A carboxylic ester, commonly referred to as an ester, is a derivative of a carboxylic acid in which the hydrogen of the carboxyl group is replaced by a carbon- containing group.
Functional group
.. ..
C O C
..O..
Methyl acetate (an ester)
C O O
CH3 CH3
Carboxylic ester
A derivative of a carboxylic acid in which H of the carboxyl group is replaced by a carbon.
Example 1.12
The molecular formula of methyl acetate is C3H6O2. Draw the structural formula of another ester with this same molecular formula.
Solution
There is only one other ester with this molecular formula. Its structural formula is
C O O
CH2 CH3 H
(we will usually write this HCOOC2H5)
Problem 1.12
Draw structural formulas for the four esters with the molecular formula C4H8O2.
F. Carboxylic Amides
A carboxylic amide, commonly referred to as an amide, is a derivative of a carboxylic acid in which the !OH of the carboxyl group is replaced by an amine. As the model shows, the group is planar, something we will explain later.
Functional group
Dimethylacetamide (an amide)
C CH3
CH3 H3C
N C N
O O
1.4 Bond Angles and Shapes of Molecules
In Section 1.2, we used a shared pair of electrons as the fundamental unit of a covalent bond and drew Lewis structures for several molecules and ions contain- ing various combinations of single, double, and triple bonds. We can predict bond angles in these and other molecules and ions in a very straightforward way using a concept referred to as valence-shell electron-pair repulsion (VSEPR). VSEPR is based on the electrons in an atom’s valence shell. These valence electrons may be involved in the formation of single, double, or triple bonds, or they may be
Carboxylic amide
A derivative of a carboxylic acid in which the !OH is replaced by an amine.
VSEPR
A method for predicting bond angles based on the idea that electron pairs repel each other and keep as far apart as possible.
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22 Chapter 1 Covalent Bonding and Shapes of Molecules
unshared (lone pair). Each combination creates a negatively charged region of space, and, because “like” charges repel each other, the various regions of electron density around an atom will spread out so that each is as far away from the others as possible.
Figure 1.2
A methane molecule, CH4. (a) Lewis structure and (b) shape.
H H
H
H C
(a) (b)
109.5°
109.5°
We use VSEPR in the following way to predict the shape of a methane mol- ecule, CH4. The Lewis structure for CH4 shows a carbon atom surrounded by four regions of electron density, each of which contains a pair of electrons forming a bond to a hydrogen atom. According to VSEPR, the four regions ra- diate from carbon so that they are as far away from each other as possible. This occurs when the angle between any two pairs of electrons is 109.5°. Therefore, we predict all H!C!H bond angles to be 109.5°, and the shape of the mol- ecule to be tetrahedral (Figure 1.2). The H!C!H bond angles in methane have been measured experimentally and found to be 109.5°, identical to those predicted.
We predict the shape of an ammonia molecule, NH3, in exactly the same man- ner. The Lewis structure of NH3 shows nitrogen surrounded by four regions of electron density. Three regions contain single pairs of electrons forming covalent bonds with hydrogen atoms. The fourth region contains an unshared pair of elec- trons (Figure 1.3). Using VSEPR, we predict that the four regions of electron den- sity around nitrogen are arranged in a tetrahedral manner, that H!N!H bond angles are 109.5°, and that the shape of the molecule is pyramidal (like a triangular pyramid). The observed bond angles are 107.3°. This small difference between the predicted and observed angles can be explained by proposing that the unshared pair of electrons on nitrogen repels adjacent electron pairs more strongly than do bonding pairs.
Figure 1.4 shows a Lewis structure and a ball-and-stick model of a water mol- ecule. In H2O, oxygen is surrounded by four regions of electron density. Two of these regions contain pairs of electrons used to form single covalent bonds to the two hydrogens; the remaining two contain unshared electron pairs. Using VSEPR, we predict that the four regions of electron density around oxygen repel each other and are arranged in a tetrahedral manner. The predicted H!O!H bond angle is 109.5°. Experimental measurements show that the actual bond angle is 104.5°, a value smaller than that predicted. This difference between the predicted and observed bond angles can be explained by proposing, as we did for NH3, that unshared pairs of electrons repel adjacent pairs more strongly than do bonding pairs. Note that the distortion from 109.5° is greater in H2O, which has two unshared pairs of electrons, than it is in NH3, which has only one unshared pair.
A general prediction emerges from this discussion of the shapes of CH4, NH3, and H2O molecules. If a Lewis structure shows four regions of electron density around a central atom, VSEPR predicts a tetrahedral distribution of electron den- sity and bond angles of approximately 109.5°.
In many of the molecules we shall encounter, an atom is surrounded by three regions of electron density. Figure 1.5 shows Lewis structures and ball-and-stick models for formaldehyde, CH2O, and ethylene, C2H4.
According to VSEPR, a double bond is treated as a single region of electron density. In formaldehyde, carbon is surrounded by three regions of electron den- sity: two regions contain single pairs of electrons forming single bonds to hydrogen atoms while the third region contains two pairs of electrons forming a double bond to oxygen. In ethylene, each carbon atom is also surrounded by three regions of
Tetrahedral
A bonding arrangement in which an atom is bonded to four atoms located at the corners of a tetrahedron.
Pyramidal
A bonding arrangement in which an atom is bonded to three atoms in a triangular pyramid.
Unshared electron pair
..
..
H H
H N (a)
(b)
107.3°
Figure 1.3
An ammonia molecule, NH3. (a) Lewis structure and (b) shape.
O
H H
(a)
(b)
104.5°
Unshared electron pairs
Figure 1.4
A water molecule, H2O. (a) Lewis structure and (b) shape.
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1.4 Bond Angles and Shapes of Molecules 23 Figure 1.5
Shapes of formaldehyde, CH2O, and ethylene, C2H4. Molecules shown from (a) top view and (b) side view. Note that chemists commonly use solid wedges to represent bonds projecting toward the viewer, and broken wedges for bonds projecting away from the viewer.
C H H
Top view Side view
Top view Side view
Formaldehyde
Ethylene
C C H
H
H
H H
H
H H O
(a) (b)
(a) (b)
C
C C
121.8⬚
121.4⬚ 117.2⬚
116.5⬚ H
H
Indicates bond is projecting away from viewer
Indicates bond is projecting toward viewer O
electron density: two contain single pairs of electrons, and the third contains two pairs of electrons.
Three regions of electron density about an atom are farthest apart when they are coplanar (in the same plane) and make angles of 120° with each other. Thus, the predicted H!C!H and H!C!O bond angles in formaldehyde and the pre- dicted H!C!H and H!C!C bond angles in ethylene are all 120° and the atoms are coplanar. The experimentally measured angles are quite close to this predic- tion, as shown in Figure 1.5.
In still other types of molecules, a central atom is surrounded by only two regions of electron density. Figure 1.6 shows Lewis structures and ball-and-stick models of carbon dioxide, CO2, and acetylene, C2H2.
In carbon dioxide, carbon is surrounded by two regions of electron density:
each contains two pairs of electrons and forms a double bond to an oxygen atom.
In acetylene, each carbon is also surrounded by two regions of electron density.
One contains a single pair of electrons and forms a single bond to a hydrogen atom, and the other contains three pairs of electrons and forms a triple bond to a carbon atom. In each case, the two regions of electron density are farthest apart if they form a straight line through the central atom and create an angle of 180°.
Both carbon dioxide and acetylene are linear molecules. Predictions of VSEPR are summarized in Table 1.9.
O C
C C
H H
(a) O
Carbon dioxide
(b)
Acetylene
Figure 1.6
Shapes of (a) carbon dioxide, CO2, and (b) acetylene, C2H2, molecules.
Table 1.9 Predicted Molecular Shapes (VSEPR) Regions of Electron
Density Around Central Atom
Predicted Distribution of Electron Density
Predicted
Bond Angles Examples
4 Tetrahedral 109.5° N O
O
O O
C H H C C
H H H
H
H C C H C
C H
H H
H H H
H H
H
3 Trigonal planar 120°
2 Linear 180°
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24 Chapter 1 Covalent Bonding and Shapes of Molecules