C. Melting or Boiling Point of Hydrocarbons. Example: Cyclooctane Cyclopentane 1. ���µÑ…�°�¿�µ Octane 2. Cyclohexane ���µÑ…ane 3. Methane Ethane 4. Pentane Cyclooctane 5. Cyclobutane Cycloheptane 6. 2-���µÑ…�µ�¿e 2-Octene 7. Cyclobutene Cyclooctene
Melting or Boiling Point of Hydrocarbons
Procedure:ÂÂ
In the picture below, ENCIRCLE the compound that has higher boiling point or melting point for each of the following pairs. ÂÂ
The longer the hydrocarbon chain or ring size the higher the melting or boiling points. 3. Cyclic structures have higher boiling or melting points than their non-cyclic counterparts with the same number of carbon atoms. These differences are due in large part to cyclic systems having more rigid and more symmetrical structures.ÂÂ
Example:ÂÂ
o Propane has higher melting or boiling point compared to ethane because propane has longer hydrocarbon chain which is composed of 3 carbon atoms in contrast to ethane which is composed only of 2 carbon atoms.
o Cyclopentene has higher melting or boiling point compared to cyclobutene because cyclopentene has larger ring size which is composed of 5 carbon atoms in contrast to cyclobutene which is composed only of 4 carbon atoms.ÂÂ
o Cyclopropane has higher melting or boiling point compared to propane because cyclic structures have higher boiling or melting points than their non-cyclic counterparts with the same number of carbon atoms. These differences are due in large part to cyclic systems having more rigid and more symmetrical structures.
The Correct Answer and Explanation is :
The picture and procedure you referred to discuss the comparison of boiling or melting points of hydrocarbons. The general principles for determining the compound with a higher boiling or melting point in the pairs you listed are based on chain length and structural differences. Here’s how you can approach the given pairs:
- Cyclooctane vs Cyclopentane:
- Cyclooctane has 8 carbon atoms in a ring, while Cyclopentane has 5. According to the rule that cyclic structures have higher boiling or melting points than their non-cyclic counterparts with the same number of carbon atoms, cyclooctane has a higher boiling/melting point due to its larger ring size and increased symmetry and rigidity.
- Octane vs Cyclohexane:
- Octane (C8H18) is a straight-chain alkane, while Cyclohexane (C6H12) is a cyclic compound. Even though octane has more carbon atoms, cyclohexane’s cyclic structure makes it more rigid and symmetric, increasing its boiling/melting point. Therefore, Cyclohexane has a higher boiling/melting point.
- Methane vs Ethane:
- Ethane (C2H6) has a longer carbon chain compared to Methane (CH4), so Ethane has a higher boiling/melting point.
- Pentane vs Cyclooctane:
- Cyclooctane is a cyclic compound with 8 carbon atoms, while Pentane (C5H12) is a straight-chain alkane. As noted, cyclic compounds have higher boiling/melting points than non-cyclic ones with the same number of carbon atoms. Therefore, Cyclooctane has a higher boiling/melting point.
- Cyclobutane vs Cycloheptane:
- Cycloheptane (C7H14) has a larger ring size compared to Cyclobutane (C4H8), and cyclic structures are more stable and symmetrical. Therefore, Cycloheptane has a higher boiling/melting point.
- 2-Octene vs Cycloheptene:
- Cycloheptene is a cyclic compound, while 2-Octene is a straight-chain alkene. Following the trend, Cycloheptene has a higher boiling/melting point due to its cyclic structure.
- Cyclobutene vs Cyclooctene:
- Cyclooctene has a larger ring size (8 carbon atoms) compared to Cyclobutene (4 carbon atoms), so Cyclooctene has a higher boiling/melting point.
Explanation: In hydrocarbons, several factors determine the melting or boiling points: the size of the molecule, whether the structure is cyclic or acyclic, and the symmetry and rigidity of the molecule. Cyclic compounds generally have higher melting and boiling points because their structures are more rigid and symmetric, allowing for stronger intermolecular forces. Additionally, longer chains or larger rings increase the surface area for van der Waals forces, which also contributes to higher boiling or melting points. Thus, when comparing hydrocarbons, cyclic structures tend to have higher melting or boiling points than their acyclic counterparts with the same number of carbon atoms, and longer hydrocarbon chains or larger ring sizes also lead to higher melting/boiling points.