Chapter 12 Unsaturated Hydrocarbons 1 UNSATURATED HYDROCARBONS contain

Chapter 12: Unsaturated Hydrocarbons 1

UNSATURATED HYDROCARBONS • contain carbon-carbon multiple bonds. Alkenes C=C double bonds Alkynes triple bonds Aromatics benzene rings 2

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 lowest-numbered carbon. Examples: 1 2 3 4 CH 3 -CH=CH-CH 3 2 -butene 1 2 3 4 5 6 CH 3 -CH 2 -CH=CH-CH 3 2 -hexene 3

Step 4: Locate and name attached groups. Step 5: Combine all the names. 4

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: Click here to play Coached Problem 5

THE GEOMETRY OF ALKENES • In C=C bonds, sp 2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2 p orbital. A representation of sp 2 hybridization of carbon: 6

• During hybridization, two of the 2 p orbitals mix with the single 2 s orbital to produce three sp 2 hybrid orbitals. One 2 p orbital is not hybridized and remains unchanged. 7

• 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. 8

• The planar geometry of the sp 2 hybrid orbitals and the ability of the 2 p electron to form a “pi bond” bridge locks the C=C bond firmly in place. Click here to play Chemistry Interactive 9

• 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. 10

PHYSICAL PROPERTIES OF ALKENES • Similar to alkanes • Nonpolar • Insoluble in water • Soluble in nonpolar solvents • Less dense than water • Unpleasant, gasoline-like odors 11

ALKENE REACTIONS • Alkenes are quite chemically reactive • Alkene reactions follow the pattern: • These reactions are called addition reactions. 12

HALOGENATION • Halogenation (addition) reactions produce haloalkanes or alkylhalides. 13

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. 14

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. ” Click here to play Coached Problem 15

ADDITION OF SIMPLE ACIDS • Addition of simple acids following Markovnikov’s rule: CH 2 = CH – CH 3 + HBr CH 2 – CH 3 | | H Br 16

HYDRATION • Hydration (addition of water) reactions follow Markovnikov’s rule: H 2 SO 4 CH 2 = CH – CH 3 + H 2 O CH 2 – CH 3 | H | OH • This reaction requires an acid catalyst. Click here to play Coached Problem Click here to play Chemistry Interactive 17

ADDITION POLYMERIZATION • An addition polymer is a polymer formed by the linking together of many alkene molecules through addition reactions. 18

POLYMERIZATION • Polymers – very large molecules made up of repeating units • Monomer – the starting material that becomes the repeating units of a polymer 19

COPOLYMER • An addition polymer formed by the reaction of two different monomers is a copolymer. 20

ALKYNES • Ethyne (commonly called acetylene) is the simplest alkyne and is used as a fuel for torches and in making plastics. 21

• Alkynes are named in exactly the same ways as alkenes, except the ending –yne is used. Examples: 22

THE GEOMETRY OF ALKYNES • In C≡C bonds, sp hybrid orbitals are formed by the carbon atoms, with two electrons left in unhybridized 2 p orbitals. A representation of sp hybridization of carbon: 23

• During hybridization, one 2 p orbital mixes with the single 2 s orbital to produce two sp hybrid orbitals. Two 2 p 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. 24

• 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 1 s 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. Click here to play Coached Problem 25

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, HCl, H 2 O • Require twice as many moles of addition reagent as alkenes in reactions that go on to completion 26

BENZENE • Aromatic compounds contain the benzene ring or one of its structural relatives. • Aliphatic compounds don’t contain this structure. 27

• 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: Click here to play Chemistry Interactive 28

• 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! 29

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. 30

Guideline 2: Some common names are IUPAC-accepted and used preferentially. 31

Guideline 3: When the benzene ring is part of a more complex hydrocarbon, the benzene ring is referred to as a phenyl group. 32

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). 33

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. Click here to play Coached Problem 34

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 35

POLYCYCLIC AROMATIC COMPOUNDS Contain two or more fused benzene rings and are often known to be carcinogenic 36