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.

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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

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.
Transcribed Image Text: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.
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