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
Hydrogenation
Halogenation
Hydrohalogenation
hydration
Polymers
Aromatic compounds
Properties of aromatic compounds

2. Unsaturated Hydrocarbons
Compare the following structures of ethane, ethene and ethyne.
What difference do you notice among the first and the last two structures? (Ethane is saturated and ethene and ethyne are unsaturated?
How would you define unsaturated hydrocarbons?
Ethane

3. Bond Angles in Alkenes and Alkynes
According 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.
The groups attached to a triple bond are at 180° angles.
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4. Naming Alkenes Look and study the names of the following alkenes
Alkene IUPAC Common
H2C=CH ethene ethylene
H2C=CH─CH3 propene propylene
cyclohexene
CH2=CH─CH2─CH butene
CH3─CH=CH─CH butene
How is the naming of alkenes done? Talk over with your group members

5. Naming Alkynes
Now, let’s look at the case of alkynes. Naming is done in a fashion similar to that of alkenes.
Alkyne IUPAC Common
HC≡CH ethyne acetylene
HC≡C─CH3 propyne
4-Bromo-2-pentyne
cycloheptyne

6. Naming Alkenes and Alkynes with Substituents
Now what if the alkene or alkyne has a substituent? Here is an example. Write the IUPAC name for
CH3 │
CH3─CH─CH=CH─CH3
STEP 1 Name the longest carbon chain pentene
STEP 2 Number the chain from the double bond
STEP 3 Give the location of each substituent: methyl- 2-pentene
2-pentene

7. Naming Alkynes with Substituents
Write the IUPAC name for CH3 │
HC≡C─CH─CH3

8. Summary: Naming Alkenes and Alkynes
Okay, so let’s summarize the rules for naming alkenes and alkynes. Here is what we do:
Name the longest carbon chain with a double or triple bond
Indicate the location of the double or triple bond in the main chain by number, starting at the end closer to the double or triple bond
Cycloalkenes do not require the numerical prefix but the bonds are given numbers 1 and 2
Give the location and name of each substituent (alphabetical order) as a prefix to the name

9. Learning Check Write the IUPAC name for each of the following:
1. CH2=CH─CH2─CH3
2. CH3─CH=CH─CH3
CH3 |
3. CH3─CH=C─CH3
4. CH3─CC─CH3
Solution
______________

10. Learning Check Write the structural formula for each of the following:
A. 2-pentyne
3-methyl-2-pentene
Solution
_______________
________________

11. Cis and Trans Isomers In an alkene, cis and trans isomers are possible
because the double bond
Is rigid.
Cannot rotate.
Has groups attached to the carbons of the double bond that are fixed relative to each other.
CH3 CH CH3
CH = CH CH = CH
cis trans CH3

12. Cis-Trans Isomers Cis-trans isomers occur when
different groups are attached to
the double bond.
In a cis isomer, groups are attached on the same side of the double bond.
In the trans isomer, the groups are attached on opposite sides.
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13. Cis-Trans Isomerism
Cis-trans isomers do not occur if a carbon atom in the double bond is attached to identical groups.
Identical Identical
2-bromopropene ,1-dibromoethene
(not cis or trans) (not cis or trans) H Br H H

14. Naming Cis-Trans Isomers
The prefixes cis or trans are placed in front of the alkene name when there are cis-trans isomers.
cis trans
cis-1,2-dibromoethene trans-1,2-dibromoethene

15. Learning Check
Name each, using cis-trans prefixes when needed.

16. Lecture 3 Unsaturated Hydrocarbons
Reactions of Alkenes and Alkynes
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17. Addition Reactions
There are four addition reactions we will study, summarized in table 12.2
TABLE 12.2

18. Hydrogenation
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)
Your conclusion about hydrogenation:

19. Hydrogenation of Oils Adding H2 to double bonds in vegetable
oils produces
Compounds with higher melting points.
Solids at room temperature such as margarine,
soft margarine,
and shortening.
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20. Learning Check
Write an equation for the hydrogenation of 1-butene using a platinum catalyst.

21. Trans Fats In vegetable oils, the unsaturated fats usually contain
cis double bonds.
During hydrogenation, some cis double bonds are converted to trans double bonds (more stable) causing a change in the fatty acid structure
If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.

22. Learning Check Write the product of each the following reactions: Pt
CH3─CH=CH─CH3 + H2
+ H2

23. Halogenation
Question: What happens in halogenation reaction? Think about the following reactions.
Your conclusion about halogenation of alkenes and alkynes is that …

24. Learning Check Write the product of the following addition reactions:
1. CH3─CH=CH─CH3 + Cl2 2.

25. Testing for Unsaturation
Br2
When bromine (Br2) is added to an alkane, the red color of bromine persists.
When bromine (Br2) is added to an alkene or alkyne, the red color of bromine disappears immediately.
Br2
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26. Hydrohalogenation
What happens in hydrohalogenation reactions? Again, think about where atoms of a hydrogen halide end up.

27. 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 .

28. 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

29. 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)

30. Learning Check Write the product for the hydration of each of the
following: H+
1. CH3─CH2─CH=CH─CH2─CH HOH
CH3
│ H+
2. CH3─C=CH─CH2─CH HOH
HOH

31. Learning Check
Write the products of each reaction

32. 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.

33. Polymerization
In polymerization, small repeating units called monomers join to form a long chain polymer.
monomer unit repeats n

34. Common Synthetic Polymers
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35. Polymers from Addition Reactions
TABLE 12.3
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36. More Monomers and Polymers
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37. Learning Check
What is the starting monomer for polyethylene?

38. Learning Check Name the monomer used to make Teflon and write a
portion of a Teflon polymer using four monomers.

39. Recycling Plastics Recycling is simplified by using codes found
on plastic items.
1 PETE Polyethyleneterephtalate
2 HDPE High-density polyethylene
3 PV Polyvinyl chloride
4 LDPE Low-density polyethylene
5 PP Polypropylene
PS Polystyrene
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40. Lecture 3 Unsaturated Hydrocarbons
Aromatic Compounds
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41. Benzene Structure Benzene
Has 6 electrons shared equally among the 6 C atoms.
Is also represented as a hexagon with a circle drawn inside.
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42. Aromatic Compounds in Nature and Health
Vanillin Aspirin
Ibuprofen Acetaminophen

43. Naming Aromatic Compounds
Aromatic compounds are named
With benzene as the parent chain.
With one side group named in front of benzene.
methylbenzene chlorobenzene

44. Some Common Names Some substituted benzene rings
Have common names used for many years.
With a single substituent use a common name or are named as a benzene derivative.
toluene aniline phenol
(methylbenzene) (benzenamine) (hydroxybenzene)

45. Aromatic Compounds with Two Groups
Two naming systems are used when two groups are
attached to a benzene ring.
Number the ring to give the lowest numbers to the side groups.
Use prefixes to show the arrangement:
ortho(o-) for 1,2-
meta(m-) for 1,3-
para(p-) for 1,4-

46. Aromatic Compounds with Two Groups
3-chlorotoluene ,4-dichlorobenzene chlorophenol
m-chlorotoluene p-dichlorobenzene o-chlorophenol

47. Learning Check Select the correct name for each compound:
1) chlorocyclohexane
2) chlorobenzene
3) 1-chlorobenzene
1) 1,2-dimethylbenzene
2) m-xylene
3) 1,3-dimethylbenzene

48. Learning Check
Write the structural formulas for each of the following:
A. 1,3-dichlorobenzene
B. o-chlorotoluene

49. Learning Check Identify the organic family for each: A. CH3─CH2─CH=CH2B.
C. CH3─C≡CH D.

50. Properties of Aromatic Compounds
Have a stable aromatic bonding system.
Are resistant to many reactions.
Undergo substitution reactions, which retain the stability of the aromatic bonding system.

51. Substitution Reactions
In a substitution reaction, a hydrogen atom on a benzene ring is replaced by an atom or group of atoms.
Type of substitution H on benzene replaced by
Halogenation chlorine or bromine atom
Nitration nitro group (—NO2)
Sulfonation —SO3H group

52. Halogenation In a halogenation
An H atom of benzene is replaced by a chlorine or bromine atom.
A catalyst such as FeCl3 is needed in chlorination.
A catalyst such as FeBr3 is needed in bromination.

53. Nitration In the nitration of benzene
An H atom of benzene is replaced by a nitro (-NO2) group from HNO3.
An acid catalyst such as H2SO4 is needed.

54. Sulfonation In a sulfonation
An H atom on benzene is replaced by a —SO3H group from SO3.
An acid catalyst such as H2SO4 is needed.

55. Learning Check
Write the equation for the bromination of benzene including catalyst.