Arrange the melting points of following compounds in decreasing order
1. n - butane
2. cis - 2- butene
3. trans -2- butene
4. 1 - butyne

Options

A. `1 gt 2 gt 3 gt 4`
B. `4 gt 2 gt 3 gt 1`
C. `4 gt 3 gt 2 gt 1`
D. `3 gt 2 gt 1 gt 4`

Answer

To arrange the melting points of the compounds in decreasing order, we will analyze the structures and properties of each compound:

1. **n-butane (C4H10)**: This is a straight-chain alkane. It has a symmetrical structure, which allows for efficient packing in the solid state. Alkanes generally have lower melting points compared to alkenes and alkynes due to weaker van der Waals forces.

2. **cis-2-butene (C4H8)**: This is a cis isomer of butene. The presence of a double bond introduces some rigidity and reduces symmetry compared to n-butane. The cis configuration creates a dipole moment, which can affect its melting point.

3. **trans-2-butene (C4H8)**: This is a trans isomer of butene. The trans configuration is more symmetrical than the cis configuration, resulting in a lower dipole moment. This increased symmetry generally leads to a higher melting point than its cis counterpart.

4. **1-butyne (C4H6)**: This is an alkyne with a triple bond. Alkynes typically have higher melting points than alkenes and alkanes due to the linear geometry of the triple bond, which allows for better packing and stronger intermolecular forces.

### Step-by-step Solution:

1. **Identify the Compounds**:
- n-butane (C4H10)
- cis-2-butene (C4H8)
- trans-2-butene (C4H8)
- 1-butyne (C4H6)

2. **Analyze Symmetry and Dipole Moments**:
- n-butane has high symmetry and no dipole moment, leading to a relatively low melting point.
- cis-2-butene has lower symmetry and a dipole moment, resulting in a higher melting point than n-butane.
- trans-2-butene has higher symmetry and no dipole moment, leading to a higher melting point than cis-2-butene.
- 1-butyne, with a triple bond, has a linear structure and higher melting point due to stronger intermolecular forces.

3. **Rank the Melting Points**:
- 1-butyne (highest melting point due to triple bond and linear structure)
- trans-2-butene (higher symmetry and no dipole moment)
- cis-2-butene (lower symmetry and dipole moment)
- n-butane (lowest melting point due to weaker van der Waals forces)

4. **Final Order**:
- 1-butyne > trans-2-butene > cis-2-butene > n-butane

### Final Answer:
The order of melting points in decreasing order is:
1. 1-butyne
2. trans-2-butene
3. cis-2-butene
4. n-butane