7주차 1회차할로겐화합물

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7주차“할로겐화합물”

“Alkyl Halides”

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Alkyl Halides

Organic Chemistry, 2011

Alkyl Halides

•Halogen-substituted organic compounds

•1. Naming Alkyl Halides

Step 1

•1. Naming Alkyl Halides

Step 2

•1. Naming Alkyl Halides

Step 3

•1. Naming Alkyl Halides

•2. Preparing Alkyl Halides

•2. Preparing Alkyl Halides

� Reactivity

� Carbocation intermediate

•2. Preparing Alkyl Halides

� These reactions normally take place in high yield

� Reactive species

•2. Preparing Alkyl Halides

� N-bromosuccinimide (NBS) selectively brominates allylic positions

� Requires light for activation

–Preparing Alkyl Halides from Alkenes: Allylic Bromination

•2. Preparing Alkyl Halides

•2. Preparing Alkyl Halides

–Allylic Stabilization

� More stable than typical alkyl radical by 40 kJ/mol (9 kcal/mol)

� Allylic radical is more stable than tertiary alkyl radical

•2. Preparing Alkyl Halides

–Stability of the Allyl Radical: Resonance Revisited

� Three electrons are delocalized over three carbons� single electron is dispersed

•2. Preparing Alkyl Halides

–Use of Allylic Bromination

� Allylic bromination with NBS creates an allylic bromide� Reaction of an allylic bromide with base produces a conjugated diene, useful

in synthesis of complex molecules

•3. Reaction of Alkyl Halides: Grignard reagents

� Both basic and nucleophilic

•3. Reaction of Alkyl Halides: Grignard reagents

� R─X → R ─ MgX → R ─ H

•4. Nucleophilic Substitution Reactions

� Walden� Inversion of configuration� Nucleophilic Substitution Reaction (Most common)� Remember: (+)/(-) signs do not indicate configurations

•4. Nucleophilic Substitution Reactions

� Mechanisms (SN2, SN1)� Nucleophile, Substrate, Leaving group

•4. Nucleophilic Substitution Reactions

� An acetylide anion

•4. Nucleophilic Substitution Reactions

� Nucleophiles

•5. The SN2 Reaction

� Nu attacks the substrate from a direction 180o away from the leaving group

� TS� Partially formed Nu-C bond� Partially broken C-X bond� Negative charge is shared� Negative charge is shared� Simultaneous bond making and breaking� Octet rule is not violated

� Umbrella inversion

•5. The SN2 Reaction

� Reaction rates depends on� concentrations of both Nu and Substrate� Bimolecular reaction (SN2)� SN2 reaction takes place in a single step when substrate and nucleophile

collide and react

– Probing the mechanism of SN2 Reactions

•5. The SN2 Reaction

� Complete inversion

– Stereochemistry of SN2 Reactions

Tetrahedral Carbon

Planar

Tetrahedral (Inverted)

•5. The SN2 Reaction

� Spatial accessibility (Bulkiness of substrate)� determines how easy an SN2 reaction will proceed

– Steric Effects in SN2 Reactions

Me 1o 2o 3o> > >

•5. The SN2 Reaction

� Why?� Trajectory is blocked� Remember! 180o away from the leaving group!

– Vinylic halide & Aryl halide

•5. The SN2 Reaction

� Expelled with a negative charge� Stable anion is a good leaving group� I, Br, Cl� The most common leaving groups

– The Leaving Group in SN2 Reactions

•5. The SN2 Reaction

� If a group is very basic or very small, it is a bad leaving group.

� Alkyl fluorides, alcohols, ethers, and amines do not typically undergo SN2 reactions.

–Poor Leaving Groups

•5. The SN2 Reaction

–Alternatively; make it better leaving group

•5. The SN2 Reaction

– The Solvent

� Solvents that can donate hydrogen bonds (-OH or –NH) slow SN2 reactions by associating with reactants

� Energy is required to break interactions between reactant and solvent� Polar aprotic solvents (no NH, OH, SH) form weaker interactions with

substrate and permit faster reaction

•5. The SN2 Reaction

– The Solvent