Biological Chemistry FIRST YEAR ORGANIC CHEMISTRY Lecture Two Alkanes and Cycloalkanes Convenor : Dr. Fawaz Aldabbagh

C1- Petroleum Refining Distillation – Separation of Fractions according to Bpt Cold water is used to condense fractions below 70 C C5-C10 is Gasoline C12-C18 is Kerosene or Jet Fuel - 40 – 200 C C20 + (300 C) Heating Oil Non-Volatiles = Paraffin wax, asphalt, tar C + Catalytic Cracking and Octane Rating

Physical Properties of Alkanes Less dense than water – Non-Polar Forms two layers with water Bpt increases with chain length, C 5 -C 17 are liquids, higher are solids Branched molecules have lower Bpt Cycloalkanes have higher Bpt Chemical Properties change little – Homologous Series C 5 -C 8 : Can dissolve human tissues, especially lipid molecules of cell membranes, “drying out effect on skin” Solids include “Vaseline” Laxatives

Cycloalkanes Cyclopropane C n H 2n Cyclobutane Cyclopentane

Angle Strain in Cyclopropane and Cyclobutane – weaker “Bent” C-C bonds C-C Bond angles 60 and 88 o respectively Eclipsed hydrogens – Torsion Angle Reduced in Cyclobutane by folding or bending Pentane has C-C bond angles of 108 o C-C bonds slightly bent out of planarity in order to reduce torsion angle The most stable cycloalkane with o C-C bond angles

“Chair” is the most stable Conformation Free of Torsional Strain Non-Planar H’s are as far apart as possible “Boat” is free of angle strain, Not torsional strain Higher Energy Conformer C-C rotation Eclipsed H’s and Flag-pole effect between C1 and C4 H’s Conformation of the Boat is quite flexible so to reduce torsional strain and flag-pole effect and so PE

Energy Barriers between conformations is low Separation of conformers is impossible at room temp. Estimated that 99% of cyclohexane is a chair at any given moment Higher Cycloalkanes are non-planar very puckered so to reduce torsional strain Van der Waals repulsion between H’s becomes greater – transannular strain Draw a P.E. plot of the conformations of cyclohexane

Sir D.H.R. Barton, Nobel Prize 1969

How to draw Cyclohexane ? put in axial H’s put in equitorial H’s

Fused Bicyclic Systems cis is the same side trans opposite sides

Chair Conformation is very important in nature Cholesterol is the most abundant steroid in the human body 25% cell membranes, 10% dry weight of brain And main component of gallstones STEROIDS Biosynthetic precursor to many steroid hormones including vitamin D Liver makes cholesterol So not essential in the diet High Blood levels lead to CVD

Diamond One giant molecule with millions of strong covalent bonds