Chapter 5OrganohalidesCompounds that contain one ormore halogen atoms
Naming alkyl halides• Commonly called alkyl halides- identifying firstalkyl group and then the halogen• Named systematically as haloalkanes (halogenas substituent)• find longest chain and name parent• Number carbon of parent chain beginning at endnearer the first substituent whether alkyl or halo.• If numbering can be made from either end beginat end nearer first substituent.
Exercise• Name the following alkyl halides• Draw structures corresponding to thefollowing IUPAC names– 2,3-dichloro-4-methylhexane– 4-bromo-4-ethyl-2-methylhexane– 3-iodo-2,2,4,4-tetramethylpentane– Cis-1-bromo-2-ethylcyclopentane
Structure of alkyl halides• Increase in size down periodic table• Length of carbon-halogen bonds increaseaccordingly and C-X bond strengthdecrease• C-X bond polar• Chemistry of alkyl halides dominated bytheir electrophilic behavior
Preparing alkyl halides• Preparing alkyl halides from alkanes– simple alkyl halides can be prepared byradical reaction of alkane with Cl2 or Br2 inpresence of UV light– initiation, propagation, termination– reaction continues in a self sustaining cycle– mixture of products result– Br more selective because it depends onstability of radical
• Preparing alkyl halides from alkenes– Reaction of alkenes with HX or X2– Reaction of N-bromosuccinimide (NBS) inpresence of light to give products resultingfrom substitution of hydrogen by bromine atallylic position (analogous to alkanehalogenation)– Allylic radical- greater stability becausegreater number of resonance, electronsattracted to more nuclei.– Allylic bromination of unsymmetrical alkenesgives mixture of products• Preparing alkyl halides from alcohols– Treat alcohols with HCl, HBr, HI.
Reactions of alkyl halides• Grignard reagent– React with Mg metal in ether/THF solvent toyield alkylmagnesium halides, RMgX(Grignard reagent)– Halogens: Cl, Br, I– C-Mg polarized making C nucleophilic andbasic
Oxidation and reduction inorganic chemistry• Oxidation: decreases electron density oncarbon by:– Forming one of these; C-O, C-N, C-X– Or breaking this; C-H• Reduction: increases electron density oncarbon by:– Forming this; C-H– Or breaking one of these; C-O, C-N, C-X
Exercise• Reaction below in not favorable to beapply in industry. Why?• A large amount of 1-bromopent-2-ene isneeded for a synthesis reaction. Thefollowing reaction was carried out. What iswrong with the following reaction.explain
• Predict the product(s) of the followingreactions
SN1• SN1 reaction (substitution, nucleophlic,unimolecular)– Loss of leaving group before nucleophileapproaches– Characteristic of reaction• Substrate: yield most stable carbocation• Nucleophile: nonbasic prevent competition withelimination. Does not affect rxn rate. Neutral nucleophileswork well• Leaving group: good leaving group• Solvent: polar solvent
SN2• SN2 reaction (substitution, nucleophlic,bimolecular)– Single step without intermediate– Incoming nucleophile reacts with the substrate(alkyl halide or tosylate) from opposite direction ofleaving group.– Invert stereochemical configuration– Characteristic of reaction• Substrate: steric hindrance decreases rxn rate. Bestmethyl, 10 substrates, 2o react slowly, 3o do not react.• Nucleophile: basic,-ve charge• Leaving group: good leaving group more stable• Solvent: polar aprotic solvent
Elimination reaction• E2 reaction (elimination, bimolecular)– Anti periplanar geometry for p orbital overlap– Non zaitsev’s rule• E1 reaction (elimination, unimolecular)– E1 and SN1 reactions occur together– No geometric requirement– zaitsev’s rule• E1cB reaction– Carboanion intermediate– Common in substrates with poor leaving group
Summary• Primary alkyl halides:– SN2 reaction occur if good nucleophile is used– E2 reaction occur if strong base is used– E1cB reaction occur if the leaving group is 2 C away fromcarbonyl group• Secondary alkyl halides:» SN2 reaction occur if weakly basic nucleophile is used in apolar aprotic solvent» E2 reaction occur if strong base is used» E1cB reaction occur if the leaving group is 2 C away fromcarbonyl group» 2o allylic and benzylic alkyl halides can undergo SN1 andE1 reactions if weakly basic nucleophile is used in a proticsolvent
• Tertiary alkyl halides:• E2 reaction occur if base is used• SN1 and E1 reactions occur togetherunder neutral conditions• E1cB reaction occur if the leaving group is2 C away from carbonyl group