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The synthesis of an alkene can be done by dehydration of alcohol. Water is eliminated to form a double bond in the process of dehydration of alcohol by using E1 and E2 mechanisms. The alkene is produced when the alcohol reacts with the protic acid, leading to the loss of water molecules. This is the best example of an elimination reaction.
The relative ease of dehydration of alcohol is different for primary, secondary and tertiary alcohols. In the case of tertiary alcohols, the carbonation is very stable. Therefore, the dehydration rate is the highest for it compared to secondary and primary alcohols.
Tertiary > Secondary > Primary
The temperature required for the reaction to occur decreases with the increase in the substitution of the -OH containing carbon:
Primary alcohols: 170°–180°C
Secondary alcohols: 100°–140 °C
Tertiary alcohols: 25°–80°C
If the reaction does not occur at the required temperature, it will not get dehydrated, and it will lead to the formation of ethers by reacting with one another instead of forming an alkene.
Alcohols are amphoteric in nature—they act as both acid and base. The -OH group is weakly basic because of the lone pair of electrons present in the oxygen. To an electron-deficient proton, oxygen can donate two electrons. Therefore, R—OH acts as a base and is protonated into the acidic alkyloxonium ion +OH2 in the presence of a strong acid. For dehydration reaction with an acid to form alkenes, this kind of basic characteristic of alcohol is essential.
Reaction to alcohol dehydration
A dehydration process is a chemical reaction that produces water by extracting its elements from a single source. The production of alkene occurs when alcohol is dehydrated.
The following is a simple structural equation for alcohol dehydration:
C2H5OH → C2H4 +H2O
Elimination reactions in the mechanism of dehydration, which are opposite to substitution and addition processes, include dehydration of alcohol.
Two groups or two atoms on neighbouring carbon atoms are eliminated or removed from the molecule in an elimination process, leaving numerous bonds between the carbon atoms.
The E1 technique dehydrates secondary and tertiary alcohols in acidic conditions. The leaving group from the hydroxide ion is easily removed by protonation of the hydroxyl group and replaced with water. The hydronium ion H3O+ is more stable than H2O, and its conjugate surface is a better leaving group than an OH.
An E1 elimination may occur when a protonated alcohol is dehydrated to produce a highly stable carbocation.
Acid-catalysed E1 elimination by such a carbocation is exceedingly slow due to the instability of the main carbocation formed during E1 dehydration on primary alcohol and may require additional procedures.
In an E2 reaction, a proton is lost from carbon, while water is lost from the surrounding carbon. This prevents the production of an unstable carbocation and allows the creation of an alkene.
A protonated primary alcohol
When an adjacent double bond is established, dehydration is relatively simple. The E1cB mechanism allows hydroxyl carbonyl compounds to be eliminated because the carbonyl group is near the hydroxyl group.
Mechanism of alcohol dehydration
Dehydration techniques differ across alcohols, even when the same catalyst is employed. There are two types of selectivity in the E2 mechanism — anti and syn elimination. Syn elimination products are made when groups are removed from the same side of the molecule, whereas anti-elimination products are formed when groups are removed from the opposite side.
The presence of boron phosphorus oxide catalyses the dehydration of alcohol. Under dehydration, alcohol’s reactivity diminishes in the following order — Superior → 3 pentanols superior → 2 propanols superior → 1 pentanol superior → ethanol Tert-amyl alcohol, which is superior → 3 pentanols superior → 2 propanols superior → 1 pentanol superior → ethanol.
The catalytic activity of the oxides for propanol dehydration is made up of a particular quantity of boron and phosphorus, proportional to the total number of acid sites.
Butanol is dehydrated using boron phosphorus oxide. All of these reactions employ the carbonium ion mechanism, and their activity is proportional to the total number of Lewis and Bronsted acid sites.
The creation of carbanion is the first step in the E1cB dehydration process, which requires breaking a C-H bond. The production of a carbonium ion by the abstraction of an OH group is the first step in dehydration.
The E2 method involves removing a proton and a hydroxyl group from alcohol simultaneously without forming an ionic intermediate. Alumina is a particular kind of E2 oxide. While the three processes may be separated in various ways, they cannot be recognised using the kinetic method, unlike liquid-phase reactions. At the carbonium ion stage, isomerisation happens through the E1 process.
Secondary Dehydration by Alcohol
The breaking of a C-O bond and removal of a proton from the beta position are required for the dehydration of alcohol. Dehydration produces a single alkene or a mixture of alkenes and occurs in the following order — tertiary, secondary, and primary dehydration.
Alcohol-Induced Dehydration at the Tertiary Level
Since tertiary alcohols’ carbocations are more stable and simpler to synthesise than primary and secondary carbocations, they are the simplest to dehydrate.
For the mechanism of dehydration, tertiary alcohol must be heated to about 500° C in 5% H2SO4. Secondary alcohol can be dehydrated at about 1000° C in 75% H2SO4, but primary alcohol can only be dehydrated at 1700C in 95% H2SO4, both at very high temperatures. A molecule known as the carbocation intermediate is generated during the dehydration of secondary and tertiary alcohols. This phase reorganises the carbocation if the outcome is a more stable carbocation.
Mechanism of dehydration of ethanol to ethene
Generally, dehydration of alcohol takes place in the 3 steps mentioned below:
Formation of protonated alcohol: This step is reversible, where ethanol reacts with protic acid. As there are two lone pairs of electrons present in oxygen, it acts as a Lewis base.
Formation of carbocation: In the mechanism of dehydration of alcohol, it is the slowest step, and hence it is also known as the rate-determining step. In this step, the carbocation is formed on the breaking of the C-O bond. This C-O bond is weakened by the presence of a positive charge on a highly negative oxygen atom. The protonated ethanol, therefore, eliminates water molecules to form the carbocation of ethyl.
Ethene formation: In dehydration of alcohol, it is the last step where the proton formed is removed. The C-H bond by the carbon atom near carbocation leads to the formation of ethene (alkene, i.e., C=C).
The acid used in the 1st step is released in the 3rd step. Ethene is removed as it is formed to drive the equilibrium to the right.
Conclusion
A dehydration reaction, also known as Zimmer’s Hydrogenesis in chemistry, is a chemical process in which the reacting molecule or ion loses water. Condensation reactions of this type are the most common. Dehydration reactions, on the other hand, are widespread processes that are the inverse of hydration reactions. Sulfuric acid and alumina are two common dehydration agents used in organic synthesis. Frequent heating affects dehydration reactions.
E1 and E2 processes are used to dehydrate alcohol. The E1 mechanism is used by secondary and tertiary alcohols, whereas the E2 mechanism is used by primary alcohols.
Dehydration synthesis is a process that helps serve as a chemical foundation for the construction of larger macromolecules, in addition to connecting molecules and producing new products like alcohols and ethers. Another application of dehydration reaction is in food processing. Food processing is a method of preserving a variety of foods for an infinite period of time by removing moisture and limiting the growth of microbes. Prehistoric people employed dehydration to sun-dry seeds, which is one of the oldest ways of food preservation.
