14.1 Introduction to Haloalkanes
Learning Objectives:
1. Be able to name halogenalkanes.
2. Describe the physical properties of
halogenalkanes.
3. Explain the trend in bond polarity.
4. Explain the trend in reactivity.
Task
Draw the skeletal formula and structural
formula for the following:
-
1-iodopentane
2-bromobutane
3-chloropentane
2-chloro-3-methylbutane
1-iodopentane
I
2-bromobutane
3-chloropentane
2-chloro-3-methylbutane
Cl
tetrachloromethane
iodomethane
dibromodifluoromethane
1,1-diiodoethane
1-chloro-3-fluoro-2-iodopropane
Primary, secondary and tertiary
A chain of carbon atoms can be represented by R when drawing the structure. This is
referred to as an R group.

Primary (1°) halogenoalkanes have one R
group attached to the carbon linked to the
halogen.

Secondary (2°) halogenoalkanes have two R groups
attached to the carbon linked to the halogen.

Tertiary (3°) halogenoalkanes have three R
groups attached to the carbon linked to the
halogen.
Physical Properties – Boiling Point
• Explain the trend in boiling points of haloalkanes:
Why are
fluorides
not on the
chart?
Physical Properties – Boiling Pt.
• Explain the trend in boiling points of haloalkanes:
Physical Properties – Solubility
• The carbon-halogen bond is polar (halogens
more electronegative than carbon).
• But not polar enough to be soluble in water.
• Haloalkanes are…
–soluble in organic solvents
–insoluble in water
Bond Polarity
• Difference in
electronegativity
makes the C-X
bond polar.
Carbon
Reactivity:
Bond Polarity vs. Bond Enthalpy
• Two factors determine the reactivity of
haloalkanes
– Bond polarity
– Bond enthalpy
Bond enthalpy is
the more
important factor,
reactivity increases
DOWN the group.
14.2 Nucleophilic Substitution
Learning Objectives:
1. Define the term nucleophile and name some
examples.
2. Describe the nucleophilic substitution
reaction with haloalkanes.
3. Draw the mechanism for a nucleophilic
substitution reaction.
Carbon-Halogen Bond is Polar
• The partial positive charge on the carbon
atom (electron deficient) attracts species with
a negative charge (electron rich).
Nucleophiles
• Nucleophiles are electron rich and are attracted to
the partial positive charge on the carbon. Have lone
pairs attached to an electronegative atom.
• Nucleophiles = electron pair donors
Examples:
δOH¯
CN¯
NH3
δH 2O
Nucleophilic Substitution Mechanism
1. Lone pair on nucleophile “attacks” the δ+ carbon
and forms a dative covalent bond.
2. The halogen takes both electrons from the C-X
bond and becomes the leaving group (X-).
“curly arrow” represent
movement of electron pairs
NUCLEOPHILIC SUBSTITUTION
AQUEOUS SODIUM HYDROXIDE
ANIMATED MECHANISM
Nucleophilic Substitution – OH• Haloalkanes react with hydroxides to produce
alcohols and halide ions.
Nucleophilic Substitution – CN• Haloalkanes react with cyanide ions to
produce nitriles and halide ions.
Nucleophilic Substitution – NH3
• Haloalkanes react with ammonia to produce
amines and halide ions.
1)
2)
14.3 Elimination Reactions
Learning Objectives:
1. Describe the elimination reaction of
haloalkanes.
2. Draw the mechanism for an elimination
reaction.
3. Describe the conditions that would favor
elimination over substitution.
Elimination Mechanism
1. OH- removes a H+ ion from the haloalkane to
form water.
2. The electrons in the C-H bond now form a CC double bond.
3. The C now has 5 bonds so the halogen takes
the electrons from the C-X bond and is
eliminated as a halide ion.
ELIMINATION
ANIMATED MECHANISM
Mixture of elimination products
If the carbon chain is four or more carbons in length and the halogen is not attached
to a terminal carbon, a mixture of positional isomers may be formed.
attack at A
but-2-ene
A
B
attack at B
but-1-ene
Conditions:
Substitution vs. Elimination
Substitution
• Cold hydroxide
• Aqueous solution
• Primary
haloalkanes
• OH- acts as a
nucleophile
Elimination
• Hot hydroxide
• Ethanolic solution
(NO WATER)
• Tertiary
haloalkanes
• OH- acts as a base
(proton acceptor)
14.4 Formation of Haloalkanes
Learning Objectives:
1. Define the term free radical.
2. Describe the reaction mechanism for freeradical substitutions.
Free Radical
• Free radical = species with an UNPAIRED
electron
.
Cl
Free Radical Substitution
1. Initiation (free radicals are formed)
2. Propagation (free radicals react to
form additional free radicals)
3. Termination (free radicals react and
are removed)
Initiation
• UV light provides the energy needed to break
the Cl-Cl bond.
Propagation
• Free radicals are EXTREMELY reactive. They
react with molecules to form new free
radicals. This is a chain reaction.
Termination
• When two free radicals react it removes the free
radicals and terminates the chain reaction.
• There are multiple termination products.
Example: CFC’s and the Ozone Layer
• Chlorofluorocarbons
(CFC’s) are haloalkanes.
• They are known to cause
damage to the ozone layer
(which protects us from
harmful UV radiation).
• Ozone is destroyed through
a free radical substitution
reaction.
Read pg. 193 and 195
and make notes on CFCs
and the ozone layer
including the chemical
equations for the free
radical reactions.