Final answer:
C-C single bonds in alkanes such as 1,2-dichloroethane and butane can rotate freely, which can cause different conformers as the molecule changes shape. However, alkenes like 1,2-dichloroethene have restricted rotation due to the presence of a double bond which makes those molecules rigid.
Step-by-step explanation:
Free rotation about a carbon-to-carbon single bond (C-C) is a characteristic feature of alkanes, as opposed to alkenes, where the presence of a double bond restricts rotation, making the molecule rigid. For example, in 1,2-dichloroethane, which is an alkane, there is free rotation around the C-C bond. This means that while you can draw structural formulas that might look different due to the rotation, they still represent the same molecule. However, with molecules that have a double bond, like 1,2-dichloroethene, rotation is not possible without breaking the bond. In Figure 1.3.4 (c), the rotation around C-C in a CH₃CH₂ molecule is illustrated, showing different orientations of H atoms on one C relative to the H atoms on the other C, which are called conformers of each other. In molecules like C4H10, or butane, although the carbon chain can bend as groups rotate freely about the C-C bonds, this does not change the identity of the compound.
When examining the hybridization and bond angles in such molecules, carbon atoms typically exhibit sp³ hybridization and have bond angles close to 109.5°. Lastly, understanding molecular structures, like the linear structure mentioned, is crucial in predicting how molecules will behave, especially in reactions and interactions with other molecules.