Classification, Naming, and preparation of Alcohols

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Alcohols

 The compounds in which hydroxyl groups are attached to an alkyl group are called alcohols.

Classification

         On basis of the number of hydroxyl groups, these are classified as:

Monohydric Alcohols

These are containing one hydroxyl group in their molecules called monohydric alcohols.

Polyhydric Alcohols

        Alcohols containing more than one hydroxyl group in their molecules are called polyhydric alcohols.

  • Dihydric Alcohols: They containing two hydroxyl groups in their molecules are called dihydric R-OH.
  • Trihydric Alcohols: they containing three hydroxyl groups in their molecules are called trihydric R-OH.

alcohols

Classification of Monohydric Alcohols

  • Primary Alcohols

These in which carbon attached to the hydroxyl group is further directly linked with only one carbon atom are called primary alcohols.

  • Secondary Alcohols

             These in which carbon attached to the hydroxyl group is further directly linked with two carbon atoms are called secondary R-OH.

  • Tertiary Alcohols

            Alcohols in which carbon attached to the hydroxyl group is further directly linked with three carbon atoms are called tertiary R-OH.

Naming

     Common or Trivial System: In this system, alcohols are named by naming the alkyl group attached to the hydroxyl group and adding alcohol as a separate word.

           The position of the other substituents in the alkyl groups is indicated by the Greek letters (alpha), (beta), (gamma), and (delta). The carbon atom bearing the hydroxyl group is named as, next to carbon atoms named as, and so on respectively.

Amyl alcohol

ROH, C5H11OH containing five carbons is commonly called amyl alcohol and end carbon is generally named as (omega).

        IUPAC System:

  1. Select the longest chain containing the hydroxyl group as the parent alkanes.
  2. Change the ‘e’ of the corresponding alkane to ‘ol’. Thus the these are named alkanols.
  3. Number the carbon atoms in such a way so that the hydroxyl group gets the smallest possible number.
  4. The position of the hydroxyl group is indicated by the number of carbon to which it is attached, and is written before the its name.
  5. Other substituents are numbered, named, and placed as prefixes in alphabetical order.
  1. In unsaturated, hydroxyl groups are given lower numbers rather than points of unsaturation.

When the hydroxyl group is not a principal functional group, it is named substituent hydroxy.             

                         
Diols and Triols have both common and IUPAC names. The IUPAC names are obtained by adding the suffix diols or triols to the name of the parent alkanes containing two or three hydroxyl groups respectively. The position of the hydroxyl group is indicated by the numbers.

Industrial Preparation 

Manufacture of Methanol: At one time, most methanol was produced by the destructive distillation of wood and therefore, was called “wood alcohols”. Today most methanol is manufactured from methane obtained from natural gas. Methane gas is passed with steam over Ni under pressure and at a temperature of 900˚C

An equimolar mixture of CO and H2 is called water gas. This mixture is passed over heated zinc and chromium oxide at 450-500˚C under 200 atm.

Manufacture of Ethanol: Ethanol is the alcohol of wine, beer, whiskey, and similar beverages and can be prepared by the fermentation of sugars in the presence of yeast. Fermentation is a biochemical process that occurs in the presence of certain enzymes secreted by micro-organism such as yeast. The optimum temperature for this process is 25-35˚C. Proper aeration, dilution of the solution, and absence of any preservative are essential conditions for this fermentation.

Collection of Ethanol: An alcohol obtained by fermentation is only up to 12%. It never exceeds 14% because above this enzyme becomes inactive. This crude thus obtained is distilled again and again to obtain 95% ethanol which is called ‘rectified spirit’.To obtain 100% pure ethanol, called absolute alcohol, it is mixed with benzene.

Denaturing of Alcohol: Sometimes ethanol is denatured by the addition of 10% methanol to make it unfit for drinking. Such alcohol is called Methylated spirit. A small quantity of pyridine or acetone may also be added for this purpose.

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