14.1 introduction to haloalkanes learning objectives: 1.be able to name halogenalkanes. 2.describe...
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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.
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Task
Draw the skeletal formula and structural formula for the following:
- 1-iodopentane- 2-bromobutane- 3-chloropentane- 2-chloro-3-methylbutane
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1-iodopentane
I
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2-bromobutane
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3-chloropentane
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2-chloro-3-methylbutane
Cl
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tetrachloromethane
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iodomethane
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dibromodifluoromethane
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1,1-diiodoethane
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1-chloro-3-fluoro-2-iodopropane
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A chain of carbon atoms can be represented by R when drawing the structure. This is referred to as an R group.
Primary, secondary and tertiary
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.
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Physical Properties – Boiling Point• Explain the trend in boiling points of haloalkanes:
Why are fluorides
not on the chart?
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Physical Properties – Boiling Pt.• Explain the trend in boiling points of haloalkanes:
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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
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Bond Polarity
• Difference in electronegativity makes the C-X bond polar.
Carbon
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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.
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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.
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Carbon-Halogen Bond is Polar
• The partial positive charge on the carbon atom (electron deficient) attracts species with a negative charge (electron rich).
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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:
OH¯ CN¯ NH3 H2O
δ-δ-
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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
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AQUEOUS SODIUM HYDROXIDE
ANIMATED MECHANISM
NUCLEOPHILIC SUBSTITUTION
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Nucleophilic Substitution – OH-
• Haloalkanes react with hydroxides to produce alcohols and halide ions.
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Nucleophilic Substitution – CN-
• Haloalkanes react with cyanide ions to produce nitriles and halide ions.
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Nucleophilic Substitution – NH3
• Haloalkanes react with ammonia to produce amines and halide ions.
1)
2)
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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.
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Elimination Mechanism1. OH- removes a H+ ion from the haloalkane to
form water.2. The electrons in the C-H bond now form a C-
C 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.
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ELIMINATION
ANIMATED MECHANISM
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Mixture of elimination productsIf 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 Aattack at BA B
but-2-ene but-1-ene
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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)
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14.4 Formation of Haloalkanes
Learning Objectives:1. Define the term free radical.2. Describe the reaction mechanism for free-
radical substitutions.
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Free Radical
• Free radical = species with an UNPAIRED electron
Cl.
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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)
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Initiation
• UV light provides the energy needed to break the Cl-Cl bond.
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Propagation• Free radicals are EXTREMELY reactive. They
react with molecules to form new free radicals. This is a chain reaction.
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Termination• When two free radicals react it removes the free
radicals and terminates the chain reaction. • There are multiple termination products.
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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.