Cpt 3' ALKANES

1
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

2
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 Hs. Conclusion
  molecules are Saturated Hydrocarbons, made using
  C-H single bonds
• Methyl bromide, ethyl Chloride, ... Conclusion
  molecules are made using single bonds

3
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.

4
3.1. Structure

• General Formulas
• a.         Alkanes
• 1. Open Chain Compounds
• Formula CnH2n2
• Types of chains         
• Straight (normal) all C's in one line
• Ex Pentane
• branched C's branch off other nonterminal C's
• Ex isopentane

5
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

6
Alkanes (Continued 2)

• Classes of C's
• primary (1o) connected to only 1 C
• secondary (2o)                          2 Cs
• Tertiary (3o) Connected to only 3 Cs
• quaternary(4o)                            
  four 
• Note to qualify as 1o-3o, an alkyl halide must
  have the appropriate C-X bond

7
Classes of C's (Example)
8
Alkanes (Continued)

• 2. Cyclic alkanes
• General formula CnH2n
• Can be
• Fully cyclic
• Cyclic with side-chains

9
Cycloalkanes (Examples)
10
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 Cs for Alkyl halides primary,
  secondary, tertiary

11
Alkyl Halides (examples)
12
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

13
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

14
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

15
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 subs names fluoro, chloro, bromo,
  iodo.

16
Open Chain Alkanes (Examples)
17
Open Chain Alkanes (Examples names)

• A 3-Ethyl-2,6-dimethyl-4-propyloctane
• B

18
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

19
Complex substituents (Examples)
20
Alkyl Substituents with special names

•  

21
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

22
Cycloalkanes (Continued)

• Substituents must have less Cs 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-butylhex
  ane

23
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.

24
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.

25
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

26
a. Analysis of Open chain compounds

• Ex2 Butane
• conformation groups involved         Energy
  level   (kcal/mol) 
• Anti (staggered)       CH3s 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

27
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)

28
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

29
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. 

30
Cycloalkanes (Examples)
31
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.

32
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

33
Cyclohexane Chair conformations (Continued 2)
34
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

35
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

36
Twist-boat conformation

• Intermediate between chair and boat conformations
• Adopted to relieve partially boat strain
• ex 1,4-ditertiobutylcyclohexane