) The formula C2H2Br2 and
(b) The formula C2Br2Cl2. In each instance designate compounds that are cis-trans isomers of each other. Predict the dipole moment of each one.
The Correct Answer and Explanation is:
To address this question, we will examine two chemical formulas:
(a) C₂H₂Br₂
(b) C₂Br₂Cl₂
For each, we will identify the possible cis-trans isomers (geometric isomers) and predict their dipole moments.
(a) C₂H₂Br₂
This compound is a dibromoethene with two hydrogen atoms and two bromine atoms on a carbon-carbon double bond. The basic structure is:
H–C=C–H, with two Br atoms substituting for two of the H atoms.
There are two geometric isomers:
- Cis-1,2-dibromoethene
- Structure: Br and Br on the same side of the double bond.
- The two dipole moments from Br atoms add up partially, resulting in a net dipole moment (non-zero).
- Trans-1,2-dibromoethene
- Structure: Br atoms on opposite sides.
- The dipoles from the two Br atoms cancel each other out, leading to a zero or very small dipole moment.
Thus, these are cis-trans isomers due to the restricted rotation of the C=C double bond and different spatial arrangements of the substituents.
(b) C₂Br₂Cl₂
This is a tetra-substituted ethene where all substituents are halogens. Isomerism depends on how Br and Cl atoms are arranged around the double bond.
Two cis-trans isomers possible:
- Cis-isomer (cis-1,2-dibromo-1,2-dichloroethene)
- Br and Cl are on the same side respectively.
- This arrangement leads to net dipole moments, making it polar.
- Trans-isomer (trans-1,2-dibromo-1,2-dichloroethene)
- Each carbon has one Br and one Cl, arranged across from each other.
- Due to vector cancellation of bond dipoles, the dipole moment is lower, possibly close to zero.
Summary of Dipole Moments
| Compound | Isomer | Dipole Moment Prediction |
|---|---|---|
| C₂H₂Br₂ | Cis | Non-zero (polar) |
| Trans | ≈ 0 (non-polar) | |
| C₂Br₂Cl₂ | Cis | Non-zero (polar) |
| Trans | Low or ≈ 0 |
Conclusion (300 words)
The compounds C₂H₂Br₂ and C₂Br₂Cl₂ are examples of molecules that can exhibit geometric (cis-trans) isomerism due to the presence of a carbon-carbon double bond, which prevents free rotation. In C₂H₂Br₂, the two bromine atoms can be either on the same side (cis) or opposite sides (trans) of the double bond. In the cis-isomer, the molecular dipoles of the Br atoms reinforce each other, leading to a net dipole moment and hence a polar molecule. In contrast, the trans-isomer has its dipoles canceling out, resulting in a non-polar molecule with a near-zero dipole moment.
Similarly, C₂Br₂Cl₂ has four halogen atoms around the double bond: two bromines and two chlorines. Depending on their spatial arrangement, the molecule can exist in cis and trans forms. In the cis-isomer, each pair of like atoms (Br-Br and Cl-Cl) are on the same side, producing a net dipole moment due to their unequal electronegativities and vector directions—making the molecule polar. In the trans-isomer, opposing positions of Br and Cl on each carbon lead to cancellation of dipoles, resulting in a lower overall dipole moment or potentially a non-polar structure.
In both cases, cis-isomers are generally polar, and trans-isomers are less polar or non-polar, a principle that helps in predicting physical properties like boiling points and solubility based on molecular geometry and polarity.
