Organic Chemistry - Part VIII

Methanol
The first of the homologous series of alcohols is methanol (CH₃OH). This is the simplest of all alcohols. When hydrogen from methane (CH₄), the first member of the homologous series of alkanes, is replaced by one –OH or hydroxyl group, methanol is formed. Traditionally methanol was made from destructive distillation of wood and was therefore known as wood alcohol. Methanol is also known as methyl alcohol.  

Manufacture of methanol
Mixture of carbon monoxide (CO) and hydrogen gas (H₂) in the proportion of 1 : 1 is used as a starting material for the preparation of methanol. This is known as water gas. Passing steam over red heated coke powder produces water gas. The carbon in the coke reacts with H₂O  in from steam to become CO and H₂ is released. This mixture of CO and H₂ is mixed with double the volume of pure H₂ gas and the entire mixture is pressurized to 300 atmospheres. When this compressed mixture is forced over heated (up to 300°C) zinc oxide-chromium oxide catalyst, methanol is produced. The equations below show the process.  

Properties of methanol
1. Methanol is a colourless liquid at room temperature.
2. Methanol has a slight fruity odour.
3. The boiling point of methanol is 64.5°C.
4. Methanol is completely miscible in water.
5. The density of methanol is less than water.
6. Methanol is a neutral solution and shows negative results for all acid tests.
7. Methanol is poisonous and if ingested can cause blindness.
8. Methanol is very flammable and burns with a pale blue flame. It forms carbon dioxide and water on complete oxidation or combustion.
9. Methanol reacts with Na to give off hydrogen gas.
10. Methanol reacts with ethanoic acid to give methyl ethanoate which is an ester.  

Ethanol
Ethanol or ethyl alcohol is the most common of all alcohols. Colloquially when one uses the word alcohol, it is generally meant to be ethanol. When one replaces hydrogen from ethane (C₂H₆), the second member of the homologous series of alkanes, by –OH or hydroxyl group, ethanol (C₂H₅OH) is  formed.  

Manufacture of ethanol
Ethanol is made by fermentation of sugar or by hydration of ethene.

Ethanol preparation from fermentation of sugar
Fermentation is a process whereby an organic compound is broken into smaller molecules by enzymes such as yeast. Ethanol is prepared by fermentation of molasses. Molasses, a by product of sugar industry, has traces of sugar, which gets broken down into ethanol. The fermentation is initiated by yeast, which secretes two enzymes called invertase and zymase. These enzymes act as catalyst for converting sugar into ethanol. The reaction is exothermic and carbon dioxide is released in the process. The reaction is shown as:

Molasses is heated to about 20 to 30°C in an airtight container. Yeast is mixed in the molasses. Presence of air will tend to oxidize the mixture whose resultant is vinegar (acetic acid or ethanoic acid) CH₃COOH. This is undesirable. The airtight container has outlets for removal of carbon dioxide released during the fermentation process. Ethanol and water vapours that are formed are collected and separated by fractional distillation.

Ethanol can also be made by fermentation of starchy materials such as barley, potatoes and maize. The starch is first converted to sugar by fermentation and then to ethanol.

Ethanol preparation by hydration of ethene
The molecular formula of ethene is C₂H₄. An addition of a water molecule will make ethene into ethanol (C₂H₅OH). Addition of water to a molecule and its assimilation into the molecular formula is known as hydration. Ethene obtained from cracking of petroleum along with water is pressurized to 60 atmosphere and passed over heated (300°C) phosphoric acid (H₃PO₄). Ethanol is directly produced in this reaction. The double bonded ethene molecule breaks its pi bond to accommodate one H and one OH group. The phosphoric acid is a catalyst. The reaction is shown below.  

Properties of ethanol
1. Ethanol is a colourless liquid at room temperature.
2. Ethanol has a slight fruity odour.
3. The boiling point of ethanol is 78°C
4. Ethanol is completely miscible in water.
5. The density of ethanol is less than water.
6. Ethanol is a neutral solution and shows negative results for all acid tests.
7. Ethanol is very flammable and burns with a pale blue flame. It forms carbon dioxide and water on complete oxidation or combustion. Incomplete oxidation gives rise to ethanoic acid and water.

8. Ethanol reacts with Na to give off hydrogen gas.

9. Ethanol reacts with ethanoic acid to give ethyl ethanoate which is an ester.  

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