Concept: Dipole moment arises from the unequal sharing of electrons between atoms in a molecule, leading to a net separation of positive and negative charges. For a molecule to have a net dipole moment, it must contain polar bonds, and its molecular geometry must be such that the individual bond dipoles do not cancel each other out. Symmetrical molecules often have zero net dipole moment even if they contain polar bonds.

Why (B) is correct:

Option Analysis:

• A) 2,2-dimethyl propane: This molecule is neopentane, $(C{H}_3{)}_{4}C$. It has a tetrahedral geometry around the central carbon atom. All C-H and C-C bonds are either nonpolar or have very small bond dipoles. Due to its highly symmetrical tetrahedral structure, the small individual bond dipoles cancel each other out, resulting in a net dipole moment of zero.
• B) trans-2-pentene: The structure of trans-2-pentene is $C{H}_3-CH=CH-C{H}_2-C{H}_3$. The C=C double bond is nonpolar. However, the methyl group ($-C{H}_3$) and ethyl group ($-C{H}_{2}C{H}_3$) attached to the double bond are electron-donating groups (due to hyperconjugation and inductive effect). These groups have different electron-donating abilities, and their positions in the trans configuration do not perfectly cancel out the small bond dipoles, leading to a small but non-zero net dipole moment. Specifically, the C-CH3 and C-CH2CH3 bonds have small dipoles, and their arrangement in the trans isomer often results in a net dipole moment, unlike highly symmetrical trans isomers where identical groups are opposite.
• C) cis-3-hexene: The structure of cis-3-hexene is $C{H}_{3}C{H}_2-CH=CH-C{H}_{2}C{H}_3$. In this molecule, two identical ethyl groups are attached to the same side of the double bond. While the C-C bonds have small dipoles, the symmetry of the cis arrangement of identical groups often leads to a cancellation of these small dipoles, resulting in a very small or effectively zero net dipole moment. For cis isomers with identical groups, the dipoles often cancel or are very small.
• D) 2,2,3,3-tetramethyl butane: This molecule is $(C{H}_3{)}_{3}C-C(C{H}_3{)}_3$. It is a highly symmetrical molecule with a central C-C bond and numerous methyl groups. Due to its high symmetry, all individual bond dipoles (C-C and C-H) cancel each other out, resulting in a net dipole moment of zero.

Comparing the options, trans-2-pentene is the most likely to possess a net dipole moment due to the asymmetry introduced by the different alkyl groups and their arrangement around the double bond.

Correct Answer: (B)