3ELIMINATION REACTIONElimination reactions involve the loss of elements from the starting material to form a new bond in the product.The product of elimination reaction is an Alkene
4E1 AND E2 REACTION E2 E1 E-Elimination E-Elimination 2-Bimolecular 1-Unimolecular
5E2 REACTIONHydroxide cannot act as a nucleophile in this reaction because of the bulky tertiary halide. Rather, hydroxide acts as a base and abstracts a proton.
6MECHANISM OF E2 REACTION The removal of a proton and a halide ion is called dehydrohalogenation
7MECHANISM OF E2 REACTION Carbon attached to halogenCarbon adjacent to α-carbonAn E2 reaction is also called a b-elimination or a 1,2-elimination reaction
8The weaker the base, the better it is as a leaving group
92 structurally β-carbon The Regioselectivity of the E2 Reaction2 product produce2 structurally β-carbonbaseThe major product of an E2 reaction is the most stablealkene
10Reaction coordinate diagram for the E2 reaction 2-Butene formed faster than 1-butene
11The Zaitsev RuleThe more substituted alkene product is obtained when aproton is removed from the b-carbon that is bondedto the fewest hydrogens2-β-Hydrogens3-β-HydrogensMonosubstitutedDisubstituted
12The more substituted alkene is not always the most stable
13Conjugated alkene products are preferred over the more substituted alkene product:Do not use Zaitsev’s rule to predict the major productIf the alkylhalide has a double bond or a benzene ring.
14The anti-Zaitsev Rule (Hoffmann Product) Bulky Alkyl halideZaitsev productHofmann productBulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkeneLess sterically hinderedMajor product- Most stable product
16Another exception to Zaitsev’s rule When the halogen is fluorine-The major product of the E2 reaction of alkyl Fluoride is the less substituted alkene.STRONGEST BASE-POOREST LEAVING GROUPThe Fluoride ion does not have as strong a propensity to leave as another halide ion.
17Consider the elimination of 2-fluoropentane… Major product-Less substituted alkeneA carbanion-like transition state
19CLASS EXERCISE 1-(E2)Which of the alkyl halides is more reactive in an E2 reaction?Producing more stable alkeneBr better leaving groupProducing more stable alkene
20CLASS EXERCISE 2-(E2) ANSWER: Give the major elimination product obtained from an E2 reaction of the following alkyl halides with hydroxide ion:ANSWER:
21E1 REACTION First order elimination reaction Must have at least two step
22Mechanism of E1 reaction Alkylhalide dissociates,forming carbocationA Base removes a proton from β-carbon
23How does a weak base like water remove a proton from an sp3 carbon?1) The presence of a positive charge greatly reduces thepKa2) Hyperconjugation weakens the C-H bond by electrondensity
24The Regioselectivity of the E1 Reaction Fewest β-Hydrogen (According to Zaitsev rule)The major product in an E1 reaction is generally the more substituted alkene
25Reaction coordinate diagram for the E1 reaction 09_02
26Because the first step is the rate-determining step, the rate of an E1 reaction depends both on the ease with which the carbocationis formed and how readily the leaving group leaves
27E1 AND E2 REACTION Major product generally the most stable alkene Tertiary alkyl halides are the most reactive and the primary alkyl halides are the least reactive.For alkyl halide with the same alkyl group, alkyl iodide are the most reactive and alkyl fluoride are the least reactive.
28Because the E1 reaction forms a carbocation intermediate, we need to consider carbocationrearrangement
291° Carbocation are too unstable to be formed in E1 reaction Competition Between E2and E1 Reactions1° Carbocation are too unstable to be formed in E1 reactionAn E2 is favored by a high concentration of strong base and anaprotic polar solventAn E1 is favored by a weak base and a protic polar solvent
30Parallel on the opposite side Parallel on the same side Stereochemistry of the E2 ReactionThe bonds to the eliminated groups (H and X) must bein the same planeParallel on the opposite sideParallel on the same sideSyn eliminationAnti eliminationThe anti elimination is favored over the syn elimination
31Consider the stereoselectivity of the E2 reaction (E)- : the higher priority groups are on opposite sides of the double bond.(Z)- : the higher priority groups are on the same side of the double bond.Syn-eliminationAnti-eliminationThe alkene with the bulkiest groups on opposite sides of thedouble bond will be formed in greater yield, because it is the morestable alkene
33When only one hydrogen is bonded to the b-carbon, the major product of an E2 reaction depends on the structureof the alkene
34Stereochemistry of the E1 Reaction The major stereoisomer obtained from an E1 reaction isthe alkene in which the bulkiest substituents are onopposite sides of the double bondBoth syn and anti elimination can occur in an E1 reaction,both E and Z formedIn contrast,E2 forms both E and Z only if the β-carbon is bondedto 2 Hydrogen.If β-carbon bonded to only 1 Hydrogen,E2 form only 1 productbecause anti elimination favored.
35Substitution and Elimination Competition BetweenSubstitution and EliminationAlkyl halides can undergo SN2, SN1, E2, and E11) HO- is called nu in a substitution reaction (Attack carbon) and a base in elimination reaction (removes proton).2) Decide whether the reaction conditions favor SN2/E2 or SN1/E1SN2/E2 reactions are favored by a high concentration of a good nucleophile/strong baseSN1/E1 reactions are favored by a poor nucleophile/weak base3) Decide how much of the product will be the substitution productand how much of the product will be the elimination product