1. CHAPTER 7
Haloalkanes

2. Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX

3. Timberlake LecturePLUS 1999
Haloalkanes
An alkane in which one or more H atoms is replaced with a halogen (F, Cl, Br, or I)
CH3Br 1-bromomethane
Br (methyl bromide)
CH3CH2CHCH3 2-bromobutane Cl
chlorocyclobutane
Timberlake LecturePLUS 1999

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: 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 1999
Name the following:
Timberlake LecturePLUS 1999

8. Timberlake LecturePLUS 1999
Name the following:
bromocyclopentane
1,3-dichlorocyclohexane
Timberlake LecturePLUS 1999

9. SN2 – 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. SN2 – 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. SN2
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 SN2 ANIMATION R .. Cl: .. H CH3 ENERGY PROFILE
Press the slide show button to see the animation. Press ESC to finish.

14. SN2 ANIMATION
ENERGY PROFILE R
:Br: .. .. C
Cl: .. H
CH3

15. SN2 ANIMATION
ENERGY PROFILE R
:Br: .. .. C
Cl: .. H
CH3

16. SN2 ANIMATION
ENERGY PROFILE R
:Br: .. .. C
Cl: .. H
CH3

17. SN2 ANIMATION
ENERGY PROFILE R
:Br: .. .. C
Cl: .. H
CH3

18. C SN2 ANIMATION R .. .. :Br Cl: H CH3 ENERGY PROFILE Transition Stated- d- C H
CH3
Activated
Complex

19. SN2 ANIMATION
ENERGY PROFILE R
:Br ..
:Cl: .. C H
CH3

20. SN2 ANIMATION
ENERGY PROFILE R
:Br ..
:Cl: .. C H
CH3

21. SN2 ANIMATION
ENERGY PROFILE R
:Br ..
:Cl: .. C H
CH3

22. SN2 ANIMATION
ENERGY PROFILE R
:Br .. C H
CH3

23. SN1 – Substitution Nucleophilic, Unimolecular
S = substitution
N = nucleophilic
1 = 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 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. SN1
Step 1: ionization of the C-X bond gives a carbocation intermediate

26. SN1
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. SN1
For an SN1 reaction at a stereocenter, the product is a racemic mixture

28. SN1
the nucleophile attacks with equal probability from either face of the planar carbocation intermediate

29. SN2 and SN1
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: a kinetic property measured by the rate at which a Nu attacks

31. Nucleophilicity
Table 7.2

32. 2. Structure of the Haloalkane
SN1 reactions
governed by electronic factors, namely the relative stabilities of carbocation intermediates
relative rates: 3° > 2° > 1° > methyl
SN2 reactions
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. SN1
SN1 will be favored if a tertirary carbocation is involved, sometimes if a secondary carbocation is involved
SN1 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 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: 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 solvents
aprotic solvents favor SN2 reactions

38. Summary of SN1 and SN2

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)

42. b-Elimination
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