Chapter 12Unsaturated Hydrocarbons

Spencer L. SeagerMichael R. Slabaugh

www.cengage.com/chemistry/seager

Jennifer P. Harris

UNSATURATED HYDROCARBONS• Unsaturated hydrocarbons contain carbon-carbon multiple

bonds.• Alkenes contain carbon-carbon double bonds (C=C ).• Alkynes contain carbon-carbon triple bonds (C≡C).• Aromatics contain benzene rings.

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: Locate the C=C bond with the lower-numbered

carbon. 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

NAMING ALKENES (continued)• Step 4: Locate and name attached groups.• Step 5: Combine all the names.

NAMING ALKENES WITH MULTIPLE DOUBLE BONDS• Step 1: Follow the same naming instructions for alkenes with

one double bond, except use the endings –diene, – triene, and the like to denote the number of double bonds.

• Step 2: Indicate the location of all the multiple bonds.

• EXAMPLES:

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:

THE GEOMETRY OF ALKENES (continued)

• 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.

THE GEOMETRY OF ALKENES (continued)

• 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.

THE GEOMETRY OF ALKENES (continued)

• 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.

THE GEOMETRY OF ALKENES (continued)

• 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.

THE GEOMETRY OF ALKENES (continued)

• Geometric isomers have different physical properties.

PHYSICAL PROPERTIES OF ALKENES

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

PHYSICAL PROPERTIES OF ALKENES

ALKENE REACTIONS• Alkenes are quite chemically reactive.• Alkene reactions follow the pattern:

• These reactions are called addition reactions.

ALKENE REACTIONS (continued)

ALKENE REACTIONS (continued)• HALOGENATION• Halogenation (addition) reactions produce haloalkanes or

alkyl halides.

ALKENE REACTIONS (continued)•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.•Polyunsaturated molecules contain several double bonds.•Hydrogenation of polyunsaturated molecules raises their melting points.

ALKENE REACTIONS (continued)• 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.”

ALKENE REACTIONS (continued)• ADDITION OF SIMPLE ACIDS• Addition of simple acids when Markovnikov’s rule is not

required:

• Addition of simple acids following Markovnikov’s rule:

ALKENE REACTIONS (continued)• HYDRATION• Hydration (addition of water) reactions follow

Markovnikov’s rule:

• This reaction requires an acid catalyst.

ALKENE REACTIONS (continued)• Hydration reactions are believed to occur in three steps.• Step 1: • H+ from acid catalyst is attracted to the electrons in the

carbon-carbon double bond.• It becomes bonded to one of the carbon atoms by a

sharing of electrons.• The other carbon atom from the double bond becomes an

extremely reactive carbocation (positively charged carbon atom with only three bonds).

• The carbocation attracts the oxygen atom (with two unshared pairs of electrons) in a water molecule.

ALKENE REACTIONS (continued)• Step 2:• One pair of oxygen electrons form a covalent bond with

the carbocation.

ALKENE REACTIONS (continued)• Step 3:• H+ is lost to produce the alcohol. • Note: Catalyst is regenerated in this step.

ALKENE REACTIONS (continued)• ADDITION POLYMERIZATION• An addition polymer is a polymer formed by the linking

together of many alkene molecules through addition reactions.

POLYMERIZATION• Polymers are very large molecules made up of

repeating units.• A monomers is the starting material that becomes

the repeating units of a polymer.

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

monomers is a copolymer.

COMMON ADDITION POLYMERS

COMMON ADDITION POLYMERS

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

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

ALKYNE NOMENCLATURE• Alkynes are named in exactly the same ways as alkenes,

except the ending –yne is used.

• Examples:

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:

THE GEOMETRY OF ALKYNES (continued)

• 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.

THE GEOMETRY OF ALKYNES (continued)

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

• The other sp hybrid orbital of each carbon atom overlaps with a 1s orbital of a hydrogen atom 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.

ALKYNE PROPERTIES• 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