Download 3. Alkanes

Dr. Peter Wipf
Chemistry 0310 - Organic Chemistry 1
Chapter 3. Reactions of Alkanes
The heterolysis of covalent bonds yields anions and cations, whereas the homolysis creates
radicals. Radicals are species with unpaired electrons that react mostly as electrophiles, seeking a
single electron to complete their octet. Free radicals are important reaction intermediates and are
formed in initiation reactions under conditions that cause the homolytic cleavage of bonds. In
propagation steps, radicals abstract hydrogen or halogen atoms to create new radicals.
Combinations of radicals are rare due to the low concentration of these reactive intermediates and
result in termination of the radical chain.
!CHAIN REACTION SUMMARY
initiation
Cl2
hn
reactant
product
PhCH3
HCl
DH = -16 kcal/mol
or D
Cl
.
PhCH2Cl or Cl2
termination
PhCH2 .
propagation
PhCH2 .
or Cl
chain-carrying intermediates
(low concentrations)
PhCH2 .
or Cl
.
DH = -15 kcal/mol
.
PhCH2Cl
Cl2
product
reactant
PhCH2Cl
or PhCH2CH2Ph
termination
Alkanes are converted to alkyl halides by free radical halogenation reactions. The relative
stability of radicals is increased by conjugation and hyperconjugation:
CH2 .
R
>
R C
R
H
. >
R C
R
H
. >
H C
R
H
. >
H C.
H
Oxygen is a diradical. In the presence of free-radical initiators such as metal salts, organic
compounds and oxygen react to give hydroperoxides. These autoxidation reactions are responsible
for the degradation reactions of oils, fatty acids, and other biological substances when exposed to air.
Antioxidants such as hindered phenols are important food additives. Vitamins E and C are biological
antioxidants. Radical chain reactions of chlorinated fluorocarbons in the stratosphere are responsible
for the "ozone hole".
The Hammond postulate states that the structure of the transition state of organic reactions is
related to the ground state (starting material or product) that is closest in energy. A catalyst lowers
the energy of the transition state and thus speeds up the establishment of an equilibrium.
Some nomenclature rules:
The underlying principle for nomenclature is that each different compound should have a
different name. The rules for nomenclature are organized by the International Union of Pure and
Applied Chemistry (IUPAC). The following list summarizes the rules for the nomenclature of alkanes:
1. The longest contiguous chain of carbon atoms determines the parent name of the compound.
2. All substituents that branch off the main chain are named and numbered so that the lowest
possible numbers result.
3. Prefixes di-, tri-, tetra-, penta-, etc. are used for substituents of the same kind.
4. In naming the compound, all compounds are listed in alphabetical order, ignoring all prefixes.
5. Numbers of substituents that are grouped together are separated by commas. The name of the
last substituent is merged with the name of the straight chain alkane that forms the parent name of the
compound.
In bicycloalkanes, we name the compound according to the total number of carbons
encompassed by the two rings. Numbering starts at one of the bridgehead carbons and proceeds
along the longest bridge to the other bridgehead, then along the next longest bridge back to the fist
bridgehead. The shortest bridge is numbered last. Interposed in the name in brackets is an
expression that denotes the number of carbon atoms in each bridge, in order of decreasing length
and separated by periods.
The systematic names of alkyl halides and alcohols are assigned in the same way. The
prefixes fluoro-, chloro-, bromo-, and iodo- are used to indicate the presence of halogens. Alcohols
are named by changing the e ending of the name of the alkane to ol and using a number to indicate
the position of the hydroxyl group. In naming an alcohol, the carbon chain is numbered so that the
carbon atom bearing the hydroxyl group has the lowest possible number.
Examples:
Cl
5-sec-butyl-2,7-dimethylnonane
2-methylbicyclo[2.2.1]heptane
OH
4-chloro-1-butanol