Hydrocarbon isomers

by Nathan Hickey & André Gagnier

 

Curricular expectations (SCH 4U): B. Organic Chemistry B3 demonstrate an understanding of the structure, properties, and chemical behaviour of compounds within each class of organic compounds.  

B2.3 build molecular models for a variety of simple organic compounds. B3.5 explain the concept of isomerism in organic compounds, and how variations in the properties of isomers relate to their structural and molecular formulae.

Less interesting, (but crucial) stuff first...

… now the more interesting stuff.

You have 2 minutes: 

In groups of two, Use a molecular kit and

build C6H14

in as many different ways as you can.

Then draw your results.   

GO!

Challenge! (the hook, targeting the students’ competitive natures)

Follow-up question: Are these the same?

What is their chemical formula?

C3OH8          and       C3OH8 

They both have the same chemical formula!  

So again, are they the same?

How about these?

 They're all C5H12.

So are they all the same?  All different?In groups of 2,use a molecular kit

to discover.

• Begin with the Isomer Challenge on slide #3 followed by the questions on the next three slides. (Inquiry-based learning)

 • Between every new question will be in-class discussions

where the students must propose hypotheses and take a stand.

  • Following this is an introduction to the term "isomer“ and the

theory of constitutional isomers.  • End the period with the on-line Isomer Construction Set,

with a few questions assigned as homework.  

Day 1: Introduction to constitutional isomers

To demonstrate that isomers have different physical and chemical properties, the students will evaluate the boiling point and density of the following compounds: • Propanol• Isopropanol

 After the labs, the rest of the period will be spent discussing why it is that isomers react differently (different electron density, different polarity, etc.).

Day 2: Lab

Using molecular kits, explore the nature of stereoisomers.

  

 Practice exercises are posted on the class Wiki.

Students are also asked to determine whether alkynes also have stereoisomers. 

Day 3: Alkene isomers (stereoisomers)

PROBLEM SOLUTION

Students may think that changing the shape (ie.

rotating a C-C bond) means a new isomer is created.

Using molecular models, show that for it to be an isomer,

bonds need to be broken and replaced. Rotations do not

create isomers.Understanding that cis- and

trans- geometric isomers are, in fact 2 different substances.

Using pictures and molecular models to allow students to see

the 3D configuration.Students think that isomers are always members of the same class of compound. (eg. the student can think that any

isomer of an alcohol must also be an alcohol, when in fact it

could be an ether).

Using inquiry-based discovery, students will have to construct all the possible isomers. Using

pictures of 2 molecules (alcohol and ether) and asking students to count the atoms. They will realize that the formulae are

the same.

Potential Student Difficulties

• the Bunsen burners used to determine the compounds' boiling points (obvious fire and burn hazards). Proper care must be taken and safety goggles must always be worn.

• the organic compounds that we handle will have to be

dealt with carefully. Some of them are toxic and all of them are flammable! Proper disposal must be ensured afterwards.

  

Safety Concerns

 

Evaluations

Assessment FOR learning

•Questioning based on answers given during isomer identification challenge

Assessment AS learning

•Group-based work during isomer identification challenge and online activity

Assessment OF learning

•Homework: Problem sets•Laboratory: Properties of

different isomers.

Cis- and trans- fatty acids in foods◦ trans-fatty acids are generally associated with health risks

including coronary heart disease ◦ The mechanisms through which these compounds contribute to

these health risks are under investigation

Octane rating of gasoline◦ Simply, this rating can be used as a rough measure of the

isomeric content in the fuel

Chiral drug industry◦ Exploiting isomers in pharmaceuticals to gain better efficacy.◦ Not to be discussed in depth as it includes classes of isomers

outside the scope of the class

Practical Applications and Societal Implications

 

References:•http://www.flatworldknowledge.com/pub/basics-general-organic-and-bio/421680#web-421674

(This site has general information on organic chemistry and is very simple and straightforward)

•http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html

(A more advanced site with information on organic chemistry)

•http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi

(A web-based isomer tutorial. This site contains the virtual isomer construction set)

•www.a-levelchemistry.co.uk/(The isomerism exercises were obtained from this site)