Chapter 12 Unsaturated Hydrocarbons Spencer L. Seager Michael R. Slabaugh 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: CH 3 -CH=CH-CH 3 2-butene CH 3 -CH 2 -CH 2 -CH=CH-CH 3 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, sp 2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp 2 hybridization of carbon:

THE GEOMETRY OF ALKENES (continued) During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp 2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

THE GEOMETRY OF ALKENES (continued) This gives a planar shape for the sp 2 bonding orbitals with the unhybridized p orbital perpendicular to the plane of the three sp 2 hybridized orbitals.

THE GEOMETRY OF ALKENES (continued) The planar geometry of the sp 2 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)

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

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 Br 2, H 2, HCl, H 2 O Require twice as many moles of addition reagent as alkenes in reactions that go on to completion

BENZENE Aromatic compounds contain the benzene ring or one of its structural relatives. Aliphatic compounds don’t contain this structure.

BENZENE (continued) In benzene, the six p orbital bonding electrons of the sp 2 hybridized carbon atoms can move freely around the ring. A hybrid orbital view of the benzene structure:

BENZENE (continued) 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!

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

NAMING BENZENE DERIVATIVES (continued) Guideline 2: Some common names are IUPAC-accepted and used preferentially.

NAMING BENZENE DERIVATIVES (continued) Guideline 3: When the benzene ring is part of a more complex hydrocarbon, the benzene ring is referred to as a phenyl group.

NAMING BENZENE DERIVATIVES (continued) 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).

NAMING BENZENE DERIVATIVES (continued) 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.

PROPERTIES OF AROMATIC COMPOUNDS 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.

IMPORTANT AROMATIC COMPOUNDS

IMPORTANT AROMATIC COMPOUNDS (continued) A number of aromatic compounds must be present in our diet for proper nutrition. A vitamin is an organic nutrient that the body cannot produce in the small amounts needed for good health.

POLYCYCLIC AROMATIC COMPOUNDS Polycyclic aromatic compounds contain two or more fused benzene rings and are often known to be carcinogenic.