Alkanes Alkanes • Alkanes are fully saturated hydrocarbons hydrocarbons

					                    Alkanes
• Alkanes are fully saturated hydrocarbons
  - hydrocarbons have only C’s and H’s
  - saturated = all single bonds (max. # of H’s)
• Alkanes have the general formula CnH2n+2
  - example: C4H10
• They can be straight-chained or branched
  - example: CH3CH2CH2CH3 vs. (CH3)2CHCH3
• Alkanes, and all other organic compounds, are
  named according the the IUPAC system
  (International Union of Pure and Applied
  Chemistry)
 Naming Straight-Chain Alkanes and Alkyl Groups
• The names of all alkanes end in -ane
• The number of carbons in a straight-chain alkane is
  indicated by putting a prefix before the -ane
• Examples:          CH4 = methane
               CH3-CH3      = ethane
          CH3-CH2-CH3       = propane
• In branched alkanes the substituents (groups attached
  to the carbon chain) are called alkyl groups
• An alkyl group = an alkane with one H removed
• Alkyl groups are named by changing the ending of the
  alkane name to -yl
• Example: CH3-CH2- = ethyl
     Classification of Carbons in Alkanes
• Carbons can be classified by how many other C’s
  are attached to them:
  - No C’s = methyl            CH4
  -1C        = primary (1)    CH3-CH3
  - 2 C’s = secondary (2) CH3-CH2-CH3
  - 3 C’s = tertiary (3)      CH3-CH(CH3)2
  - 4 C’s = quaternary (4) C(CH3)4
            Naming Branched Alkanes
•   First find the longest chain of C’s (parent)
•   Number the C’s in parent chain (begin at end
    nearest 1rst branch point)
•   Identify substituents and number by point of
    attachment to parent chain
•   Write full name
       Example:
    CH3-CH(CH3)-CH2-CH3 = 2-methylbutane
•   If there is more than one of a substituent a prefix is
    used and a number is given for each substituent
•   Example:
•   CH3-CH2-C(CH3)3           = 2,2-dimethylbutane
            Conformations of Alkanes
•   Because alkanes have only single bonds they have
    free rotation about those bonds
•   This allows for different spatial arrangements of
    the atoms called conformations
•   Conformations are not the same as isomers
•   Some conformations are more stable than others,
    so the compound spends more time in those
•   For ethane(CH3-CH3):
    - when all H’s are lined up as you sight down the
    C-C bond, called eclipsed, this is highest energy
    - when none of the H’s are lined up, called
    staggered, this is lowest energy
                  Haloalkanes
•  Haloalkanes have one or more halogens replacing
   hydrogen on an alkane
• The halogens are numbered and named as
   substituents
   -F       =    fluoro
   - Cl     =    chloro
   - Br     =    bromo
   -I       =    iodo
• If more than one halogen is present, they are
   named in alphabetical order
• Example:
CH3-CH(Br)-CH(Cl)-CH3 = 2-bromo-3-chlorobutane
                   Cycloalkanes
•   Carbons can also bond together to form rings
•   Rings with only C’s, H’s and single bonds are
    called cycloalkanes
•   Cycloalkanes have the general formula CnH2n
•   The smallest is cyclopropane (C3H6)
    - cyclopropane is an unstable molecule
    - it’s forced to have bond angles of 120between
    the C’s, while they would normally be 109 (the
    carbons each have 4 electrons groups and should
    be tetrahedral)
•   The only cycloalkanes with little or no strain are
    cyclopentane (C5H10) and cyclohexane (C6H12)
•   Most cycloalkanes are not flat because they prefer
    tetrahedral geometry
              Naming Cycloalkanes
•   Cycloalkanes are named by adding cyclo- to the
    parent alkane name
•   Substituents are numbered when there is more than
    one of them
•   They are numbered starting at the one that gives
    the others the lowest numbers
•   Or, they are numbered alphabetically when there
    are only two, or both directions are equal
         Cl


                    =    2-chloro-1,4-d imethylcyclohexane
              Cis and Trans Isomers
•   Because cycloalkanes do not have free rotation
    around the carbons, it matters on which side of the
    ring a substituent is relative to other substituents
•   Two substituents on the same side (top or bottom)
    of the ring are called Cis
•   Two substituents on opposite sides of the ring are
    called Trans
•   Cis and Trans isomers are stereoisomers; they
    have the same molecular formula, and the atoms
    are connected in the same order but are arranged in
    a different spacial orientation
Physical Properties of Alkanes and Cycloalkanes
•   Alkanes are nonpolar and are not soluble in water
•   They have low melting and boiling points due to
    their weak intermolecular forces (dispersion)
•   They are also less dense than water
•   Alkanes are mostly obtained from crude oil
    - the crude oil is fractionated based on boiling pt.
    - heavier fractions are often “cracked”, put under
    high heat and pressure, to obtain more gasoline
•   Alkanes are used extensively as fuels of various
    types (for heating, cooking, driving, etc.)
    Reactions of Alkanes and Cycloalkanes
•   Because they are nonpolar, and their covalent
    bonds are strong, alkanes and cycloalkanes are
    relatively unreactive
•   The two types of reactions that they do undergo
    are combustion and halogenation
•   In combustion reactions, alkanes react with
    oxygen to form carbon dioxide, water and heat
      CH4 + 2O2  CO2 + 2H2O + Heat
•   In halogenation reactions, halogens replace one or
    more H’s on an alkane (a substitution reaction)
     CH4 + Cl2 (+ light or heat)  CH3Cl + HCl
                      Mechanism of Alkane Halogenation
•       These halogenations are radical reactions
•       Radicals are molecules (or atoms) that have one or more unpaired
        electrons (the half-headed arrows represent movement of single electrons

    1. Initiation:   Cl       Cl           +        Light             Cl            +       Cl



                                   H                                                    H


    2. Propagation:       H        C       H            +    Cl            H            C        +        H-Cl

                                   H                                                    H



              H                                                       H


       H      C       +            Cl          Cl                 H   C        Cl           +        Cl

              H                                                       H

                                       H                                                H


      3. Termination:         H        C            +       Cl             H            C       Cl

                                       H                                                H

				
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