A common error that students make is to account for stereochemistry on every question, regardless of the context. One such context where stereochemistry is not always applied is resonance. Let's look at the different resonance structures in q.25:

Resonance refers to the delocalization of electrons between atoms within a molecule. Thus, the specific atoms involved in this delocalization matter, not just the overall appearance. Here, the negative charge is distributed across two carbons and a nitrogen.

Now, let's compare the middle resonance structure with Option A:

Here, the middle resonance structure looks identical to Option A. The structures are superimposable via a 180 degree rotation about the vertical axis. But be careful, these structures are not the exact same since the position of their atoms differ. The two carbons that can carry the charge are highlighted in green and blue (see above). The middle resonance structure carries the charge on the green carbon. When we rotate this structure to match Option A, the charge is still carried on the green carbon. In contrast, Option A carries the charge on the blue carbon.

No matter how you rotate the two molecules in space, the charge still resides on different atoms. This holds true for any resonance structures that are superimposable (see below). Although we can rotate one structure to perfectly match the other, the charge still resides on fundamentally different atoms, making them distinct resonance structures.

*Note: As a quick rule of thumb, assume that the molecule is never rotated or moved in questions about resonance structures.

There are two trends that describe the effect of branching on the melting point of hydrocarbons:

These trends seem to contradict. After all, a molecule with extensive branching, such as neopentane, will have a high degree of symmetry. In short, for highly branched molecules, Trend #2 trumps Trend #1. To understand this, we must look at the structure of solids: crystal lattices.

Crystal lattices are highly ordered structures composed of repeating arrangements of molecules. These crystal lattices are held together by various intermolecular forces (e.g. London Dispersion forces). These crystal lattices become stronger when molecules pack more tightly together, which maximizes the strength and number of intermolecular forces. Highly symmetrical molecules can pack more tightly together, and due to their symmetry, allow interactions to form in several directions. Thus, symmetrical molecules form stronger crystal lattices which require more energy (e.g. heat) to disrupt, resulting in higher melting points.

Consider the following image of blocks from the popular video game Tetris. These blocks provide a rough analogy for the crystal lattice arrangement of hydrocarbons.