Why is the melting point of 4-bromoacetanilide so high? Does it have any reason to do with the chemical structure, or the para aspect?

Question

We used melting point to determine the type of bromoacetanilide, and it was closest to 4-bromoacetanilide. How can I explain why the melting point was what it is?

Answer 1

Melting points of solid organic compounds can be affected by several factors. Symmetry is one of these factors. Acetanilide, 2-bromoacetanilide, and 3-bromoacetanilide have melting points in similar range, respectively about $\pu{114}$, $96$-$\pu{101}$, and $87$-$\pu{89 ^\circ C}$. Yet, melting points of 2-bromoacetanilide and 3-bromoacetanilide are significantly lower than that of acetanilide, even without extra bromine atom. Apart from this, 4-bromoacetanilide has the highest melting point among them, $177$-$\pu{181 ^\circ C}$. Thus, you can safely say bromine have nothing to do with their melting point differences.

Both acetanilide and 4-bromoacetanilide have a symmetry element (plane of symmetry) that allows increased $\pi-\pi$ stacking among individual molecules during their crystallization (see the correct educational guess of Mithoron in his comment above). That may be the reason for higher melting points for them compared to compounds lacking that symmetry element. For example, if you replace the bromine with chlorine or iodine, the resulting compounds shows the same trend:

$$ \begin{array}{lr} \text{Compound} & \text{Melting poing} \\\hline \text{4-Chloroacetanilide} & \pu{178-179 ^\circ C} \\ \text{3-Chloroacetanilide} & \pu{77-78 ^\circ C} \\ \text{2-Chloroacetanilide} & \pu{87-88 ^\circ C} \\ \text{4-Iodoacetanilide} & \pu{184-185 ^\circ C} \\ \text{3-Iodoacetanilide} & \pu{119 ^\circ C} \\ \text{2-Iodoacetanilide} & \pu{109-111 ^\circ C} \\\hline \end{array} $$

Even if the substitution pattern, not the substitution group, affected the melting point of above compounds, there is a peer review paper (Ref.1) showing both substitution pattern and the substitution group have affected on resulting compounds' melting points (interestingly). In an useful table, the result describe th decreasing melting points in 1,4-dihalogenated phenyl derivatives when a halogen or an electrondonating group is introduced as the third substitution (Ref.1). This is actually supporting our caurse here. The parent compound is 1,4-dihalogenated phenyl derivative, which have plane of symmetry element. The introduction of third substitution distrupt this symmetry element and hence lower the melting point. For instance, the melting point of 1,4-dichlorobenzene is listed as $\pu{53.5 ^\circ C}$. The melting point of 2,5-dichloroaniline (when amine is introduced to ortho-position of one chloro group) has been decreased to $47$-$\pu{50 ^\circ C}$.

References:

1. G. B. Heisig, "Anomalous Effect of Ortho-Para Orienting Groups on the Melting Points of Dihalogenated Benzene Derivatives," J. Am. Chem. Soc. 1928, 50(1), 139–145 (DOI: https://doi.org/10.1021/ja01388a017).