Considere el para-azoxianisol, que es un cristal líquido...

Considere el para-azoxianisol, que es un cristal líquido nemático en el intervalo de temperatura de $21^{\circ} \mathrm{C}$ a $47^{\circ} \mathrm{C}$ : GRAPH CAN'T COPY. (a) Escriba la estructura de Lewis para esta molécula, mostrando todos los pares de electrones solitarios asi como los enlaces. (b) Describa los orbitales hibridos que utiliza cada uno de los dos nitrógenos. ¿Qué ángulos de enlace cabe esperar alrededor del átomo de nitrógeno que está unido a oxigeno? (c) La sustitución de uno de los grupos $-\mathrm{OCH}_3$ del para-azoxianisol por un grupo $-\mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3$ hace que el punto de fusión de la sustancia baje; el intervalo de cristal líquido cambia de 19-76 ${ }^{\circ} \mathrm{C}$. Explique por qué esta sustitución produce los cambios de propiedades que se observan. (d) ¿Cómo cabe esperar que cambie la densidad del para-azoxianisol al fundirse a $117^{\circ} \mathrm{C}$ ? $2 \mathrm{Al}$ pasar del estado liquido nemático al isotrópico a $137^{\circ} \mathrm{C}$ ? Explique.

Considere el para-azoxianisol, que es un cristal líquido nemático en el intervalo de temperatura de $21^{\circ} \mathrm{C}$ a $47^{\circ} \mathrm{C}$ :
GRAPH CAN'T COPY.
(a) Escriba la estructura de Lewis para esta molécula, mostrando todos los pares de electrones solitarios asi como los enlaces. (b) Describa los orbitales hibridos que utiliza cada uno de los dos nitrógenos. ¿Qué ángulos de enlace cabe esperar alrededor del átomo de nitrógeno que está unido a oxigeno? (c) La sustitución de uno de los grupos $-\mathrm{OCH}_3$ del para-azoxianisol por un grupo $-\mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3$ hace que el punto de fusión de la sustancia baje; el intervalo de cristal líquido cambia de 19-76 ${ }^{\circ} \mathrm{C}$. Explique por qué esta sustitución produce los cambios de propiedades que se observan. (d) ¿Cómo cabe esperar que cambie la densidad del para-azoxianisol al fundirse a $117^{\circ} \mathrm{C}$ ? $2 \mathrm{Al}$ pasar del estado liquido nemático al isotrópico a $137^{\circ} \mathrm{C}$ ? Explique.

Key Concepts

Phase Transition Effects on Density

Phase transitions, such as melting from a crystalline or liquid crystalline state to an isotropic liquid, involve changes in molecular order and packing. Typically, as a substance melts, the density decreases because the molecules become less tightly packed and more disordered. Similarly, further transitions from an ordered mesophase to an isotropic phase often involve subtle changes in density due to differences in molecular alignment and free volume.

Liquid Crystal Phases and Nematic Order

Liquid crystals are a state of matter that exhibit properties between those of conventional liquids and solid crystals. In the nematic phase, molecules have no positional order but tend to align along a common axis. This directional order and fluidity are crucial for technologies such as displays, and the behavior of liquid crystals is sensitive to temperature and molecular structure.

Substituent Effects on Phase Behavior

Substituent effects refer to the influence that different functional groups attached to a molecule have on its physical properties, such as melting point and phase stability. Changes in the size, shape, or polarity of substituents can disrupt molecular packing and intermolecular interactions, thereby altering phase transition temperatures and the stability of phases like the nematic liquid crystalline state.

Orbital Hybridization and Molecular Geometry

Orbital hybridization involves the mixing of atomic orbitals to form new hybrid orbitals that are used to form bonds in a molecule. This concept explains the observed molecular geometries and bond angles. For instance, a nitrogen atom that forms bonds with electronegative atoms might adopt sp2 hybridization, leading to bond angles that are characteristic of a trigonal planar arrangement, though slight deviations can occur due to the presence of lone pairs.

Electron Dot Structures (Lewis Structures)

Lewis structures are diagrams that depict the bonding between atoms of a molecule and the lone electron pairs that may exist in the molecule. This concept is fundamental in chemistry because it visually represents how atoms share or exchange electrons in bonding, allowing for the prediction of molecular structure, reactivity, and properties.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the geometry of molecules based on the repulsion between electron pairs in the valence shell of an atom. This theory helps in understanding the spatial arrangement of bonds and lone pairs, explaining deviations in bond angles from idealized geometries based on electron pair repulsions.

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