Types of Structural Isomerism

If molecules have the same molecular formula but different structural formulae then they are called structural isomers and are said to display structural isomerism.
But there are different types of structural isomerism:

• chain isomerism (also known as skeletal isomerism)
• position isomerism (also known as regioisomerism)
• functional group isomerism (functional isomerism)

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Chiral Centres and Chiral Molecules

What is a chiral molecule?
How do you find the chiral centre, or chiral carbon, in an organic molecule?
What makes a molecule chiral?
Why is chirality important?

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Azobenzene: cis-trans isomers

A new soluble, light sensitive polymer has been created at the University of Helsinki. The polymer partially dissolves in water or certain alcohols, and, when exposed to light, the polymer switches to its cis isomer and dissolves completely.

This new polymer chain is known to contain azo compounds, and azo compounds are known for this ability to switch between cis and trans isomers using light.

Azo compounds are organic molecules in which two benzene rings (known as phenyl groups) are linked by two nitrogen atoms.
Azobenzene has the IUPAC name diphenyldiazene and is the simplest of the azo compounds.
When the 2 benzene rings (phenyl groups) are on the same side of the N=N bond this is known as the cis isomer. The structural formula for cis-azobenzene is shown below:

When the 2 benzene rings (phenyl groups) are on different sides of the N=N bond this is known as the trans isomer. The structural formula for trans-azobenzene is shown below:

The cis isomer is less stable than the trans isomer. In the presence of light (hν) , the trans isomer will convert to the cis isomer:

hν →

but because the cis isomer is less stable, it will relax back to the trans isomer over time:

→

Cis-azobenzene is more polar than trans-azobenzene so cis-azobenzene is more water soluble than trans-azobenzene.

Reference:
Szymon Wiktorowicz, Heikki Tenhu, Vladimir Aseyev. Using Light To Tune Thermo-Responsive Behavior and Host–Guest Interactions in Tegylated Poly(azocalix[4]arene)s. Macromolecules, 2013; 46 (15): 6209 DOI: 10.1021/ma4011457

Further Reading:
Cis-trans (geometric) Isomers

Suggested Study Questions:

1. Give the molecular formula for cis-azobenzene and trans-azobenzene.
2. Explain why azobenzene displays cis-trans (geometric) isomerism.
3. If the N=N bond in azobenzene was replaced with N-N, would the resulting molecule display cis-trans (geometric) isomerism? Explain your answer.
4.  On a drawing of the structural formula of trans-azobenzene, circle a phenyl group.
5. Diazene (diimine) has the molecular formula (NH)₂ . It contains the same N=N bond as azobenzene or diphenyldiazene. Diazene also displays cis-trans (geometric) isomerism. Draw both cis-diazene and trans-diazene.
6. Hydrazine has the molecular formula N₂H₄. Draw a possible structural formula for hydrazine. 
7. Do you expect hydrazine to display cis-trans (geometric) isomerism? Explain your answer.
8. Draw the molecular formula for the molecule trans-dibutyldiazene.
9. Dibutyldiazene undergoes an addition reaction with hydrogen gas. Draw the structural formula for the product of this reaction.

Isobutene

Isobutene and isobutylene are other names used for the organic compound with the IUPAC name 2-methylpropene. 2-methylpropene is one of the four structural isomers of butene and it exists as a highly flammable, colourless gas at standard temperature and pressure. Isobutene can be converted into fuel additives that increase the octane rating and prevent engine knocking.
Addition polymerization of isobutene results in polyisobutene (PIB) a rubbery substance which is used in the manufacture of tires, adhesives, ball bladders, caulks and sealants, cling film, rubber modification, fuel additives, and chewing gum.

Researchers at the Department of Energy's Pacific Northwest National Laboratory and at Washington State University have developed a new catalyst material composed of zinc oxide and zirconium oxide that will convert bio-ethanol into isobutene in one production step.
If the catalyst was composed only of zinc oxide, the ethanol was mostly converted into acetone (the chemical used in nail polish remover).
If the catalyst was composed only of zirconium oxide, the ethanol was mostly converted into ethylene (the chemical made by plants that ripens fruit).
When the catalyst is composed of a 1:10 ratio of zinc oxide to zirconium oxide, 83% of the ethanol was converted into isobutene.

Reference
Junming Sun, Kake Zhu, Feng Gao, Chongmin Wang, Jun Liu, Charles H. F. Peden, Yong Wang. Direct Conversion of Bio-ethanol to Isobutene on Nanosized ZnxZryOzMixed Oxides with Balanced Acid–Base Sites. Journal of the American Chemical Society, 2011; 133 (29): 11096 DOI: 10.1021/ja204235v

Further Reading
Naming Carbon Compounds
Structural Isomers of Alkenes
Ideal Gas Law
Polymers and Polymerization
Fuel Definitions
Energy Profiles

Study Questions

1. Give the molecular formula and structural formula for 2-methylpropene.
2. Draw the structural formula for all four structural isomers of butene.
3. Write an equation to represent the polymerization of 2-methylpropene to polyisobutene.
4. Use diagrams to show why the polymerization reaction above is considered to be an addition polymerization reaction.
5. On the diagram produced in response to question 4, clearly label the monomer and polymer compounds.
   
6. Give likely formulae for both zinc oxide and zirconium oxide.
7. Give the IUPAC name, the molecular formula and the structural formula, for each of the following:
   • isobutene
     
   • acetone
   • ethylene
8. Write equations for each of the following catalyzed reactions:
   • the conversion of ethanol into acetone
     
   • the conversion of ethanol into ethylene
9. Which of the following compounds could undergo addition polymerization? Justify your answer.
   
   • acetone
   • ethylene
10. Name the products of any successful addition polymerization in question 8.
    
    
    

PBDEs

Polybrominated diphenyl ethers, PBDEs, have been used as flame retardants in many common household products such as blankets, couches, food wrappers, as well as in motor vehicles and airplanes.
A diphenyl ether molecule has the following structure:

One of the polybrominated diphenyl ether compounds is 2,2',4,4',5-pentabromodiphenyl ether and it has following structure
There are 209 possible substances which are included in the family of polybrominated diphenyl ethers (PBDEs) and these are called congeners. Congeners include dibromodiphenyl ethers, tribromodiphenyl ethers, tetrabromodiphenyl ethers, pentabromodiphenyl ethers, hexabromodiphenyl ethers, heptabromodiphenyl ethers, octabromodiphenyl ethers, nonabromodiphenyl ethers and decabromodiphenyl ethers.

PBDEs with 1-5 bromine atoms per molecule are considered to be more dangerous than PBDEs with more bromine atoms per molecules because they are more efficient at accumulating in living things. In humans they effect hormone levels in the thyroid which can lead to alterations in behaviour. Studies have shown that children in developed countries have higher levels of blood PBDEs than adults, ranging from 24 to 114 parts per billion.

Most PBDEs have either been banned or are in the process of being phased out, eg,:

• Penta and octa PBDEs are no longer manufactured in the USA because of the health risks while the European Union banned their use in 2004.
• Deca PBDE has commonly been used in the electronics industry but has now been banned in Europe.

There is, however, growing concern that PBDEs may have a long environmental life.

Reference
Crystal Y. Usenko, Eleanor M. Robinson, Sascha Usenko, Bryan W. Brooks, Erica D. Bruce. PBDE developmental effects on embryonic zebrafish. Environmental Toxicology and Chemistry, 2011; DOI: 10.1002/etc.570

Further Reading
Nomenclature
Haloalkanes
Structural Isomers
Functional Groups
Percentage Composition
Parts per Million Concentration

Study Questions

1. For the diphenyl ether molecule:
   • identify the ether functional group
   • give the molecular formula
   • calculate the molecular mass (formula weight)
   • calculate the percentage composition
2. For the 2,2',4,4',5-pentabromodiphenyl ether molecule:
   • identify the ether functional group
   • give the molecular formula
   • calculate the molecular mass (formula weight)
   • calculate the percentage composition
3. How many bromine atoms would be found in a molecule of each of the following:
   
   • dibromodiphenyl ether
   • tribromodiphenyl ether
   • tetrabromodiphenyl ether
   • pentabromodiphenyl ether
   • hexabromodiphenyl ether
   • heptabromodiphenyl ether
   • octabromodiphenyl ether
   • nonabromodiphenyl ether
   • decabromodiphenyl ether
4. Draw one structural isomer of each of the molecules above.
5. Name each of the molecules you have drawn in question 4 above.
6. Give the molecular formula for the molecules you have drawn that have been considered to be the least dangerous.
7. The blood of a child is found to have 70ppb PBDE. Convert this to:
   • ppm
   • mg/g
   • g/L
8. Assuming the child above has 4.7L of blood in his/her body, calculate the mass of PBDE present.
9. In the USA, the EPA set a "safe" limit of 7 micrograms per kilogram of body weight for PBDE. If the child above has a mass of 18kg, is the amount of PBDE in his/her blood considered safe? Explain your answer.
   
   
   
   

Pentane from Oil

Crude oil is refined by "cracking", the process in which large molecules are broken up into smaller molecules. The products of the cracking process include gasoline (petrol), kerosene, heating oil and lubricants. Catalysts can be used to further refine these hydrocarbons.

Rice University scientists have discovered that sub-nanometer clusters of active tungsten oxide lying on top of inert zirconium oxide (zirconia) are a highly efficient catalyst that turns straight-chain molecules of pentane, one of the many hydrocarbons present in gasoline (petrol), into better burning branched-chain hydrocarbons. This process of rearranging the carbon and hydrogen atoms in a molecule is referred to as isomerization.

Reference:
Nikolaos Soultanidis, Wu Zhou, Antonis C. Psarras, Alejandro J. Gonzalez, Eleni F. Iliopoulou, Christopher J. Kiely, Israel E. Wachs, Michael S. Wong. Relatingn-Pentane Isomerization Activity to the Tungsten Surface Density of WOx/ZrO2. Journal of the American Chemical Society, 2010; : 100903140709054 DOI: 10.1021/ja105519y

Further Reading
Organic Nomenclature
Naming Straight Chain Alkanes
Naming Branched-Chain Alkanes
Isomers of Alkanes
Uses of Hydrocarbons
Ethene

Study Questions

1. What is meant by the term hydrocarbon?
2. Give the names of 4 hydrocarbons.
3. Give the molecular formula for each of the hydrocarbons named above.
4. Give the molecular formula and the condensed molecular formula for pentane.
5. Draw the structural formula for pentane.
6. Draw the structural formula for all the possible isomers of pentane.
7. Name each of the isomers drawn above.
8. Why do you think that the branched-chain isomers of pentane are referred to as "better burning" hydrocarbons? Explain your answer.