Upload
garrett-wolaver
View
249
Download
4
Tags:
Embed Size (px)
Citation preview
• Haloalkane (alkyl halide): a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX
Timberlake LecturePLUS 1999 3
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
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
3-Bromo-2-methyl-pentane
12
34
5Br
4-Bromocyclohexene
1
23
4
56Br
OH
Cl
trans-2-Chloro-cyclohexanol
4
56
1
23
Nomenclature– examples
• Common names: name the alkyl group followed by the name of the halide
Br
2-Bromo-4-methyl-pentane
12
34
5
4-Bromo-cyclohexene
1
234
56
trans-2-Chloro-cyclohexanol
Br Cl
OH
BrCl Cl
2-Bromobutane(sec-Butyl bromide
Cnhloroethene(Vinyl chloride)
3-Chloropropene(Allyl chloride)
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
CHCl3CH2Cl2 CCl2=CHClCH3CCl3 Dichloromethane(Methylene chloride)
Trichloromethane (Chloroform)
Trichloroethyne (Trichlor)
1,1,1-Trichloroethane (Methyl chloroform)
PerchloroethylenePerfluoropropanePerchloroethane
C CCl
ClClCl
ClCl
F C C C FF
F
F
F
F
F ClC C
Cl
ClCl
Timberlake LecturePLUS 1999 8
Name the following:
bromocyclopentane
1,3-dichlorocyclohexane
Br
Cl
Cl
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)
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.
SN2 – Substitution Nucleophilic, Bimolecular
C XNu CNu X
The nucleophile attacks the reactive center from the side opposite the leaving group; in other words it involves a backside attack by the nucleophile.
nucleophilicsubstitution
Nucleophile
++ C NuC XNu - X-
leavinggroup
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
C Br
H
HH
HO + C
H
H H
HO Br
- -
Transition state withsimultaneous bond breaking
and bond forming
C
H
HH
HO + Br -
C Cl:
CH3
H
..
..
SN2 ANIMATIONENERGY PROFILE
R
Press the slide show button to see the animation. Press ESC to finish.
SN1 – Substitution Nucleophilic, Unimolecular
– S = substitution– N = nucleophilic– 1 = unimolecular (only one species is involved in
the rate-determining step)
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
SN1
• Step 1: ionization of the C-X bond gives a carbocation intermediate
C
CH3
CH3H3C
+C
H3C
H3C
Br
H3C
slow, ratedetermining
A carbocation intermediate; carbon is trigonal planar
+ Br
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
CH3O
H H3C
C
CH3
CH3
OCH3
H
C
CH3
CH3
CH3
O
H3C
HH3C
H3C
C
H3C
O
CH3
H
fast ++ ++
++++ fastC
H3CH3C
O
H3C
OCH3
HOHO
CH3CH3
H
H
CH3
H3C
CH3C
H3C
SN1
• For an SN1 reaction at a stereocenter, the product is a racemic mixture
(R)-EnantiomerPlanar carbocation (achiral)
C
H
Cl
C6H5
Cl
C+
C6H5
H
Cl
CH3OH-Cl-
-H+
SN1– the nucleophile attacks with equal probability
from either face of the planar carbocation intermediate
+
A racemic mixture
Cl
C6H5 C6H5
C OCH3
H
CH3O CH
Cl(R)-Enantiomer(S)-EnantiomerPlanar carbocation
(achiral)
C+
C6H5
H
Cl
CH3OH
-H+
SN2 and SN1
• Are 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
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
Nucleophilicity
• Table 7.2
good
poor
Br-, I -
HO-, CH3O-, RO-
CH3S-, RS-
H2O
CH3OH, ROH
CH3COH, RCOH
O O
NH3, RNH2, R2NH, R3N
CH3SH, RSH, R2S
Effectiveness Nucleophile
moderateCH3CO-, RCO-
O O
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°
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
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
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!!!
I- > Br- > Cl- >> F- > CH3CO- > HO- > CH3O- > NH2-
Greater ability as leaving group
Greater stability of anion; greater strength of conjugate acid
Rarely act as leaving groups in nucleophilic substitution and -elimination reactions
O
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
CH3COOHCH3CH2OHCH3OHHCOOH
H2OStructure
Acetic acid
Formic acid
EthanolMethanol
Water
ProticSolvent
Polarity of Solvent
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
(CH3CH2)2O
CH2Cl2
OCH3CCH3
OCH3SCH3
Diethyl ether
Dichloromethane
AproticSolvent Structure
Dimethyl sulfoxide (DMSO)
Acetone
Polarity ofSolvent
Summary of SN1 and SN2
CH3X
RCH2X
R2CHX
R3CX
Type of Haloalkane
Methyl
Primary
Secondary
Tertiary
SN2 SN1
Substitutionat a stereocenter
SN2 is favored. SN1 does not occur. The methylcation is so unstable that it is never observed in solution.
SN1 does not occur. Primary carbocations are so unstable thatthey are never observed in solution.
SN1 is favored in protic solventswith poor nucleophiles.
SN2 is favored in aproticsolvents with goodnucleophiles.
SN2 does not occur becauseof steric hindrance aroundthe substitution center.
SN1 is favored because of the ease of formation of tertiary carbocations.
Inversion of configuration.The nucleophile attacksthe stereocenter from theside opposite the leavinggroup.
Racemization. The carbocationintermediate is planar, and attack bythe nucleophile occurs with equalprobability from either side.
SN2 is favored.
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)
b-Elimination• Zaitsev rule: the major product of a -
elimination is the more stable (the more highly substituted) alkene
Br CH3CH2O-Na+
CH3CH2OH2-Methyl-2-butene (major product)
2-Bromo-2-methylbutane
2-Methyl-1-butene
+
Br CH3O-Na+
CH3OH+
1-Methyl-cyclopentene
(major product)
1-Bromo-1-methyl-cyclopentane
Methylene-cyclopentane
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
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)
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
CH2-C-CH3
Br
CH3
CH3-C-CH3
CH3
Br –slow, rate
determining
+(A carbocation intermediate)
+
HO
H3CH-CH2-C-CH3
CH3
HOH
H3CCH2=C-CH3
CH3fast+
+ ++
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
E2 Mechanism
• A one-step mechanism; all bond-breaking and bond-forming steps are concerted
CH3CH2OCH3
H-CH-CH2-Br
CH3CH2O-H CH3CH=CH2 Br
+
+ +