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  • An Overview of Organic ReactionsWhy 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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    Kinds of Organic ReactionsIn general, we look at what occurs and try to learn how it happensCommon patterns describe the changes

    Addition reactions two molecules combine

    Elimination reactions one molecule splits into two

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  • *Substitution parts from two molecules exchangeRearrangement reactions a molecule undergoes changes in the way its atoms are connected

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  • What kind of reaction is the transformation shown below? an elimination reactiona rearrangement reactiona substitution reactionan addition reactionnone of theseLearning Check:

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  • What kind of reaction is the transformation shown below? an elimination reactiona rearrangement reactiona substitution reactionan addition reactionnone of theseSolution:

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  • *How Organic Reactions Occur? MechanismsIn a clock the hands move but the mechanism behind the face is what causes the movementIn an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observeReactions occur in defined steps that lead from reactant to product

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  • *Steps in MechanismsWe classify the types of steps in a sequenceA step involves either the formation or breaking of a covalent bondSteps can occur in individually or in combination with other stepsWhen several steps occur at the same time they are said to be concerted

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  • *Types of Steps in Reaction MechanismsBond formation or breakage can be symmetrical or unsymetrical Symmetrical- homolyticUnsymmetrical- heterolytic

    Bond BreakingBond Making

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  • *Indicating Steps in MechanismsCurved 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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  • *Radical ReactionsNot as common as polar reactionsRadicals 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 moleculeA radical can add to an alkene to give a new radical, causing an addition reaction

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  • *Three types of steps

    Initiation homolytic formation of two reactive species with unpaired electronsExample formation of Cl atoms form Cl2 and lightPropagation reaction with molecule to generate radicalExample - reaction of chlorine atom with methane to give HCl and CH3.

    Termination combination of two radicals to form a stable product: CH3. + CH3. CH3CH3Steps in Radical Substitution

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  • *Steps in Radical Substitution: Monochlorination of MethaneInitiationPropagation

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  • *Steps in Radical SubstitutionTerminationWith excess concentration of Cl2 present continued reaction is probable with formation of dichloro, trichloro, and tetrachloro methanes.

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  • In a radical chain reaction, what would be the best description of the following reaction?H3C + Cl CH3ClpropagationeliminationinitiationterminationsubstitutionLearning Check:

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  • In a radical chain reaction, what would be the best description of the following reaction?H3C + Cl CH3ClpropagationeliminationinitiationterminationsubstitutionSolution:

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  • *Radical Substitution: With >1 kind of HWhen there is >1 type of H then there is >1 option for radical formation and therefore >1 option for a monohalogenation product.

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  • In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced?23456Learning Check:

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  • In the reaction of Cl2 with 2-methylbutane, how many monochlorinated isomers are produced?23456Solution:

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  • *Polar ReactionsMolecules can contain local unsymmetrical electron distributions due to differences in electro negativities

    This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom

    The more electronegative atom has the greater electron densityElements such as O, F, N, Cl more electronegative than carbon

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  • *PolarizabilityPolarization is a change in electron distribution as a response to change in electronic nature of the surroundings

    Polarizability is the tendency to undergo polarization

    Polar reactions occur between regions of high electron density and regions of low electron density

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  • p. 144PolarizabilityBonds inherently polar already can be made more polar by reactions with acids or bases.

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  • p. 144PolarizabilityBonds not inherently polar can be polarizable as interactions with solvent or other polar molecules effect the electron distribution.Large atoms with loosely held electrons are more polarizable than small atoms with few tightly held electrons.So: S is more polarizable than OI is more polarizable than Cl

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  • *Generalized Polar ReactionsAn electrophile, an electron-poor species, combines with a nucleophile, an electron-rich speciesAn electrophile is a Lewis acidA nucleophile is a Lewis baseThe combination is indicate with a curved arrow from nucleophile to electrophile

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  • *Figure 5.1: Some nucleophiles and electrophiles. Electrostatic potential maps identify the nucleophilic (red; negative) and electrophilic (blue; positive) atoms.

  • p. 146Learning Check:Which of the following is likely to be a nucleophile and which an electrophile?

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  • p. 146Solution:Which of the following is likely to be a nucleophile and which an electrophile?EENN

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  • p. 146Is BF3 is likely to be a nucleophile or an electrophile?Learning Check:

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  • p. 146Is BF3 is likely to be a nucleophile or an electrophile?Solution:E

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  • Which of the following is expected to be the worst nucleophile?NH3H2OBH3ethylene(CH3) 3PLearning Check:

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  • Which of the following is expected to be the worst nucleophile?NH3H2OBH3ethylene(CH3) 3PSolution:

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  • *An Example of a Polar Reaction: Addition of HBr to EthyleneHBr adds to the part of C-C double bondThe bond is e- rich, allowing it to function as a nucleophileH-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile

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  • *Mechanism of Addition of HBr to EthyleneHBr electrophile is attacked by electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ionBromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bondThe result is that ethylene and HBr combine to form bromoethaneAll polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

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  • *Figure 5.2: A comparison of carboncarbon single and double bonds. A double bond is both more accessible to approaching reactants than a single bond and more electron-rich (more nucleophilic). An electrostatic potential map of ethylene indicates that the double bond is the region of highest negative charge (red).

  • p. 142Learning Check:What product would you expect?

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  • p. 142Solution:What product would you expect?

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  • *Using Curved Arrows in Polar Reaction MechanismsCurved arrows are a way to keep track of changes in bonding in polar reactionThe arrows track electron movement Electrons always move in pairsCharges change during the reactionOne curved arrow corresponds to one step in a reaction mechanism

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  • *Rules for Using Curved ArrowsThe arrow (electrons) goes from the nucleophilic reaction site (Nu: or Nu:- ) to the electrophilic reaction site (sink, E or E+)

    The nucleophilic site can be neutral or negative

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  • *The nucleophilic site can be negative or neutral

    Rules for Using Curved Arrows

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  • *The electrophilic site can be positive or neutral

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    The octet rule must be followed

    The hydrogen already has two e-s so when another pair moves in the 2 already owned have to leave.

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  • What is the role of the alkene in the reaction above? electrophilenucleophilefree radicalcatalystLewis acidLearning Check:

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  • What is the role of the alkene in the reaction above? electrophilenucleophilefree radicalcatalystLewis acidSolution:

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  • Learning Check:Add curved arrows to indicate the flow of electrons:

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  • Solution:Add curved arrows to indicate the flow of electrons:

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  • Add curved arrows to indicate the flow of electrons:Learning Check:

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  • *Add curved arrows to indicate the flow of electrons:Solution:

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  • p. 142Learning Check:What carbocation intermediate is consistent with the product formed? Propose a mechanism. (Add curved arrows to indicate the flow of electrons.)

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  • Reaction Mechanism

  • Bond Cleavage+vely charged ion carbocation-vely charged ion carbaanion Heterolytic CleavageHomolytic Cleavage Free radicals.

  • Carbonium ionPlanar sp2 hybridised bond angle 120oHas six electronsStabilized by resonance or inductive effect or hyperconjugation

  • Examples of carbonium ion