Test for Hydroxyl Group

Theoretical Background / Context

• Aliphatic alcohols are non-aromatic hydrocarbons possessing at least one alcohol hydroxyl group within their structure. 
• They can be either cyclic or acyclic compounds. Alcohols are considered to be neutral compounds.

   Aliphatic Cyclic Alcohol             Aliphatic Acyclic Alcohol

• Alcohols are also classified as primary, secondary, and tertiary alcohols according to the number of carbon atoms attached to the carbon atom linked to the hydroxyl group.

First: Preparation of Aliphatic Alcohols: 

• Alcohols can be prepared through some chemical routes such as reduction of the corresponding aldehydes and ketones using some reducing agents such as lithium aluminum hydride or sodium borohydride. 
• They can be also obtained from the hydration of the corresponding alkenes. 
• Some primary alcohols can be synthesized through the nucleophilic substitution of corresponding alkyl halides using potassium or sodium hydroxides.
• Alcohols can be also obtained through some biological routes such as ethanol and butanol through fermentation processes in presence of glucose. 
• Glucose is obtained from starch hydrolysis in the presence of yeast.

Second: General Properties of Aliphatic Alcohols: 

• Aliphatic alcohols are polar compounds and can form hydrogen bonds easily due to hydroxyl group chemistry.
• Simple alcohols such as methanol, ethanol, and propanol are highly miscible in water. Longer ones such as butanol are moderately miscible with water. 
• In general, alcohols are more soluble than their corresponding alkanes in water due to the formation of hydrogen bonds with water molecules.
• Alcohols have higher boiling points than that of their corresponding alkanes or ethers owing to the ability of alcohols to form hydrogen bonds.

Third: Occurrence of Aliphatic Alcohols in Nature: 

• Aliphatic alcohols such as ethanol are dominant in nature as a byproduct of fermentation processes.

Forth: Uses of Aliphatic Alcohols

• Many aliphatic alcohols have been used in different industrial and pharmaceutical applications. For instance, methanol is used as fuel and in production of formaldehyde.
• Ethanol is also used in beverages industries, and as a solvent in the preparation of some pharmaceutical products such as elixirs. 
• In addition, it has been used as fuel. Ethanol is also used as a sanitizer and antibacterial in gels and wipes.
• Longer alcohols and fatty alcohols have been used in the industries of plasticizers and detergents.

Principle of Work

• In the test for hydroxyl group, aliphatic alcohols are tested by detecting their hydroxyl groups using chemical tests. 
• In addition, these tests can be used to differentiate between alcohols and other organic classes, especially those containing hydroxyl groups. 
• This is done by using specific reagents.

Acetyl Chloride Test: 

• In this test for OH group, acetyl chloride is added to a sample containing alcohol. 
• The acetyl chloride reacts with the alcohol producing esters and hydrochloric gas. 
• The hydrochloric acid gas is then detected using ammonium hydroxide where white fumes of ammonium chloride are formed as shown in the following reactions. 
• Ammonia gas then can be detected using hydrochloric acid as they react together forming white fumes of ammonium chloride as shown in the following chemical reactions.

C2H5OH + CH3COCl  → CH3COOC2H5 + HCl ↑

HCl + NH4OH → H2O + NH4Cl ↑