Section 1.3: Reactions of Hydrocarbons
Reactions of Alkanes
Combustion in the presence of O2 (Oxidation) Review Grade 11
Eg. Complete Combustion
2 C6H14 + 19 O2 
12 CO2 + 14 H2O
Eg. Incomplete Combustion
C6H14
+ 7 O2 
2 CO2 + 7 H2O + C + 3 CO
Substitution reactions with Halogens (Halogenation pg. 25)
 C-C bonds are hard to break but H atoms may be substituted for by halogens: F2, Cl2 or Br2
 F2 will react without catalyst
 Cl2 and Br2 require heat or UV radiation to break apart the diatomic molecules
Eg.
General Equation
Reactions of Alkenes and Alkynes
 C=C and CC are weak bonds that are easily broken making molecules that contain them more
reactive than alkanes
Addition Reactions
 double or triple bond is broken and replaced by atoms adding onto the molecule
a) Halogenation
 F2, Cl2 or Br2 are all able to react with alkenes and alkynes without the use of catalysts
Eg.
General Equation
b) Hydrogenation (with H2)
 requires heat, pressure, or a catalyst
 process used on oils to turn them into solid fats Eg. Margarine and Peanut Butter
Eg.
General Equation
Markovnikov’s Rule
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”
c) Hydrohalogenation
 reactions occur at room temperature
Eg.
General Equation
d) Hydration (with H2O)
 involve the addition of water (HOH)
 require a H2SO4 catalyst
Eg.
General Equation
New symbols:
Alkyl Groups
Halogen
Phenyl group
R, R’, R’’, etc. (R, R-prime, R-double prime) (R = radical)
X
Ø
New prefixes:
bromoBrchloroClfluoroFhydroxyl-OH
* organic and hydroxy - halides are polar (and therefore can be dissolved into water)
nitro-
-NO2
phenyl -
-
(benzene)
Reactions with Aromatic Hydrocarbons
Reactivity is intermediate between alkanes and alkenes.
The Benzene structure is particularly stable. We show Benzene as having 3 C=C (double bonds) and 3 C-C
(single bonds). This would suggest shorter, weaker C=C bonds alternating with longer, stronger C-C bonds.
In actuality X-ray diffraction shows 6 equal length, equal strength bonds. The 18 valence electrons in
Benzene are equally shared in a delocalized arrangement. – a unique bonding situation. Hence, the benzene
ring does NOT undergo addition reactions that break apart double bonds – benzene rings DO NOT BREAK
APART in hydrocarbon reactions.
a) halogenation - substitution Rx of a halogen with cycloalkane
 requires heat or UV
eg.
halogenation – substitution Rx of a halogen with benzene (no addition)
 requires FeBr3 catalyst
eg.
Halogenation - substitution Rx of a halogen with cyclohexene
 occurs at room temperature (due to easy to break double bond)
eg.
Note that the benzene structure is not broken in the second reaction
b) Nitration – substitution Rx of nitric acid with benzene
 requires H2SO4 catalyst
eg.
c) Alkylation – substitution Rx of an alkyl halide with benzene
 requires AlCl3 catalyst
eg.
BENZENE DOES NOT UNDERGO ADDITION REACTIONS