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The conversion of ethanol to ethene and water involves an elimination reaction, often referred to as dehydration. This process requires heating ethanol to about 180°C in the presence of concentrated sulphuric or phosphoric acid, or alternatively by passing ethanol vapor over alumina at 350°C. The mechanism starts with a covalent bond formation between a hydroxyl oxygen and a hydrogen ion, leading to the creation of a carbocation and the elimination of water. This reaction yields ethene and water, with the acids acting as catalysts, remaining unchanged at the end.
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The reaction is also called dehydration because water is eliminated from the ethanol.
The ethanol must be heated to about 180 °C with concentrated sulphuric acid or phosphoric acid.
Alternatively, ethanol vapour can be passed over alumina, Al2O3, at 350 °C.
Although the reaction looks simple, it happens in more than one step.
This is the start of the mechanism for the elimination of water from ethanol using concentrated sulphuric acid.
A covalent bond begins to form between the oxygen atom in the hydroxyl group and a hydrogen atom in a sulphuric acid.
The bond between the hydrogen atom and the rest of the sulphuric acid molecule begins to break.
The pair of electrons in the C-O bond is attracted to the positive charge and the bond begins to break.
The remainder of the ethanol molecule is now a positively charged ion, called a carbocation.
A bond begins to form between the hydrogensulphate ion and one of the hydrogen atoms.
Notice that, overall, sulphuric acid has not been used up in the reaction so it behaves as a catalyst.
This is the start of the simplified mechanism for the elimination of water from ethanol.
A covalent bond begins to form between the oxygen atom in the hydroxyl group and a hydrogen ion.
The pair of electrons in the C-O bond is attracted to the positive charge and the bond begins to break.
The remainder of the ethanol molecule is now a positively charged ion, called a carbocation.
Notice that, overall, the hydrogen ion has not been used up in the reaction so it behaves as a catalyst.
If it is heated to 180 °C with concentrated sulphuric acid, two alkenes form.
In the first stage of the reaction mechanism a protonated alcohol forms.
There are two hydrogen atoms next to the positively charged carbon atom in the carbocation.
