Every K+ that is carried into benzene brings a fluoride ion (F–) with it, resulting in a solution containing strongly complexed potassium ions and relatively unsolvated fluoride ions.  In polar matrix such as water and alcohols, fluoride ion is strongly solvated by hydrogen bonding and is neither very basic nor very nucleophilic. On the other part, the poorly solvated, or “naked,” fluoride ions that are present when potassium fluoride dissolves in benzene in the presence of a crown ether are better able to express their anionic reactivity. Thus, alkyl halides react with ionic potassium fluoride in covalent benzene solvent like 18-crown-6, easily gives a method of the preparation of different floride.  Preparation (i) Willamson’s continous ethrification Diethyl ether and many others, for example, are prepared by acid-catalyzed reaction of the corresponding alcohols, as we already studuied in alkenes, known as williamson contineous etherification.  In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary. Isopropyl alcohol, however, is readily available at low cost and gives yields of diisopropyl ether high enough to justify making (CH3)2CHOCH(CH3)2 by this method on an industrial approach. (ii) Williamson synthesisA method of long standing for the preparation of ethers is the Williamson ether synthesis. Nucleophilic substitution of an alkyl halide by an alkoxide gives the carbon-oxygen bond of an ether.  Preparation of ethers by the Williamson ether synthesis is most successful when the alkyl halide is one that is reactive toward SN 2 substitution. Methyl halides and primary alkyl halides are the best reactants (least reactive in elimination).  Secondary and tertiary alkyl halides are not suitable reactants, because they tend to react with alkoxide base by E2 elimination rather by SN 2 substitution. Whether the alkoxide base is primary, secondary, or tertiary is not very important than the nature of the alkyl halide. Thus benzyl terbutyl ether is prepared in high yield from benzyl chloride, a primary chloride that is incapable of undergoing elimination, with potassium ter. butoxide (base).  The alternative synthetic route using the sodium salt of benzyl alcohol and an terbutyl halide would be much less effective, because of increased competition from elimination, as the alkyl halide becomes 3° than it easily undergo elimination. We take some following example also, always prefer alkyl halide as primary.  The alternative combination, cyclohexyl bromide and sodium ethoxide, is not correct, because elimination will be the major product.  (iii) Oximercuration – demercuration of alkenes Addition of ROH according to marknownikoff’s rule without any molecular rearrangement occurs.  (iv) Alcohol with diazomethane
(v) Alkyl halide with dry Ag2O
This reaction is not carried out with moist Ag2O because moist Ag2O is actually AgOH where substitution occurs and formation of alcohols from alkyl halide takes place. Chemical properties (i) Oxidation Ether are less reactive due to absence of polarity, along with an ability to soluble in nonpolar substances like CCl4, that makes ethers so often used as solvents when carrying out many organic reactions. Nevertheless, most ethers are explosive and hazardous materials, and precautions must be taken when using them. Diethyl ether is extremely flammable and because of its high volatility can form explosive mixtures in air very quickly. Open flames must never be present in laboratories where diethyl ether is being used regularly. A second dangerous property of ethers is when they undergo oxidation in presence of air to form explosive peroxides. Where air oxidation of diethyl ether gives explosive strained peroxide proceeds according to the following equation.  The reaction is a free-radical type, and oxidation occurs at the carbon that contain the ether oxygen to form a hydroperoxide, a compound of the type ROOH. Hydroperoxides tend to be unstable and shock-sensitive due to molecular strain. On standing, they form related peroxidic derivatives, which are also. Due to this, one should never use old bottles of dialkyl ethers, and extreme care must be taken in their uses. (ii) Acid–catalyzed fission of ethers When the carbon-oxygen bond of alcohols is undergo fission on reaction with hydrogen halides, just similar to an ether linkage undergo breaking with hydrogen halides.  The fission of ethers is generally carried out under conditions like excess hydrogen halide, cold or hot HX, such that the alcohol formed as one of the original products and by heating converted to an alkyl halide. Thus, the reaction with hot HX finally gives two alkyl halide molecules :  Cyclic ethers yield one molecule of a dihalide :  The order of hydrogen halide reactivity is HI>HBr >HCl. Hydrogen fluoride is not effective so normally not considered in organic chemistry. A mechanism for the cleavage of diethyl ether by hydrogen bromide is given below where the key is an SN2-like attack on a dialkyloxonium ion by bromide. Overall Reaction :  Mechanism of reaction Step A : Ether reacts with proton to give a dialkyloxonium ion (oxonium salt)  Step B : Halide attack on carbon of the dialkyloxonium ion. This step gives each molecule of an alkyl halide and an alcohol.  Step C : In last step where an alcohol is converted to an alkyl halide.  (iii) Reaction with HI (fission reaction) Reaction with HI is an example of fission reaction where cleavage is decided by structure of ether. If there is smaller alkyl group up to secondary than fission is S N 2 so back attack of nucleophile occurs while for large alkyl group always fission by S N 1 mechanism because they prefer carbocation mechanism.
When ethers reacts with cold HI to form one mole of alkyl iodide. This when reacts with silver nitrate to form yellow precipitate of AgI. By finding out the weight of AgI we can find out the structure of ether. This method is known as of zeisal’s method for estimation of alkoxy group in the molecule for example. Problem 0.74 gm of methyl ether forms 2.35 gm of AgI at NTP find out sturcture of methyl ether ?  (iv) Reaction with dil H2SO4
(v) Reaction with CO
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