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 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

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl trichloromethane

CHCl3 + Cl2 CCl4 + HCl tetrachloromethane 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 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 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 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