Chemical processes of dehydration and dehydrogenation
What are the main characteristics of these processes?
In organic chemistry, reactions of dehydrogenation and dehydration are often used for obtaining unsaturated compounds – for example alkenes. To conduct these processes, substances of different structure and composition, conditions and catalysts, but in both reactions the same unsaturated products may be obtained.
Dehydration of organic compounds
Dehydration is the reaction of a water molecule separating from an organic molecule. Alcohols are often subjected to dehydration, as they have one or several OH groups. Reagents may assist in separating water from an alcohol molecule, thus forming an unsaturated compound. Under different conditions, dehydration may affect both bonds within a molecule and bonds between them. This is the basis for the classification of dehydration – it can be intermolecular and intramolecular accordingly. Products of these reactions are different.
So that dehydration does not affect the adjacent atoms of the alcohol, the temperature of the reaction must exceed 140 ᵒC (284 ᵒF). Concentrated sulfuric (phosphoric) acid is used as a dehydrating substance. The optimum temperature is 180 ᵒC (356 ᵒF).
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- CH₃-CH₂-CH₂-OH = CH₃-CH=CH₂ + H₂O (a double bond forms between carbon atoms, from which a hydroxyl group and hydrogen separate; propylene forms);
If the alcohol is secondary (the hydroxyl group is not located at the final carbon atom), the reaction may go in two directions:
CH₃-CH₂-CH(OH)-CH₃ = CH₃-CH=CH-CH₃ + H₂O (butene-2);
CH₃-CH₂-CH(OH)-CH₃ = CH₃-CH₂-CH=CH₂ + H₂O (butene-1).
The reaction takes place according to Zaitsev’s rule (in the dehydration of alcohols, hydrogen separates from the less hydrated carbon atom). In the center of the molecule, is carbon with two hydrogen atoms, and at the end with three; this means that separate to a large extent will take place from the group where there is less hydrogen – in the middle.
With low heating (below 284 ᵒF or 140 ᵒC) alcohols dehydrate with the formation of an ester:
СН₃-ОН + НО-СН₃ = СН₃-О-СН₃ + Н₂О.
Dehydration takes place as follows:
R-[ОН + Н]О-R = R-О-R + Н₂О (one hydroxyl group separates entirely, and only hydrogen from the second).
Nonsymmetrical esters can also be obtained:
С₃Н₇-ОН + НО-СН₃ = С₃Н₇-О-СН₃ + Н₂О (the reagents contain a mixture of alcohol).
Simultaneously, secondary symmetric ester products form:
С₃Н₇-ОН + НО-С₃Н₇ = С₃Н₇-О-С₃Н₇ + Н₂О;
СН₃-ОН + НО-СН₃ = СН₃-О-СН₃ + Н₂О.
Primary alcohols enter into these reactions more often. Tertiary and secondary alcohols do not form esters, but turn into alkenes by the mechanism of intramolecular dehydration.
The dehydrogenation reaction is the process of an even number of hydrogen atoms separating from a molecule of an organic compound. This reaction does not usually take place without a catalyst. Products of dehydrogenation may be alkenes, alkines, dienes – various unsaturated compounds. In the dehydrogenation of lower alkanes (their carbon chain is not longer than C₂-C₄), the molecule remains acyclic. Closing into the cycle takes place for alkanes with a carbon chain longer than 4 carbon atoms.
In the dehydrogenation process, a break of the C-H bond takes place, hydrogen atoms in adjacent carbon atoms move away and form molecular hydrogen, and in their place a double bond closes in the molecule. In dehydrogenation of hydrocarbons (for example butane of normal structure), a mixture of isomers may form (substance with different structure or position of atoms in space, but with an identical atomic composition and weight). 2 isomers of the product are obtained – butene-1 and butene-2.
2CH₃-CH₂-CH₂-CH₃ = CH₃-CH=CH-CH₃ + CH₂=CH-CH₂-CH₃ + 2Н₂ (in the presence of a nickel catalyst at a temperature of 500 ᵒC (932 ᵒF)).
At a temperature of 450-650 ᵒC (842-1202 ᵒF) on an alumo-chrome catalyst 2 molecules of water separate and the alkadiene butadiene-1,3 forms:
CH₃-CH₂-CH₂-CH₃ = CH₂=CH-CH=CH₂ + 2H₂.
Dehydration and dehydrogenation are reactions that are often used in practice in organic synthesis. It is important when carrying them out to take into account that often it is not a pure substance that forms as a product, but a mixture of isomeric substances – if a certain substance must be obtained in pure form, it must be separated from the isomers and side products of reaction.