* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Organic Halides (Haloalkanes) (Alkyl Halides)
Enantioselective synthesis wikipedia , lookup
Aromaticity wikipedia , lookup
Elias James Corey wikipedia , lookup
Kinetic resolution wikipedia , lookup
Woodward–Hoffmann rules wikipedia , lookup
Cracking (chemistry) wikipedia , lookup
Discodermolide wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Organosulfur compounds wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Baylis–Hillman reaction wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Tiffeneau–Demjanov rearrangement wikipedia , lookup
Diels–Alder reaction wikipedia , lookup
George S. Hammond wikipedia , lookup
Marcus theory wikipedia , lookup
Petasis reaction wikipedia , lookup
Wolff rearrangement wikipedia , lookup
Ene reaction wikipedia , lookup
Asymmetric induction wikipedia , lookup
Hydroformylation wikipedia , lookup
Stille reaction wikipedia , lookup
Functional Group #1 Dr Seemal jelani Chem-261 (Haloalkanes) (Alkyl Halides) 5/23/2017 Organic Halides 1 Structure Dr Seemal jelani Cl | CH=C-CH2-CH3 | Cl 1,2-dichloro-1-butene Chem-261 5/23/2017 • R-X where R = alkyl group and X = halide (F, Cl, Br, I) bromoethane 2 Chlorine Chloro- Bromine Bromo- Iodine Iodo- Prefixes for the first four members of the halogen family 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 Chem-261 Fluoro- Dr Seemal jelani Fluorine 5/23/2017 Nomenclature 3 Chem-261 Dr Seemal jelani • 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 5/23/2017 Properties 4 Chem-261 Dr Seemal jelani • 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 5/23/2017 Uses 5 Reactions – Preparing Alkyl Halides • Substitution reactions with Halogens Ex. benzene + Br → bromobenzene + HBr Chem-261 • Halogenation (by addition) Ex. C2H4 + HCl → C2H5Cl Dr Seemal jelani C=C and C≡ C are weak bonds that are easily broken making molecules that contain them more reactive than alkanes 5/23/2017 • Alkenes/Alkynes readily react with halides of acids to form alkyl halides. 6 Chem-261 Dr Seemal jelani • 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 5/23/2017 Markovnikov’s Rule 7 Markovnikov’s Rule Dr Seemal jelani Chem-261 5/23/2017 Two more examples: 8 Reactions - Reactions with aromatics Chem-261 C6H6 + Cl2 ===========> C6H5Cl + HCl Chlorobenzene Dr Seemal jelani FeCl3 catalyst 5/23/2017 Substitution 9 Rapid. 3° C+ is stabilized by protic solvent (H2O) 3º .. (Lucas Test) (CH3)3C OH + SN1 .. H Cl + (CH3)3C OH .. 2 - H2O (CH3)3C + .. (CH3)3C Cl : + H2O .. Dr Seemal jelani Chem-261 • Alkyl halides can be prepared from alcohols by reaction with HX, i.e., the substitution of a halide on a protonated alcohol. 5/23/2017 Preparation of Alkyl Halides from Alcohols .. :Cl : .. 10 Very slow. Protic solvent inhibits the nucleophile 1º SN2 CH3CH2 .. OH .. . + H Cl D CH3CH2 + OH2 .. : Cl : .. CH3CH2Cl + Dr Seemal jelani Chem-261 5/23/2017 • 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. H2O 11 Br :P 1º SN2 .. CH3(CH2)4CH2 OH .. Br Br ether Chem-261 Dr Seemal jelani •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. 5/23/2017 Preparation of Alkyl Halides from Alcohols + PBr2 CH3(CH2)4CH2 O .. H CH3(CH2)4CH2Br Br - 12 Preparation of Alkenes from Alkyl Halides KOH in ethanol + Br KBr + KOH cyclohexene (80 % yield) Dr Seemal jelani Bromocyclohexane + Chem-261 -HBr H2O 5/23/2017 2° and 3° alkyl halides can be dehydrohalogenated with a strong base such as OH- producing an alkene. Clearly, this is an E2 reaction. 13 Summary of SN /Elimination Reactions • Alkyl Halide Substrate Reactivity: H C H Br CH3 C H unhindered substrates favor S N2 do not form a stable C + do not react by S N1 or E1 3º CH3 Br CH3 C H Br CH3 CH3 C 5/23/2017 H 2º Br CH3 Chem-261 1º hindered substrates. S N2 increasingly unfavorable, E2 is OK form increasingly stable C + favors SN1 and E1. E2 is OK Dr Seemal jelani H methyl E2 reactions possible with strong bases E2 reactions possible with strong bulky bases (t-butoxide) 14 Reactivity of Nucleophiles HS- CN- I- CH3O- HO- NH3 Cl- H2O 125,000 125,000 100,000 25,000 16,000 1000 700 1 good nucleophiles which are also strong bases favor elimination 5/23/2017 good nucleophiles which are weak bases favor SN reactions pKb = 23 pKb = 22 pKb = 21 pKb = 11 pKb = -1.7 pKb = -2 pKb = -21 I- Br - Cl- F- HO- RO- H2N- 30,000 10,000 200 1 0 0 0 good leaving groups favor both substitution and elimination reactions poor leaving groups make both substitution and elimination reactions unfavorable Dr Seemal jelani Chem-261 Leaving Group Activity 15 Chem-261 Dr Seemal jelani 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. 5/23/2017 Note 16