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Chapter 12:Unsaturated Hydrocarbons

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UNSATURATED HYDROCARBONS• contain carbon-carbon multiple bonds.

Alkenes C=C double bonds

Alkynes triple bonds

Aromatics benzene rings

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NAMING ALKENESStep 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: Locate the C=C bond with the lowest-numbered carbon.

Examples:1 2 3 4

CH3-CH=CH-CH3

2-butene

1 2 3 4 5 6

CH3-CH2-CH2-CH=CH-CH3

2-hexene

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Step 4: Locate and name attached groups.

Step 5: Combine all the names.

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NAMING ALKENES WITH MULTIPLE DOUBLE BONDS

Step 1: Follow the same naming instructions for alkenes with one double bond, except use the endings ﾐ diene, -triene, and the like to denote the number of double bonds.

Step 2: Indicate the location of all the multiple bonds, including those with rings.

EXAMPLE:

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THE GEOMETRY OF ALKENES• In C=C bonds, sp2 hybrid orbitals are formed by the carbon

atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon:

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• During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

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• This gives a planar shape for the sp2 bonding orbitals with the unhybridized p orbital perpendicular to the plane of the three sp2 hybridized orbitals.

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• The planar geometry of the sp2 hybrid orbitals and the ability of the 2p electron to form a “pi bond” bridge locks the C=C bond firmly in place.

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• Because there is no free rotation about the C=C bond, geometric isomerism is possible.

• cis- isomers have two similar or identical groups on the same side of the double bond.

• trans- isomers have two similar or identical groups on opposite sides of the double bond.

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PHYSICAL PROPERTIES OF ALKENES• Similar to alkanes• Nonpolar• Insoluble in water• Soluble in nonpolar solvents• Less dense than water • Unpleasant, gasoline-like odors

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ALKENE REACTIONS• Alkenes are quite chemically reactive

• Alkene reactions follow the pattern:

• These reactions are called addition reactions.

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HALOGENATION• Halogenation (addition) reactions produce haloalkanes or

alkylhalides.

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HYDROGENATION• 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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MARKOVNIKOV’S RULE• Unsymmetrical alkene addition reactions follow

Markovnikov’s rule which states that when a molecule of H-X adds to an alkene, the H predominantly attaches to the carbon already bonded to the most hydrogens. “The rich get richer.”

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ADDITION OF SIMPLE ACIDS• Addition of simple acids following Markovnikov’s rule:

CH2 = CH – CH3 + HBr CH2 – CH – CH3

H Br||

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HYDRATION• Hydration (addition of water) reactions follow

Markovnikov’s rule:

• This reaction requires an acid catalyst.

CH2 = CH – CH3 + H2O CH2 – CH – CH3

H OH||

H2SO4

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ADDITION POLYMERIZATION• An addition polymer is a polymer formed by the linking

together of many alkene molecules through addition reactions.

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POLYMERIZATION• Polymers – very large molecules made up of repeating

units• Monomer – the starting material that becomes the

repeating units of a polymer

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COPOLYMER• An addition polymer formed by the reaction of two different

monomers is a copolymer.

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ALKYNES• Ethyne (commonly called acetylene) is the simplest alkyne

and is used as a fuel for torches and in making plastics.

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• Alkynes are named in exactly the same ways as alkenes, except the ending –yne is used.

Examples:

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THE GEOMETRY OF ALKYNES• In C≡C bonds, sp hybrid orbitals are formed by the carbon

atoms, with two electrons left in unhybridized 2p orbitals. A representation of sp hybridization of carbon:

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• During hybridization, one 2p orbital mixes with the single 2s orbital to produce two sp hybrid orbitals. Two 2p orbitals are not hybridized and remain unchanged.

• This gives a linear shape for the sp bonding orbitals with the unhybridized p orbitals perpendicular to the line of the two sp hybridized orbitals.

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• A carbon-carbon sigma bond forms by the overlap of one sp hybrid orbital of each carbon.

• The other sp hybrid orbital of each carbon overlaps with a 1s orbital of a hydrogen to form a carbon-hydrogen sigma bond.

• The remaining pair of unhybridized p orbitals of each carbon atom overlap sideways to form a pair of pi bonds between the carbon atoms.

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PHYSICAL PROPERTIES OF ALKYNES• Similar to alkanes and alkenes• Nonpolar• Insoluble in water• Soluble in nonpolar solvents• Less dense than water • Low melting and boiling points

CHEMICAL PROPERTIES OF ALKYNES• Similar to alkenes

• React by addition reaction with Br2, H2, HCl, H2O

• Require twice as many moles of addition reagent as alkenes in reactions that go on to completion

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BENZENE• Aromatic compounds contain the benzene ring or one of

its structural relatives.

• Aliphatic compounds don’t contain this structure.

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• In benzene, the six p orbital bonding electrons of the sp2 hybridized carbon atoms can move freely around the ring.

• A hybrid orbital view of the benzene structure:

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• This gives rise to two possible benzene structures called Kekulé structures in honor of the German chemist who suggested that benzene might be represented by a ring arrangement of carbon atoms with alternating single and double bonds between the carbon atoms:

• Which are better represented by:

Note: that there is only 1 available bonding site on each carbon atom!

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NAMING BENZENE DERIVATIVESGuideline 1: When a single hydrogen attached to the benzene ring is replaced, the compound can be named as a derivative of benzene.

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Guideline 2: Some common names are IUPAC-accepted and used preferentially.

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Guideline 3: When the benzene ring is part of a more complex hydrocarbon, the benzene ring is referred to as a phenyl group.

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Guideline 4: When two groups are attached to the benzene ring, their positions can be designated by the prefixes ortho (o), meta (m), and para (p).

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Guideline 5: When two or more groups are attached, their positions can be indicated by numbering the ring so as to obtain the lowest possible numbers for the attachment positions.

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PHYSICAL PROPERTIES OF AROMATIC COMPOUNDS• Similar to alkanes and alkenes• Nonpolar• Insoluble in water• Hydrophobic

CHEMICAL PROPERTIES OF AROMATIC COMPOUNDS• Aromatic rings are relatively stable chemically and often

remain intact during reactions• Benzene does not react like alkenes and alkynes• Benzene does undergo substitution reactions, in which a ring

hydrogen is replaced by some other group

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POLYCYCLIC AROMATIC COMPOUNDS

Contain two or more fused benzene rings and are often known to be carcinogenic