CHEM 30B  Dr. R. Rinehart      Chapter 11  ALKANES  

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I. The nature of “Organic” Chemistry
A.     Inorganic C:   CO, CO_2,  (HCO₃^-, CO₃^-2),  CN^-,  OCN^-,  SCN^-,  C^-4  

B. Organic C: originally believed to  necessarily originate from (once-)LIVING sources  BUT, Wohler (1828) converted ammonium cyanate  NH₄OCN  to  urea  H₂NCONH₂  by heating, forcing a total rethinking of the concept. NOW, organic chemistry  is defined as the branch of chemistry dealing with all compounds of carbon except  those listed above.  

            C. Organic vs. Inorganic Compounds   
                        Inorganic: 2.5x10⁵ known;  Organic: >6x10⁶ known.   
                                    Q. Why the big difference??

  II. It’s the BONDING, folks!  

A. Covalent bonds: shared electron pairs

B. . Hybrid orbitals: ₆C  1s²2s²2p² ;   {2s+2p_x+2p_y+2p_z} form 4 equivalent sp³ orbitals

C.    Bond geometry: sp³ hybridization shows tetrahedral geometry

D.    Chains of carbons with multiple possible arrangements  [S&S table 1.5, p. 13]  

            E. Hydrocarbons: just with C and H alone, >10⁶ possible compounds .
                        Five major classes of hydrocarbons listed below:

                        1. ALKANES                           C_nH_2n+2          “RH” 
                        2. CYCLOALKANES              C_nH_2n 
                        3. Alkenes:                         contain C=C double bonds   C_nH_2n 
                              4. Alkynes:                         Contain C≡C triple bonds     C_nH_2n-2   
                        5. Aromatics:     e.g. benzene, C₆H₆ :  cyclic, “conjugated” systems”  ’nuff said for now!   

III. Types of Chemical FORMULAS we’ll be using 

A.  Empirical: relative atomic ratios; result of elemental analysis
B.  MOLECULAR: actual numbers of each type of atom e.g. glucose C₆H₁₂O₆  
C.  STRUCTURAL: show how the atoms are arranged and connected together.    
            1.      Lewis structures:  yes,  they’re baaaack!  
            2.      Kekule structures:  a short line indicates each covalent bond  [we’ll use them a lot]
                             Each atom is shown. 
            3.  Condensed:  hexane, C₆H₁₂ is CH₃(CH₂)₄CH₃ 
                           easily incorporated into text;  
            4.  Skeletal or bond-line:  
                            H usually omitted when attached to C [exception: aldehyde], 
                                    H is generally shown when attached to heteroatoms like O or N;  
                            lines used to indicate C-C and C-E bonds, 
                                where E = any element other than C or H; 
                            symbol for C usually omitted  
                                    [end of line segment assumed to be C unless otherwise indicated;  
                                         if you can count to 4, you can figure the # of H’s on each C];  
                             other elements or functional groups [N, O, S, CO₂H, Cl, etc.] 
                                 indicated with  symbol and line representing bond to appropriate
                                 carbon atom.  

 Once you get used to them, these are often the most convenient way to quickly draw
                                     complex structures; we’ll be using them extensively.  

IV.  Alkanes: the simplest hydrocarbons, & the foundation for everything else that follows!

           A.   Methane  CH₄ :   structure,  bonding,  and geometry  
           B.   Higher “straight-chain” alkanes  [S&S table 1.4, p.16] : conformation = shape
 

           C.   Structural  Isomers: the possibilities increase due to branching