1 Lecture 3 Unsaturated Hydrocarbons Lecture Outline Class odds and ends Defining unsaturation in hydrocarbons Naming alkenes and alkynes Cis and trans isomers Reactions of alkenes and alkynes o Hydrogenation o Halogenation o Hydrohalogenation o hydration Polymers Aromatic compounds Properties of aromatic compounds
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1 Lecture 3 Unsaturated Hydrocarbons Lecture Outline Class odds and ends Defining unsaturation in hydrocarbons Naming alkenes and alkynes Cis and.
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Lecture 3 Unsaturated Hydrocarbons
Lecture Outline Class odds and ends Defining unsaturation in hydrocarbons Naming alkenes and alkynes Cis and trans isomers Reactions of alkenes and alkynes
o Hydrogenationo Halogenationo Hydrohalogenationo hydration
Polymers Aromatic compounds Properties of aromatic compounds
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Compare the following structures of ethane, ethene and ethyne.
1. What difference do you notice among the first and the last two structures? (Ethane is saturated and ethene and ethyne are unsaturated?
2. How would you define unsaturated hydrocarbons?
Unsaturated Hydrocarbons
Ethane
C C
H
H
H H
H
H
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Bond Angles in Alkenes and AlkynesAccording to VSEPR theory: Three groups in a double
bond are bonded at 120° angles.
Alkenes are flat, because the atoms in a double bond lie in the same plane.
There are four addition reactions we will study, summarized in table 12.2
TABLE 12.2
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Question: what happens in hydrogenation? Study the examples given below to answer this question?(Side note: a catalyst such as Pt or Ni is used to speed up the reaction)
What happens in hydrohalogenation reactions? Again, think about where atoms of a hydrogen halide end up.
ClH
CH3 CHCHCH3+ HClCH3CHCHCH3
Br
H + HBr
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Markovnikov’s Rule
When an unsymmetrical alkene undergoes hydrohalogenation, the H in HX adds to the carbon in the double bond that has the greater number of H atoms .
HCl
CH2CHCH3
CH2CHCH3 + HCl
ClH
CH2CHCH3 Does not form
C with the most H
Product that forms
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Hydration
In the addition reaction called hydration An acid H+ catalyst is required. Water (HOH) adds to a double bond. An H atom bonds to one C in the double bond. An OH bonds to the other C.
H OH
H+ │ │CH3─CH=CH─CH3 + H─OH CH3─CH─CH─CH3
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Hydration
When hydration occurs with a double bond that has an
unequal number of H atoms, The H atom bonds to the C in the double bond with
the most H. The OH bonds to the C in the double bond with the
fewest H atoms.
OH H H+ │ │CH3─CH=CH2 + H─OH CH3─CH─CH2
(1H) (2H)
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Learning Check
Write the product for the hydration of each of the
following:
H+
1. CH3─CH2─CH=CH─CH2─CH3 + HOH
CH3
│ H+
2. CH3─C=CH─CH2─CH3 + HOH
H+
3. + HOH
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Learning Check
Write the products of each reaction
C.
B.
A.
HOH +CH3CHCHCH3
Pt
+ Cl2 CH2CHCH3
+ H2
H+
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Polymers
Polymers are Large, long-chain molecules. Found in nature, including cellulose in plants,
starches in food, proteins and DNA in the body.
Also synthetic such as polyethylene and polystyrene, Teflon, and nylon.
Made up of small repeating units called monomers.
Made by reaction of small alkenes.
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Polymerization
In polymerization, small repeating units called monomers join to form a long chain polymer.