# Molecular Geometries and Isomers

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Molecular Geometries and Isomers

Topics VSEPR The Different Geometries Electronic and Molecular Bond Angles Isomers

VSEPR The Valence Shell Electron Pair Repulsion Theory states that electron pairs will take up postions in space, as far apart from one another as possible. This is because like charges repel each other.

Nonbonded vs. Bonded Pairs
Nonbonded pairs will take up more space around the central atom than bonded pairs. This will affect the bond angles and the shape of the molecule.

Ex. In a linear molecule such as CO2, the bond angles are 180o
Bond Angles are the angles formed between the atoms bonded around the central atom. Ex. In a linear molecule such as CO2, the bond angles are 180o O = C = O 180o

Names of Molecular Geometries
Linear has 180o bond angles Angular (Bent)~ 105o or ~120o Triangular Planar ~ 120o Pyramidal ~ 107o Tetrahedral ~ 109o

There are exceptions to the octet rule
There are exceptions to the octet rule. The names of these Geometries are... Square Planar Triangular Bipyramidal Octahedral Irregular Tetrahedral or see-saw Square Pyramidal T - Shaped

Don’t be disappointed but...
We will not be covering the exceptions to the octet rule.

Two Types of Geometries
1. The electronic geometry is the shape of the particle, due to electron pairs around the central atom.

Types of Geometries... 2. The Molecular geometry is the shape of the particle, made by the atoms bonded to the central atom.

How To Determine The Shape Of a Particle.
1. Draw the Lewis Dot Structure. 2. Count up how many positions are occupied around the central atom, (Atoms and / or electron pairs).

Use the following guide lines to draw a starting shape:
Two positions occupied = Linear. Three positions occupied = Triangular Planar. Four Positions occupied = Tetrahedral.

This is only how to start the drawing. There is more to come !
Ex. Consider H2O The dot structure of water will show that there are 4 positions occupied around the O atom. Two positions are Hydrogens and two positions are nonbonding pairs of electrons.

The dot structure of water...
4 1 : H O H : .. 2 3 So we start by drawing a tetrahedral shape.

Tetrahedral Shape Two lines are on the same plane, the third is pointing back and the fourth points forward. O

H H Now put the bonded atoms on the shape. O 105o
Bond angles between bonding atoms are ~105o O H H 105o

What this diagram shows is...
A structure that has 4 positions around it has an electronic geometry of TETRAHEDRAL. If only Two of the Four positions are bonded, then the molecular geometry is ANGULAR.

Ex. 2 Four Positions Occupied
Consider NH3 The dot structure of NH3 has 4 positions around the Nitrogen. Notice that there are only 3 bonds.

107o Draw the structure for 4 positions. add the bonded atoms N H H H
Bond angles between bonding atoms are ~ 107o

What this diagram shows is...
Since there are 4 positions occupied, the electronic geometry is still tetrahedral. Only 3 of the 4 positions are bonded atoms, so the molecular geometry is PYRAMIDAL.

Ex. 3 Four positions occupied and all four are bonded.
H H C H CH4 .. : : .. Four positions occupied and all four bonded make a tetrahedral electronic and molecular geometry.

Some models of molecules with 4 occupied positions...
Tetrahedral electronic and molecular geometries ~109o angles

Pyramidal - 4 positions, 3 bonded (Molecular geometry)
Ex. NH3

Angular Molecular Geometry - 4 positions, 2 are bonded
Ex. H2O

Structures with 3 occupied positions - Electronic geometry = triangular Planar
Ex. SO2 The dot structure shows that only 2 of the positions are bonded. .. .. .. O S O : : : : .. ..

S O O Draw the structure with 3 occupied positions...
resonance and add the bonded atoms. The molecular geometry is ANGULAR

Three occupied positions with all three positions bonded...
Ex. NO31- The dot structure shows that the electronic and molecular geometries are triangular planar.

Some molecular diagrams with three occupied positions...
Ex. SO2

3 Occupied Positions... Ex. NO3 ~120o bond angles

Two Occupied Positions Are Always Linear Electronic and Molecular
Ex. CO2 The dot structure shows that there are two positions occupied and both positions are bonded.

.. : :: :: : .. O C O Two Positions ...
Note that the CENTRAL ATOM has no nonbonded electrons. If it did, they would cause the molecule to bend. ..

Two positions occupied with one postion bonded...
Ex. CN- C N : ::: : When ever there are only TWO Atoms, both the electronic and molecular geometries are Linear.

Molecular Diagrams With Two Occupied Positions...
Ex. CN-

Two Positions... 180o bond angles
Two resonance structures of CO2 - LINEAR 180o bond angles

Isomers Isomers are two or more compounds with the same molecular formula but different bonding arrangements. Some of them have different physical and chemical properties while others only have reactive differences.

Constitutional or Structural Isomers
These isomers have their atoms bonded in a different order. The order of bonding may change the functional group. It may involve branching in the carbon chain. The double bonds may be in different locations.

Structural Isomers .. .. .. .. : : : : : .. .. .. .. .. .. Ex. C3H8O
H H H H C C C O H .. .. .. .. : : : : : Propanol--> .. .. .. .. .. H H H H C O C C H <--Methoxy ethane ..

Example; Branching C 4 H10 H H H H H C C C C H .. .. .. .. : : : : : .. .. .. .. continued...

C 4 H10 continued... H H H H C C C H H H H C H H

C4H8 Double bond locations differ... H H H H C C C C H H H H H H H H

How do these structures differ?
The top structure (a) is called 1-butene and the bottom structure (b) is called 2-butene. They have different physical and chemical properties; b.p., m.p. etc.

Stereoisomers Cis and Trans isomers Optical isomers

Cis and Trans Isomers The atoms are bonded in the same order but they are oriented differently in space.

A trans isomer... H CH3 C C CH H Trans-2-butene C 4 H 8

A cis isomer... CH CH 3 C C H H cis-2-butene C4H8

Optical Isomers Optical isomers are substances that rotate plane polarized light. An optical isomer contains a chiral carbon in its structure.

Rotation of plane polarized light...
Visible light travels in scattered waves. A polarizer concentrates the light so that it travels in only one direction. It is then polarized.

Rotation of plane polarized light continued...
When polarized light is passed through an optical isomer and viewed through an instrument called a polarimeter, the angle of the light wave is rotated.

A Chiral Carbon A chiral carbon is an asymmetrical carbon, that is, one that has four different groups bonded to it.

Ex. of chiral carbons CH2 OH This is NOT a chiral carbon because there are Two H’s bonded to it. It doesn’t have 4 different groups.

Chiral Carbons continued...
HCBrFOH The carbon in this compound is chiral because it DOES contain 4 different groups. H F C Br OH

Enantiomers Entiomers are pairs of optical isomers that are non-superimposible mirror images.

Enantiomers Mirror images are like your right and left hands. They are the same but opposite and no matter how hard you try, you cannot lie them flat on top of each other in the same direction.

Enantiomers Non-superimposible means just that, you cannot lie them flat on top of each other in the same direction and have them fit.

Enantiomers Many drugs (medications) are enantiomers (non-superimposible mirror images). One of the mirror images will be more active or beneficial than the other.

What To Know, What To Know
VSEPR THE GEOMETRIES -draw and name them, give real examples Types of isomers - draw examples of each. (The book has several examples). Bond angles