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Biological Chemistry FIRST YEAR ORGANIC CHEMISTRY. Lecture Two Alkanes and Cycloalkanes. Convenor : Dr. Fawaz Aldabbagh. C1-. Distillation – Separation of Fractions according to Bpt. Petroleum Refining. Cold water is used to condense fractions below 70 C. - 40 – 200 C. C5-C10 is Gasoline. - PowerPoint PPT Presentation
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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
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