Chapter 1 : VSEPR

Chapter 1: Chemical Bonding

VSEPR - Predicting the Shapes of Molecules

Looking first at simple molecules, where a central atom is surrounded by two or more other atoms, there are only six unique electronic geometries that an organic chemist needs to be readily familiar with. The electronic geometry is determined by how many bonding and non-bonding electron groups surround a central atom.

For each electronic geometry, there may be a number of different molecular geometries (the shape of a molecule when only bonded atoms are considered). Representations are given below, manipulate the molecular geometries and see how they relate to the parent electronic geometry.

Though not important when determining the structure of most organic compounds, the trigonal bipyramidal and octahedral geometries are important when considering transition states during S_N2 mechanisms or the geometry about transition metals in reagents or proteins.

Electronic Geometry

Molecular Geometries

Linear

BeH₂

Linear

Trigonal planar


BF₃	SO₂
Trigonal planar	Bent

Tetrahedral


CH₄	NH3	H2S
Tetrahedral	Trigonal pyramidal	Bent

Trigonal bipyramidal


PF5	TeCl4	BrF3	XeF2
Trigonal bipyramidal	See-saw	T-shaped	Linear

Octahedral


SF6	IF5	XeF4
Octahedral	Square pyramidal	Square planar

Representing Molecular Shapes on a Piece of Paper:

Consider the molecule CH4. Below left can be found its Chime image. Figure I shows how this Chime image would be represented on a piece of paper. Unbroken lines are used to represent bonds in the plane of the paper, hashed wedges represent bonds going back out of the plane of paper and solid wedges represent bonds coming out of the plane of the paper. See if you can manipulate the molecule in the Chime image below to match the drawing in figure II.

CH₄

Can you manipulate the trigonal bipyramidal transition state, given in the Chime image below, so that it matches the drawings in each of the figures immediately to the right?

C₂H₇OBr

Notice that bonds that are in the process of being formed or broken are given as dashed lines. The representations that are clearest then are those that place those bonds in the plane of the paper so that they do not get confused with the dashed wedges that represent bonds coming out of the plane of the paper (Figure I III).

The most desirable drawings are usually the simplest, generally they place as many atoms as possible in the plane of the paper.

Consider the molecule n-butane, whose Chime image is given below. Manipulate the molecule so that it matches the drawing in Figure IX.

C₄H₁₀
Figure IX

So what happens when lone pairs of electrons are involved? The most desirable drawings still place the most bonding sites in the plane of the paper even though lone pairs of electrons are most easily represented as being in the plane of the paper (it is not easy to wedge and hash a lone pair).

Consider the Chime image for NH3 given below. Figure X would be the best drawing since it considers the lone pair on nitrogen as being in a bonding site in the plane of the paper. As you can see in Figure XI it is much more confusing if you choose to represent the lone pair as coming out of the paper.

NH₃

Ethyl methyl ether is a structure where the opposite is true. Drawing the lone pairs in the plane of the paper would be far more confusing than if you consider them as being out of the plane.

C₃H₈O
Figure XII

You need to learn to discriminate which drawings most clearly represent the structure of a molecule.