Which of the following compounds can give E1cB reaction ?

Options

A. `CF_3-CHCl_2`
B. `C_6H_5-undersetunderset(NO_2)(|)CH-CH_2F`
C. `CH_3-CH_2-CH_2Br`
D. `C_6H_5-undersetunderset(OH)(|)CH-CH_2-CHO`

Answer

To determine which of the given compounds can undergo an E1cB reaction, we first need to understand the mechanism of E1cB reactions. E1cB stands for "Elimination Unimolecular via Conjugate Base." This mechanism typically occurs when a compound has a beta hydrogen that can be abstracted by a base, leading to the formation of a stabilized carbanion intermediate.

### Step-by-Step Solution:

1. **Identify the Compounds:**
We have four compounds to analyze:
- A: CH3-CH(Br)-CH2-CH3
- B: C6H5-CH(NO2)-CH2F
- C: CH3-CH2-CH2-Br (Bromopropane)
- D: C6H5-CHOH-CH2-CHO

2. **Check for Beta Hydrogens:**
For an E1cB reaction to occur, the compound must have a beta hydrogen that can be abstracted by a base.
- **A:** CH3-CH(Br)-CH2-CH3 has beta hydrogens.
- **B:** C6H5-CH(NO2)-CH2F has beta hydrogens.
- **C:** CH3-CH2-CH2-Br has beta hydrogens.
- **D:** C6H5-CHOH-CH2-CHO has beta hydrogens.

3. **Assess Stabilization of the Carbanion:**
The carbanion formed after the abstraction of the beta hydrogen must be stabilized by resonance or inductive effects.
- **A:** After abstraction, the carbanion is stabilized by the -I effect of bromine. Thus, it can undergo E1cB.
- **B:** The carbanion can be stabilized by resonance from the nitro group and the -I effect of fluorine. Thus, it can undergo E1cB.
- **C:** The carbanion formed here is not stabilized by any strong electron-withdrawing groups or resonance. Therefore, it will not undergo E1cB but will likely proceed via E2.
- **D:** The carbanion can be stabilized by resonance with the phenyl group and the adjacent carbonyl. Thus, it can undergo E1cB.

4. **Conclusion:**
The compounds that can undergo E1cB reactions are:
- A: CH3-CH(Br)-CH2-CH3
- B: C6H5-CH(NO2)-CH2F
- D: C6H5-CHOH-CH2-CHO
- **C** (Bromopropane) cannot undergo E1cB.

### Final Answer:
The compounds that can give E1cB reactions are A, B, and D.