CHEM 30B Dr. R Rinehart
Chapter 12 

Unsaturated  Hydrocarbons

Unsaturated Compounds -- Slides by Warren Gallagher at the University of Wisconsin, Eau Claire
view > Lecture 2

See learning objectives

            The term “unsaturated” refers to hydrocarbons containing less than the theoretical maximum number of hydrogen atoms for a given number of carbons, due to the presence of  double or triple bonds between carbon atoms. Alkanes, with the general formula CnH2n+2 ,are said to be “saturated” [with hydrogen]. Cycloalkanes, even though containing fewer H than the maximum, are also considered saturated, because they normally do not react with hydrogen and can not react without destroying the characteristic ring structure.  There are three types of unsaturated hydrocarbons: alkenes, alkynes, and aromatics. Each can be forced to add 1 or more molecules of  H2 under certain conditions. 

I. ALKENES:  contain 1 or more C=C double bondsCnH2n (for one C=C)
            A.  Structure: involved C have sp2 hybridization; sigma and pi bonds between C

If you haven't used the hybrid orbital tutor at Colby College, now would be a very good time to go to: -- [you need to have Shockwave];

see my table of selected alkenes
Chime structures of a large number of alkenes by Dave Woodcock at Okanagan University College
Use Netscape 4.7x with Chime installed for these!! 

            B.  Nomenclature: the suffix -ene plus a locant;

For Dave Woodcock's lessons on alkene nomenclature, see
Use Netscape 4.7x with Chime installed for these!! 
(my IE version of Chime also worked!)

            C.  Geometry: bond angles = 120o ; planar;  no free rotation means the possibility of
                        distinct geometric isomers [cis/trans].
            D.  Physical properties: similar to alkanes C1 to C4 (g); C5 to C15 (l) sp. gr. ~0.7 [float on H2O]
            E.  Chemical properties:  the double bond is quite reactive toward “electrophiles”
                        like Br2, I2, and H+ {acids}, forming alcohols,  monohalides, and other classes of products;
                            and H2 (with catalysts like Ni or Pt), forming alkanes or saturated compounds;
                    `        the characteristic reaction of alkenes is addition.

A classic test for alkenes [and alkynes] is that they decolorize added solutions of
Br2 [orange
ΰ colorless] or potassium permanganate [purple soln ΰ brown/black ppt.]

            F.  Polymerization  is the linking of many small molecules [monomers] to form enormous
                        “macromolecules” or polymers. Alkenes do this by addition reactions; this is a
                        mainstay of the plastics industry and explains why ethylene is THE  prime
                        industrial organic chemical.      

II.  ALKYNES: contain C≡C triple bonds  CnH2n-2 (for one C≡C)
            A.  Structure: sp-hybridized C; one sigma plus two pi bonds
            B. Nomenclature: -yne with locant;
            C.  Geometry; bond angles 180o; linear
            D.  Properties and reactions similar to alkenes  

Chime structures of alkynes by Dave Woodcock at Okanagan University College
Use Netscape 4.7x with Chime installed for these!! 
Alkyne nomenclature by Dave Woodcock at Okanagan University College
Use Netscape 4.7x with Chime installed for these!! 
(my IE version of Chime also worked!)

III.  AROMATIC compounds- the name is historical, not descriptive-all volatile hydrocarbons have odors --  
        sometimes they’re even nice (pinene,cedrene, limonene) but usually not.

     Aromatic compounds contain rings
of six carbon atoms with alternating single and double bonds.

see my table of aromatic compounds

Chime structures of aromatic compounds by Dave Woodcock at Okanagan University College

            A.  Structure: cyclic, planar, conjugated, “magic #” of p-electrons; all ring atoms sp2
                        1. Benzene and alkylbenzenes, (monocyclic)
                        2. Bi-, tri-, and polycyclic aromatic hydrocarbons
                        3. Replacing one or more of the carbon atoms in the ring with another element
                                    [usually nitrogen] yields a heterocyclic aromatic compound. These are
                                    critical components of living systems-- the “bases” of DNA and RNA,
                                    the vitamins thiamine B1, niacin B3, pyridoxine B6, and folic acid, etc.
            B.  Nomenclature of benzene derivatives
                        1. Common substituents: halo-, nitro-, alkyl-, hydroxy-, amino-, carboxyl-
                        2. monosubstituted: alkylbenzene vs. phenylalkane
disubstituted: ortho-, meta-, and para-  structural (positional) isomers
                        4. trisubstituted and higher: use number locants

Nomenclature of aromatic compounds by Dave Woodcock at Okanagan University College:
monosubstituted benzenes
disubstituted benzenes
multisubstituted benzenes
fused-ring [polycyclic] aromatics

            C.  Geometry: planar, hexagonal rings
            D.  Phys. props: liq or solid; nonpolar; insol in H2O;
absorb UV light
            E.  Chemical props. : resistant to addition, but undergo substitution [of H by other groups];
                        more stable than alkenes or alkynes; flammable
Biological properties: can be quite toxic; benzene is known carcinogen and bone
                        marrow depressant; toluene as inhalant is a drug of abuse.
                        While aromatic hydrocarbons are truly nasty critters, aromatic groups within
                        other classes of compounds can be essential for life-- the aromatic amino acids
                        phenylalanine, tyrosine, and tryptophan, and the vitamin riboflavin are examples.


How can you tell from a molecular formula if a hydrocarbon is not an alkane?

SODAR”= the Sum Of Double bonds And Rings

[also known as the "Index of Hydrogen Deficiency" or the "units of unsaturation"]

            is a way to tell if a given molecular formula is possibly unsaturated.


. formula of alkane with same # of carbons    CnH2n+2    has (2n+2) hydrogens


b. formula of given compound                             CnHx     has  hydrogens

c. SODAR  =  [(2n+2) − x] / 2

(this is the number of hydrogen
molecules that would have to be used to reduce every double or triple bond and break open every ring in the structure, thereby producing an alkane of some type)

d. example: benzene is C6H6 : compare with  C6H14 (hexane)

                    SODAR = (146)/2 = 8/2 = 4;   in this case,  4  =  1 ring + 3 C=C

e. For compounds other than hydrocarbons
(i.e., those containing heteroatoms),
use the following modifications:

●  REPLACE all halogens (and alkali metals) in the formula
with the same number of hydrogens

        C3H4Cl2 = C3H6    SODAR = 1
        C4H9Li = C4H10    SODAR = 0

●  IGNORE oxygen and sulfur (and alkaline earth metals) in the formula;
they have no effect on the # of H required for a given number of C

        C15H31CO2H = C16H32O2 = C16H32    SODAR = 1
        C6H5MgBr = C6H6    SODAR = 4

●  for each nitrogen (or phosphorus) present in the formula,
SUBTRACT one hydrogen

        C7H14N2 = C7H12    SODAR = 2




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© Ronald W. Rinehart, 2002, 2003