Unit 4 Alkyl Halides: Nucleophilic Substitution and Elimination Reactions

Nomenclature of Alkyl Halides  Apply the same rules you learned for the alkanes.  IUPAC: Name the halogen as a substituent. bromoethane  Common name: The alkyl group is named as the substituent. Ethyl bromide CH 3 CH 2 Br

Nomenclature of Alkyl Halides 3-(iodomethyl)pentane 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 C is bonded only to H atoms.  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 Br CH 3 CH 2 Br

Nomenclature of Alkyl Halides methyl halide 1° halide 2° halide 3° halide

Nomenclature of Alkyl Halides  A vicinyl dihalide has two halogens bonded to neighboring sp 3 C atoms.  A geminal dihalide has two halogens on the same sp 3 C.

Nomenclature of Alkyl Halides  A allylic halogen is one that is bonded to the sp 3 C atom immediately next to a double bond.  FYI: The allylic position can be occupied by a substituent other than a halogen.

IR Spectrum of an Alkene (for comparison)

IR Spectrum of an Allylic Chloride The C-Cl str is cm -1.

Nomenclature of Other Halides  A vinyl halide has a halogen bonded to an sp 2 C of an alkene.  An aryl halide has a halogen bonded to one of the sp 2 C atoms of the aromatic ring.

toluene chlorobenzene

Nomenclature of Other Halides  A benzylic halide has a halogen bonded to an sp 3 C substituent of benzene.

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. trans-9-phenyl-2-methylundec-3-ene TNT

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<Br<Cl<F.  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 E.N.bond length partial charge dipole moment F D Cl D Br D I D for comparisondipole moment C-H0.3 D N-H1.31 D O-H1.53 D C=O2.4 D C≡NC≡N3.6 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  2Br hυhυ 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

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  H 2 C=CH-CHBr-CH 3 + H 2 CBr-CH=CH 2 -CH 3 NBS, hv

Allylic Bromination - NBS ++ 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.