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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

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Biological ChemistryFIRST YEAR ORGANIC CHEMISTRY

Lecture Two

Alkanes and Cycloalkanes

Convenor: Dr. Fawaz Aldabbagh

slide2
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

C12-C18 is Kerosene or Jet Fuel

- 200 C +

C20 + (300 C) Heating Oil

Non-Volatiles = Paraffin wax, asphalt, tar

Catalytic Cracking and Octane Rating

physical properties of alkanes
Physical Propertiesof Alkanes

Less dense than water – Non-Polar

Forms two layers with water

Bpt increases with chain length,

C5-C17 are liquids, higher are solids

Branched molecules have lower Bpt

Cycloalkanes have higher Bpt

Chemical 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

cycloalkanes
Cycloalkanes

CnH2n

Cyclopropane

Cyclobutane

Cyclopentane

slide5
Angle Strain in Cyclopropane and Cyclobutane – weaker “Bent” C-C bondsC-C Bond angles 60 and 88o respectively

Eclipsed hydrogens – Torsion Angle

Reduced in Cyclobutane by folding or bending

Pentane has C-C bond angles of 108o

C-C bonds slightly bent out of planarity in order

to reduce torsion angle

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

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“Chair” is the most stable Conformation

Free of Torsional Strain

Non-Planar

H’s are as far apart as possible

C-C

rotation

Higher Energy Conformer

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

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

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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

Draw a P.E. plot of the conformations of cyclohexane

Higher Cycloalkanes are non-planar

very puckered so to reduce torsional strain

Van der Waals repulsion between H’s becomes greater – transannular strain

how to draw cyclohexane
How to draw Cyclohexane ?

put in axial H’s

put in equitorial H’s

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Fused Bicyclic Systems

cis is the same side

trans opposite sides

chair conformation is very important in nature
Chair Conformation is very important in nature

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

Cholesterol is the most abundant steroid in the human body

25% cell membranes, 10% dry weight of brain

And main component of gallstones

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Diamond

One giant molecule with millions of strong covalent bonds