Lecture 3

Lecture 3 - Properties and reactions of alkanes, alkanes, and alkynes

1. Properties of Alkanes
   1. Odorless or mild odder; colorless, tasteless, non-toxic
   2. Nonpolar, insoluble in water, soluble in nonpolar organic solvents
   3. Flammable
   4. Now very reactive
   5. Examples: mineral oils, petroleum jelly, paraffin wax
   6. Physical properties
      1. Intermolecular forces are many London dispersion forces
      2. Boiling and melting points increase with chain length.
      3. Boiling point tends to decrease with increased branching
2. Properties of Alkenes & Alkynes
   1. Similar in physical properties to Alkanes
   2. Insoluble in water and non-polar
   3. flammable
   4. boiling points tend to raise with size
   5. Are much more reactive than alkanes
   6. Cis-trans isomerism
3. Kinds of Organic reactions
   1. Addition reactions
      1. A+B -> C
      2. Two reactants "add" together to give a single product.
      3. No atoms are "left over"
   2. Elimination Reactions
      1. A -> B + C
      2. Opposite of an addition reaction.
      3. Single atom splits into two
   3. Substitution reactions
      1. AB + CD -> AC + BD
      2. Reactants exchange part to give two new products
      3. Book's example is a radical substitution reaction
   4. Rearrangements
4. Reactions of Alkanes
   1. Combustion
      1. Reaction of hydrocarbon with oxygen
      2. Produces carbon dioxide and water
      3. Incomplete oxidation produces CO as well
   2. Halogenation
      1. Radical reactions
         1. Initiation - start reaction
         2. Propagation - produces another radical
         3. Termination - End reaction chain no radical produced
         4. Chain reaction
      2. Chlorine and Bromine the most reactive. Fluorine very unreactive and Iodine is less reactive than Cl and Br, but is used.
      3. initiated by heat or light
      4. Halogen attaches to a carbon
      5. Can have multiple halogens on one alkane
      6. Many side products
      7. Some selectivity is found in radical reactions. Favored products come from most stable radicals
      8. In general the most stable radicals and 3>2>1
   3. Pyrolysis/Cracking
   4. Elimination
      1. Removal of a hydrogen and halogen to produce an alkene
      2. First hydrogen removed by a base
      3. Halogen is the leaving group
      4. Carbanion intermediate present.
      5. Primary carbanion most stable and tirtary is the least.
5. Reactions of alkenes and alkynes
   1. Hydrogenations
      1. Hydrogen placed across the double bone
      2. Must be done with a catalyst
   2. Hydrohalogenation
      1. Many times will see two possible products, but only one is formed
      2. Markovnikov's Rule - hydrogen attaches to most subsitiuted carbon on the alkene or alkyne.
      3. Intermediate is the more stable carbocation. This leads to the final product.
   3. Halogenation
      1. Mechanism differs from that if hydrohalogenation
      2. Intermediate can be a carbocation as in a hydrohalogenation or can be an cyclic intermediate
      3. Reactions involving bromine tend to have the cyclic intermediate (bromonium ion)
   4. Homework