Dehydration bimolecular

Can you explain why is bimolecular dehydration not appropriate for the preparation of ethyl methyl ether ... 

Bimolecular dehydration is generally used for the synthesis of symmetrical ethers from unhindered 1° alcohols. Industrially, diethyl ether is obtained by heating ethanol at 140 °C in the presence of H2SO4. In this reaction, ethanol is protonated in the presence of an acid, which is then attacked hy another molecule of ethanol to give diethyl ether. This is an acid-catalysed Sn2 reaction. If the temperature is too high, alkene is formed via elimination. 

The difficulty in this mechanism is the highly unfavorable endothermic nature of the a elimination of methanol to carbene [Eq. (3.51)] in contrast with the thermodynamically favored bimolecular dehydration [Eq. (3.52)] ... 

It has been shown that only those alcohols that form detectable surface alco-holate species on alumina undergo bimolecular dehydration with ether and water as reaction products (340). Thus, ether formation is the dominant reaction direction of methanol and ethanol at low temperatures, and the tendency toward ether formation is reduced as the chain length increases and chain branching occurs (28, 340). The same trends are observed for the stability and surface concentrations of the surface alcoholate species (27, 28, 47, 340). Alcoholate formation is due to a dissociative chemisorption step of the alcohol that occurs on A1—O pair sites (47, 340, 354-358). One is, thus, led to the conclusion that incompletely coordinated Al3+ ions and O2- ions are both important sites in the ether formation from alcohols and that their participation should be detectable by specific poisoning. 

In some cases, a protonated primary alcohol may be attacked by another molecule of the alcohol and undergo an Sn2 displacement. The net reaction is a bimolecular dehydration to form an ether. For example, the attack by ethanol on a protonated molecule of ethanol gives diethyl ether. 

This bimolecular dehydration of alcohols is a type of condensation, a reaction that joins two (or more) molecules, often with the loss of a small molecule such as water. This method is used for the industrial synthesis of diethyl ether (CH3CH2—O—CH2CH3) and dimethyl ether (CH3—O—CH3). Under the acidic dehydration conditions, two reactions compete Elimination (dehydration to give an alkene) competes with substitution (condensation to give an ether). 

An important principle of synthesis is to avoid mixtures of isomers wherever possible minimizing separations increases recovery of products. Bimolecular dehydration is a random process. Heating a mixture of ethanol and methanol with acid will produce all possible combinations dimethyl ether, ethyl methyl ether, and diethyl ether. This mixture would be troublesome to separate. 

The unknown has molecular weight 134 this is double the weight of methyl cellosolve, minus 18 (water). The IR shows no OH, only ether C—O. The NMR shows no OH, only H—C—O in the ratio of 3 2 2. Apparently, two molecules of methyl cellosolve have combined in an acid-catalyzed, bimolecular dehydration. 

The dehydration of alcohols is one of the oldest catalytic reactions and studied over kaolin, a-alumina, silica, silica-alumina, aluminium phosphate, W2O5 etc. The mononmolecular dehydration of ethyl alcohol produces ethylene and the bimolecular dehydration due to condensation of ethyl alcohol produces diethyl ether. The reported [11] apparent activation energies for the dehydration of ethyl alcohol to ethylene and condensation to diethyl ether are 13.8 and 14.2 K cal/mole respectively. These values have been precisely measured by pulse technique in the temperature range 190 to 275 C. Because of the closeness of the apparent activation energies, attainment of cent percent selectivity for ethylene by inhibition of the condensation reaction poses a great selectivity problem. However, the users are very rigid about cent percent selectivity for ethylene at a reactor temperature of 350 C. 

Bimolecular dehydration can be used to synthesize symmetrical dialkyl ethers from simple, unhindered primary alcohols. This method is used for the industrial synthesis of... 

The least expensive method for synthesizing simple symmetrical ethers is the acid-catalyzed bimolecular dehydration, discussed in Section 11-IOB. Unimolecular dehydration (to give an alkene) competes with bimolecular dehydration. To form an ether, the alcohol must have an unhindered primary alkyl group, and the temperature must be kept low. If the alcohol is hindered or the temperature is too high, the delicate balance between substitution and elimination shifts in favor of elimination, and very little ether is formed. Bimolecular dehydration is used in industry to make symmetrical ethers from primary alcohols. Because the dehydration is so limited in its scope, it finds little use in the laboratory synthesis of ethers. 

If the conditions are carefully controlled, bimolecular dehydration is a cheap synthesis of diethyl ether. In fact, this is the industrial method used to produce millions of gallons of diethyl ether each year. 

Bimolecular dehydration of alcohols is generally a poor synthetic method. 

Explain why bimolecular dehydration is a poor method for making unsymmetrical ethers such as ethyl methyl ether. 

Bimolecular dehydration of alcohols industrial synthesis (Section 11-lOBand 14-7)... 

It is therefore reasonable to suggest that in the zeolite-catalyzed process too condensation proceeds via bimolecular dehydration of methyl alcohol to dimethyl ether,... 

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