chemical reactions of alkanes 1. combustion 2. desaturation- forms alkenes 3. halogenation- forms...
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Chemical Reactions of Alkanes
1. Combustion
2. Desaturation- forms alkenes
3. Halogenation- forms alkyl halides
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Chemical Reactions of Alkanes: Combustion
CH4 + 2 O2 CO2 + 2 H2O + energy
2 C8H18 + 25 O2 8 CO2 + 9 H2O + energy
.
.
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Chemical Reactions of Alkanes: Desaturation- forms alkenes
.
.
C C
H
H
H
H H
H
Pd catalyst
H2
C CH H
HH
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Chemical Reactions of Alkanes: Halogenation- forms alkyl halides
.
.
C C
H
H
H
H H
H
C C
H
H
H
H H
ClCl2 HCl
h
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Mechanism of Halogenation is Free Radical Based
.
Cl2 2 Clh
.
.
Cl Clh
2 Cl
.
.
C C
H
H
H
H
H
C C
HH
H
H
H
H Hh
.
+
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2nd Step in Mechanism of Halogenation is Elongation
.
C C
H
H
H
H
H
C C
H
H
H
H H
H
C C
HH
H
H
H
Cl
Cl
.
HCl
or
C C
H
H
H
H
H ClCl2
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Various Combinations of Free Radicals Represent Termination
.
.
C C
H
H
H
H
H
C C
H
H
H
H C
H
C
H
H H
H
H
C C
HH
H
H
H
Cl
.
or
C C
H
H
H
H
H Cl
+ C C
H
H
H
H
H
or . . .
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Nomenclature of Alkyl Halides
1. Chain as alkanes
2. -ine of halogen changed to -o
3. Identify attachment site
e.g. 2-chloroethane
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Physical Properties of Alkyl Halides
1. Polar
2. Liquid instead of gas
3. Tend to be toxic, taken internally
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Predominant Product
Determined by radical stability
Direction of stability
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Stereochemistry
C
H
CH2CH3
CH3 Br C
Br
CH2CH3
CH3 H
.
.
• Chiral Carbon- a carbon which has four different groups bonded to it
• Dashed wedge: behind; solid wedge, in front• Enantiomers- nonsuperimposable, mirror images
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The RS system of nomenclature
CH CH3
CH2CH3
Cl
.
.
1. Position the molecule so the H is in back
2. Number highest mol. wt. (1) to lowest (3)
3. Clockwise R, counterclockwise S
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Alkenes: Names and Structures
Alkenes- hydrocarbons that have one or more double bonds between carbon atoms
CnH2n
Monounsaturated- one double bond
Polyunsaturated- two or more double bonds
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Naming Alkenes
1. Find the longest continuous carbon chain that contains the double bond(s), even if it is not the longest carbon chain, and number from the end nearer the double bond.
2. Write the alkane name corresponding to the longest carbon chain, then change the -ane suffix to -ene.
3. Identify by number the lower-numbered carbon atom containing a double bond, followed by a hyphen directly before the alkene name; precede this by naming substituents in the usual way.
4. If two or more double bonds occur, specify each double bond by number and use an appropriate prefix before -ene to indicate the number of double bonds.
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Cis-Trans Isomerism
1. “R groups” must be attached to different carbons
2. Larger “R groups” get priority
C
C
H3C H
HH3C
C
C
H CH3
HH3C
C
C
H3C CH3
HH
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Practice
C C
CH3
CH3
H
CH3
.
.
C C
CH3CH2CH2
H
CH2CH2CH3
H
.
.
• Give the IUPAC name for the above molecules
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More Practice
• Draw the structure of the following compounds, for which a correct IUPAC name is given:
cis-3-hexene
3-ethylcyclohexene
C C
CH3CH2
H
CH2CH3
HCH2CH3 .
.
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Properties of Alkenes and Alkynes
1. nonpolar
2. less dense than alkanes
3. trend toward gas rather than liquid
4. used to make polymers
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Reactions of Alkenes
Addition reaction- since H has been taken away to create a double bond, H or other molecules can be added across the double bond, yielding a substituted alkane.
1. catalytic hydrogenation
2. halogenation
3. addition of HCl and HBr (Markovnikonv's rule)
4. hydration
5. oxidation
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Reactions of Alkenes: Catalytic Hydrogenation
.
.
C C
H
H
H
H H
H
Pd catalyst
H2
C CH H
HH
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Reactions of Alkenes: Halogenation
.
.
C C
H
Br
H
H H
Cl
C CH H
HH
Br2 or Cl2
C CH H
HHBr Br C C
H H
HH
BrBr
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Reactions of Alkenes: Acid-Catalyzed Hydration (Markovnikov’s Rule)
.
.
CH3CH2CH CH3HCH3CH2CH CH2 +
O H
H
CH3CH2CH CH3 + CH3CH2CH CH3
OH
H+
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Reactions of Alkenes: Addition of HCl and HBr (Markovnikov’s Rule)
CH3CH CH3
Br
CH3CH CH3 + Br
CH3CH CH2 H Br+ CH3CH CH3 + Br
.
.
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Reactions of Alkenes: Catalytic Hydrogenation
Addition reaction- since H has been taken away to create a double bond, H or other molecules can be added across the double bond, yielding a substituted alkane.
1. catalytic hydrogenation
2. halogenation
3. addition of HCl and HBr (Markovnikonv's rule)
4. hydration
5. Others exist
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Alkynes: Names and Structures
Alkynes- hydrocarbons that have one or more triple bonds between carbon atoms
CnH2n-2
Named as alkenes, with the exception that -yne is used instead of -ene
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Reactions of Alkynes
Since H has been taken away to create the triple bond, just as for a double bond, the same reactions will occur for alkynes as alkenes.
The difference is, 2 moles will be added instead of 1.