Hydrocarbons: Isomers

SCH3UAvital Stopnicki and Laura McLennanHydrocarbons: Isomers

Isomers: introductionHow 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.

Lets Take A Look:This drug causes serious birth defects when pregnant women take it during their first trimester. Known as a teratogenThese compounds have the same molecular formula but are different in structure: they have different physical properties and behave differently:

This drug does not cause birth defects and has therapeutic uses eg. alleviates nausea and vomiting

R limoneneS limoneneThese are naturally occurring isomersSmells like lemonSmells like orange These isomers interact differently with receptors on the tongue, enabling a person to perceive different tastes. Why are isomers important?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)

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 groupsBig IdeasOrganic 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 isomersOverall Expectations

Students willB2. Investigate physical and chemical properties of elements and compounds, and use various methods to visually represent them*B2.3 build molecular models for a variety of simple organic compounds [PR, AI, C]*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 B2.7 write chemical formulae and name the compounds using the International Union of Pure and Applied Chemistry (IUPAC) nomenclature system.* 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.Prior knowledge:

Organic compounds areCompounds 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

Organic Compoundscompounds that contain carbon (but not CO, CO2 or ionic carbon compounds)Hydrocarbonsorganic compounds containing only carbon and hydrogen atomsAlkanesContain single bonds only between carbon atomsCnH2n+2Aromaticscontain aromatic rings e.g. benzeneAliphaticstraight chains and non-aromatic ringsAlkenesContain at least one double bond between carbon atomsAlkynesContain at least one triple bond between carbon atomsAcyclic straight chainsCyclic Non-aromatic ringsNaming Hydrocarbons (a review)# of C atomsAlkaneCnH2n+2AlkeneAlkyneAlkyl Group Prefix 1methaneCH4methenemethynemethyl--CH32ethaneCH3-CH3Etheneethyneethyl--CH2-CH33propaneCH3CH2-CH3propenepropynepropyl--(CH2)2-CH34butaneCH3(CH2)2-CH3butenebutynebutyl--(CH2)3-CH35pentaneCH3(CH2)3-CH3pentenepentynepentyl--(CH2)4-CH36hexaneCH3(CH2)4-CH3hexenehexynehexyl--(CH2)5-CH37heptaneCH3(CH2)5-CH3hepteneheptyneheptyl--(CH2)6-CH38octaneCH3(CH2)6-CH3octeneoctyneoctyl--(CH2)7-CH39nonane CH3(CH2)7-CH3nonene nonyne nonyl--(CH2)8-CH3Isomers and Isomerism:an introductionIsomers are molecules that have the same molecular formula but a different arrangement of the atoms in space, and different properties (e.g. melting point)

IsomersStereo- or Geometric IsomersStructural Isomers (aka Constitutional Isomers)Atoms are connected in a different orderAtoms are connected in the same order, but arranged differently in space10Structural IsomersSame molecular formula, but atoms are connected in a different order

Example: C4H10 might be 2-methylpropane: (aka isobutane)

or might be butane:

Possible Student MisconceptionsStructural isomers are notmolecules 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).Teaching StrategiesTo 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 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.

Naming Structural IsomersThe 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. IUPAC namecompound structureNaming Aliphatic Compounds and Their IsomersFind 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.

1 2 3 4 5 6CH3CH2 -CH3CH3 CH CH CH2 CH2 CH3Longest chain of carbons is 6 carbons parent chain is hexanemethyl group on carbon 2ethyl group on carbon 3Name of compound: 3-ethyl-2-methylhexane.Student MisconceptionStudents 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 CH3CH2 CH2 CH2 CH3 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.Teaching StrategiesReplay 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 Alkenes and AlkynesAlkenes 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

CycloalkanesForm a closed ring (non-aromatic)May be cycloalkanes (all single bonds) or cycloalkenes (at least one double bond)Cyclopentane C5H10Cyclopentene C5H8CH2CH2CH2CH2CH2CH2CH2CH2CHCHNaming Alkenes and AlkynesThe 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 CH3 CH3 CH = CH CH3Note: The parent chain is the longest chain which contains the double or triple bond. Geometric Isomers of AlkenesIn 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: HCH3CCH3CHH3CCCCH3HHcis-2-butene(cis substituent on the same side)trans-2-butene(trans substituents on opposite sides)Teaching StrategiesStudents to use molecular model kits to investigate double and triple bonds, specifically that double bonds dont twist or rotate propene

Lab activity Reactivity of alkanes and alkenes identifying saturated and unsaturated compounds in fats and oils (McGraw-Hill, pp. 554-555)

Post Lab Extension 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 resourcesStudents will be evaluated on: accuracy of information (TI) Making connections (A) Communication and Presentation (C)

Teaching StrategiesKhan Academy video on Isomers

http://www.youtube.com/watch?v=457xnJv80O0Student MisconceptionsStudents will often miss the existence of geometric isomers. Theyll write 2-butene as:CH3-CH=CH-CH3and miss the cis- and trans- isomers. If theres a carbon-carbon double bond, students should always draw the full structural diagrams.HCH3CCH3CHH3CCCCH3HHMore Applications of isomerscis- 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:

butanemelting point -140 Cboiling point -1 C2-methypropanemelting point -159.6 Cboiling point -11.7 CApplications of isomersNatural 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 gutta percha, 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.

Resourceshttp://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi online 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.htm This site from ASCP discusses isomers at length through a symposium of medical researchers and doctors.