Dehydration of Alcohol

Dehydration of Alcohol Mechanism

When alcohol reacts with protic acids it tends to lose a molecule of water in order to form alkenes. These reactions are generally known as dehydration of alcohols. It is a basic example of an elimination reaction. The rates differ for the primary, secondary and tertiary alcohols. The carbonation is very much stable in the case of tertiary alcohols; hence the rate of dehydration is highest for tertiary alcohols as compared to secondary and primary alcohols. Dehydration can be performed in a 3 step mechanism:

  • 1. Formation of protonated alcohol

  • 2. Formation of carbonation

  • 3. Formation of alkenes

  • Mechanism of Dehydration of Alcohols:


    Dehydration of alcohols follows E1 or E2 mechanism. The primary alcohols, elimination reactions follow the E2 mechanism whereas the secondary and tertiary alcohols elimination reaction follows the E1 mechanism. Basically, it follows a 3 step mechanism. The steps that are involved are explained below:

    1.       Formation of protonated alcohol:
     
    In this particular step, the alcohol is reacted upon by a protic acid. Due to the single pair present on the oxygen atom, it acts as a Lewis base. This step is easily and quickly reversible.

    2.       Carbocation formation:

    In this step, the C-O bond breaks which generate a carbocation. This is the slowest step in the mechanism of dehydration of an alcohol. Therefore, the formation of carbocation is said to be the rate determining step.

    3.       Alkene formation:

    This is the final step in the dehydration of alcohols. In this step, the proton generated is eliminated with the help of a base. The carbon atom neighboring to the carbocation breaks the present C-H bond to form C=C. Therefore, an alkene is formed.


    Dehydration is generally a condition when a person loses a lot of water by sweating or less intake of water, but such type of dehydration is different than dehydration that we see in chemical reactions.

    Alcohol Dehydration Reaction


    Dehydration reaction is a type of chemical reaction wherein water is formed from the extraction of the components of water from a single reactant. An alkene is produced when dehydration of an alcohol is performed.
     A basic structural equation for alcohol dehydration is as follows:

    C2H5OH →→ C2H+ H2O
     
    Alcohol dehydration is an example of an elimination reaction which is quite the opposite of substitution reaction and addition reaction.

    An elimination reaction is a type of reaction wherein 2 groups or 2 atoms on neighboring carbon atoms are eliminated or removed from a molecule which leaves multiple bonds between the carbon atoms.
     
    Dehydration of secondary and tertiary alcohols in acidic conditions follows the E1 method. The protonation of the hydroxyl group successfully converts the leaving group from hydroxide ion to water. The hydronium H3Ois way stronger than H2O, the conjugate surface of the former H2O is a better leaving group than that of a latter OH.
     
    When a relatively stable carbocation is produced by dehydration of a protonated alcohol an E1 elimination can take place.



     
     
     Because an unstable primary carbocation would be structured in the E1 dehydration on primary alcohol, acid catalyzed E1 elimination through such a carbocation is so slow that different pathways are followed.
     
    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. Alcohol that bears a carbonyl group two carbons away readily goes through dehydration and this finally yields α, β- unsaturated carbonyl compound.




     

    The location of the carbonyl group to the hydroxyl group in β hydroxyl carbonyl compounds opens the way for elimination under general conditions by the E1cB mechanism.
     
    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.
     
    The base-catalyzed loss of water from β hydroxy carbonyl compounds is one of the examples of elimination reaction which involves a sphybridized carbon atom that follows the E1cB pathway.

    Alcohol Dehydration Mechanism


     The mechanism of dehydration may vary from alcohol to alcohol even when the same catalyst is being used.
    The dehydration of alcohol series done by Thomke over BPO4, Ca3(PO4)2 and Sm2O3 determined the mechanism by 2 precise criteria, uptake of deuterium from deuterated catalysts into produced olefin and un-reacted alcohol. 

    The alcohols presented E1 on BPO4, E2 on Ca3(PO4)2 and E1cB on Sm2O3.

    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 reactivity of alcohol in the dehydration decreases in the order of: Tert- amyl alcohol> 3 pentanol > 2 propanol> 1 pentanol> ethanol.

    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 butane-2-ol depicts the E1B mechanism.  The dehydration of isobutyl alcohol over SiO2–Al2O3 yields a combination of butene in which the fractions of n-butane is around 33%. 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.

    Secondary Alcohol Dehydration
     
    Dehydration of alcohol requires a cleavage of a C-O bond with loss of a proton from the beta position. The result of dehydration is either an alkene or a mixture of the alkenes and the order of dehydration is first tertiary, then secondary, and finally primary.



    Tertiary Alcohol Dehydration
     
    Tertiary forms of alcohol are easiest to dehydrate as the carbocations are more stable and thus easier to form compared to primary and secondary carbocations
     
    For dehydration to take place, the alcohol must be heated to roughly 50 0C in 5% H2SO4. Secondary alcohol needs about 100 0C in 75% H2SO4 and primary can only be dehydrated at 170 0C in 95% H2SO4 which are under extreme conditions. The dehydration process of both secondary and tertiary alcohols involves the formation of a product called the carbocation intermediate. Here the carbocation gets rearranged if the result is a more stable carbocation.
     
    Dehydration of Primary Secondary and Tertiary Alcohols
     
    Alcohol and ethers possess leaving groups which are stronger Lewis bases than halide ions (is a halogen atom which has a negative charge). This makes alcohols and ethers less reactive than the alkyl halides (compounds where one or more hydrogen atoms in an alkane get replaced by halogen atoms). They need to be protonated before undergoing an elimination or substitution reaction.
     
    The primary, secondary, and tertiary alcohol undergo a process called the nucleophilic substitution reactions (in chemistry a nucleophilic substitution is class of reactions where the nucleophile bonds with or attacks the positive charge of an atom or a group of atoms to substitute a leaving group) with HI, HBr, and HCl to form alkyl halides.  Secondary alcohol gets oxidized to ketones and primary are oxidized to carboxylic acids by the chromic acid.
     
    They are categorized as SN2 reactions in primary alcohols and SN 1reactions in secondary as well as tertiary alcohols. Tertiary alcohols tend to be easier to dehydrate and primary alcohols to be the hardest.
     
    The dehydration of either a tertiary or secondary alcohol is known as an E1 reaction (two-step mechanism), the dehydration of primary alcohol is an E2 (one step mechanism) reaction because of the difficulty encountered in forming primary carbocations.