rxnstoknow - Reactions You Should Know When You Begin...

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Reactions You Should Know When You Begin Organic II 1. Free radical halogenation of alkanes. Alkane + halogen + uv, ROOR, or 300 ° C+. R-H + F 2 Not useful, too fast, not specific R-H + Cl 2 Useful, controllable, somewhat nonspecific (3°>2°>1°>CH 4 ) R-H + Br 2 Useful, controllable, specific for 3° if present (3°»2°> 1°>CH 4 ) R-H + I 2 Not useful, too Slow Determine the number of constitutional isomers produced when a halogen is reacted with a higher alkane (more complex than simple alkane). 2. Combustion of alkanes. 2CH 4 + 2O 2 2CO 2 + 4 H 2 O 3. Dehydrogenation of an alkane to give an alkene. Alkane + metal catalyst (Pt, Ni, Pd) + heat yields an alkene. C 2 H 6 + Pt and heat C 2 H 4 + H 2 Usefull for ethane, propane, and isobutane only. Others give multiple products. 4. Addition to alkenes. Adding agent can be symmetrical or asymmetrical. Ex. H 2 vs. HCl Symmetrical: H 2 , C1 2 , Br 2 , and I 2 (I 2 slow and readily reversible) Asymmetrical: HCl, RBr, HOH (H 2 O) Addition of symmetrical agents may be anti or syn depending on mechanism or catalyst. Addition of asymmetrical agents follows Markovnikov's Rule except for addition of HBr in the presence of peroxides which adds anti-Markovnikov (only works with HBr). CH 4 + Cl 2 CH 3 Cl + Cl 2 CH 2 Cl 2 + Cl 2 CHCl 3 + Cl 2 CH 3 Cl + HCl CH 2 Cl 2 + HCl CHCl 3 + HCl CCl 4 + HCl CH 2 CH CH 3 + HX CH 2 C CH 3 X H
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5. Dehydrohalogenation of alkyl halides to give alkenes. Elimination reactions favored by strong bases and heat. Halogen and hydrogen must be anticoplanar. Hindered bases favor elimination (E2) over substitution (SN 1 and SN 2 ). Higher temperatures favor elimination. Bases that favor elimination: Alcoholic KOH, alkoxides (t-butoxide commonly used), NH 2 - will cause elimination. 6. Dehydration of alcohols to yield alkenes. Acid catalyzed and reversible reaction. Proceed through a carbocation intermediate. Skeletal rearrangements possible. 3° ROH easier than 2° ROH easier than 1° ROH. Typical conditions include: 20% H 2 SO 4 at 30-40°C 85% H 3 PO 4 at 60-100°C Conc. H 2 SO 4 at +140 ° C 7. Halohydrin Reaction.
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