Hydrocarbons: Isomers

1. Hydrocarbons: Isomers SCH3U AvitalStopnicki and Laura McLennan

2. How can compounds with identical molecular formulas have distinctly differing properties? The answer lies in the unique structure of each compound. Structure is the key to unlocking the mystery of how chemicals behave. Compounds with the same molecular formula but different structures are called isomers. Isomers: introduction

3. These compounds have the same molecular formula but are different in structure: they have different physical properties and behave differently: These are naturally occurring isomers R limonene S limonene This drug causes serious birth defects when pregnant women take it during their first trimester. Known as a teratogen This drug does not cause birth defects and has therapeutic uses eg. alleviates nausea and vomiting Let’s Take A Look: Smells like lemon Smells like orange These isomers interact differently with receptors on the tongue, enabling a person to perceive different tastes.

4. Look at these two molecules, they have the same molecular formula. Can you explain why they might behave differently or have different properties? Teaching Strategies: Brainstorm ideas on the board, have students discuss in groups Think about this question! This will be researched and discussed at the end of the unit, and we will come back to answer why isomers are important. (specifically in the food and drug industry) Why are isomers important?

5. Organic compounds are often identified by their molecular formula (e.g. C6H14), but the molecular formula alone does not tell you how the atoms in the compound are connected to each other. • Compounds with the same molecular formula but the atoms connected in a different order – or orientated differently in space – will have different chemical and physical properties. The connectivity of the atoms in a compound and the orientation of those atoms in space determines the physical and chemical properties of that compound. • Compounds which have the same molecular formula but different molecular structures are called isomers. • Since isomers have different properties, different uses can be made of the isomers Big Ideas

6. Students will B2. Investigate physical and chemical properties of elements and compounds, and use various methods to visually represent them • *B2.3build molecular models for a variety of simple organic compounds [PR, AI, C] • *B3.5explain the concept of isomerism in organic compounds, and how variations in the properties of isomers relate to their structural and molecular formulae • B2.7 write chemical formulae… and name the compounds using the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system. Overall Expectations * These expectations come from SCH4U curriculum documents. The new curriculum documents do not include hydrocarbons or isomers as part of the expectations, however, it is indeed introduced in the grade 11 curriculum and is included in both McGraw and the newer Nelson texts.

7. Organic compounds are… • Compounds that contain carbon, but not CO(g), CO2 (g), and ionic compounds containing carbon Hydrocarbons are … • Organic compounds that contain only carbon and hydrogen atoms in their molecular structure Prior knowledge:

8. Organic Compoundscompounds that contain carbon (but not CO, CO2 or ionic carbon compounds) Hydrocarbons organic compounds containing only carbon and hydrogen atoms Aliphatic straight chains and non-aromatic rings Aromatics contain aromatic rings e.g. benzene Acyclic straight chains Cyclic Non-aromatic rings Alkenes Contain at least one double bond between carbon atoms Alkynes Contain at least one triple bond between carbon atoms Alkanes Contain single bonds only between carbon atomsCnH2n+2

9. Naming Hydrocarbons (a review)

10. Isomers are molecules that have the same molecular formula but a different arrangement of the atoms in space, and different properties (e.g. melting point) Isomers Stereo- or Geometric Isomers Structural Isomers (aka Constitutional Isomers) Isomers and Isomerism:an introduction Atoms are connected in the same order, but arranged differently in space Atoms are connected in a different order

11. Same molecular formula, but atoms are connected in a different order Example: C4H10 might be 2-methylpropane: (aka isobutane) or might bebutane: Structural Isomers

12. Structural isomers are not…molecules that are in apparently different arrangements which result from the molecule rotating as a whole or from a portion of the molecule rotating about a particular bond or bonds  These are all 2-methylbutane (a straight chain of 4 carbons, with a methyl- group on carbon 2). Possible Student Misconceptions

13. To find all possible structural isomers of a given molecular formula: • Use molecular model kits to have students investigate isomers and non-isomers of hydrocarbons. The teacher should demonstrate and students should investigate how rotation around a single bond does not create a new isomer. • Online interactive isomer builder: http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi • Card game: distribute cards - one per student - with different hydrocarbons (formula or drawing) on each card. Students are instructed to find their classmate(s) with cards containing isomers of their card. Once found, the group has to figure out which structures are true isomers and which are simply the same structure, drawn differently. Teaching Strategies

14. Teaching Strategy: Have students find and draw all possible isomers of a given molecular formula using molecular models or online applet. Differentiation: Provide the expanded molecular formula (set out here) as scaffolding to struggling students. For advanced students, do not tell them the total number of possible isomers.

15. The IUPAC naming system provides a unique name for each compound. The name provides the structure and the structure provides the IUPAC name of the compound. In other words, if you know one, you can find the other. Naming Structural Isomers IUPAC name compound structure

16. Find the longest continuous chain – this is the parent chain that gives the hydrocarbon its name. • Number the carbons in the parent chain. Start numbering at the end closest to the first branch. • Name the branches, in alphabetical order, using the numbered carbons of the parent chain to identify the branch location. methyl group on carbon 2 Longest chain of carbons is 6 carbons  parent chain is hexane CH3 Naming Aliphatic Compounds and Their Isomers CH3 – CH – CH – CH2 – CH2 – CH3 1 2 3 4 5 6 CH2 -CH3 ethyl group on carbon 3 Name of compound: 3-ethyl-2-methylhexane.

17. Students will often fail to find the longest carbon chain and will name the parent chain as the longest straight chain in the provided diagram. CH3 – CH2 – CH – CH2 – CH2 – CH3 CH2 – CH2 – CH2 – CH3 Student Misconception Many students will name this compound 3-buytlhexane, when its proper name is 3-propylheptane. • Teaching Strategy: proper modeling by the teacher and reinforcement of this concept is necessary. Students will need practice in correctly naming compounds.

18. Replay the card game activity, but add in cards with names of hydrocarbons. Students with structure cards must find their match with the correct name card. • Role play activities where students must ‘be the carbon’ and form a hydrocarbon by holding hands. The rest of class names the hydrocarbon. (Consider having two students back to back act as a single carbon since carbon bonds four times and a single student only has two hands.) • Pen and paper worksheets naming and drawing hydrocarbon isomers Teaching Strategies

19. Alkenes have one or more double bonds between two carbon atoms e.g. ethene • Alkynes have one or more triple bonds between two carbon atoms e.g. ethyne Alkenes and Alkynes

20. Form a closed ring (non-aromatic) • May be cycloalkanes (all single bonds) or cycloalkenes (at least one double bond) CH CH2 CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 Cycloalkanes Cyclopentane C5H10 Cyclopentene C5H8

21. The location of the double or triple bond affects the properties of the compound. The IUPAC name tells you the location of the double or triple bond in the compound. Structural Isomers of butene (C4H8): 1-butene2-butene CH2 = CH – CH2 – CH3CH3 – CH = CH – CH3 Naming Alkenes and Alkynes Note: The parent chain is the longest chain which contains the double or triple bond.

22. H3C CH3 H H3C C C C C H H H CH3 • In addition to structural isomers, alkenes and alkynes can have stereo- or geometric isomers. Geometric isomers have their atoms connected in the same order, but arranged differently in space. Geometric isomers result from the fact that a double bond does not rotate. Geometric Isomers of 2-butene: Geometric Isomers of Alkenes cis-2-butene(cis– substituent on the same side) trans-2-butene(trans– substituents on opposite sides)

23. Students to use molecular model kits to investigate double and triple bonds, specifically that double bonds don’t twist or rotate propene • Lab activity – Reactivity of alkanesand alkenes – identifying saturated and unsaturated compounds in fats and oils (McGraw-Hill, pp. 554-555) Teaching Strategies

24. Students can further investigate the properties of saturated and unsaturated compounds in various fats and oils, including making a comparison of geometric isomers, the unsaturated cis and trans fatty acids, using both print and electronic resources. • Students will explain, based on their research and in understanding the structure of the compounds, why unsaturated fats are said to be healthier than saturated fats. They will connect this to the question asked at the beginning of the unit: Why are isomers important in the food and drug industry? • Teacher to provide suitable resources • Students will be evaluated on: accuracy of information (TI) Making connections (A) Communication and Presentation (C) Post Lab Extension

25. Khan Academy video on Isomers Teaching Strategies http://www.youtube.com/watch?v=457xnJv80O0

26. H3C H3C CH3 H C C C C H H H CH3 • Students will often miss the existence of geometric isomers. They’ll write 2-butene as: CH3-CH=CH-CH3 and miss the cis- and trans- isomers. If there’s a carbon-carbon double bond, students should always draw the full structural diagrams. Student Misconceptions

27. cis- and trans- isomers behave differently. Pharmaceuticals take advantage of different behaviours of geometric isomers. Amphetamine and dextroamphetamine are stereoisomers of one another and have different effects on humans. • Stereoisomers have different physical properties: More Applications of isomers butane melting point -140° C boiling point -1° C 2-methypropane melting point -159.6° C boiling point -11.7° C

28. Natural rubber from the rubber tree is a polymer of isopropene, in the cis configuration. It is flexible and elastic. • trans-1,4-isopolypropene, better known as guttapercha, is found in tropical trees of Southeast Asia. It has similar, but not identical properties, to its stereoisomer natural rubber, but is harder and tougher. Applications of isomers

29. http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgionline tutorial to build structural isomers • Schmidt, H.J. (1995). Student Misconceptions – Looking for a Pattern. Science Education, vol. 81, Issue 2, pp.123-135. • American Society of Consultant Pharmacists http://www.cmecorner.com/macmcm/ascp/ascp2002_01.htmThis site from ASCP discusses isomers at length through a symposium of medical researchers and doctors. Resources