Organic Chemistry Chapter 7: Substitution Reactions

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Substitution Reaction
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One group attacks and one group leaves
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A leaving group ALWAYS ______
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takes a pair of electrons with it
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Characteristics of a good leaving group:
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•electronegative leaving group creates a partial charge on the site of attack to attract the negative charge of the nucleophile. •Be able to stabilize the electrons it leaves with •A good leaving group is a WEAK BASE because it handles the negative charge better • ex: halogens
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Alkyl halides
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compounds where a carbon group (alkyl) is bonded to a halide (F, Cl, Br, or I).
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When naming alkyl halides
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The halide group is the key substituent we will name and locate.
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Common Name: Ethyl Chloride
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/cl
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What is the alpha carbon? What are beta carbons?
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Alpha: where the halide group is attached Beta: the adjacent carbons to the alpha carbon
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3 types of alkyl halides:
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Primary (1º) Secondary (2º) Tertiary (3º)
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EVERY nucleophilic substitution reaction will
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involve NUCLEOPHILIC ATTACK and the LOSS OF A LEAVING GROUP, but rearrangements and proton transfer are possible
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SN2 Mechanism:
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•Concerted (One step) •second order: the rate is linearly dependent on the concentrations of two different compounds •the rate is dependent on the concentrations of the nucleophile and substrate (leaving group)
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SN2 Stereospecificity
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•Stereospecific (product configuration is dependent on starting material configuration): SN2 will have inversion of configuration and reaction proceeds through back-side attack because the nucleophile will encounter a node if it attacks from the front side and will not have a net bond
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SN2 Kinetics Most Reactive (fast) <——————- Least Reactive (slow)
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Methyl, Primary, Secondary, Tertiary due to steric hindrance
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SN1 Mechanism
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•Stepwise process 1st step: loss of leaving group to form carbocation 2nd step: nucleophilic attack on carbocation
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SN1 Kinetics
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•Rate determining step (the slow step) is loss of leaving group •First Order: rate is linearly dependent on the concentration of only one compound Least reactive/least stable———–> Most reactive/stable methyl, primary, secondary, tertiary •rate is governed by the stability of the carbocation that is formed
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Tertiary substrates typically undergo ____ Primary substrates typically undergo ____ Secondary substrates ________
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3º–> SN1 1º–> SN2 2º–> SN2 or SN1 depending on other factors
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SN1 Stereochemistry
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The carbocation that is formed can be attacked from either side (back or front), leading to both inversion and retention of configuration (slight preference for inversion due to ion pairs)
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Some SN1 processes are accompanied with extra steps 1. Proton transfer at the beginning of SN1 2. Proton transfer at the end of SN1 3. Carbocation rearrangement during SN1
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1. occurs when the substrate is an alcohol (OH-) 2. also called Solvolysis–occurs when the nucleophile is neutral (water–H2O) 3. occurs so that the carbocation is more stable
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Determining the mechanism: Step 1: Determine Substrate • 1. Methyl or Primary? • 2. Tertiary? •3. Secondary, allylic, benzylic? •4. Vinyl and aryl? Step 2: Nucleophile •5. Strong nucelophile? •6. Weak nucleophile? Step 3: Leaving Group •7. Good LG? •8. Excellent LG? Step 4: Solvent Effects •9. Polar Aprotic? (contains no H+ atoms directly connected to an electronegative atom) •10. Protic? (contains atleast 1 H+ connected directly to an electronegative atom)
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1. SN2 2. SN1 3. Either– move to next factor 4. Neither 5. SN2 6. disfavors SN2 therefore allows SN1 to compete 7. SN2 8. SN1 9. SN2 10. SN1
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Weak nucleophiles?
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F-, H2O, R-OH
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Strong nucleophiles?
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I-, HS-, HO-, Br-, H2S, RO-, Cl-, RSH, N(trip bond)C-
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Good LG’s:
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I->Br->Cl->tosylate>water
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Bad LG’s:
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HO<CH3CH2O-<(CH3)3O-<NH2-
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Polar Aprotic solvents:
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DMSO, DMF, Acetonitrile, HMPA
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Protic solvents:
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water: H2O; Methanol: Me-O-H; Ethanol: Et-O-H Acetic acid: ROOH; Ammonia: NH3
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In POLAR APROTIC SOLVENTS, what is the reactivity of halides?
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F>Cl>Br>I
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In PROTIC solvents what is the reactivity of halides?
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I>Br>Cl>F
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Some SN2 processes are accompanied with extra steps 1. Proton transfer at the beginning of SN2 2. Proton transfer at the end of SN2 3. Proton transfer before and after SN2
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1. occurs when the substrate is an alcohol (OH-) (very rare in SN2 reactions) 2. also called Solvolysis–occurs when the nucleophile is neutral (water–H2O) 3. When the nucleophile is the hydronium ion (H3O+)
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Choosing reagents necessary for substitution Step one: Analyze substrate and stereochemistry •1. Tertiary? •2. Primary? •3.Secondary- inversion of stereochemistry? •4. Secondary- inversion and retention? Step 2: Analyze leaving group SN1–> weak nucleophile in protic solvent SN2–>strong nucleophile in aprotic solvent •5. Bad leaving group? (-OH) Step 3: Use conditions that favor the identified mechanism
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1. SN1 2. SN2 3. SN2 4. SN1 5. SN1: use acid to protonate OH group SN2: convert to tosylate using TsCl and pyridine

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