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4. Haloalkanes-Organic Chemistry.docx

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Chemistry 1027A/B
Mel Usselman

Organic Chemistry October 15, 2012 4. Haloalkanes- Pre midterm - Friday lecture will be like a mass tutorial - Pi bonds exists in many things – wherever they occur they do the reactions we have studied - Halogens on molecules act like hydrogen’s and have a valence of 1 - General formula: CnH2n +1x o Count x as an H for units of unsaturation calculation - Ex. C3H4Br2 = 2 bromines  1 unit of unsaturation - 2-chlorobutane or sec-butyl chloride common name been in use for so long that they are accepted as well - Alkyl halide o Essential to recognize the type of alkyl halide o Primary alkyl halide- carbon hosting the halogen is bonded to only one other carbon o Secondary alkyl halide- carbon bearing the halogen is bonded to two alkyl groups bonded to 2 carbons o Tertiary alkyl halides- carbon bearing he halogen is bonded to 3 carbon groups - Physical Properties o And sp3 C-X bond is polar- want a good dipole  Sp2 C-X bonds are much less polar  Sp C-X bonds are the least polar o Nature does not like halogens - Preparation of haloalkanes o Prepare halides by addition reactions to pi binds - Substitution reactions o Nucleophilic substitution in which the halogen atom is replaced by a nucleophilic group  Because of the dipole the saturated carbon does not react  If replace hydrogen with halogen- halogen wants the electrons  Polar bond introduces reactivity to it  This is only for sp3 C-X bonds o Leaving groups  Groups that can be displaced by a nucleophile- leaves with their electron pair  Leaving groups are all things that generate a polar bond- anything that does this is a potential leaving group  Leaving groups are all electronegative groups that create a partial +ve charge on a sp3 C atom  Nu groups all bear and electron pair o Everything that has polar covalent bonds you can always predict the electronegativity between two atoms o The major reaction type for saturated compounds bearing polar covalent bonds o There are 2 substitution mechanisms  Same starting material – same product  2 different routes for getting there  Nature can substitute polar bonds in any type of molecule  Reactions  1) SN1= unimolecular nucleophilic reaction  2) SN2= bimolecular nucleophilic substitution o SN2: When you have a leaving group with a dipole bond…  Strong nucleophile- running around looking for something (any carbon)  Nucleophile comes crashing in on opposite side of the carbon  Sometimes there are big groups blocking the carbon meaning the nucleophile doesn’t always come in crashing at the right place all the time  Because the leaving group is blocking the other side  Nu starts to form a bond and the carbon starts to loose the bond with the leaving group  This is known as the transition state- has high energy  This is the high spot on the graph  Delta H here is negative exothermic reaction  When this is completed the Nu is attached to the carbon and the leaving group has left  The other substituents on the carbon are now inverted think of an umbrella  Easy question?  How many molecules does it take to make the transition state? o 2  If the leaving group gets more and more electronegative, is the reaction going to be better or worse?  Better.  If the electron pair gets more and more available, further out, does the reaction go faster?  Yes it does  If the substituents are great big huge groups? Will they speed the reaction up or slow it down?  This will slow down the reaction because it will block more of the molecule making it more difficult for the nucleophile to come in attack the carbon  The mechanistic step is bimolecular (2 molecule involved)  Reaction will be a second order one that depends on the initial concentration of both reagents  Stereochemical inversion at the reaction center  Nu attacks the substrate (R-LG) directly opposite from the leaving group (termed backside attack)  Sometimes the label of the stereocenter remains the same even though the inversion has occurred  Can have an inversion where an S enantiomer is converted to an S enantiomer  ALWAYS HAVE INVERSION  Steric Bulk around the reaction center will slow the reaction  Goes fastest when the groups are smallest  Rate of an SN2 depends critically on the structure of R- LG molecule  ** CH3-LG > (primary) C-LG > (secondary) C-LG >>> (Tertiary) C-LG**  Molecules with a (tertiary) C-LG react so slowly by an SN2 mechanism that they in fact do not normally occur  Aprotic solvents (ones lacking a positively charged H atom) making the reaction go faster  Reactions without a positive hydrogen make the reaction go faster  Protic slows it down  The Better the nucleophile the better the reaction  The strength of a Nu varies inversely with the stability of the non-boned electron pair  Remember the strongest base is the least stable electron pair  What makes me a better nucleophile? o An unstable electron pair- better at attaching yourself to a hydrogen and better attaching yourself to an electron pair
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