7.6. Ethers

1. By dehydration of alcohols Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K. At 413 K, ethoxyethane is the main product.

The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below:

Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether. The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows SN1 pathway when the alcohol is secondary or tertiary about which you will learn in higher classes. However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.

This chunk discusses two methods for preparing ethers: dehydration of alcohols and the conditions needed for successful ether formation. When alcohols are subjected to dehydration (removal of water) in the presence of acids like sulfuric acid, they can convert to ethers under specific conditions. For example, ethanol can become ethoxyethane if the temperature is controlled properly. If the temperature is too high or if secondary or tertiary alcohols are used, the reaction may favor the formation of alkenes instead of ethers. This is important in synthetic chemistry as it shows how conditions influence the products formed during chemical reactions.

Think of making a cocktail: if you follow the recipe precisely (correct temperature and mixing time), you end up with a delicious drink (ether). However, if you alter the ingredients (like using a high temperature or wrong kind of drink), you might end up with a completely different beverage (alkene). Just like in cooking, following the correct procedure and conditions in chemistry is crucial for achieving the desired results.

2. Williamson synthesis It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide. R–X + R’–O Na → R–O–R’ + NaX

Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method. The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.

The Williamson synthesis is a key reaction in organic chemistry for producing ethers, whether symmetrical (having the same alkyl groups on both sides of the ether bond) or unsymmetrical (different alkyl groups). This method utilizes an alkyl halide with a sodium alkoxide. In this reaction, the alkoxide ion, which is a strong nucleophile, attacks the primary alkyl halide, leading to the formation of the ether. It’s worth noting that for best results, the alkyl halide should be primary to avoid competing elimination reactions, which could yield alkenes instead.

Imagine you're building a new house (the ether) and need to connect two pieces of land (the alkyl groups) together with a sturdy bridge (the ether bond). You use quality materials (sodium alkoxide) and make sure the land (alkyl halide) you are attaching to is solid and suitable for building (primary, unbranched). If you try to attach to unstable or complex ground (a tertiary alkyl halide), your bridge may collapse (result in an unwanted alkene). Just like ensuring a strong foundation is essential in construction, using the right conditions and materials in chemistry guarantees successful chemical reactions.

7.6.2 Physical Properties The C-O bonds in ethers are polar and thus, ethers have a net dipole moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases:

The large difference in boiling points of alcohols and ethers is due to the presence of hydrogen bonding in alcohols. The miscibility of ethers with water resembles those of alcohols of the same molecular mass. Both ethoxyethane and butan-1-ol are miscible to almost the same extent i.e., 7.5 and 9 g per 100 mL water, respectively while pentane is essentially immiscible with water.

This chunk discusses the physical properties of ethers, particularly how polarity affects their characteristics, like boiling points and solubility in water. Ethers have C-O bonds that are polar, which means they can interact with other polar substances, but their polarity is not as strong as that in alcohols. As a result, ethers typically have lower boiling points than their corresponding alcohols due to the absence of strong hydrogen bonds which are present in alcohols and contribute to their higher boiling points. Furthermore, ethers can still dissolve in water like alcohols, but their solubility is adjusted based on their molecular weight; both are similar in this regard, making them somewhat comparable in their behavior in aqueous environments.

Think of ethers as a light sweater compared to alcohols as a heavy winter coat. Both can keep you warm (are miscible with water), but the heavy coat (alcohol) provides much more insulation (higher boiling point) due to its thickness (hydrogen bonds). If you were to try to fit both types of clothing into a suitcase of limited space, the heavy coat would take up more room and weigh more than the light sweater (the water doesn't mix as well with hydrocarbons).

7.6.3 Chemical Reactions

1. Cleavage of C–O bond in ethers Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules.

Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide.

Ethers with two different alkyl groups are also cleaved in the same manner. The order of reactivity of hydrogen halides is as follows: HI > HBr > HCl. The cleavage of ethers takes place with concentrated HI or HBr at high temperature.

Here, we explore the chemical reactions of ethers, particularly how they can be cleaved or broken down. Ethers are generally stable; however, when treated with strong acids like hydrogen halides, especially in high concentrations and temperatures, they can undergo cleavage at their C-O bonds. This reaction primarily produces alkyl halides. The ability for different types of ether (alkyl versus aryl) to cleave differently is highlighted; for example, alkyl-oxygen bonds tend to be weaker than aryl-oxygen bonds, which influences the products formed during cleavage reactions. Reactivity varies by the type of hydrogen halide used, highlighting that HI is the most effective for breaking these bonds.

Think of ethers as robust bridges over a river. Usually, they stand strong (least reactive), but during a storm (the addition of strong acids), they may collapse (cleavage) depending on the conditions. Storms with more intense winds (HI versus HCl) tend to damage the bridges more effectively, causing them to fall apart and may yield various materials (alkyl halides and phenols) depending on how they were constructed (the structure of the ether). Just like in construction, understanding the stability and weak points can help in effectively managing and utilizing the structures.

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