UT Arlington CHEM 2321 - Chap.8 notes (15 pages)

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Chap.8 notes

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15
School:
University of Texas at Arlington
Course:
Chem 2321 - Organic Chemistry
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Chap 8 notes Introduction to elimination reactions Elimination when a proton from the beta position is removed together with the leaving groups forming a double bond this is called a beta elimination or 1 2 elimination Dehydrohalogenation name of the process that involves a halide as a leaving group Dehydration when the leaving group is water Alkenes Are usually the end products of an elimination process Alkenes possess double bonds Nomenclature of Alkenes 1 2 3 4 Identify the parent is the longest chain that includes the Identify the substituents Assign a locant to each substituent Arrange the substituents alphabetically When naming the parent replace the suffix ane with ene Example bonds Pentane pentene When numbering the parent chain the double bonds must have the lowest numbers in other words start numbering form the double bonds Example 4 ethyl 3 isopropyl 2 5 dimethyl 2 heptene Number associated with substituents Number associated with the position of the double bond hept 2 ene Common names Ethylene Propylene Styrene Vinyl Allyl Phenyl Methylene Alkenes are also classified by their degree of substitution Substitution refers to the number of alkyl groups connected to a double bond A double bond is comprised of a sigma bond and a pi bond Using cis and trans designations Cycloalkenes comprised of fewer than seven carbon atoms cannot accommodate trans pi bonds rather cis pi bonds There is no need to identify the stereoisomerism of the double bond when naming each of these compounds because the stereoisomerism is inferred For example there is no need to call cyclohexene as cis cyclohexene it is understood An 8 membered ring is the smallest ring to ever contain a trans double bond and be stable at room temperature Bredt s Rule it is not possible for a bridgehead carbon of a bicyclic system to possess a double bond if it involves a trans pi bond being incorporated in a small ring Using E and Z designations Recall Cahn ingold prelog naming 1 Evaluate each carbon separately 2 Highest atomic number is given highest largest priority 3 Compare the highest priority groups Example Z is a representation of the word Zussamen which means together in German in other words cis E reference to the German word Entgegen meaning opposite in other words trans Alkene Stability Factors 1 Steric hindrance cis is less stable that trance due to steric hindrance 2 Degree if substitution tetra is more stable than tri substituted alkenes due to hyperconjugation of alkyl groups Hence the more alkyl bonds there are the more the stability Possible mechanism for elimination Recall All elimination reactions exhibit at least 2 of the four patterns Every elimination reaction features a proton transfer as well as loss of a leaving group In the figure above the reaction occurs simultaneously concerted The figure above depicts a stepwise mechanism where the leaving group leaves generating an intermediate carbocation which is then deprotonated by a base resulting in an alkene Concerted process a base abstracts a proton and the leaving group leaves simultaneously Stepwise process first the leaving group leaves and then the base abstracts a proton Kinetic evidence for a concerted mechanism Show that many elimination reactions exhibit second order kinetics Rate k substrate base Rate is directly proportional to the concentrations of the two different compounds This suggests that in the mechanism the substrate and nucleophile collide with each other This step involved two chemical entities it is therefore said to be bimolecular in other words E2 Effect of substrate Tertiary substrate undergo E2 reactions rapidly Tertiary substrates undergo E2 but not SN2 due to the role played by the reagent A substitution reaction occurs when the reagent functions as a nucleophile and attacks an electrophile whereas an elimination reaction occurs when the reagent functions as a base and abstracts a proton In a tertiary substrate the steric hindrance prevents the reagent from functioning as a nucleophile Figure above shows the rate of reactivity of the E2 mechanism in terms of degree of substitution In terms of energy the transition state is lowest in energy when a tertiary substrate is used and therefore the energy of activation is lowest for tertiary Regioselectivity of E2 reactions When there are two different places that double bonds can form in the product resulting in 2 different regiochemical outcomes The more substituted alkene is generally observed to be the major product Example The reaction above is said to be regioselective Zaitsev product the more substituted alkene Hoffman product the lest substituted alkene The regiochemical outcome of an E2 reaction can often be controlled by carefully choosing the base The more sterically hindered the base and reactant is the greater the chance of the Hoffman product being the major product Stereoselectivity when in a reaction the substrates produce two stereoisomers in unequal amounts Trans is the major product since it is more stable than cis Stereospecificity when there is only one stereoisomeric product obtained To understand how this is obtained when forming a double bond pi orbitals must overlap and for that to occur the proton at the beta position the leaving group and the carbon that will ultimately bear the pi bond must all be coplanar lie in the same plane Before a double bond is made the carbon carbon bond can be rotated as it is a sigma bond When rotating it there are two ways we can achieve a coplanar arrangement anti coplanar and syn coplanar In the Newman projection the anti coplanar conformation is staggered and the syn coplanar conformation is eclipsed Elimination via syn coplanar involves a transition state of higher energy as a result of the eclipsed geometry hence elimination occurs more rapidly via anti coplanar It is not necessary for the molecule to be entirely coplanar if there are some deviations in angle from 180 say 178 or 179 the reaction can still proceed as long as the angle is close to 180 In this case the term periplanar is used rather than coplanar In such a conformation the orbital overlap is significant enough for an E2 reaction to occur Instead it is good enough to say that it is sufficient enough for the proton and the leaving group to be anti periplanar The stereoisomeric product of an E2 process depends on the configuration of the starting alkyl halide The product obtained depends on the configuration of the starting alkyl halide The only way to


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