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chapter5 - 5 An Overview of Organic Reactions Based on...

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5. An Overview of Organic Reactions Based on McMurry’s Organic Chemistry , 7 th edition
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2 Why this chapter? To understand organic and/or biochemistry, it is necessary to know: -What occurs -Why and how chemical reactions take place We will see how a reaction can be described
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3 5.1 Kinds of Organic Reactions In general, we look at what occurs and try to learn how it happens Common patterns describe the changes Addition reactions – two molecules combine Elimination reactions – one molecule splits into two
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4 Video Presentation Addition Reaction – Real Video - Additions
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5 Substitution – parts from two molecules exchange
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6 Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected
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7 5.2 How Organic Reactions Occur: Mechanisms In a clock the hands move but the mechanism behind the face is what causes the movement In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observe Reactions occur in defined steps that lead from reactant to product
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8 Steps in Mechanisms We classify the types of steps in a sequence A step involves either the formation or breaking of a covalent bond Steps can occur in individually or in combination with other steps When several steps occur at the same time they are said to be concerted
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9 Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymetrical Symmetrical- homolytic Unsymmetrical- heterolytic
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10 Indicating Steps in Mechanisms Curved arrows indicate breaking and forming of bonds Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’)
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11 5.3 Radical Reactions Not as common as polar reactions Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule A radical can add to an alkene to give a new radical, causing an addition reaction
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12 Three types of steps Initiation – homolytic formation of two reactive species with unpaired electrons Example – formation of Cl atoms form Cl 2 and light ` Cl 2 + heat 2Cl (D E = 243 kJ/mole) H 3 C-H + heat H 3 C + H (438 kJ/mole) [Too slow to be important] Propagation – reaction with molecule to generate radical Example - reaction of chlorine atom with methane to give HCl and CH 3 . Steps in Radical Substitution CH 4 + Cl 2 + light or heat CH 3 Cl + HCl + CH 3 CH 3 (trace amount)
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13 5.4 Polar Reactions Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.

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chapter5 - 5 An Overview of Organic Reactions Based on...

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