Geometry - Molecular Shapes
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Geometry
Now that we can draw organic compounds, let’s take a closer look at common geometries we will encounter. The simple theory that helps us understand the shapes of molecules is the valence shell electron pair repulsion theory, VSEPR (pronounced “Vesper”) theory. The idea is that all of the electron groups on an atom (single bonds, double bonds, triple bonds, lone pairs of electrons) are all negatively charged groups. Since like charges repel each other, these groups get as far away from each other as they can. The farthest two groups can get away from each other on an atom is 180°, a linear shape.
Linear molecular geometry
The farthest three groups can get away from each other on an atom is 120°, a trigonal planar shape.
Trigonal planar molecular geometry
The farthest four groups can get away from each other is 109.5°, a tetrahedral shape. We use wedges and dashes to help us draw these shapes. A regular line means that atom lies in the plane of the paper. A wedge indicates an atom is coming towards you, in front of the page. A dashed line indicates an atom is going away from you, behind the page.
Tetrahedral molecular geometry
Even though these all have the same basic shape, we call them different things. Whenever we name a shape, we treat lone pairs of electrons like they are invisible. You can eve cover them up with your hand and ignore them. We just look at the atoms and name the shape we see. On a side note, notice “trigonal pyramid” is the name of a pyramid with a triangle as its base. This is different than a pyramid with a square pyramid as its base (named obviously “square pyramid”).
17. State the geometric shape of the molecule with the following bond angles between atoms (assume no lone pairs of electrons).
a) 109° b) 120° c) 180°
18. Name the geometric shape of the following molecules.
a) CCl4
b) PH3
c) H2C=O
d) O=C=O
e) F2O
f) AlCl3
Dipole Moment
Once we know the geometry of a molecule, we can use that information, along with what we know about electronegativity to determine if a molecule is polar or nonpolar. If a molecule is polar, it has one side of the molecule that is more positive and one side that is more negative. The charges are lopsided. Having a positive and negative side means we have two poles, or a dipole. The strength of this positive-negative separation is called the dipole moment of the molecule. The positive and negative sites are balanced and symmetrical if a molecule is nonpolar.
To determine if a molecule is polar or nonpolar, we look at the molecule’s shape and the electronegativity of the atoms at each bond. We also remember that lone pairs of electrons are very negative spots.
Let’s look at some nonpolar molecules first. They all have a dipole moment of zero. There are some more positive and some more negative spots in the molecules, but overall nonpolar molecules have a symmetrical distribution of charge. Notice the symmetry of the following nonpolar molecules. They all have a mirror plane of symmetry or an inversion point of symmetry.
Nonpolar molecules
Let’s look at some polar molecules. These all have a dipole moment.The dipole moment of each bond is shown with the red dipole arrows. The overall dipole moment for the molecule is shown with the blue arrow. The vector sum of the individual bond dipole moments is the overall molecular dipole moment. Another way to think about that is if you pulled or pushed individually in the direction of all of the little red arrows, how would the molecule move? This direction, the blue arrow, is the overall molecular dipole moment.
Polar molecules
19. Circle the following compounds that are polar (have a dipole moment).
20. Indicate whether each of the following molecules contain polar bonds? Which molecules are polar overall?
a) Cl2
b) HCl
c) H2O
d) CH4
e) CHBr3
f) CH3OCH3
Solubility
But, why do we care?
Some very practical chemistry comes out of knowing the polarity of molecules. For instance, we can begin to predict what solvent is best to dissolve something. Polar solvents tend to dissolve polar compounds and nonpolar solvents dissolve nonpolar compounds. One can remember this by remembering the phrase “Like dissolves like.” For instance, dirt and grease are nonpolar. So, good spot removing cleaners are nonpolar solvents like carbon tetrachloride or hexane.
Some nonpolar solvents
More polar compounds, like glucose, and ionic compounds like sodium chloride (table salt) are more soluble in more polar solvents like water. Glucose and salt do not dissolve in nonpolar solvents at all!
Water soluble compounds
21. Which compound is more soluble in water?
Answers
17. State the geometric shape of the molecule with the following bond angles between atoms (assume no lone pairs of electrons).
a) 109° (tetrahedral) b) 120° (trigonal planar) c) 180° (linear)
18. Name the geometric shape of the following molecules.
a) CCl4 (tetrahedral)
b) PH3 (trigonal pyramidal)
c) H2C=O (trigonal planar)
d) O=C=O (linear)
e) F2O (bent)
f) AlCl3 (trigonal planar)
19.
20.
a) Cl2 No polar bonds, nonpolar molecule
b) HCl Yes polar bonds, polar molecule
c) H2O Yes polar bonds, polar molecule
d) CH4 Yes polar bonds, nonpolar molecule
e) CHBr3 Yes polar bonds, polar molecule
f) CH3OCH3 Yes polar bonds, polar molecule (because it is bent)
21. Which compound is more soluble in water?
a) CH3CH2OH or CH3CH2CH2CH3
b) CH3CH2OCH2CH3 or (CH3)2NH