Chapter 6 alkyl halide a derivative of an alkane in which one or more of the hydrogen atoms has been replaced by a halogen vinyl halide a derivative of an alkene in which one or more of the hydrogen atoms on the double bonded carbon atoms has been replaced by a halogen aryl halide an aromatic compound benzene derivative in which a halogen is bonded to one of the carbon atoms of the aromatic ring alkyl halides are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 but it 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 halides 1 London forces 2 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 group from 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 and forms a new pi bond the reaction is an elimination these eliminations are called dehydrogenations because a hydrogen halide has 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 as the 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 product