chapter_2_structure_and_bonding

chapter_2_structure_and_bonding - Chapter 2 Structure and...

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Chapter 2: Structure and Properties of Organic Molecules
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Atomic Orbitals (2.1-2.5) Wave functions that represent the probability of finding electrons in a specific region of space s, p, d, f orbitals 33 In organic chemistry, need to concentrate only on s and p orbitals
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p orbitals have a nodal plane Area of space where the probability of finding electrons is almost zero 34
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head2down Molecular orbitals (2.2) are produced when atomic orbitals (either native or hybridized) of different atoms interact boxshadowdwn Produces bonding and anti-bonding orbitals * antibonding molecular orbital Molecular Orbitals for H 2 35 bonding molecular orbital relative energy s atomic orbital of hydrogen s atomic orbital of hydrogen
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head2down In the case of p orbitals, the overlap can take place in two different forms 36
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37
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Single Bonds (2.3) All single bonds are sigma bonds and all double or triple bonds contain only one sigma bond head2down Sigma bonds can be formed from 38 atomic orbitals, hybridized or native or a combination
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head2down 2 other types of bonds in organic molecules: head2down Pi ( π ) bond 39 head2down Hydrogen-bond (H-bond)
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head2down π -bond : cannot exists if a σ -bond is not already present head2down Result from the overlap of p orbitals of two atoms. The π -bond is always perpendicular to the sigma bond connecting the nuclei. 40 head2down Occur in sp 2 and sp hybridized atoms (double/triple bonds)
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head2down Double bond 4 electrons in the bonding region between the nuclei * first pair forms the sigma bond * second pair forms the pi bond Normal combination for a double 41 bond such as the one found in ethylene Example: Ethene or ethylene C C H H H H
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head2down Triple Bond 6 electrons in the bonding region between the nuclei * first pair forms the sigma bond * second/third pairs form the pi bonds Normal combination for a triple bond such as the one found in acetylene 42
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Hybridization and Molecular shapes (2.4) head2down Molecular shapes are related to hybridization of central atom 43 sp 3 : only single bonds; 109.5 o angle (methane) sp 2 : double bond; 120 o angle (ethylene) sp: triple bond; 180 o angle (acetylene)
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head2down Atomic orbitals can combine to generate new orbitals: * Hybridization: combination of atomic orbitals of the same atom producing new orbitals of lower energy * sp, sp 2 and sp 3 These orbitals explain the geometry of molecules Shape of sp Hybrid Orbitals 44 in phase out-of-phase one s orbital combines with 3 p orbitals to form 4 new sp 3 hybrid orbitals s p sp 3
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45
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46
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head2down Only 3 general shapes are normally found in organic molecules: 47
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Rotation of Single Bond vs Rigidity of Double Bond (2.7) head2down Consider ethane: CH 3 -CH 3 Each carbon is sp 3 48 Many structures of ethane are possible. They differ only by the position of one methyl group in relation to the other one:
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head2down CONFORMATIONS : Structures that differ only by rotation along a single bond Ex: ethane Both forms exist, and all other structures in between. In fact a real ethane molecule rotates through all the conformations, 49 but the staggered conformation is favoured.
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