Part 2

Occurs in Alkanes. Nucleophiles are electron rich species that attracted to positively polarized atoms. Basicity: Stronger the base, stronger the nucleophile. Size/polarizability: dependson solvent. Protic = larger atoms. Aprotic = more basic atoms are better.

I > Br > Cl > F Weak bases make good leaving groups. Can accept electron pair and dissociate to form stable species.

Rate is dependent on ONE species. Rate determining step is the carbocation. Favored in POLAR protic solvents (water or acetone) rate = k[RX] Want stable carbocations: 3 > 2 > 1 > CH3 (favored with use of bulky nucleophiles) Produces Racemic products (lose optical activity) Leaving group = weak bases are best.

ONE step (no carbocation) rate = k[Nu][RX] (substrate and nucleophile) Usually attacks from the backside. Best Reactant: 1 > 2 > 3 Favored in polar APROTIC solvents. Optically active and inverted products. Need strong nucleophile and transition state.

Used in synthesis of alkenes. Elimination reactions of either alcohols or alkyl halides. In these reactions, the carbon skeleton loses HX (X = halide) or a molecule of water, to form double bond. Two types E1 and E2

TWO step process, proceeding through a carbocation intermediate. k = [RX] (substrate) Elimination of leaving group plus proton = double bond. • Leaving group departs, producing carbocation. • Proton is removed by a base. E1 is favored by same factors as SN1: highly polar solvents, Weak Nu, highly branched carbon chains (3), and good leaving groups. However, HIGH temps favor E1.

ONE step process. k = [RX][Base] Strong base such as ethoxide ion (C2H5O-) removes a proton, while halide ion anti to proton leaves = double bond. Often two possible products, but more substituted preferred. Steric hindrance does NOT affect E2. Strong base favors E2 over SN2. SN2 favored over by E2 by weak Lewis bases. (Strong Nu)

Reductive process of adding hydrogen to a double bond with aid of a metal catalyst. (platinum, palladium, and nickel) Product: Alkane with syn addition of H. Reaction takes place on metal surface, thus H atoms are added to same face (syn addition).

An electrophilic addition that occurs in alkenes. Electron of double bond acts as Lewis base and reacts with electrophilic HX molecules. • Yields carbocation after double bond reacts with H+ • Halide ion combines with carbocation to give alkyl halide. Follows Markovnikov rule (add to the most substituted carbon) Product: Alkyl halide

Addition of halogens to double bond. Rapid process. Nucleophile is the double bond, which attacks an X2 molecule, displacing X-. Forms intermediate cyclic halonium ion, which is then attacked by X- to make a dihalo compound. Anti-addition (attacks SN2). Product: dihalo alkane

Water can be added to alkenes under acidic conditions. Protonated according to markovnikov. Performed at LOW temperatures. Product = alcohol

Alternate mechanism for addition of HX to double bond. Occurs when PEROXIDES, O2, or other impurities are present. Disobeys Markovnikov rule. Product: terminal alkyl halides.

Diborane (B2H6) adds readily to double bonds. • Boron atoms is the Lewis Acd and attaches to less sterically hindered C atom. • Oxidation-hydrolysis with PEROXIDE/Aq. base produces alcohol with ANTI-markovnikov, syn orientation. Product: Alcohol with anti-Markovnikov.

Involved in Oxidation. • Cold, dilute KMnO4 produces 1, 2 diols (vicinal) with syn orientation. • Hot, basic KMnO4 plus acid produces: • Nonterminal alkenes = 2M COOH • Terminal - 1M COOH and 1 CO2 (if disubsituted it makes a ketone).

Involved in Oxidation. Treatment of alkenes with OZONE, followed by reduction with Zn/H20 results in CLEAVAGE of double bond. Product: Aldehyde. If reduced with NaBH4 instead of Zn/H2) will result in Alcohols.

Alkenes can be oxidized with peroxycarboxylic acids. e.g. Peroxyacetic acid (CH3CO3H) OR m-chloroperoxybenzoic acid (mcpba). Products = Epoxides or Oxiranes.

Creation of long, MW chains composed of repeating subunits in alkenes. Occurs through radical mechanism, requires HIGH TEMP and PRESSURE.

• Elimination of HX from geminal and vicinal dihalides with HEAT and BASE. • Add existing Triple bond to Nucleophile by removing acidic proton with STRONG BASE.

Can be hydrogenated with a catalyst. Two Ways: 1. Lindlar’s catalyst (H2, Pd/BaSO4, Quinoline) Product: Alkane (cis) • Na, NH3 (liquid) -33 degrees C. Product: Trans alkene.

Electrophilic addition to alkynes occurs in same manner as it does alkenes. (X2 added) Occurs according to Markovnikov. Can be stopped at alkene or proceed further. Product: Dihalo alkene or alkane

Radicals add to triple bonds as with double bonds with ANTI-markovnikov (X-). Product: trans isomer. Because intermediate vinyl radical can isomerize to its more stable form.

Addition to triple bonds is same as double bonds. Addition is syn and BORON adds first. Boron can be replaced with proton from acetic acid = cis alkene. Product: syn alkene.

Alkynes can be oxidatively cleaved with either basic KMnO4 followed by acidification or Ozone. KMnO4 Product: 2 M COOH Ozone Product: 2 M COOH

4n + 2 pi electrons Important indicator of aromaticity. n = any nonnegative integer; thus can be 2, 6, 10, 14, 18, etc.