Transcript Unit 4 - Alkyl Halides
Unit 4
Alkyl Halides: Nucleophilic Substitution and Elimination Reactions
Nomenclature of Alkyl Halides
Apply the same rules you learned for the alkanes.
CH 3 CH 2 Br
IUPAC: Name the halogen as a substituent. bromoethane
Common name: The alkyl group is named as the substituent. Ethyl bromide
Nomenclature of Alkyl Halides 3-(iodomethyl)pentane I F 4-(2-fluoroethyl)heptane
Nomenclature of Alkyl Halides
An alkyl halide has a halogen bonded to an sp 3 C. Methyl halides, CH 3 X: the sp 3 bonded only to H atoms.
CH 3 C is Br X denotes a halogen: F, Cl, Br, I.
Primary (1°) halides, RCH 2 X: the sp 3 C is bonded to one other C atom.
CH 3 CH 2 Br
Nomenclature of Alkyl Halides methyl halide 1° halide 2° halide 3° halide
Nomenclature of Alkyl Halides
A vicinyl dihalide two halogens bonded to neighboring sp has 3 C atoms.
A geminal dihalide two halogens on the same sp 3 C.
has
Nomenclature of Alkyl Halides
A allylic halogen is one that is bonded to the sp 3 immediately next to a double bond. C atom
FYI: The allylic position than a halogen. can be occupied by a substituent other
IR Spectrum of an Alkene (for comparison)
IR Spectrum of an Allylic Chloride The C-Cl str is 600-800 cm -1 .
Nomenclature of Other Halides
A vinyl halide has a halogen bonded to an sp 2 C of an alkene.
An aryl halide halogen bonded to one of the sp 2 C atoms of the aromatic ring.
has a
toluene chlorobenzene
Nomenclature of Other Halides
A benzylic halide bonded to an sp 3 benzene. has a halogen C substituent of
Nomenclature: Nitro, Phenyl
A benzene ring as a substituent is called phenyl, -C 6 H 5 . Another substituent to know is the nitro group, -NO 2 .
TNT trans-9-phenyl-2-methylundec-3-ene
Uses of Alkyl Halides
Solvents
1,1,1-trichloroethane and methylene chloride Starting materials for syntheses.
Anesthetics
Halothane Freons Pesticides
DDT, lindane (lice), chlordane (termites)
C-X is a Polar Bond
Polar bonds have dipole moments which are proportional to both partial charge and bond length.
Partial charge increases with electronegativity: I
Bond length increases down the group.
These two trends are in opposite directions, and results in the C-Cl bond being the most polar, with C-F and C Br close behind.
C-X is a Polar Bond
F Cl Br I
E.N.
4.0
3.2
3.0
2.7
bond length
1.38
1.78
1.94
2.14
partial charge
0.23
0.18
0.16
0.13
for comparison dipole moment
C-H 0.3 D N-H O-H C=O C≡N 1.31 D 1.53 D 2.4 D 3.6 D
dipole moment
1.51 D 1.56 D 1.48 D 1.29 D
Intermolecular Forces in Alkyl Halides
The strongest intermolecular force in alkyl halides is the London force (instantaneous dipole-induced dipole). This force increases with molecular weight and with surface area.
Since the C-X bond is polar, there will also be a contribution from the dipole dipole attraction.
Physical Properties of Alkyl Halides - Boiling Points
Here the surface area has more effect than the C-X dipole. Since atomic size increases down the halogen group, boiling points will, too, if the alkyl part of the compound is the same.
For compounds with similar molecular weights, branched compounds will have lower boiling points than compounds with straight chains.
Physical Properties of Alkyl Halides - Densities
Alkyl fluorides are comparable to alkanes and are less dense than water.
Alkyl chlorides with one Cl are less dense than water.
Alkyl bromides and iodides are denser than water.
Preparation of Alkyl Halides Allylic Bromination
We will learn other ways to prepare alkyl halides in later units.
Free-radical halogenations often give a mixture of products.
Allylic bromination, a free-radical process, can be selective.
Allylic Bromination Mechanism
Initiation Propagation, step 1 Br 2
hυ
2Br • allylic free radical
Allylic Bromination Mechanism
The allylic free radical is stabilized by resonance and will form two products.
Allylic Bromination Mechanism Propagation, step 2.
+ Br• + Br•
Allylic Bromination - Overall Reaction
NBS, n-bromosuccinimide, is often used as the source of bromine. That way, the amount of bromine is kept low…why?
H 2 C=CH-CH 2 -CH 3 NBS, hv
H 2 C=CH-CHBr-CH 3 + H 2 CBr-CH=CH 2 -CH 3
Allylic Bromination - NBS
O O N Br + HBr Br 2 + N H O O
Br 2 is present in trace amounts in NBS (for initiation), and HBr is formed as soon as any products are formed, and so is available to produce more Br 2 .