The concept of valence-shell electron-pair repulsion (VSEPR) is presented in introductory organic chemistry as a way to predict molecular geometries. The idea be- hind VSEPR is that areas of electron density repel each other so that the geometry of bonds and/or lone pairs of electrons around any one atom places these areas as far apart as possible. For four areas of electron density, a tetrahedral geometry is predicted. For three areas of electron density, a trigonal planar geometry is predicted. Two areas of electron density lead to a linear geometry. VSEPR is simply a predictive tool, but in some cases, it gives an incorrect prediction. In these instances, additional insights into bonding are necessary, such as resonance. Interestingly, several important situations are critical to biochemistry where VSEPR breaks down. Two examples are shown here. :O: CH3 H,N H3N CH3 H :0: :0: Histidine The circled nitrogen atoms are predicted by VSEPR to be tetrahedral in geometry because each appears to have four areas of electron density: three bonds and a single lone pair of electrons. However, in both cases, structural analysis has revealed that the atoms actually have a trigonal planar geometry.
Formal Charges
Formal charges have an important role in organic chemistry since this concept helps us to know whether an atom in a molecule is neutral/bears a positive or negative charge. Even if some molecules are neutral, the atoms within that molecule need not be neutral atoms.
Polarity Of Water
In simple chemical terms, polarity refers to the separation of charges in a chemical species leading into formation of two polar ends which are positively charged end and negatively charged end. Polarity in any molecule occurs due to the differences in the electronegativities of the bonded atoms. Water, as we all know has two hydrogen atoms bonded to an oxygen atom. As oxygen is more electronegative than hydrogen thus, there exists polarity in the bonds which is why water is known as a polar solvent.
Valence Bond Theory Vbt
Valence bond theory (VBT) in simple terms explains how individual atomic orbitals with an unpaired electron each, come close to each other and overlap to form a molecular orbital giving a covalent bond. It gives a quantum mechanical approach to the formation of covalent bonds with the help of wavefunctions using attractive and repulsive energies when two atoms are brought from infinity to their internuclear distance.
What is the hybridization state of the circled nitrogens? What kind of orbital contains the lone pairs identified in these circles?
1. sp, 2p
2. sp2, sp2
3. sp3, 2p
4. sp2, 2p
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