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:
ISOMERS Learning Goals: I will understand the difference between structural isomers and stereoisomers (diastereomers and enantiomers) and be able to name/draw them accordingly
TYPES OF ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS STRUCTURAL ISOMERISM (CONSTITUTIONAL ISOMERS) STEREOISOMERISM (CONFORMATIONAL & CONFIGURATIONAL ISOMERS) GEOMETRICAL ISOMERISM (Diastereomers) OPTICAL ISOMERISM (Enantiomers) Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space.
caused by different arrangements of the carbon skeleton similar chemical properties slightly different physical properties more branching = lower boiling point There are two structural isomers of C 4 H 10. 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. BUTANE straight chain 2-METHYLPROPANE branched C 4 H 10 STRUCTURAL ISOMERS
DIFFERENCES IN PROPERTIES ChemicalIsomers show similar chemical properties because the same functional group is present. PhysicalProperties such as density and boiling point show trends according to the of the degree of branching Boiling Point“straight” chain isomers have higher values than branched ones the greater the degree of branching the lower the boiling point branching decreases the effectiveness of intermolecular forces less energy has to be put in to separate the molecules - 0.5°C straight chain - 11.7°C branched greater branching = lower boiling point
Number of Possible Isomers of Alkanes Section 14.3 The more carbon atoms there are, the greater the number of possible isomers
Drawing Structural Isomers: You Try Section 14.3 There are 5 structural alkane isomers for C 6 H 14. Can you draw and name them all? Use molecular model kits to help.
STEREOISOMERS Molecules 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)
GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES an example of stereoisomersim found in some, but not all, alkenes occurs due to the RESTRICTED ROTATION OF C=C bonds get two forms.... CIS Groups/atoms are on the SAME SIDE of the double bond TRANS Groups/atoms are on OPPOSITE SIDES across the double bond Isomers- have different physical properties - e.g. boiling points, density - have similar chemical properties - in most cases
DIASTEREOMERS RESTRICTED ROTATION OF C=C BONDS Single 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’ ROTATION Animation doesn’t work in old versions of Powerpoint
RESTRICTED ROTATION OF C=C BONDS C=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. This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space. Animation doesn’t work in old versions of Powerpoint DIASTEREOMERS
How to tell if it exists Two different atoms/group s attached Two similar atoms/group s attached Two different atoms/group s attached GEOMETRICAL ISOMERISM Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism DIASTEREOMERS
Isomerism in butene There are 3 structural isomers of C 4 H 8 that are alkenes *. Of these ONLY ONE exhibits geometrical isomerism. BUT-1-ENE2-METHYLPROPENE trans BUT-2-ENEcis BUT-2-ENE * YOU CAN GET ALKANES WITH FORMULA C 4 H 8 IF THE CARBON ATOMS ARE IN A RING DIASTEREOMERS
RESTRICTED ROTATION OF CYCLOALKANES a)Cis or Trans? b)Name it! c) Cis or Trans? d) Name it! DIASTEREOMERS
OPTICAL ISOMERS (ENANTIOMERS) Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it. There are four different colours arranged tetrahedrally about the carbon atom 2-chlorobutane exhibits optical isomerism because the second carbon atom has four different atoms/groups attached CHIRAL CENTRES
ENANTIOMERS two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other non-superimposable means you you can’t stack one form exactly on top of the other ENANTIOMERS MUST BE: non-superimposable mirror imagesex. hands
What is a non-superimposable mirror image? Animation doesn’t work in old versions of Powerpoint ENANTIOMERS
17 Learning Check: Classify each of the following pairs as enantiomers or not enantiomers a) b) c)
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 “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...” A Quote from Lewis Carroll’s ‘Alice Through the Looking Glass’…
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)