CHEM 30B Dr. R Rinehart
Chapter 12 
Unsaturated  Hydrocarbons

Unsaturated Compounds -- Slides by Warren Gallagher at the University of Wisconsin, Eau Claire view  http://www.chem.uwec.edu/Chem150_S06/Pages/lecture-materials.html > 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 C_nH_2n+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 bonds;  C_nH_2n (for one C=C)
            A.  Structure: involved C have sp² hybridization; sigma and pi bonds between C

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

            C.  Geometry: bond angles = 120^o ; 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.

            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  C_nH_2n-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 180^o; linear
            D.  Properties and reactions similar to alkenes  

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.

            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
                        3. disubstituted: ortho-, meta-, and para-  structural (positional) isomers
                        4. trisubstituted and higher: use number locants

            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
            F.  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.
 

HERE'S HOW IT WORKS:

a. formula of alkane with same # of carbons    C_nH_2n+2    has (2n+2) hydrogens
 

b. formula of given compound                             C_nH_x     has  x  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 = (14 − 6)/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

 

IF YOU STILL DON'T BELIEVE IT, TRY IT OUT!!!