Primary alcohols undergo bimolecular elimination E2 mechanism while secondary and tertiary alcohols undergo unimolecular elimination E1 mechanism. The relative reactivity of alcohols in dehydration reactions is ranked as follows:. Primary alcohols dehydrate through the E2 mechanism. The hydroxyl oxygen donates two electrons to a proton from sulfuric acid H 2 SO 4 , forming an alkyloxonium ion.
Then the conjugate base, HSO 4 — , reacts with one of the adjacent beta hydrogen atoms while the alkyloxonium ion leaves in a concerted process, forming a double bond. Secondary and tertiary alcohols dehydrate through the E1 mechanism.
Similarly to the reaction above, secondary and tertiary —OH protonate to form alkyloxonium ions. However, in this case the ion leaves first and forms a carbocation as the reaction intermediate. The water molecule which is a stronger base than the HSO 4 - ion then abstracts a proton from an adjacent carbon to form a double bond. Notice in the mechanism below that the alkene formed depends on which proton is abstracted: the red arrows show formation of the more substituted 2-butene, while the blue arrows show formation of the less substituted 1-butene.
Recall that according to Zaitsev's Rule, the more substituted alkenes are formed preferentially because they are more stable than less substituted alkenes.
Additinally, trans alkenes are more stable than cis alkenes and are also the major product formed. For the example below, the trans diastereomer of the 2-butene product is most abundant. The dehydration mechanism for a tertiary alcohol is analogous to that shown above for a secondary alcohol.
Examples of these and related reactions are given in the following figure. The predominance of the non-Zaitsev product less substituted double bond is presumed due to steric hindrance of the methylene group hydrogen atoms, which interferes with the approach of base at that site.
The first uses the single step POCl3 method, which works well in this case because SN2 substitution is retarded by steric hindrance. The second method is another example in which an intermediate sulfonate ester confers halogen-like reactivity on an alcohol.
An E2 reaction takes place, in which a proton is lost from carbon at the same time as water is lost from the neighboring carbon. This allows for the formation of an alkene without any in-between formation of an unstable carbocation.
A Protonated Primary Alcohol Alkene. Dehydration is mainly easy when a neighboring double bond is formed.
The carbonyl group plays 2 vital roles, helps in stabilizing the transitional carbanion and it gives additional driving force for elimination in giving improved stability to the neighboring product. Alcohol Dehydration Mechanism. The mechanism of dehydration may vary from alcohol to alcohol even when the same catalyst is being used.
The dehydrogenation of alcohol accompanied the dehydration of alcohol over some basic oxides. In the E2 mechanism, there is a different kind of selectivity, anti and syn elimination. The syn elimination products are made when the groups are eliminated from the same side of the molecule and anti-elimination products are made when the groups are removed from the opposite side.
The dehydration of an alcohol is catalyzed with the help of boron phosphorus oxide. The catalytic activities of the oxides are made up of a different number of Boron and phosphorus for propanol dehydration which shows a relation with the total amount of acid sites.
Butanol goes through dehydration on boron phosphorous oxide. The activity shows a relation with the total of Lewis and Bronsted acid sites and in all of these reactions, the carbonium ion mechanism is in service. In the E1cB mechanism, the initial step of dehydration is the formation of carbanion, which means that a C-H bond is broken in the first step.
The starting step of dehydration is the formation of a carbonium ion by abstraction of an OH group. This mechanism takes place with strongly acidic catalysts like aluminosilicate.
E2 mechanism includes the elimination of a proton and hydroxyl group from alcohol which is concerted without formation of ionic intermediate. Alumina is a basic E2 oxide. The 3 mechanisms can be differentiated in various ways but unlike the liquid phase reactions, the kinetic method cannot be used. With the E1 mechanism, the isomerization occurs in carbonium ion stage. The formation of 2 butenes from 1 butanol depicts the E1 mechanism. High selectivity for 1 butene from butaneol depicts the E1B mechanism.
Since the rate of skeletal isomerization of isobutene to n-butene is comparatively lower than the rate of formation of n-butene in dehydration, the n-butene is a primary product. This shows the reaction goes through the E1 mechanism.
The formation of n-butene is related with the formation of the least stable isopropyl carbonium ions which are already rearranged by hydride or methyl transfer to from more stable tertiary or secondary carbonium ions. The dehydration reaction of alcohols to generate alkene proceeds by heating the alcohols in the presence of a strong acid, such as sulfuric or phosphoric acid, at high temperatures.
The required range of reaction temperature decreases with increasing substitution of the hydroxy-containing carbon:. If the reaction is not sufficiently heated, the alcohols do not dehydrate to form alkenes, but react with one another to form ethers e.
Alcohols are amphoteric; they can act both as acid or base. The lone pair of electrons on oxygen atom makes the —OH group weakly basic. Oxygen can donate two electrons to an electron-deficient proton. This basic characteristic of alcohol is essential for its dehydration reaction with an acid to form alkenes. Different types of alcohols may dehydrate through a slightly different mechanism pathway.
This ion acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid the nucleophile then attacks the hydrogen adjacent to the carbocation and form a double bond. Primary alcohols undergo bimolecular elimination E2 mechanism while secondary and tertiary alcohols undergo unimolecular elimination E1 mechanism. The relative reactivity of alcohols in dehydration reaction is ranked as the following. Oxygen donates two electrons to a proton from sulfuric acid H 2 SO 4 , forming an alkyloxonium ion.
Then the nucleophile HSO 4 — back-side attacks one adjacent hydrogen and the alkyloxonium ion leaves in a concerted process, making a double bond. Similarly to the reaction above, secondary and tertiary —OH protonate to form alkyloxonium ions. However, in this case the ion leaves first and forms a carbocation as the reaction intermediate. The water molecule which is a stronger base than the HSO 4 - ion then abstracts a proton from an adjacent carbon, forming a double bond.
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