Presentation on theme: "Molecular Geometries and Isomers. Topics å VSEPR å The Different Geometries å Electronic and Molecular å Bond Angles å Isomers."— Presentation transcript:
Molecular Geometries and Isomers
Topics å VSEPR å The Different Geometries å Electronic and Molecular å Bond Angles å 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 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. Nonbonded vs. Bonded Pairs
Bond Angles are the angles formed between the atoms bonded around the central atom. Ex. In a linear molecule such as CO 2, the bond angles are 180 o O = C = O 180 o
Names of Molecular Geometries Linear has 180 o bond angles Angular (Bent) ~ 105 o or ~120 o Triangular Planar ~ 120 o Pyramidal ~ 107 o Tetrahedral ~ 109 o
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 The electronic geometry is the shape of the particle, due to electron pairs around the central atom. 1. The electronic geometry is the shape of the particle, due to electron pairs around the central atom.
Types of Geometries... TheMolecular geometry is the shape of the particle, made by the atoms bonded to the central atom. 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: 3Two positions occupied = Linear. 3Three positions occupied = Triangular Planar. 3Four Positions occupied = Tetrahedral.
This is only how to start the drawing. There is more to come ! Ex. Consider H 2 O 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... H O H:.. : 1 2 3 4 So we start by drawing a tetrahedral shape.
Tetrahedral Shape O Two lines are on the same plane, the third is pointing back and the fourth points forward.
Now put the bonded atoms on the shape. O H H Bond angles between bonding atoms are ~105 o 105 o
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. 2Four Positions Occupied Consider NH 3 The dot structure of NH 3 has 4 positions around the Nitrogen. Notice that there are only 3 bonds.
Draw the structure for 4 positions. N add the bonded atoms H H H Bond angles between bonding atoms are ~ 107 o 107 o
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. CH 4 H H C H H :.. : 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 ~109 o angles
Angular Molecular Geometry - 4 positions, 2 are bonded Ex. H 2 O
Structures with 3 occupied positions - Electronic geometry = triangular Planar Ex. SO 2 The dot structure shows that only 2 of the positions are bonded. O S O :.. :::
Draw the structure with 3 occupied positions... and add the bonded atoms. S O O The molecular geometry is ANGULAR resonance
Three occupied positions with all three positions bonded... Ex. NO 3 1- The dot structure shows that the electronic and molecular geometries are triangular planar.
Some molecular diagrams with three occupied positions... Ex. SO 2
3 Occupied Positions... Ex. NO 3 ~ 120 o bond angles
Two Occupied Positions Are Always Linear Electronic and Molecular Ex. CO 2 The dot structure shows that there are two positions occupied and both positions are bonded.
Two Positions... :: : :.. Note that the CENTRAL ATOM has no nonbonded electrons. If it did, they would cause the molecule to bend. OCO
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... Two resonance structures of CO 2 - LINEAR 180 o 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.
Ex. C 3 H 8 O Structural Isomers H H H H C C C O H H H H :::::.. H H H H C O C C H H H H.. Propanol--> <-- Methoxy ethane
Example; Branching C 4 H 10 H H H H H C C C C H H H H H.. : :::: continued...
C 4 H 10 continued... H H H H C C C H H H H C H H
Double bond locations differ... C4H8C4H8 H H H H C C C C H H H H H H H H H C C C C H H H (a) (b)
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 4 Cis and Trans isomers 4Optical isomers
Cis and Trans Isomers The atoms are bonded in the same order but they are oriented differently in space.
H CH 3 C C CH 3 H Trans-2-butene A trans isomer... C 4 H 8
A cis isomer... CH 3 C C H cis-2-butene C4H8C4H8
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.
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. Rotation of plane polarized light continued...
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 CH 2 OHThis 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 H F C Br OH The carbon in this compound is chiral because it DOES contain 4 different groups.
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