Answer:
No net dipole moment: 1,2-dibromoethene (trans-isomer)
Net dipole moment: 1,2-dibromoethene (cis-isomer), 1,1-dibromoethene
Explanation:
A Lewis Structure is a simplified representation of the valence shell electrons in a molecule. It shows arrangement of the electrons around individual atoms in a molecule. Electrons are shown as "dots" or for bonding electrons as a line between the two atoms (double line for a double bond).
Isomers are compounds with the same chemical composition but different physical arrangement of atoms. Isomers can be split into two main types, structural isomers and stereo-isomers. Structural isomers can be split into chain (different arrangement of atoms/molecules), position (different position of functional group) and functional (different functional group) isomers. Stereo-isomers can be split into geometric (different atoms around a bond with limited rotation) and optical (non-super-imposable mirror images) isomers.
Dipole moments occur when there is a separation of charge. It can occur between two ions in an ionic bond or between atoms in a covalent bond due to a difference in electro-negativity. When one atom is more electro-negative than another, that atom pulls more tightly on the shared pair of electrons, or when one atom has a lone pair of electrons and the difference of electro-negativity vector points in the same way a net dipole moment occurs. One of the most common examples is the water molecule, which consists of one oxygen and two hydrogen atoms. The differences in electro-negativity and lone electrons gives oxygen a partial negative charge and each hydrogen a partial positive charge.
In the attached image, 3 isomers of dibromoethene can be seen. Isomer A and B are geometric stereo-isomers. Isomer C is a position structural isomer. All 3 isomers have polar C-Br bonds as bromine is more electro-negative than carbon. Isomer A has the bromine atoms pulling the electron cloud in opposite directions (180° to each other) thus there is no net dipole moment. Isomers B and C have the bromine atoms pulling electron cloud in a direction not 180° to each other so they do not cancel each other out resulting in a net dipole moment.
Trans -1,2 - dibromobutane has no net dipole moment because the two dipoles cancel out in the molecule.
Isomers are compounds that have the same molecular formula but different structural formulas. In the case of dibromoethene, the isomers are stereoisomers because they differ in the way the atoms and groups are arranged in space.
The three isomers of dibromoethene are; trans -1,2 - dibromobutane, cis- 1,2- dibromobutane and 1,1 - dibromobutane. These are shown as structures I, II and III in that order.
Trans -1,2 - dibromobutane has no net dipole moment because the two dipoles cancel out in the molecule. The two bromine atoms exert an equal pull on the electron cloud in opposite direction thereby cancelling out the dipoles.
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