Question

Phenols are added in gasoline to

• A. improve the octane number
• B. reduce its viscosity
• C. act as an antioxidant
• D. increase its pour point

Hint:

Phenolic compounds (and aromatic amines) are standard antioxidants in petroleum fuels and lubricants.
Their antioxidant action relies on donating a hydrogen atom to unstable free radicals, converting them into stable, non-reactive resonance-stabilized phenoxy radicals.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the primary function of adding phenolic compounds (such as alkylphenols) as additives in commercial gasoline formulations.
Gasoline contains unstable unsaturated hydrocarbons that can undergo chemical degradation during storage.

Step 2: Key Formula or Approach:
1. Gasoline blends often contain alkenes (olefins) and dienes, which are highly susceptible to oxidation by atmospheric oxygen.
2. This liquid-phase oxidation proceeds via a free-radical chain reaction, leading to the formation of peroxides, polymers, and insoluble sticky organic deposits known as "gums".
3. These gums can clog fuel filters, carburetor jets, and fuel injectors, causing engine performance issues.
4. To prevent this degradation, antioxidants are added to terminate the free-radical chain reactions.

Step 3: Detailed Explanation:
Let us analyze the chemical role of phenols as antioxidants:
- Phenols, especially sterically hindered phenols (like 2,6-di-tert-butylphenol), possess a highly reactive hydroxyl (\(-\text{OH}\)) group.
- When a peroxide radical (\(\text{ROO}^\bullet\)) is formed in the fuel, the phenol molecule readily donates its hydrogen atom to the radical:
\[ \text{ROO}^\bullet + \text{ArOH} \to \text{ROOH} + \text{ArO}^\bullet \] - The resulting phenoxy radical (\(\text{ArO}^\bullet\)) is highly stable due to the resonance delocalization of the unpaired electron across the aromatic ring, as well as steric hindrance from bulky alkyl groups.
- Because the phenoxy radical is stable and unreactive, it cannot propagate the chain reaction, effectively terminating the oxidation process.
- Therefore, phenols act as highly efficient chain-terminating antioxidants, extending the storage shelf-life of gasoline and preventing gum formation.
- They do not significantly affect fuel viscosity, pour point, or octane rating at the low concentrations (parts per million) in which they are added.

Step 4: Final Answer
Thus, phenols are added to act as an antioxidant, corresponding to option (C).