Chapter 12: Unsaturated Hydrocarbons

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

NAMING ALKENES Examples: 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 CH3-CH=CH-CH3 2-butene 1 2 3 4 5 6 CH3-CH2-CH2-CH=CH-CH3 2-hexene

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 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 Click here to play Coached Problem

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:

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.

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.

Click here to play Chemistry Interactive 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. Click here to play Chemistry Interactive

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.

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

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

HALOGENATION Halogenation (addition) reactions produce haloalkanes or alkylhalides.

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.

Click here to play Coached Problem 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

ADDITION OF SIMPLE ACIDS Addition of simple acids following Markovnikov’s rule: CH2 = CH – CH3 + HBr  CH2 – CH – CH3 | | H Br

HYDRATION This reaction requires an acid catalyst. Hydration (addition of water) reactions follow Markovnikov’s rule: This reaction requires an acid catalyst. H2SO4 CH2 = CH – CH3 + H2O  CH2 – CH – CH3 | | H OH Click here to play Coached Problem Click here to play Chemistry Interactive

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

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:

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.

Click here to play Coached Problem 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. Click here to play Coached Problem

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

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

Click here to play Chemistry Interactive 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: Click here to play Chemistry Interactive

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

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

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

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).

Click here to play Coached Problem 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

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

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