Identifying Lone Pairs

12 For each of the compounds below determine whether any of the nitrogen atoms bear a formal charge  

Recall that the appropriate number of valence electrons for a carbon atom is four. In order to have a positive formal charge, a carbon atom must be missing an electton. In other words, it must have only three valence electrons. Such a carbon atom can only form three bonds. This must be taken into account when counting hydrogen atoms  

In summary, both and C will have only three bonds. The difference between them is the nature of the fourth orbital. In the case of C, the fourth orbital is empty. In the case of C, the fourth orbital holds a lone pair of electrons. 

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Distribute the remaining electrons as oo pairs (to identify lone pairs of electrons). As H only requires two electrons, the Lewis structure of NH, is complete, total = 8, but the Fs of BF require three further electron pairs to complete their octets, total = 32. 

Distribute the remaining electrons as 00 pairs to identify lone pairs of electrons). As hydrogen only requires two electrons, only one 00 electron pair is added to the nitrogen atom, and two 00 electron pairs to the oxygen atom. This completes the octet, in both cases, of 8 electrons. 

Now let s get some practice identifying lone pairs when they are not drawn. The following example will demonstrate the thought process ... 

We must learn to identify lone pairs in allylic positions. Here are several examples ... 

Drawing and Interpreting Bond-Line Structures (Section 2.2) Identifying Lone Pairs (Section 2.5)... 

SkillBuilder 2.4 Identifying Lone Pairs on Oxygen Atoms... 

NAOs for an atomic block in the density matrix which have large occupancy numbers (say >1.90) are identified as lone pair orbitals. Their contributions to the density matrix are also removed. 

Each pair of atoms (AB, AC, BC,...) is now considered, and the two-by-two subblocks of the density matrix (with the core and lone pair contributions removed) are diagonalized. Natural bond orbitals are identified as eigenvectors which have large eigenvalues (occupation numbers larger than say 1.90). 

Problem 1.12 1 Following is a molecular model of aspirin (acetylsalicylic acid). Identify the hybridization of each carbon atom in aspirin, and tell which atoms have lone pairs of elections (gray = C, red = O, ivory = LI). 

Identify all nonbonding lone pairs of electrons in the following molecules, and tell what geometry you expect for each of the indicated atoms. 

Complete the electron-dot structure of caffeine, showing all lone-pair electrons, and identify the hybridization of the indicated atoms. 

Look for any lone-pair electrons, and identify any atom with an electronegativity substantially different from that of carbon. (Usually, this means O, N, P, Cl, or Br.) Electron densjty will be displaced in the general direction of the electronegative atoms and the lone pairs. 

First, look at the reaction and identify the bonding changes that have occurred. In this case, a C—Br bond has broken and a C-C bond has formed. The formation of the C-C bond involves donation of an electron pair from the nucleophilic carbon atom of the reactant on the left to the electrophilic carbon atom ol CH Br, so we draw a curved arrow originating from the lone pair on the negatively charged C atom and pointing to the C atom of CH3Br. At the same time the C—C bond forms, the C-Br bond must break so that the octet rule is not violated. We therefore draw a second curved arrow from the C-Br bond to Br. The bromine is now a stable Br- ion. 

FIGURE 3.1 The names of the shapes of simple molecules and their bond angles. Lone pairs of electrons are not shown because they are not included when identifying molecular shapes. 

Once we have identified the arrangement of the most distant locations of these regions, which is called the electron arrangement of the molecule, we note where the atoms lie and identify the shape of the molecule by giving it the name of the corresponding shape in Fig. 3.1. In naming the molecular shape, we consider only the positions of atoms, not any lone pairs that may be present on the central atom, even though they affect the shape. 

AX Em to identify the different combinations of atoms and lone pairs attached to... 

STRATEGY For the electron arrangement, draw the Fewis structure and then use the VSEPR model to decide how the bonding pairs and lone pairs are arranged around the central (nitrogen) atom (consult Fig. 3.2 if necessary). Identify the molecular shape from the layout of atoms, as in Fig. 3.1. 

For each of the following molecules or ions, write the Lewis structure, list the number of lone pairs on the central atom, identify the shape, and estimate the bond angles (a) PBr, ... 

Consider the bonding in CH2=CHCHO. (a) Draw the most important Lewis structure. Include all nonzero formal charges, (b) Identify the composition of the bonds and the hybridization of each lone pair—for example, by writing o(H ls,C2s/ 2). 

The following molecules are bases that are part of the nucleic acids involved in the genetic code. Identify (a) the hybridization of each C and N atom, (b) the number of a- and ir-bonds, and (c) the number of lone pairs of electrons in the molecule. 

Now that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone pair. If we are given two resonance structures and are asked to show the arrow(s) that get us from one resonance structure to the other, it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another. For example, consider the following resonance structures ... 

Whenever one compound uses its electrons to attack another compound, we call the attacker a nucleophile, and we call the compound being attacked an electrophile. It is very simple to tell the difference between an electrophile and a nucleophile. You just look at the arrows and see which compound is attacking the other. A nucleophile will always use a region of high electron density (either a lone pair or a bond) to attack the electrophile (which, by definition, has a region of low electron density that can be attacked). These are important terms, so let s make sure we know how to identify nucleophiles and electrophiles. 

The steric number identifies how many groups of electrons must be widely separated in three-dimensional space. In ammonia, for example, the nitrogen atom bonds to three hydrogen atoms, and it has one lone pair of electrons. How are the three hydrogen atoms and the lone pair oriented in space Just as in methane, the four groups of electrons are positioned as far apart as possible, thus minimizing electron-electron repulsion. 

C09-0109. Species with chemical formula X I4 can have the following shapes. For each, name the molecular geometry, identify the ideal VSEPR bond angles, tell how many lone pairs are present in the structure, and give a specific example. 

We complete the description of acetic acid by identifying the orbitals that contain the two lone pairs on the outer oxygen atom. The ffbond and the tt bond account for two valence 2 p orbitals of the oxygen atom. This leaves the third 2 p orbital and the 2. S orbital for the lone pairs. 

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