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Which of the following isomers will have the highest octane number ?
- a)n-octane
- b)2-Methylheptane
- c)2,3-Dimethylhexane
- d)2,2,4-Trimethylpentane
Correct answer is option 'D'. Can you explain this answer?
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Which of the following isomers will have the highest octane number ?a)...
The octane rating of a spark ignition engine fuel is the detonation resistance (anti-knock rating) compared to a mixture of iso-octane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. By definition, iso-octane is assigned an octane rating of 100 and heptane is assigned an octane rating of zero. An 87-octane gasoline, for example, possesses the same anti-knock rating of a mixture of 87% (by volume) iso-octane and 13% (by volume) n-heptane. This does not mean, however, that the gasoline actually contains these hydrocarbons in these proportions. It simply means that it has the same detonation resistance as the described mixture.
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Which of the following isomers will have the highest octane number ?a)...
Answer:
Octane number is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion, which is the uncontrolled ignition of the fuel-air mixture in the engine's cylinders. Higher octane numbers indicate a higher resistance to knocking.
To determine which isomer has the highest octane number, we must consider the factors that affect knocking, such as the molecular structure and the arrangement of carbon atoms in the hydrocarbon chain.
- Octane number and molecular structure:
The octane number generally increases with the branching in the hydrocarbon chain. This is because branching disrupts the regular packing of molecules, making it more difficult for the fuel-air mixture to ignite spontaneously. Therefore, isomers with more branching tend to have higher octane numbers.
- Octane number and arrangement of carbon atoms:
The arrangement of carbon atoms also plays a role in determining the octane number. Isomers with carbon-carbon double bonds (alkenes) or aromatic rings tend to have higher octane numbers because these structures are more stable and less prone to spontaneous ignition.
Now, let's analyze the given options:
a) n-octane: This is the straight-chain isomer of octane. It has no branching and a simple linear structure. Therefore, it has a relatively low octane number.
b) 2-Methylheptane: This is an isomer with a methyl group attached to the second carbon atom of the heptane chain. It has some branching, which increases its octane number compared to n-octane.
c) 2,3-Dimethylhexane: This is an isomer with two methyl groups attached to the second and third carbon atoms of the hexane chain. It has more branching compared to 2-methylheptane, resulting in a higher octane number.
d) 2,2,4-Trimethylpentane: This is an isomer with three methyl groups attached to the second, second, and fourth carbon atoms of the pentane chain. It has the highest degree of branching among the given isomers, making it the most resistant to knocking and therefore, having the highest octane number.
Therefore, option D, 2,2,4-Trimethylpentane, has the highest octane number among the given isomers.
Octane number is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion, which is the uncontrolled ignition of the fuel-air mixture in the engine's cylinders. Higher octane numbers indicate a higher resistance to knocking.
To determine which isomer has the highest octane number, we must consider the factors that affect knocking, such as the molecular structure and the arrangement of carbon atoms in the hydrocarbon chain.
- Octane number and molecular structure:
The octane number generally increases with the branching in the hydrocarbon chain. This is because branching disrupts the regular packing of molecules, making it more difficult for the fuel-air mixture to ignite spontaneously. Therefore, isomers with more branching tend to have higher octane numbers.
- Octane number and arrangement of carbon atoms:
The arrangement of carbon atoms also plays a role in determining the octane number. Isomers with carbon-carbon double bonds (alkenes) or aromatic rings tend to have higher octane numbers because these structures are more stable and less prone to spontaneous ignition.
Now, let's analyze the given options:
a) n-octane: This is the straight-chain isomer of octane. It has no branching and a simple linear structure. Therefore, it has a relatively low octane number.
b) 2-Methylheptane: This is an isomer with a methyl group attached to the second carbon atom of the heptane chain. It has some branching, which increases its octane number compared to n-octane.
c) 2,3-Dimethylhexane: This is an isomer with two methyl groups attached to the second and third carbon atoms of the hexane chain. It has more branching compared to 2-methylheptane, resulting in a higher octane number.
d) 2,2,4-Trimethylpentane: This is an isomer with three methyl groups attached to the second, second, and fourth carbon atoms of the pentane chain. It has the highest degree of branching among the given isomers, making it the most resistant to knocking and therefore, having the highest octane number.
Therefore, option D, 2,2,4-Trimethylpentane, has the highest octane number among the given isomers.
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Which of the following isomers will have the highest octane number ?a)n-octaneb)2-Methylheptanec)2,3-Dimethylhexaned)2,2,4-TrimethylpentaneCorrect answer is option 'D'. Can you explain this answer?
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