The Chemistry of Alkanes Physical Properties of Alkanes as molecular size increases so does the boiling point of the alkane increased size increased dispersion forces Alkanes Boiling Point ˚C H Methane CH 4 H C H -161.6 H H H Ethane C2H6 H C C H -88.6 H H
Propane C3H8 CH3 (CH2)1 CH3 -42.1
Butane C4H10 CH3 (CH2)2 CH3 -0.5
Pentane C5H12 CH3 (CH2)3 CH3 36.1 hexane C6H14 CH3 (CH2)4 CH3 68.7 Chemical Reactions and Alkanes because the C-C and C-H bonds are relatively strong , the alkanes are fairly unreactive their inertness makes them valuable as lubricating materials and as backbone material in the construction of other hydrocarbons Combustion of Alkanes
At high temperatures alkanes combust ΔH˚
CH4 + O2 CO2 + 2H2O -890.4 kJ
C4H10 + 13/2O2 4CO2 + 5H2O -3119 kJ
these reactions are all highly exothermic Halogenation of Alkanes at temperatures above 100 ˚C
CH4 + Cl2 CH3Cl + HCl chloromethane
CH3Cl + Cl2 CH2Cl2 + HCl dichloromethane
CH2Cl2 + Cl2 CHCl3 + HCl trichloromethane chloroform
CHCl3 + Cl2 CCl4 + HCl tetrachloromethane Carbon tetrachloride Mechanism for Halogenation of Methane
CH4 + Cl2 CH3Cl + HCl hν Cl2 • Cl + • Cl hν: energy required to break the Cl-Cl bond
• Cl H H C H H
• Cl is very reactive and able to attack the C-H bond Mechanism for Halogenation of Methane
CH4 + Cl2 CH3Cl + HCl
CH4 + • Cl • CH3 + HCl
H Cl H • C H H Mechanism for Halogenation of Methane
CH4 + Cl2 CH3Cl + HCl
• CH3 + Cl2 CH3 Cl + • Cl
Cl Cl H • Cl • C H H H Cl C H H Mechanism for Halogenation of Methane
Cl2 • Cl + • Cl chlorine free radical CH4 + • Cl • CH3 + HCl
• CH3 + Cl2 CH3 Cl + • Cl
CH4 + Cl2 CH3Cl + HCl
Side reactions • Cl + • Cl Cl2
• CH3 + • CH3 C2H6 alkyl halide
An alkane in which one or more hydrogens have been replaced with halogens
CH3Cl Example
H
CH3 CH2 CH2 C CH3 2-chloropentane Cl
H
Br CH2 CH2 C CH3 1-bromo-3-chlorobutane Cl A Functional Group a group of atoms that are largely responsible for the chemical behavior of the parent molecule some common functional groups NH2, F, Cl, Br, I, NO2 ,, ,
Alkyl groups - drived from alkanes Levels of Structure
Elemental Composition √ Empirical Formula √ Molecular Formula √ Constitution √ Conformation Configuration Stereoisomers isomers that have the same constitution but differ in the spatial arrangement of their atoms
H
H H HH CH3 CH3 C C CH3 H H H CH3 saw horse projection Stereoisomers isomers that have the same constitution but differ in the spatial arrangement of their atoms
Conformations : different spatial arrangments of a molecule generated by rotation of atoms about single bonds H
H H CH H 3 H CH3 C C CH3 CH3 H H H saw horse projection Stereoisomers H H
HH CH H CH3 3 H CH3 CH H 3 H eclipsed conformation staggered conformation H HH H CH3 H H CH3 H CH3 CH3 H Newman projection Van der Waals Strain / Steric Strain a destabilization that results when two atoms or groups approach each other too closely rotation of atoms about single bonds serves to reduce steric strain Levels of Structure
Elemental Composition √ Empirical Formula √ Molecular Formula √ Constitution √ Conformation √ Configuration Enantiomers Stereoisomers that are related as an object and its nonsuperimposable mirror image ie: being chiral bromochlorofluoromethane
Br Br
F C H H C F
Cl Cl Br Br
H C Cl Cl C H
F F
mirror images Br Br
H C Cl Cl C H
F F
nonsuperimposable Cycloalkanes Cycloalkanes Alkanes whose carbon atoms are joined in rings H H C nH 2n C H H cyclopentane C H C C 5 10 H H H C C H sp3 hybridization H H Alkane Nomenclature follows the same rules as for naming other alkanes except that the root name is preceded by the prefix cyclo-
The ring is numbered to yield the smallest substitutent number possible.
CH2 CH3 CH2 CH2
CH2 CH2 cyclobutane 1-ethylcyclobutane Example
CH3 1-Ethyl-2-methylcyclohexane
CH2 CH3
H3C 1,4-Dimethylcyclohexane
CH3 Example
1,1-Dimethylcyclohexane CH3
CH3 Cyclohexane
H H H C H H C C H
H C C H H C H H H to attain tetrahedral bond angles the cyclohexane ring must “pucker” Cyclohexane
Two stable conformations C6H12
H H H H C C H C C H H H H C C H H H Boat Cyclohexane
Two stable conformations C6H12 Cyclohexane assumes the chair conformation 99% of the time H H C H H C C H H H H C C C H H H Chair H