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UH CHEM 3331 - Structures of Organic Molecules
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CHEM 3331 1nd Edition Lecture 3Outline of Last Lecture I. Lewis Dot StructureII. Formal Chargea) Neutralb) Positivec) NegativeIII. Resonance structuresIV. Skeletal formulasV. Acids and Bases ReviewOutline of Current Lecture I. Answer to question posed in previous notesII. Linear Combination of Atomic Orbitals (LCAO)III. Bonding and anti-bondingIV. HybridizationV. Drawing compounds additional informationVI. Rotation VII. Constitutional isomersVIII. StereoisomersCurrent LectureI. Answer to previous questionIn the previous lecture the question asked was, If the anion is stable then the conjugate acid is weak or strong. The answer is strong. II. LCAOThe Linear Combination of Atomic Orbitals is the addition or subtraction of atomic orbitals. For example H2.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.These representations simply add the atomic orbitals of H2 together or subtract them. This also introduces our next topic: Bonding and Anti-Bonding.III. Bonding and Anti-BondingThe bonding orbital is the LCAO that adds the atomic orbitals while the anti-bonding orbital is the LCAO that subtracts the atomic orbitals.There is no bonding orbital without an anti-bonding orbital. For our H2 orbitals above we have σfor the bonding orbital and σ* for the anti-bonding orbital. However, this becomes more complicated when we have p orbitals that are being added together. We also have to consider the orientation they are being added together or subtracted in. With p orbitals we have σ and σ*, but when we change the orientation of the p orbitals we get π and π*. (Representation on next page)Here we have our 2px, 2py, and 2pz orbitals. 2px is the first 2 drawings. The first picture represents the bonding orbital of 2px and is σ. The second picture represents the anti-bonding orbital of 2px and is σ*. The third and fourth pictures are of 2py. The third picture is the anti-bonding orbital and is π*. The fourth picture is the bonding orbital and is π.σ is stronger than π. σ is lower energy and σ* is higher energy. IV. HybridizationHow to determine hybridization1) Add up all things bonded to an atom (including lone pairs)2) If 2 things are bonded to the atom- spIf 3 things are bonded to the atom- sp2If 4 things are bonded to the atom- sp3For example, CH4. This molecule is a tetrahedral. 4 things are bonded to C so its hybridization is sp3. V. Drawing compounds additional informationUsing our above example of CH4, there is another way to draw this compound.The thin solid lines represent the molecules resting on the same plane. The triangular solid line represents a molecule in a plane towards us. The dashed line represents a molecule in the plane away from us.VI. RotationThere are two types of rotation: Free-rotation where the bond is used as an axle and restricted rotation (resulting by double or triple bonds). For Free- rotation we will use CH3CH3 (ethane). This is the staggered free- rotation of Ethane and is more stable than the eclipsed free-rotation of ethane. Ethane changes between its eclipsed and staggered form by rotating freely around the single bond between the two carbons. This makes the two compounds the same in structure and composition. This is the eclipsed free-rotation of ethane. The two H in the plane coming towards us and going away from us are on the same side as each other. Whereas, in the first one they were on opposite sides. Restricted rotation means the compound cannot rotate on its central bond. Therefore, these two pictures of C2Cl2H2 are not the same in structure. (representation on next page)Due to the double bond between the carbons. This molecule cannot rotate between these two forms. They have different boiling points, melting points, color, etc. They have the same composition, but different structures.Compounds that have the same composition, but different structures are known as isomers.VII. Constitutional isomersConstitutional isomer simply means a molecule has the same composition but different connectivities.These are compounds such as C4H10, C5H12, C5H10.We will use C5H12 as our example. It has 3 different structures.As you can see, each arrangement of atoms has a different name because they are different compounds with the same composition. VIII. StereoisomersStereoisomers are molecules that have the same composition but differ in their 3D representation. The two different molecules are known as cis- and trans-. Cis- means the same, the atoms that are alike are on the same side of the 3D model. Trans- means opposite, the atoms that are alike are on opposite sides of the 3D model. For example,


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UH CHEM 3331 - Structures of Organic Molecules

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