Structural and Stereoisomers of Hydrocarbons Course Notes


Review of VSEPR Theory, Molecular Shapes and Bond Angles


VSEPR Theory

To be able to predict the molecular shapes of a molecule, one must use the VSEPR theory. This theory states that the electron pairs of different valence orbitals repel each other and cause the orbitals to arrange themselves so that they are as far apart as possible.


Molecular Shapes

There are five molecular shapes a molecule can have: tetrahedral, trigonal planar, trigonal pyramidal, bent, and linear.

Tetrahedral

A molecule that has a tetrahedral shape has all four pairs of electrons bonded. The four pairs of electrons are repelling each other and to minimize the repulsion the orbitals get as far apart as possible. This causes the tetrahedral shape. Because all four pairs of electrons are bonding pairs, the bond angles are all 109.5o. An example of a tetrahedral shaped molecule is methane.


Trigonal Planar

A molecule with trigonal planar shape has three bonds all of which lie in the same plane. This can include one double bond which acts as a single bond. The bond angles are 120o. An example of a trigonal planar shaped molecule is formaldehyde.


Trigonal Pyramidal

A molecule with trigonal pyramidal shape has four pairs of electrons all repelling each other. Of the four pairs of electrons, 3 are bonding pairs, and one lone pair of electrons. Bond angles for trigonal pyramidal are 109.5o. Ammonia is an example of a molecule with trigonal pyramidal shape.


Bent

Molecules with a bent shape has four pairs of electrons, but only two of the pairs are bonding pairs (two are lone pairs). Because only two pairs are bonding the bond angle is 109.5o. Water is an example of a molecule with a bent shape.


Linear

A molecule with a linear shape has two pairs of bonding electrons. Repulsions between these two pairs of electrons will cause them to be as far apart as possible which creates the linear shape. Bond angles for linear molecules are 180o. Carbon dioxide is an example of a molecule with a linear shape.


Double and Triple Bonds

Bond angles in species that contain double and triple bonds indicate that the electrons in the multiple bond behave as a single pair of electrons according to VSEPR.

Total No. of electron pairsSharedUnsharedBond AngleMolecular GeometryExample
220180linearBeF2
330120trigonal planarBF3
440109.5tetrahedralCCl4
431109.5trigonal pyramidalNH3
422109.5bentH2O

Conformational Isomers

Conformational isomers are different arrangements of atoms that are interconverted by ROTATION ABOUT SINGLE BONDS. They are molecules with the same structural formula existing as conformational isomers in 3-D due to rotation about a sigma bond. NO CHEMICAL BONDS ARE BROKEN. Specific conformational isomers require special nonmenclature such as staggered and eclipsed. Staggered conformations about carbon-carbon single bonds are more stable than corresponding eclipsed conformations because the groups in the staggered conformation are as far apart as possible.

Staggered and Eclipsed

Chair and Boat Conformations of Cyclohexane

Cyclohexane has a tetrahedral shape around each of the carbon atoms in the ring. The cyclic structure prevents free rotation around the single bonds. Chair conformation is slightly more stable than boat conformation because the hydrogen atoms in the boat conformation are very close to each other thus repelling each other. In chair conformation the hydrogen atoms are as far apart as possible.

Boat and chair configurations

Structural Isomers

Structural isomers are different arrangements of atoms that are interconverted by breaking bonds. They have the same structural formula but are connected differently. They are not stereoisomers. For example chloropropane can exist in two forms:

1-chloropropane: CH3CH2CH2Cl AND 2-chloropropane: CH3CHClCH3

In 1-chloropropane the chlorine is bonded to an end carbon while in 2-chloropropane the chlorine is bonded to the middle carbon.

Structural isomer

Stereoisomers

Stereoisomers are isomers of molecules that look very similar, have identical structural formulas but their atoms are arranged in a different order in three dimensional space, so they are not superimposable. If you find that you can visualize all the atoms of one molecule lying directly upon identical atoms in the other molecule, then the molecules are superimposable and are the very same substance and are not steriosomers.

There are two types of stereoisomers: enantiomers (mirror images) and diastereomers (not mirror images).


Enantiomers

Enantiomers are molecules(stereoisomers) that are mirror images of each other but cannot be superimposed.Human hands are an example of stereoisomers that are mirror images of each other but are non-superimposable.

chirality

They must be chiral meaning they have a structure that cannot be superimposed on its mirror image. They can exist in either the D or L configuration. One way to distinguish between a D and L form is that only the left (L handed ) form will react with enzymes. The D form doesn't have the correct shape to fit into the active site of the enzyme so it is biochemically inert.

left image right image

Diastereoisomers

Stereoisomers which are not enantiomers are called diastereoisomers. Diastereoisomers are not mirror images of one another. They do have different physical properties and can be separated from one another easily. They are distinguished by the prefix CIS and TRANS. When the molecules have identical skeletons and the carbon - carbon double bonds are in the same location in the alkene the position of of hydrogen atoms must be inspected to determine the prefix. If the hydrogen atoms are on the same side then the molecule is gien the prefix "cis". Otherwise, when the hydrogen atoms are on opposite sides the prefix is "trans".

cistrans
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