2 Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX
3 Timberlake LecturePLUS 1999 HaloalkanesAn alkane in which one or more H atoms is replaced with a halogen (F, Cl, Br, or I)CH3Br 1-bromomethaneBr (methyl bromide)CH3CH2CHCH3 2-bromobutaneClchlorocyclobutaneTimberlake LecturePLUS 1999
4 Nomenclature - IUPAC locate the parent alkane number the parent chain to give the substituent encountered first the lower numbershow halogen substituents by the prefixes fluoro-, chloro-, bromo-, and iodo- and list them in alphabetical order with other substituentslocate each halogen on the parent chain
5 NomenclatureexamplesCommon names: name the alkyl group followed by the name of the halide
6 Nomenclatureseveral polyhaloalkanes are common solvents and are generally referred to by their common or trivial nameshydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes
7 Timberlake LecturePLUS 1999 Name the following:Timberlake LecturePLUS 1999
8 Timberlake LecturePLUS 1999 Name the following:bromocyclopentane1,3-dichlorocyclohexaneTimberlake LecturePLUS 1999
9 SN2 – Substitution Nucleophilic, Bimolecular This is called a concerted reactionMeaning that the bond breaking and the bond forming occur simultaneouslyIs classified as bimolecularBecause both the haloalkane and the nucleophile are involved in the rate determining step.S = substitutionN = nucleophilic2 = bimolecular (two species are involved in the rate-determining step)
10 RecallNucleophile (nucleus loving): An electron rich species that seeks a region of low electron density (Nu).Electrophile (electron loving): A low electron-density species that seeks a region of high electron density.
11 SN2 – Substitution Nucleophilic, Bimolecular The nucleophile attacks the reactive center from the sideopposite the leaving group; in other words it involves abackside attack by the nucleophile.
12 SN2both reactants are involved in the transition state of the rate-determining stepthe nucleophile attacks the reactive center from the side opposite the leaving group
13 C SN2 ANIMATION R .. Cl: .. H CH3 ENERGY PROFILE Press the slide show button to see the animation. Press ESC to finish.
23 SN1 – Substitution Nucleophilic, Unimolecular S = substitutionN = nucleophilic1 = unimolecular (only one species is involved in the rate-determining step)
24 SN1 – Substitution Nucleophilic, Unimolecular In this reaction the bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile beginsThis is classified as unimolecularOnly the haloalkane is involved in the rate-determining stepIn other words, only the haloalkane contributes to the rate law governing the rate determining step
25 SN1Step 1: ionization of the C-X bond gives a carbocation intermediate
26 SN1Step 2: reaction of the carbocation (an electrophile, low electron density) with methanol (a nucleophile, high electron density) gives an oxonium ionStep 3: proton transfer completes the reaction
27 SN1For an SN1 reaction at a stereocenter, the product is a racemic mixture
28 SN1the nucleophile attacks with equal probability from either face of the planar carbocation intermediate
29 SN2 and SN1Are competing constantly, what determines what mechanism is a reaction going to prefer?1.The structure of the nucleophile2.The structure of the haloalkane3.The leaving group4.The solvent
30 1. The structure of the nuceophile Refer to table 7.2 page 228 from your book to see the types of nucleophiles we deal with most commonly in this semester.Nucleophilicity: a kinetic property measured by the rate at which a Nu attacks
32 2. Structure of the Haloalkane SN1 reactionsgoverned by electronic factors, namely the relative stabilities of carbocation intermediatesrelative rates: 3° > 2° > 1° > methylSN2 reactionsgoverned by steric factors, namely the relative ease of approach of the nucleophile to the site of reactionrelative rates: methyl > 1° > 2° > 3°
33 SN1SN1 will be favored if a tertirary carbocation is involved, sometimes if a secondary carbocation is involvedSN1 will never be favored if a primary cabocation or methyl are involved
34 SN2 The less crowded site will always favor the SN2 mechanism Will be favored if it involves a primary carbocation and methylSometimes will be favored if a secondary carbocation is involved
35 3.Leaving groupChlorine ion, bromine ion and Iodine ion make good leaving groups because of their size and Electronegativity help to stabilize the resulting negative chargeThe ability of a group to function as a leaving group is related to how stable is as an anionThe most stable anion and the best leaving groups are the conjugate bases of strong acids!!!
36 4. The Solvent Protic solvent: a solvent that contains an -OH group these solvents favor SN1 reactions; the greater the polarity of the solvent, the easier it is to form carbocations in it
37 4. The Solvent Aprotic solvent:does not contain an -OH group it is more difficult to form carbocations in aprotic solventsaprotic solvents favor SN2 reactions
39 Elimination reactions DehydrohalogenationThese reaction require forcing conditions like a strong base and heat.(Hydroxide ion or ethoxide ion)Halogen is removed from one carbon of a haloalkaneAnd the hydrogen from the adjacent carbonTo form a double bond(an alkene)
42 b-EliminationZaitsev rule: the major product of a -elimination is the more stable (the more highly substituted) alkene
43 E1 and E2 mechanismsThere are both examples of beta-elimination reactionsThe difference is the timing of the bond-breaking and the bond-forming steps.E1 stands for elimination and 1 for unimolecularE2 stands for elimination and 2 for bimolecular
44 E1The breaking for the halogen carbon bond has to be completely broken before any reaction occurs with the baseThis is the slow determining step (the breaking of the halogen carbon bond)
45 E1 MechanismStep 1: The breaking for the halogen carbon bond gives a carbocation intermediateStep 2: proton transfer from the carbocation intermediate to a base (in this case, the solvent) gives the alkene
46 E2The base removes a beta hydrogen at the same time that carbon halogen bond is brokenThe rate of the reaction will depend both on the haloalkane and the baseThe stronger the base the more likely it is thatthe E2 mechanism will be in operation
47 E2 MechanismA one-step mechanism; all bond-breaking and bond-forming steps are concerted
48 Table 7.6 E2 is favored if you are dealing with a primary haloalkane E2 is favored for secondary haloalkane if you have a really strong baseE1 is favored for secondary haloalkane if you have weak basesE1 is favored for tertiary haloalkanes.
49 Summary of S vs E for Haloalkanes for methyl and 1°haloalkanes
50 Summary of S vs E for Haloalkanes for 2° and 3° haloalkanes