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Chapter 6- alkyl halide: a derivative of an alkane in which one (or more) of the hydrogen atoms has been replaced by a halogen- vinyl halide: a derivative of an alkene in which one (or more) of the hydrogen atoms on the double-bonded carbon atoms has been replaced by a halogen- aryl halide: an aromatic compound (benzene derivative) in which a halogen is bonded to one of the carbon atoms of the aromatic ring- alkyl halides are used mostly as industrial and household solvents- carbon tetrachloride used to be used for dry-cleaning/ stain removing butit’s carcinogenic- chloroform and methylene chloride are also good solvents- are also used as pesticides- freons: a generic name for a group of chlorofluorocarbons used as refrigerants, propellants and solvents- just to review, a molecular dipole moment is the vetro sum of the individual bond dipole moments- 2 things that affect the boiling point of alkyl halides1. London forces2. dipole-dipole attractions (arising from the C – X bond)- free-radical halogenation isn’t very effective because it can produce a messy mixture of products- allylic: the saturated position adjacent to a carbon-carbon double bond- free-radical bromination of alkenes is highly selective- only the most stable radicals are formed- allylic shift: a rearrangement that results from reaction at either end of a resonance-stabilized allylic intermediate- halides are good leaving groups because the halogen atom can leave with its bonding pair of electrons and form a stable halide ion- when another atom replaces the halide ion, the reaction is a substitution- in a nucleophilic substitution, a nucleophile replaces a leaving groupfrom a carbon atom, using its lone pair of electrons to form a new bond to the carbon atom- when the halide ion leaves with another atom or ion (often hydrogen) andforms a new pi bond, the reaction is an elimination^ these eliminations are called dehydrogenations because a hydrogen halidehas been removed from the alkyl halide- substrate: (the compound that is attacked by the reagent);- hydroxide ion is a strong nucleophile (donor of electron pair) because the oxygen atom has unshared pairs of electrons and a negative charge- concerted reaction: a reaction in which the breaking of bonds and the formation of new bonds occur at the same time (in one step)- SN2 reactions … substitution, nucleophilic, bimolecular… the transition state is the rate-limiting step… second order overall- halogen exchange reactions are when a halide displaces another, in order to convert a halide into another halide- basicity: the equilibrium constant for abstracting a proton- nucleophilicity: the rate of attack on an electrophilic carbon atom- if the new bond is to a proton it has reacted as a base- if the new bond is to a carbon, it has reacted as a nucleophile- polarizable: having electrons that are easily displaced toward a positive charge. Polarizable atoms can begin to form a bond at a relatively long distance- steric hindrance: interference by bulky groups that slow a reaction or prevent it from occurring- protic solvent: a solvent containing acidic protons, usually O – H or N – H groups- aprotic solvents: a solvent that has no acidic protons; a solvent with no O – H or N – H groups- inversion of configuration: a process in which the groups around an asymmetric carbon atom are changed to the opposite special configuration, usually as a result of a back-side attack- sn1 reaction … substitution, nucleophilic, uni-molecular … multi-step process!- step 1: formation of carbocation (rate limiting)- step 2: nucleophilic attack on the carbocation- step 3: loss of proton to solvent^ highly substituted cations are more stable than less substituted- racemization: the loss of optical activity that occurs when a reaction shows neither clean retention - retention of configuration: formation of a product with the same configuration asthe reactant; in a nucleophilic substitution, retention of configuration occurs when the nucleophile assumes the same stereochemical position in the product as the leaving group occupied in the reactant- inversion of configuration: a process in which the groups around an asymmetric carbon atom are changed to the opposite spatial configuration, usually as a result of back-side attack- hydride shift: (symbolized as ~H) movement of a hydrogen atom with a pair of electrons from one atom (usually carbon) to another. Hydride shifts are examples of rearrangements that convert carbocations into more stable carbocations- methyl shift: symbolized (~CH3) rearrangement of a methyl group with a pair of electrons from one atom (usually carbon) to another. A methyl shift (or any alkyl shift) in a carbocation generally results in a more stable carbocation- dehydrohalogenation: an elimination in which the two atoms lost are a hydrogen atom and halogen- Zaitsev’s rule: (Saytzeff’s rule) An elimination usually gives the most substituted alkene product. Zaitsev’s rule does not always apply, but when it does, the reaction is said to give Zaitsev orientation.- anti: adding to (or eliminating from) opposite faces from a molecule- anti-coplanar: having a dihedral angle of 180 deg.- syn-coplanar: having a dihedral angle of 0 deg.- stereospecific reaction: a reaction in which different stereoisomers react to give different stereoisomers of the


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DREXEL CHEM 241 - Chapter 6

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