Organic Halides (Haloalkanes) (Alkyl Halides)

Organic Halides(Haloalkanes)(Alkyl Halides) Dr Seemal jelani Chem-261 Functional Group #1

Structure • R-X where R = alkyl group and X = halide (F, Cl, Br, I) Cl | CH=C-CH2-CH3 | Cl 1,2-dichloro-1-butene Dr Seemal jelani Chem-261 bromoethane

Nomenclature Prefixes for halides are inserted into the name of the structure in the exact same fashion as any alkyl group. Remember to write them in alphabetical order Prefixes for the first four members of the halogen family Dr Seemal jelani Chem-261

Properties • The presence of a halide makes the molecule more polar. • Since water is also polar and “like dissolves like”, alkyl halides are soluble in water. The more halides connected to the parent chain, the more polar the molecule. • The polar nature of the molecule means that boiling and melting points of alkyl halides are much higher than alkanes with a similar number of carbons Dr Seemal jelani Chem-261

Uses • CFC’s (Chlorofluorocarbons) are used in refrigerators and air conditioners (ex. Freon) (their use is now restricted due to the fact that some of them damage the ozone layer) • polymers such as Teflon (non-stick surfaces as in cookware) and PVC (polyvinylchloride) used plumbing • DDT (dichlorodiphenyltrichloroethane) – now banned pesticide • PCB’s (polychlorinated biphenyls) – electrical transformers Dr Seemal jelani Chem-261

Reactions – Preparing Alkyl Halides • Alkenes/Alkynes readily react with halides of acids to form alkyl halides. C=C and C≡ C are weak bonds that are easily broken making molecules that contain them more reactive than alkanes • Halogenation (by addition) Ex. C2H4 + HCl → C2H5Cl • Substitution reactions with Halogens Ex. benzene + Br → bromobenzene + HBr Dr Seemal jelani Chem-261

Markovnikov’s Rule • When Alkene or Alkyne reacts with a hydrogen halide or water the H bonds to the Carbon with the most H already attached. The Rich get Richer! See the Dr Seemal jelani Chem-261

Markovnikov’s Rule Two more examples: Dr Seemal jelani Chem-261

Reactions - Reactions with aromatics Substitution FeCl3 catalyst C6H6 + Cl2 ===========> C6H5Cl + HCl Chlorobenzene Dr Seemal jelani Chem-261

Preparation of Alkyl Halides from Alcohols • Alkyl halides can be prepared from alcohols by reaction with HX, i.e., the substitution of a halide on a protonated alcohol. Rapid. 3° C+ is stabilized by protic solvent (H2O) Dr Seemal jelani Chem-261

Very slow. Protic solvent inhibits the nucleophile • OH- is a poor leaving group, i.e., is not displaced directly by nucleophiles. Reaction in acid media protonates the OH group producing a better leaving group (H2O). 2 and 3 alcohols react by SN1 but Me° and 1 alcohols react by SN2. Dr Seemal jelani Chem-261 .

Preparation of Alkyl Halides from Alcohols • Alternative to using hydrohalic acids (HCl, HBr, HI), alcohols can be converted to alkyl halides by reaction with PBr3 which transforms OH- into a better leaving group allowing substitution (SN2) to occur without rearrangement. Dr Seemal jelani Chem-261

Preparation of Alkenes from Alkyl Halides 2° and 3° alkyl halides can be dehydrohalogenated with a strong base such as OH- producing an alkene. Bromocyclohexane+ KOH cyclohexene (80 % yield) Clearly, this is an E2 reaction. Dr Seemal jelani Chem-261

Summary of SN /Elimination Reactions • Alkyl Halide Substrate Reactivity: Dr Seemal jelani Chem-261

Reactivity of Nucleophiles Leaving Group Activity Dr Seemal jelani Chem-261

Note Poor nucleophiles that are also weak bases (H2O, ROH, CH3COOH, etc.) do not undergo any reaction unless a C+ is formed first If a C+ can form (as with a 2º, 3º, any benzylic, or any allylic halides), then E1 and SN1 generally occur together. Dr Seemal jelani Chem-261

Organic Halides (Haloalkanes) (Alkyl Halides)