Presentation on theme: "ISOMERS Learning Goals: I will understand the difference between structural isomers and stereoisomers (diastereomers and enantiomers) and be able to name/draw."— Presentation transcript:
1 ISOMERSLearning Goals: I will understand the difference between structural isomers and stereoisomers (diastereomers and enantiomers) and be able to name/draw them accordingly
2 TYPES OF ISOMERISM STRUCTURAL ISOMERISM (CONSTITUTIONAL ISOMERS) Same molecular formula but different structural formulaeSTEREOISOMERISM(CONFORMATIONAL &CONFIGURATIONALISOMERS)GEOMETRICAL ISOMERISM (Diastereomers)Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANSSame molecular formula but atoms occupy different positions in space.OPTICAL ISOMERISM (Enantiomers)Occurs when molecules have a chiral centre. Get two non- superimposable mirror images.
3 STRUCTURAL ISOMERS C4H10 BUTANE straight chain 2-METHYLPROPANE caused by different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling pointThere are two structural isomers of C4H10. One is a straight chain molecule where all the carbon atoms are in a single row. The other is a branched molecule where three carbon atoms are in a row and one carbon atom sticks out of the main chain.BUTANEstraight chain2-METHYLPROPANEbranchedC4H10
4 DIFFERENCES IN PROPERTIES STRUCTURAL ISOMERSDIFFERENCES IN PROPERTIESChemical Isomers show similar chemical properties becausethe same functional group is present.Physical Properties such as density and boiling point show trends accordingto the of the degree of branchingBoiling Point “straight” chain isomers have higher values than branched onesthe greater the degree of branching the lower the boiling pointbranching decreases the effectiveness of intermolecular forcesless energy has to be put in to separate the molecules- 0.5°Cstraight chain- 11.7°Cbranchedgreater branching= lower boiling point
5 Number of Possible Isomers of Alkanes The more carbon atoms there are, the greater the number of possible isomersSection 14.3
6 Drawing Structural Isomers: You Try There are 5 structural alkane isomers for C6H14. Can you draw and name them all? Use molecular model kits to help.Section 14.3
7 STEREOISOMERSMolecules have the SAME MOLECULAR FORMULA but the atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they occupy a different position in 3-dimensional space.There are two types...• GEOMETRICAL ISOMERISM (Diastereomers)• OPTICAL ISOMERISM (Enantiomers)
8 GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES an example of stereoisomersimfound in some, but not all, alkenesoccurs due to the RESTRICTED ROTATION OF C=C bondsget two forms....CISGroups/atoms are on theSAME SIDE of the double bondTRANSGroups/atoms are on OPPOSITE SIDES across the double bondIsomers - have different physical properties - e.g. boiling points, density- have similar chemical properties - in most cases
9 RESTRICTED ROTATION OF C=C BONDS DIASTEREOMERSRESTRICTED ROTATION OF C=C BONDSSingle covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same.ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATIONAnimation doesn’t work in old versions of Powerpoint
10 RESTRICTED ROTATION OF C=C BONDS DIASTEREOMERSRESTRICTED ROTATION OF C=C BONDSC=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two.Animation doesn’t work in old versions of PowerpointThis produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space.
11 DIASTEREOMERS How to tell if it exists Two different atoms/groups attachedTwo different atoms/groups attachedGEOMETRICAL ISOMERISMTwo similar atoms/groups attachedTwo similar atoms/groups attachedOnce you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerismTwo similar atoms/groups attachedTwo different atoms/groups attachedTwo different atoms/groups attachedTwo different atoms/groups attachedGEOMETRICAL ISOMERISM
12 DIASTEREOMERS Isomerism in butene There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.BUT-1-ENEcis BUT-2-ENEtrans BUT-2-ENE2-METHYLPROPENE* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING
13 RESTRICTED ROTATION OF CYCLOALKANES DIASTEREOMERSRESTRICTED ROTATION OF CYCLOALKANESCis or Trans?Name it!c) Cis or Trans?d) Name it!
14 OPTICAL ISOMERS (ENANTIOMERS) Occurrence another form of stereoisomerismoccurs when compounds have non-superimposable mirror imagesIsomers the two different forms are known as optical isomers or enantiomersthey occur when molecules have a chiral centrea chiral centre contains an asymmetric carbon atoman asymmetric carbon has four different atoms (or groups)arranged tetrahedrally around it.CHIRAL CENTRESThere are four different colours arranged tetrahedrally about the carbon atom2-chlorobutane exhibits optical isomerism because the second carbon atom has four different atoms/groups attached
15 ENANTIOMERStwo forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each othernon-superimposable means you you can’t stack one form exactly on top of the otherENANTIOMERS MUST BE:non-superimposable mirror images ex. hands
16 What is a non-superimposable mirror image? ENANTIOMERSWhat is a non-superimposable mirror image?Animation doesn’t work in old versions of Powerpoint
17 Learning Check: Classify each of the following pairs as enantiomers or not enantiomers b)c)
18 APPLICATION OF ISOMERS- THALIDOMIDE The one obvious difference between optical isomers is their response to plane polarised light. However, some naturally occurring molecules or specifically synthesised pharmaceuticals show different chemical reactivity.The drug, THALIDOMIDE is a chiral molecule and can exist as two enantiomers. In the 1960’s it was used to treat anxiety and morning sickness in pregnant women. Tragically, many gave birth to children with deformities and missing limbs.It turned out that only one of the enantiomers (the structure on the right) was effective and safe; its optically active counterpart was not. The major problem was that during manufacture a mixture of the isomers was produced. The drug was banned world- wide, but not after tens of thousands of babies had been affected.
19 A Quote from Lewis Carroll’s ‘Alice Through the Looking Glass’… “How would you like to live in Looking-glass House, Kitty?I wonder if they'd give you milk, there? Perhaps Looking-glass milk isn't good to drink...”
20 The looking-glass world Alice entered would indeed be a very different place, with mirror-image DNA and enzymes and amino acids, meaning that - as Alice suggested to her kitten - looking-glass milk would almost certainly not be good to drink. Its mirror-image molecules would all be the wrong way around for her enzymes to digest
21 LEARNING CHECK: Are the following pairs of compounds consitutional isomers or stereoisomers? Name all 6!a)b)c)