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CHAPTER 7 Haloalkanes. Haloalkane (alkyl halide): Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp 3 hybridized carbon;

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Presentation on theme: "CHAPTER 7 Haloalkanes. Haloalkane (alkyl halide): Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp 3 hybridized carbon;"— Presentation transcript:

1 CHAPTER 7 Haloalkanes

2 Haloalkane (alkyl halide): Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp 3 hybridized carbon; given the symbol RX

3 Timberlake LecturePLUS Haloalkanes An alkane in which one or more H atoms is replaced with a halogen (F, Cl, Br, or I) CH 3 Br1-bromomethane Br (methyl bromide) CH 3 CH 2 CHCH 3 2-bromobutane Cl chlorocyclobutane

4 Nomenclature - IUPAC – locate the parent alkane – number the parent chain to give the substituent encountered first the lower number – show halogen substituents by the prefixes fluoro-, chloro-, bromo-, and iodo- and list them in alphabetical order with other substituents – locate each halogen on the parent chain

5 Nomenclature – examples Common names: Common names: name the alkyl group followed by the name of the halide

6 Nomenclature – several polyhaloalkanes are common solvents and are generally referred to by their common or trivial names – hydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes

7 Timberlake LecturePLUS Name the following:

8 Timberlake LecturePLUS Name the following: bromocyclopentane 1,3-dichlorocyclohexane

9 S N 2 – Substitution Nucleophilic, Bimolecular This is called a concerted reaction – Meaning that the bond breaking and the bond forming occur simultaneously Is classified as bimolecular – Because both the haloalkane and the nucleophile are involved in the rate determining step. – S = substitution – N = nucleophilic – 2 = bimolecular (two species are involved in the rate-determining step)

10 Recall Nucleophile (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 S N 2 – Substitution Nucleophilic, Bimolecular The nucleophile attacks the reactive center from the side opposite the leaving group; in other words it involves a backside attack by the nucleophile.

12 SN2SN2 – both reactants are involved in the transition state of the rate-determining step – the nucleophile attacks the reactive center from the side opposite the leaving group

13 C Cl: CH 3 H.. S N 2 ANIMATION ENERGY PROFILE R Press the slide show button to see the animation. Press ESC to finish.

14 C Cl: CH 3 H :Br:.. S N 2 ANIMATION ENERGY PROFILE R

15 C Cl: CH 3 H :Br:.. S N 2 ANIMATION ENERGY PROFILE R

16 C Cl: CH 3 H.. :Br:.. S N 2 ANIMATION ENERGY PROFILE R

17 C Cl: R CH 3 H.. :Br:.. S N 2 ANIMATION ENERGY PROFILE

18 C R CH 3 H :Br.. Cl:.. S N 2 ANIMATION ENERGY PROFILE Activated Complex Transition State -- --

19 C :Br.. CH 3 H :Cl:.. S N 2 ANIMATION ENERGY PROFILE R

20 C :Br.. CH 3 H :Cl:.. S N 2 ANIMATION ENERGY PROFILE R

21 C :Br.. CH 3 H :Cl:.. S N 2 ANIMATION ENERGY PROFILE R

22 C :Br.. CH 3 H S N 2 ANIMATION ENERGY PROFILE R

23 S N 1 – Substitution Nucleophilic, Unimolecular – S = substitution – N = nucleophilic – 1 = unimolecular (only one species is involved in the rate-determining step)

24 S N 1 – Substitution Nucleophilic, Unimolecular In this reaction the bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile begins This is classified as unimolecular – Only the haloalkane is involved in the rate- determining step – In other words, only the haloalkane contributes to the rate law governing the rate determining step

25 SN1SN1 Step 1: ionization of the C-X bond gives a carbocation intermediate

26 SN1SN1 – Step 2: reaction of the carbocation (an electrophile, low electron density) with methanol (a nucleophile, high electron density) gives an oxonium ion – Step 3: proton transfer completes the reaction

27 SN1SN1 For an S N 1 reaction at a stereocenter, the product is a racemic mixture

28 SN1SN1 – the nucleophile attacks with equal probability from either face of the planar carbocation intermediate

29 S N 2 and S N 1 Are competing constantly, what determines what mechanism is a reaction going to prefer? 1.The structure of the nucleophile 2.The structure of the haloalkane 3.The leaving group 4.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 Nucleophilicity: a kinetic property measured by the rate at which a Nu attacks

31 Nucleophilicity Table 7.2

32 2. Structure of the Haloalkane S N 1 reactions electronic factors – governed by electronic factors, namely the relative stabilities of carbocation intermediates – relative rates: 3° > 2° > 1° > methyl S N 2 reactions steric factors – governed by steric factors, namely the relative ease of approach of the nucleophile to the site of reaction – relative rates: methyl > 1° > 2° > 3°

33 SN1SN1 S N 1 will be favored if a tertirary carbocation is involved, sometimes if a secondary carbocation is involved S N 1 will never be favored if a primary cabocation or methyl are involved

34 SN2SN2 The less crowded site will always favor the S N 2 mechanism Will be favored if it involves a primary carbocation and methyl Sometimes will be favored if a secondary carbocation is involved

35 3.Leaving group Chlorine ion, bromine ion and Iodine ion make good leaving groups because of their size and Electronegativity help to stabilize the resulting negative charge The ability of a group to function as a leaving group is related to how stable is as an anion The most stable anion and the best leaving groups are the conjugate bases of strong acids!!!

36 4. The Solvent Protic solvent Protic solvent: a solvent that contains an -OH group – these solvents favor S N 1 reactions; the greater the polarity of the solvent, the easier it is to form carbocations in it

37 4. The Solvent Aprotic solvent Aprotic solvent:does not contain an -OH group – it is more difficult to form carbocations in aprotic solvents – aprotic solvents favor S N 2 reactions

38 Summary of S N 1 and S N 2

39 Elimination reactions Dehydrohalogenation – These reaction require forcing conditions like a strong base and heat. (Hydroxide ion or ethoxide ion) – Halogen is removed from one carbon of a haloalkane – And the hydrogen from the adjacent carbon – To form a double bond (an alkene)

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42  -Elimination Zaitsev rule: Zaitsev rule: the major product of a  - elimination is the more stable (the more highly substituted) alkene

43 E1 and E2 mechanisms There are both examples of beta-elimination reactions – The difference is the timing of the bond-breaking and the bond-forming steps. E1 stands for elimination and 1 for unimolecular E2 stands for elimination and 2 for bimolecular

44 E1 The breaking for the halogen carbon bond has to be completely broken before any reaction occurs with the base This is the slow determining step (the breaking of the halogen carbon bond)

45 E1 Mechanism – Step 1: The breaking for the halogen carbon bond gives a carbocation intermediate – Step 2: proton transfer from the carbocation intermediate to a base (in this case, the solvent) gives the alkene

46 E2 The base removes a beta hydrogen at the same time that carbon halogen bond is broken The rate of the reaction will depend both on the haloalkane and the base The stronger the base the more likely it is that the E2 mechanism will be in operation

47 E2 Mechanism A 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 base E1 is favored for secondary haloalkane if you have weak bases E1 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

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