Chap. 8 notesIntroduction to elimination reactionsElimination: 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 bondsNomenclature of Alkenes1. Identify the parent: is the longest chain that includes the π bonds2. Identify the substituents3. Assign a locant to each substituent4. Arrange the substituents alphabetically- When naming the parent replace the suffix “ane” with “ene” Example: 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 ≈ hept – 2 - eneNumber associated with the position of the double bondNumber associated with substituentsCommon namesEthylenePropyleneStyreneVinylAllylPhenylMethylene- Alkenes are also classified bytheir degree of substitution : - Substitution refers to the numberof 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 thesecompounds 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 designationsRecall: Cahn – ingold – prelog naming1. Evaluate each carbon separately 2. Highest atomic number is given highest (largest) priority3. Compare the highest priority groupsExample- “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 Factors1. 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 eliminationRecall:- 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 anintermediate carbocation, which is then deprotonated by a base, resulting in an alkene.Concerted process: a base abstracts a proton and the leaving group leaves simultaneouslyStepwise 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 suggeststhat 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 substitutionreaction 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 aproton.- In a tertiary substrate, the steric hindrance prevents the reagent from functioning as anucleophile.- 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 alkeneHoffman 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 180⁰, say 178⁰ or 179⁰, the reaction can still proceed as long as the angle is close to 180⁰. Inthis case, the term periplanar, is used rather than coplanar. - In such a conformation, the orbital overlap is