Which of the following carbon-carbon bonds in the molecules below can freely rotate?H₃C−CH₃H₂C=CH₂HC≡CH

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11ecba2d_11ff_20a5_a3bf_f97b17d9ef9e_MD0008_00 Covalent bonds between atoms are formed by sharing electrons, which is made possible through the overlap of the atomic orbitals from each atom. Covalent bonds made by the end-to-end overlap of atomic orbitals are called sigma (σ) bonds whereas covalent bonds made by the side-to-side overlap of p orbitals are called pi (π) bonds. Although some atoms only participate in single covalent bonding (by a σ bond), other atoms are capable of participating in multiple bonds (double or triple bonds) by the addition of one or more π bonds. In this way, a double bond consists of one σ bond and one π bond, and a triple bond consists of one σ bond and two π bonds. Because σ bonds are made by an end-to-end orbital overlap, free rotation around the bond between the two σ-bonded atoms (such as a C−C single bond) can be achieved while maintaining the orbital overlap. Accordingly, substituents such as methyl groups (−CH₃) can fully rotate 360° around the carbon σ-bond axis (Number I). Conversely, free rotation around the bond between two π-bonded atoms (such as in a C=C double bond or a C≡C triple bond) is not possible due to side-to-side orbital overlap. Rotation would disrupt the p orbital overlap between the atoms and break the π bond; as such, π bonds lock bond rotation. Therefore, the hydrogen atoms bonded in a =CH₂ group cannot freely rotate around the double bond axis (Number II). Likewise, a C≡C bond cannot have rotation between the carbon atoms (Number III). Educational objective: Covalent bonds are formed by sharing electrons between atoms through the overlap of atomic orbitals in an end-to-end or side-to-side configuration. Multiple bonds formed by s and p orbitals consist of one σ bond and one or more π bonds. Free rotation around a bond is possible with σ bonds (single bonds) but is prevented by the structure of π bonds (double and triple bonds).

Which of the Following Carbon-Carbon Bonds in the Molecules Below