Question

Draw the structures of two compounds that have the composition \(\mathrm{CH}_{3} \mathrm{NO}_{2}\) and have all three \(\mathrm{H}\) atoms bonded to the \(\mathrm{C}\). Predict which compound has the larger ONO bond angle.

Short answer

The two structures are the nitro CH_3-NO_2and the nitrite ester CH_3-O-NO(considering resonance). The nitro compound has the larger ONO bond angle (~120 degrees), while the nitrite ester has a smaller bond angle due to lone-pair repulsion (<120 degrees).

Step-by-step solution

Identify Valence Electrons and Draw Lewis Structures

First, identify the valence electrons for C (4 valence electrons), H (1 valence electron each), N (5 valence electrons), and O (6 valence electrons each). Then, draw two possible Lewis structures for the compound CH_3NO_2,considering that all three H atoms must be bonded to the carbon atom. The two structures will differ based on the placement of the NO_2 group -- either as a nitro group with one nitrogen doubly bonded to one oxygen (N=O) and singly bonded to the other oxygen (N-O), which carries a negative charge, or as a nitrite ester with nitrogen single-bonded to both oxygens and one oxygen carrying the negative charge.

Draw the Resonance Structures for the Nitrite Ester

The nitrite ester form of CH_3NO_2has resonance structures due to the ability for the negative charge to be on either of the two oxygen atoms. Draw the two resonance structures to represent the nitrite ester.

Predict the ONO Bond Angle using Lone Pair Repulsion

To predict the ONO bond angle, consider that the nitrite ester will have bent geometry around the nitrogen due to the presence of a lone pair which repels the bonding pairs of electrons. In contrast, the nitro group has a trigonal planar geometry around the nitrogen because there are no lone pairs on the nitrogen to cause a bent shape. The trigonal planar geometry will have ONO bond angles of approximately 120 degrees, while the nitrite ester will have angles less than 120 degrees due to lone pair-bond pair repulsion.

Key concepts

Valence Electrons

Valence electrons are the outer shell electrons of an atom and are vital in determining how an element will interact with others to form chemical bonds. An element's group number on the periodic table (for groups 1, 2 and 13-18) tells us the number of valence electrons it has. For example, carbon (C), from group 14, has 4 valence electrons, hydrogen (H) has 1, nitrogen (N) 5, and oxygen (O) 6.

When drawing Lewis structures, these valence electrons are represented as dots around the elemental symbols, illustrating potential bonds. In the compound CH₃NO₂, we count the valence electrons to determine how atoms can be connected: C with 4, three H atoms each with 1, N with 5, and two O atoms each with 6, for a total of 24 valence electrons to be allotted in bonds and lone pairs.

Resonance Structures

Resonance structures are a way to represent multiple valid Lewis dot structures for a single molecule, where only the arrangement of electrons differs, not the arrangement of atoms. This is essential for molecules like CH₃NO₂ in its nitrite ester form, wherein the negative charge can reside on either of the two oxygen atoms.

These alternate structures, drawn with double-headed arrows between them, imply that the true structure is a hybrid, or average, of the resonance forms. Resonance is important because it can impact the stability of molecules and their chemical properties, including how they interact with other molecules. Properly representing all possible resonance structures allows for a more comprehensive understanding of where electrons are most likely to be found in the molecule.

Molecular Geometry

Molecular geometry refers to the three-dimensional shape of a molecule, which is determined by the spatial arrangement of bonded atoms. The geometry is crucial since it can influence physical and chemical properties like reactivity, polarity, phase of matter, color, magnetism, and biological activity.

For example, CH₃NO₂ has two major forms which have different geometries: the nitro form, with a trigonal planar geometry around the nitrogen; and the nitrite ester form, which has a bent geometry due to lone pair repulsion. The molecular geometry can be predicted using the VSEPR (Valence Shell Electron Pair Repulsion) model, which is based on the idea that electron pairs around a central atom will position themselves as far apart as possible to minimize repulsion.

Bond Angles

Bond angles are the angles between adjacent lines representing bonds that emanate from a common center, the central atom. These angles can be understood by examining the molecular geometry of a compound. In molecules where the central atom has no lone pairs, such as the nitro form of CH₃NO₂, the atoms will be spaced evenly around the central atom, resulting in a bond angle of 120 degrees indicative of trigonal planar geometry.

However, if the central atom has one or more lone pairs, as in the nitrite ester form, the lone pairs will repel the bonded atoms more strongly than the atoms repel each other. This repulsion results in smaller bond angles and a bent shape. Thus, we find that the bond angles in the bent nitrite ester form of CH₃NO₂ are less than 120 degrees due to the lone pair-bond pair repulsion around the nitrogen atom.