Chapter 3 Alkenes and Alkynes. Unsaturated Hydrocarbons Contain carbon-carbon multiple bonds....
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Transcript of Chapter 3 Alkenes and Alkynes. Unsaturated Hydrocarbons Contain carbon-carbon multiple bonds....
Chapter 3Alkenes and Alkynes
Unsaturated Hydrocarbons• Contain carbon-carbon multiple bonds.
Alkenes C=C double bonds
Alkynes C≡C triple bonds
Aromatics benzene rings
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12-3© 2006 Thomson Learning, Inc.All rights reserved
Alkenes• Structure:
• The VSEPR model predicts bond angles of 120° about each carbon of a double bond.
H
C C
H
H H
121.7°H3C
C C
H
H H
124.7°
Ethylene Propene
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12-4© 2006 Thomson Learning, Inc.All rights reserved
Alkenes• Cis-trans isomerism
• because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.
CIS TRANS
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12-5© 2006 Thomson Learning, Inc.All rights reserved
Problem 43, p. 355
(c)
(d)
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12-6© 2006 Thomson Learning, Inc.All rights reserved
Naming Alkenes
• Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending.
• Step 2: Number the longest chain so the C=C bond gets the lowest number possible.
• Step 3: Designate the C=C bond in the name with the lowest-numbered carbon.
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Examples:1 2 3 4
CH3-CH=CH-CH3
2-butene
6 5 4 3 2 1
CH3-CH2-CH2-CH=CH-CH3
2-hexene
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Naming Alkenes, cont.• Step 4: Locate and name attached groups.• Step 5: Combine all the names as you did with
alkanes.
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Alkynes - IUPAC Names• follow the same rules as for alkenes, but use the
ending -yne to show the presence of the triple bond.
CH3CHC CHCH3
CH3CH2C CCH2CCH3
CH3
CH3
3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne
1 1
2 23
3 44 5
6 7
Example 12-1, p. 333
(d)
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Cycloalkenes• To name a cycloalkene:
• number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first.
• number and list substituents in alphabetical order.
1 2
3
4
5
1
2
3
4
5
6
3-Methylcyclopentene(not 5-methylcyclopentene)
4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)
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12-12© 2006 Thomson Learning, Inc.All rights reserved
Example 12-3, p. 336
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Physical Properties• Alkenes and alkynes are nonpolar compounds.
• The only attractive forces between their molecules are London dispersion forces.
• Their physical properties are similar to those of alkanes with the same carbon skeletons.
• Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids.
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Alkene Reactions
• Alkenes are quite chemically reactive• Many reactions are addition reactions:
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Reactions of Alkenes• The most common reaction is addition
CC
C C
C C Br2
HCl
H2O
CC H2
C CBr Br
C CH Cl
C CH OH
C CHH
Descriptive Name(s )Reaction
+
+
+
+
bromination
hydrochlorination
hydration
hydrogenation(reduction)
halogenation
hydrohalogenation
Alkene Reactions, cont.• Hydrogenation (addition) reactions can occur in the presence of
a catalyst (Pt, Pd, or Ni).
The hydrogenation of vegetable oils is an important commercial process.
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Addition of H2• Virtually all alkenes add H2 in the presence of a
transition metal catalyst, commonly Pd, Pt, or Ni.
HH3C
C C
H CH3
PdCH3CH2CH2CH3
trans-2-Butene
+ H225°C, 3 atm
Butane
Pd+ H2
Cyclohexene Cyclohexane
25°C, 3 atm
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Addition of a Halogen (Halogenation)• Addition takes place readily at room temp.
• reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent
• addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond
Br2CH2Cl2
Br
Br+
1,2-DibromocyclohexaneCyclohexene
CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene
+
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12-19© 2006 Thomson Learning, Inc.All rights reserved
Addition of HX (Hydrohalogenation)• Addition of HX (HCl, HBr, or HI) to an alkene
• H adds to one carbon of the C=C and X to the other.
• Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.
CH2=CH2 HCl CH2-CH2
ClH
Chloroethane(Ethyl chloride)
Ethylene
+
CH3CH=CH2 HCl CH3CH-CH2
HClCH3CH-CH2
ClH
1-Chloropropane(not formed)
2-ChloropropanePropene
+
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•Hydration-addition of water
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Addition of H2O
• Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.
CH3CH=CH2 H2OH2SO4
CH3CH-CH2
HOH
Propene 2-Propanol+
1.
2.
3.
4.
5.
Polymerization• Polymers – long chain products made up of repeating units.• Monomer – the starting material that becomes the repeating
units of a polymer.
Table 12-2, p. 349
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Polymerization• Show the structure of a polymer by placing
parentheses around the repeating monomer unit.• Place a subscript, n, outside the parentheses to
indicate that this unit repeats n times.• The structure of a polymer chain can be reproduced
by repeating the enclosed structure in both directions.• following a section of polypropene (polypropylene)