Biological ChemistryFIRST YEAR ORGANIC CHEMISTRY

Lecture TwoAlkanes 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 GasolineC12-C18 is Kerosene or Jet Fuel

- 40 – 200 C

C20 + (300 C) Heating Oil

Non-Volatiles = Paraffin wax, asphalt, tar

- 200 C +

Catalytic Cracking and Octane Rating

Physical Propertiesof Alkanes

Less dense than water – Non-PolarForms two layers with water Bpt increases with chain length, C5-C17 are liquids, higher are solidsBranched molecules have lower BptCycloalkanes have higher BptChemical Properties change little –

Homologous Series

C5-C8 : Can dissolve human tissues,especially lipid molecules of cell membranes, “drying out effect on skin”

Solids include “Vaseline”

Laxatives

CycloalkanesH2C

CH2

CH2

Cyclopropane

CnH2n

CC

CH

H

H H

HH

H2C

H2C CH2

CH2Cyclobutane

CH2

CH2

CH2H2CH2C

Cyclopentane

Angle Strain in Cyclopropane and Cyclobutane – weaker “Bent” C-C bondsC-C Bond angles 60 and 88o respectively

Eclipsed hydrogens – Torsion AngleReduced in Cyclobutane by folding or bending

Pentane has C-C bond angles of 108oC-C bonds slightly bent out of planarity in order

to reduce torsion angle

The most stable cycloalkane with 109.5o C-C bond angles

“Chair” is the most stable ConformationFree of Torsional StrainNon-PlanarH’s are as far apart as possible

“Boat” is free of angle strain, Not torsional strain

Higher Energy Conformer

C-Crotation

Eclipsed H’s and Flag-pole effect between C1 and C4 H’sConformation of the Boat is quite flexible so to reduce torsional strain and flag-pole effect and so PE

H H

HH

Flag Pole Effect

1 4

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-planarvery puckered so to reduce torsional strainVan 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 ?H

HH

H

H

H

put in axial H’s

put in equitorial H’sH

HH

H

H

H

H H

H

HH

H

H

H

H HH HH

H

Fused Bicyclic Systems

cis is the same sidetrans opposite sidesH

H

H

H

H

H

H

H

axial

equatorial

cis-Decalin

equatorial

equatorial

trans-Decalin

H2C

H2CCH2

CH

H2C

CH2

CH2

CH2

H2C

Chair Conformation is very important in nature

H3C

CH3

CH3

HOA B

C D Cholesterol is the most abundant steroid in the human body

25% cell membranes, 10% dry weight of brainAnd main component of gallstones

STEROIDS

HO

all trans-junctions (equatorial-equatorial)

Cholestanolequatorial

CH3

H

CH3

HBiosynthetic precursor to many steroid hormones including vitamin D

Liver makes cholesterolSo not essential in the dietHigh Blood levels lead to CVD

Diamond

One giant molecule with millions of strong covalent bonds