The molecule shown on the right in the example in the right column is the amino acid histidine, and the five-membered ring is known as aromatic. An aromatic ring has 2, 6, 10, 14, etc., electrons placed in 2p orbitals around a ring. Indicate which of the following statements must therefore be true.1. There are a total of six electrons in the pi system (defined as electrons in 2p orbitals), including the lone pair on the ring N that is not circled.2. There are a total of six electrons in the pi system, including the lone pair on the ring N atom that is circled.3. The lone pair on the ring N atom that is not circled resides in an sp2 orbital on an sp2 hybridized nitrogen atom.4. Statements 2 and 3 are both true The concept of valence-shell electron-pair repulsion(VSEPR) is presented in introductory organic chemistryas a way to predict molecular geometries. The idea be-hind VSEPR is that areas of electron density repel eachother so that the geometry of bonds and/or lone pairsof electrons around any one atom places these areas asfar apart as possible. For four areas of electron density,a tetrahedral geometry is predicted. For three areas ofelectron 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 somecases, it gives an incorrect prediction. In theseinstances, additional insights into bonding arenecessary, such as resonance. Interestingly, severalimportant situations are critical to biochemistry whereVSEPR breaks down. Two examples are shown here.:O:CH3H,NH3NCH3 H:0::0:HistidineThe circled nitrogen atoms are predicted by VSEPRto be tetrahedral in geometry because each appears tohave four areas of electron density: three bonds and asingle lone pair of electrons. However, in both cases,structural analysis has revealed that the atoms actuallyhave a trigonal planar geometry.

Histidine is an aromatic amino acid. It is used for the biosynthesis of proteins. It contains an…

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Organic Chemistry: A Guided Inquiry

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ISBN:9780618974122

Author:Andrei Straumanis

Publisher:Andrei Straumanis

Chapter1: Bond Angles And Shape

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Problem 12CTQ: Consider the following flat drawing of methane (CH4) . a. What is HCH bond angle implied by this...

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The molecule shown on the right in the example in the right column is the amino acid histidine, and the five-membered ring is known as aromatic. An aromatic ring has 2, 6, 10, 14, etc., electrons placed in 2p orbitals around a ring. Indicate which of the following statements must therefore be true.

1. There are a total of six electrons in the pi system (defined as electrons in 2p orbitals), including the lone pair on the ring N that is not circled.

2. There are a total of six electrons in the pi system, including the lone pair on the ring N atom that is circled.

3. The lone pair on the ring N atom that is not circled resides in an sp2 orbital on an sp2 hybridized nitrogen atom.

4. Statements 2 and 3 are both true

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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