Chapter 6 Notes - 10/8/2010 Polarity Alkyl halides (C sp3...

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10/8/2010 1 Chapter 6 Ionic Reactions- Nucleophilic Substitution and Elimination Reactions of Alkyl Halides 1 Polarity Alkyl halides (C sp 3 hybridized bound to a halogen) are typically polarized towards the halogen, making the carbon atom it is attached to electron deficient (δ+) Carbon-halogen bonds become less polar, but longer and weaker going down the periodic table 2 Naming Vinyl group is CH 2 =CH- Vinylic halides are halides bound to an sp 2 carbon atom 3 Nucleophilic Substitution Reactions A nucleophile replaces a leaving group (bound to an electrophile) Nucleophilic substitution often occurs with the leaving group as a halogen (but this is not always the case!) 4
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10/8/2010 2 Nucleophiles Nucleophile (positive loving): Reagents which seek a positive center, have a lone pair of electrons which can form a bond Often attack electrophiles in chemical reactions What makes a GOOD nucleophile? Lone pair of electrons (usually charged), but is still quite reactive and capable of making a stable new bond Isn’t too reactive, this will often lead it to act like a base instead of a Nucleophile 5 Examples 6 Leaving Groups Leaving Group: A group which is capable of accepting an electron pair (by heterolytic bond cleavage) and leaving What makes a GOOD Leaving Group? Easily accept a pair of electrons and leave on it’s own as a STABLE species (the more stable it is when it leaves, the better the LG) Forms a weak bond to carbon (easier to break) Electron deficient (so it becomes more stable when it leaves) 7 Leaving Groups Cont A LG can leave as an anion Or a LG can leave to form a neutral species 8
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3 Kinetics of an S N 2 The initial rate of the following reaction is found to be directly proportional to the initial concentrations of both methyl chloride and hydroxide S N 2 reaction: s ubstitution, n ucleophilic, 2 nd order (bimolecular, the rate depends on two things) 9 3 Mechanism of the S N 2 Reaction S N 2 mechanism occurs in ONE STEP Nucleophile attacks electrophile and kicks out leaving group Proceeds through a TRANSITION STATE (square brackets) Transition state: VERY short lived species that cannot be isolated (usually has dashed lines) 10 Mechanism of the S N 2 Reaction As there is only one step, it is the Rate Limiting Step (slowest step of a reaction) and both the nucleophile and reactant (Electrophile/LeavingGroup) are involved, hence the RATE depends on both the Nu and E/LG 11 Mechanism Cont Note the direction of Nu attack Backside attack results in INVERSION of the carbon atoms configuration This really matters for stereochemistry!!! Because bond formation and breakage happen
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This note was uploaded on 05/16/2011 for the course CHEM BIO 20a3 taught by Professor Stover during the Fall '10 term at McMaster University.

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Chapter 6 Notes - 10/8/2010 Polarity Alkyl halides (C sp3...

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