Lecture and Chapter 6 notes

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University of Toronto St. George
Thavarajah/ Woolley/ Nitz

CHM138H1 Jasmyn Lee Chapter 6: An Overview of Organic Reactions 6.1 Kinds of Reactions: Addition  Number of bonds at carbon increases  Two atoms add together to form a single product - no atoms are “left over” Elimination  Number of bonds at a carbon decreases  Single reactant splits into two products  Often forms a small molecule such 2s H O or HBr Substitution  Number of bonds at carbon remains the same  Two reactants exchange parts to give two new products Rearrangement  Reorganization of bonds and atoms to give an isomeric product 6.2 How Organic Reactions Occur: Mechanisms Reaction Mechanisms – Absolutely Central to Organic Chemistry  Reaction Mechanism – an overall description of how a reaction occurs; describes what happens at each stage of a chemical transformation; must account for all reactants and products  Electron Accountin Arrows o Note that the arrow follows the direction of electron flow  Bond Cleavage  Bond Formation 1 CHM138H1 Jasmyn Lee  Radical Reaction – symmetrical bond-breaking and bond-making o Radical – “neutral chemical species that contains an odd number of electros and thus has a single, unpaired electron in one of its orbitals  Polar Reaction – unsymmetrical bond-breaking and bond-making o Involve species that have an even number of electrons and thus have only one electron pair in their orbitals o More common  Pericyclic Reaction 6.4 Polar Reactions  Polarity o Bond polarity is a consequence of unsymmetrical electron distribution in a bond due to electronegativity of the bonded atoms  Cl, N, O, F  more EN compared to C  Metals  less EN than C o Polar bonds can result from the interaction of functional groups with acids or bases  Polarizability o Changing interactions with solvent or other polar molecules nearby causes the electric field around a given atom to change – causes electron distribution around that atom to change (polarizability is measure of this change) o Larger atoms, loosely held electrons – more polarizable  S more polarizable than O  Polar Reactions – involve pairs of electrons moving from electron rich (nucleophile) to electron poor (electrophile) atoms  The flow of electrons in these reactions follows the polarization of the bonds involved  Writing Polar Reaction Mechanisms o Arrows represent moving electrons o Arrows go from nucleophiles to electrophiles o Nucleophiles are negative or neutral o Electrophiles are positive or neutral o Octet rule must be followed  No more than 8 electrons ins second shell  No more than 2 electrons in first shell  Nucleophiles o Are electron rich o Often have a lone pair or negative charge o Can also have an easily polarized bond double or triple bond o Form a bond by donating a pair of electrons to an electrophile o Eg/ -OH, -NH3, 2 O, -Cl o Lewis bases (electron donors)  Electrophiles o Always have an electron poor atom o Often have a vacant p orbital or a highly polarized bond o Form a bond by accepting a pair of electrons to an nucleophile o Eg/ Acids (H donors), alkyl halides, carbonyl compounds o Lewis acids (electron acceptors) 2 CHM138H1 Jasmyn Lee o Molecular Orbital calculations of electron distribution in some ions and molecules  Octet Rule o No more than 8 electrons on outer shell of second row element o Make a bond and break a bond at the same time o No more than 2 electrons in the outer shell of H  Size increases – Polarizability increases (weaker bonds more reactive)  Size decreases – Electronegativity increases (stabilizes negative charge bette
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