In which compound the C – H bond distance is longest ?

Options

A. `C_2H_2`
B. `C_2H_4`
C. `C_2H_4Br_2`
D. `C_6H_5OH`

Answer

To determine which compound has the longest C-H bond distance, we need to analyze the bond characteristics of the compounds given.

### Step-by-Step Solution:

1. **Understand Bond Distance**: The bond distance refers to the average distance between the nuclei of two bonded atoms. Generally, bond length is influenced by the type of bond (single, double, triple) and the hybridization of the carbon atom involved.

2. **Bond Multiplicity and Length**: It is important to note that bond length decreases with an increase in bond multiplicity. This means that:
- A single bond (C-H) is longer than a double bond (C=C) or a triple bond (C≡C).
- Therefore, C-H bonds in compounds with single bonds will be longer than those in compounds with double or triple bonds.

3. **Hybridization and S-character**: The hybridization of carbon also affects bond length:
- sp hybridization (triple bond) has the least bond length.
- sp² hybridization (double bond) has a shorter bond length than sp³.
- sp³ hybridization (single bond) has the longest bond length.

4. **Analyzing the Given Compounds**:
- **C2H2 (Ethyne)**: Contains a C≡C triple bond, leading to shorter C-H bonds.
- **C2H4 (Ethylene)**: Contains a C=C double bond, leading to shorter C-H bonds than single bonds.
- **C2H4Br2**: Contains C-C single bonds and C-H single bonds. The C-H bonds here are sp³ hybridized, which means they are longer than those in the previous compounds.
- **C6H5OH (Phenol)**: The C-H bond is also sp² hybridized due to the aromatic ring, which means the bond length is shorter than sp³.

5. **Conclusion**: Among the compounds analyzed, C2H4Br2 has C-H bonds that are sp³ hybridized, which results in the longest bond distance compared to the other compounds that have either double or triple bonds.

### Final Answer:
The compound with the longest C-H bond distance is **C2H4Br2**.