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Ch. 11: Unsaturated Hydrocarbons

Ch. 11: Unsaturated Hydrocarbons. Alkene – has at least one carbon-carbon double bond, C = C. Simplest alkene , C 2 H 4 , ethene. Unsaturated Hydrocarbons. Alkyne – has at least one carbon-carbon triple bond, C  C. Simplest alkyne , C 2 H 2 , ethyne. Unsaturated Hydrocarbons.

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Ch. 11: Unsaturated Hydrocarbons

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  1. Ch. 11: Unsaturated Hydrocarbons • Alkene – has at least one carbon-carbon double bond, C = C. • Simplest alkene, C2H4, ethene.

  2. Unsaturated Hydrocarbons • Alkyne – has at least one carbon-carbon triple bond, C  C. • Simplest alkyne, C2H2, ethyne.

  3. Unsaturated Hydrocarbons • Can also have double bond with ring structure. • Cycloalkene – a ring with one or more carbon-carbon double bond.

  4. Alkenes • Naming an alkene. • Find the longest chain that includes the double bond. Base name uses –ene suffix. • Number the chain so that the double bond gets the lowest numbers (has priority over other substituents). Only four carbons or longer will need a number for the double bond position. Use only the lowest number for start of double bond. • Number substituents based on this numbering. • Cycloalkenes – the double bond is ALWAYS position #1 and #2.

  5. Alkenes 5 4 3 2 1 • CH3 – CH – CH2 – CH = CH2  CH3 Correct: 4-methyl-1-pentene Not correct: 2-methyl-4-pentene

  6. Alkenes Correct = 3-methylcyclopentene Not Correct = 1-methylcyclopentene LEP #1

  7. Structure of Alkenes • The double bond in an alkene consists of a sigma and a pi bond. • Pi bonds do NOT allow free-rotation. • C2H6 (Ethane) versus C2H4 (Ethene). • The molecule around that double bond is planar (flat).

  8. Geometric Isomers • The molecule 2-butene has two unique structures. • CH3 – CH = CH – CH3 • How? • LEP #2 • The molecule 1-butene does not! • CH2= CH – CH2– CH3

  9. Geometric Isomers • Many alkenes do not have a pair of geometric isomers. • Requires two different sets of groups on each side of the double bond – one large and one small. A B C = C B A • Opposite = trans, Same side = cis • LEP #3

  10. Geometric Isomers • These types of molecules are found in many places and are very important. • Ex) Bombykol – a sex attractant (pheremone) for the silkworm moth.

  11. Addition Reactions • Both alkenes and alkynes can undergo an addition reaction where the double bond is broken and two atoms or molecule fragments are added to each side of the double bond. • Hydrogenation, adding H2 with a catalyst. • Halogenation, adding Cl2, Br2, or I2. • Hydration, adding H2O (H-OH) with an acid catalyst. • LEP #4

  12. Polymers of Alkenes • A polymer is a long chain of repeating units called monomers. • Analogy: train = monomers are the individual cars. • Monomers are typically small alkenes. • Initiated by organic peroxide, R-O-O-R’, which is split into two fragments, 2 R-O Benzoyl peroxide

  13. Polymers of Alkenes • Each fragment has an odd electron. • Peroxide fragment then reacts with monomer unit using one electron from double bond. • Monomer end now has odd electron, which is used to react with a second monomer. • Process repeats thousands of times.

  14. Common Monomers • CH2=CH2, ethylene makes polyethylene. • Two forms – low density and high density • Uses: • CH2=CHCl, vinyl chloride makes PVC. • Uses: • CH2=CHCH3, propene makes polypropylene. • Uses: • CF2=CF2, tetrafluoroethene makes Teflon. • Uses:

  15. Common Monomers • CH2=CCl2, 1,1-dichloroethene makes Saran. • Uses: • CH2=CH(C6H5), phenylethene makes polystyrene. • Uses: • Recycling – uses a series of symbols and numbers to identify the type.

  16. Aromatic Compounds • C6H6 = Benzene • A ring structure, but very different from the cycloalkanes. • Three double bonds alternate with every other C-C in ring.

  17. Aromatic Compounds • Alkenes are reactive because of the double bond. • Benzene, though, is not reactive at all. • Why? • True structure is an average of the two below.

  18. Aromatic Compounds • The six electrons from the three double bonds are delocalized (free to move). • Chemists often show this by drawing the hexagon and putting a circle inside to represent these electrons.

  19. Naming Benzene Compounds • Some common mono-substituted benzene molecules have common names that are allowed by IUPAC rules.

  20. Naming Benzene Compounds • Benzene as a substituent (C6H5-) is called Phenyl. • Mono-substituted benzenes do not require a number. • Di-substituted benzenes do require numbers. • A phenol or toluene will always have the –OH or –CH3 group as position #1. • LEP #5

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