   Chapter 8, Problem 94IL

Chapter
Section
Textbook Problem

The backbone of both DNA and RNA consists of alternating deoxyribose and phophate groups (Figure 8.14). (a) Rank the types of bonds (such as C-C C-O) in 2-deoxyribose in terms of increasing polarity. (b) What are the expected C-O-C, O-C-C, and C-C-C bond angles in the C4O ring?

(a)

Interpretation Introduction

Interpretation:

The bonds in 2-deoxyribose has to be ranked in terms of increasing polarity.

Concept introduction:

Polar covalent bond involves an unequal sharing of electrons between two bonded atoms; it creates a partial separation of charge and a dipole moment.

Covalent bonds are polar, if there is a difference in electronegativity between the bonded atoms.

Explanation

The given compound is 2-deoxyribose

The electronegativity of atoms in a bond will be the deciding factor to predict whether the bond is polar or non-polar.

Electronegativities of the atoms in 2-deoxyribose are given below:

Oxygen3.5Carbon2.5Hydrogen2.1

Electronegativity differences between bonds are given below:

C-O=3.5-2.5=1.0

C-H=2.5-2.1=0.4

O-H=3.5-2.1=1

(b)

Interpretation Introduction

Interpretation:

The expected COC, OCC and CCC bond angles in the C4O ring has to be determined.

Concept introduction:

• In VSEPR, the geometry of the molecule is explained based on minimizing electrostatic repulsion between the molecules’ valence electrons around a central atom
• Bond angle is the angle between two bonds of a molecule and it is determined based on the electron-domain geometry.

[Bond angles: tetrahedral = 109.5o, trigonal planar = 120o, T-shape = 90o]

Electron geometry is the shape of a molecule predicted by considering both bond electron pairs and lone pair of electrons.

Molecular geometry is the shape of a molecule predicted by considering only bond pair of electrons.

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