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Cpt ALKANES ALKYL HALIDES

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					    Cpt 3. ALKANES & ALKYL
            HALIDES
 Objectives:
* Name and describe the structure and
  properties of alkanes
* Name and describe the structure and
  properties of alkyl halides
CASE STUDIES
 Methane, Ethane, Butane,... Analysis
  shows
* composition of C and H only. Conclusion: the
  molecules are Hydrocarbons: made of C
  and H only
* They cannot take any more H’s. Conclusion:
  molecules are Saturated Hydrocarbons,
  made using C-H single bonds
 Methyl bromide, ethyl Chloride, ...
  Conclusion: molecules are made using single
  bonds
Introduction
 Definitions
* Alkanes = compounds of C & H bound with
  single bonds only.
 Synonyms:      * saturated hydrocarbons
                * aliphatic compounds
  (associated with fats & oils)
 Alkyl halides = compounds of C, H, and

  Halogens bound with single bonds only.
3.1. Structure
 General Formulas:
 a.      Alkanes
 *1. Open Chain Compounds

Formula: CnH2n+2
 Types of chains:

* Straight (normal): all C's in one line
Ex: Pentane
* branched: C's branch off other nonterminal C's
Ex: isopentane
Alkanes (Continued)
   Isomers: compounds with same numbers &
    kinds of atoms and different bond
    arrangements
   Constitutional isomers: differ in the way
    atoms are connected
   ex: pentane & isopentane
   Homologs: compounds differing only by one
    CH2 group
   Ex: butane & pentane
Alkanes (Continued 2)
   Classes of C's:
*   primary (1o): connected to only 1 C
*   secondary (2o):                     2 C’s
*   Tertiary (3o): Connected to only 3 C’s
*   quaternary(4o):                       four
   Note: to qualify as 1o-3o, an alkyl halide
    must have the appropriate C-X bond
Classes of C's (Example)
Alkanes (Continued)
   *2. Cyclic alkanes:
   General formula: CnH2n
   Can be
   Fully cyclic
   Cyclic with side-chains
Cycloalkanes (Examples)
b. Alkyl halides
 General structure:
 Open chain Alkyl Halides:

 CnH(2n + 1)X. X = F, Cl, Br, I

 Cyclic Alkyl Halides

CnH(2n-1)X
Types of C’s for Alkyl halides:
  primary, secondary, tertiary
          Alkyl Halides
           (examples)



          Cl          Br        F
Primary        Secondary   Tertiary
3.2. Nomenclature
   Definition: Procedure to name
    compounds
   Standard System: IUPAC
    (International Union for Pure and
    Applied Chemistry)
   Parts of name: prefix-parent-suffix
   Suffix for alkanes and alkyl halides: ane
a. Open Chain Alkanes
(and alkyl Halides)

 # of C's          Parent         Name
  1                 meth            methane
  2                 eth            ethane
  3                 prop           propane
  4                 but            butane
  5                 pent           pentane
Following chains: greek roots. More: T3-2, pg 83
Open Chains (Continued)
  Branching Chains have:
* Parent (main) Chain: the longest, or the one with
   largest # of branching points. It gives name of
   compound
* Branch chain: substituent
 Substituent name: replace suffix ane by yl name
   of hydrocarbon
# C's           prefix        name
1               meth         methyl
3               prop         propyl
10              dec           decyl
Naming branching alkanes
   *Number the main chain C's using lowest
    set of numbers for the sub's. Numbers not
    needed: 3-C chains
   * name subs in abc order (di, tri, tetra,
    ...not included)
   * use di, tri, tetra, penta,...for repeats of
    subs
   * Halogen sub’s names: fluoro, chloro,
    bromo, iodo.
Open Chain Alkanes
(Examples)




                         Br

    A                B
Open Chain Alkanes
(Examples names)
   A: 3-Ethyl-2,6-dimethyl-4-propyloctane
   B: 3-Bromo-5-Ethyl-3-propylheptane
* Complex substituents:
   Case of substituted substituents
   Name the substituent: determined by
    the longest sub chain.
   #1 position on sub: on the C that
    connects to the main chain
   Naming rules: same as for main chain
Complex substituents
(Examples)
       * Alkyl Substituents with
       special names


       *                                    *
                       *
Isopropyl      Isobutyl          sec-Butyl




   *                        *
                                       *

tertio-Butyl    Isopentyl       Neopentyl
b. Cycloalkanes.
   Thought teaser: What must be done to build
    cyclopentane from pentane?
   Definition: Cycloalkanes = Saturated cyclic
    hydrocarbons
   Synonym: alicyclic compounds
   General formula: CnH2n
   Nomenclature: place the prefix cyclo before name
    of corresponding alkane.
   #of C's Name
               Alkane             Cycloalkane
   5          Pentane            Cyclopentane
Cycloalkanes (Continued)
  Substituents: must have less C’s than cycle
* must have lowest set of #'s
* are name in abc order
* #1 = 1st sub in abc order
 Cycle as Substituent: when Side-chain has
   more C's than cycle and carries the main name
 ex:

 * 2-(1-methylcyclopropyl)pentane

 * 4-(3-isopropyl-4-methylcyclopentyl)-3-t-
   butylhexane
3.3. Conformational
analysis
   Definition: Study of energy effects on
    bond arrangements
   Basic principle: Atoms rotate around
    single bonds
   Conformation: arrangement of atoms in a
    molecule after rotation around a single bond
   Conformer (conformational isomer):
    structure obtained from a conformational
    change.
Conformational Analysis
(Continued)
   Dihedral angle: between two substituents on two adjacent
    C's, looking through the C-C bond.
   Sawhorse Representation: oblique structure representation
    used to show conformations
   Newman Projections: view of sawhorse structure with one
    carbon behind the other.
   Staggered Conformation: most stable. Atoms on neighboring
    C's are as far away from each other as possible. Dihedral angle:
    60 deg. Van Der Waals interactions: minimum
   Eclipsed conformation: least stable. Atoms on different C's:
    as close to one another as possible. DA = 0 dg. Maximum VDW
    repulsion.
Conformational Analysis
(Continued 2)
   Van der Waals repulsions: take place btw
    atoms when they are too close.
   Tortional Strain: energy barrier against
    rotation through eclipsing conformations.
   Steric Strain: VDW repulsion between bulky
    group which are too close to one another.
   Conformation energy plot: based on
    changes in steric & tortional strain
a. Analysis of Open chain
compounds
   Ex2: Butane
   conformation        groups involved        Energy level
                                               (kcal/mol)
   Anti (staggered)    CH3’s farthest apart    baseline
   Eclipses           2 x H-CH3               3.8
                       H-H                     1.0
   Gauche (staggered) CH3-CH3 DA: 60o         0.9
   Eclipses           CH3-CH3                 3.8
                       2 x H-H                 2 x 1.0
   Gauche (staggered) CH3-CH3 DA: 60o         0.9
   Eclipses           2 x H-CH3               3.8
                       H-H                     1.0
Conformationn energy plot
   Shows relation between conformations
    and energy levels.
   Based on changes in steric & tortional
    strain
   Ex: butane
   extra ex: Do conformational analysis and
    energy plots for
   * 2-methylbutane(examined through c2-c3
    bond)
b. Cyclic Compounds
 Thought teaser
* Cyclopropane: Compare C-C-C angles to the
  ideal tetrahedral angle
 Angle strain: bond distorsion due to

  difference in C-C-C angle in a cycle compared
  to the normal 109o tetrahedral angle
 Bent bonds: sgm bonds formed from

  nonaligned orbitals
Cycloalkanes Strain
Energy
   cycle           angle angle      Strain
                           strain energy
    cyclopropane 60        49        27.6
    cyclobutane     90    19         26.4
    cyclopentane 108      1          6.5
    cyclohexane     111 2            0
   Puckered ring conformations: adopted by
    rings to minimize angle, torsional and/or
    steric strains.
Cycloalkanes (Examples)



         Cyclohexane Conformations




     Chair
                                     Boat
*1. Chair Conformation of
Cyclohexane
 Cycle is puckered in form of a chair
Chair = the most stable
 Features:

* C-C-C angles: 109o.
* All dihedral angles: 60o.
* conformations: all staggered, either
  gauche or anti.
 * Strains (all kinds) : minimum.
Cyclohexane Chair
conformations (Continued)
 Positions of substituents
* Axial Position: vertically below or above
  ring
1,3-diaxial strain: steric strain between
  atoms in axial positions
* Equatorial position: slightly above or below
  horizontal plane of molecule
• Conformation inversion (cycle flipping)
Axial position become equatorial and vice-versa
ex: 1-Ethyl-3-methylcyclohexane
Cyclohexane Chair
conformations (Continued 2)
          Substituents




 Axial
                         Equatorial
Stability of disubstituted
cyclohexanes
 Thought teaser: Which isomer of 1,2-
  Dimethyl cyclohexane is most stable:
  equatorial-equatorial or equatorial-axial?
 * 1,2-Disubstituted cycles

Most stable: Trans isomer
 * 1,3-Disubstituted cycles

Most stable: Cis isomer
 Ex: 1,3-Dimethyl cyclohexane

extra ex: 1,4-dimethylcyclohexane
*2 Boat Conformation of
Cyclohexane.
   Observed:
*    Strain of 7.0 kcal/mol;
*   No angle strain;
*   Eclipsing atoms at 2 C-C bonds
   Types of strains:
*   steric, between H's on C1 & C4
*   tortional, due to eclipsing conformations
Twist-boat conformation
   Intermediate between chair and boat
    conformations
   Adopted to relieve partially boat strain
   ex: 1,4-ditertiobutylcyclohexane

				
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