20.6 Stereoisomerism

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20.6 Stereoisomerism

• 20.6.1 Describe stereoisomers as compounds with
  the same structural formula but with different
  arrangements of atoms in space.
• 20.6.2 Describe and explain geometrical
  isomerism in non-cyclic alkenes.
• 20.6.3 Describe and explain geometrical
  isomerism in C3 and C4 cycloalkanes.
• 20.6.4 Explain the difference in the physical
  and chemical properties of geometrical isomers.
• 20.6.5 Describe and explain optical isomerism in
  simple organic molecules.
• 20.6.6 Outline the use of a polarimeter in
  distinguishing between optical isomers.
• 20.6.7 Compare the physical and chemical
  properties of enantiomers.

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Structural Isomer

• Discussed in SL material.
• Have same chemical formula (same amount of
  atoms), but totally different structures (and
  therefore names)
• Ex C4H10
• Has 2 different structural isomers
• Butane CH3CH2CH2CH3
• Methyl propane CH3CH(CH3)CH3
• Can have different branches, position of
  functional groups, or completely different
  functional groups.

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Stereoisomer

• Molecules have atoms attached together in same
  order
• Different spatial arrangement (3-D)
• Two types required with IB
• Geometric
• Optical
• On exam draw full structural diagrams to show
  stereoisomers properly.

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Geometric Isomer

• Restricted rotation about the bond
• Cyclic structure (ie. Benzene)
• Double bond (ie. Alkenes)
• The atoms become fixed in place.
• Using models can be helpful in understanding
  geometric isomers.

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Not an isomer...
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This is an isomer
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Geometric isomer Cis-

• same groups on same side of the double bond or
  ring.

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Geometric isomers Trans-

• same groups on opposite sides of double bond or
  ring.

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This is not an isomer
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Physical properties

• Cis

• Trans

• Polar molecules
• Dipole-dipole interactions van der Waals
  forces between its molecules
• Higher B.P
• Lower M. P due to lack of symmetry. Not packed
  so closely together.

• Non polar molecules
• Only van der Waals forces between its molecules
• Lower B. P
• Pack together more closely because of their
  symmetry
• Higher M. P.

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Chemical Properties
Usually very similiar. One exception you need to
know. Cis and trans-butenedioic acid

• Cis-

• Trans-

• Cis-butenedioic acid heated reacts to form
  butenedioic anhydride H2O

• Trans- butenedioic acid heated sublimes, but does
  not chemically react.

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Cyclic molecules

• Cycloalkanes contain a ring of carbons, single
  bonded.
• Bond angles are different than regular single
  bonds
• Cyclopropane (60 o triangle)
• Cyclobutane (90 o square)
• Can have cis- and trans- isomers because of lack
  of movement about the bond.

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Draw on board

• Cis-1,2-dimethylcyclopropane
• Trans-1,2-dimethylcyclopropane
• Cis-1,3-dichlorocyclobutane
• Trans-1,3-dichlorocyclobutane

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Optical Isomers

• It occurs when substances have the same molecular
  and structural formulae, but one cannot be
  superimposed on the other.
• They are mirror images of each other

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• The molecule on the left will not turn into the
  molecule on the right unless you break and make
  some bonds!
• Molecules like this are said to be chiral and the
  different forms are called enantiomers.

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Optical isomers

• occur when there is an asymmetric carbon atom.
• An asymmetric carbon atom is one which is bonded
  to four different groups (may be complex (ex
  C2H5) or simple (ex Cl))
• http//cwx.prenhall.com/petrucci/medialib/media_po
  rtfolio/text_images/083_Chirality.MOV

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• Optical isomers can rotate the plane of
  polarisation of plane-polarised light
• one enantiomer rotates the polarised light
  clockwise (to the right) and is the ()
  enantiomer
• the other rotates the polarised light
  anticlockwise (to the left) and is called the ()
  enantiomer.
• A mixture containing equal amounts of both
  enantiomers is not optically active (it will not
  rotate the plane of polarisation).
• It is called a racemic mixture or racemate.

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Properties of optical isomers

• Enantiomers have identical physical and chemical
  properties with two exceptions
• Optical activity
• Reactivity with other chiral molecules

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Optical activity

• http//cwx.prenhall.com/petrucci/medialib/media_po
  rtfolio/text_images/084_OpticalActivity.MOV
• Plane polarized light If light is passed
  through a polarizer, only a single wavelength of
  light may pass through. (all other waves are
  blocked out)
• A solution of enantiomers changes the rotation of
  light being passed through a polarizer.
• They are optically active molecules

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Rotation of light

• The extent and direction of rotation can be
  deduced.
• Concentration of solutions, wavelength of light
  used and sample path length is kept constant.
• Different enantiomers (at same conc.) rotate
  plane-polarized light in equal but opposite
  directions.
• Remember racemic mixtures dont rotate any light.

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Reactivity

• When a racemic mixture is reacted with a single
  enantiomer of another chiral compound, the two
  components () and (-) enantiomers react to
  produce different products.
• The products have different chemical and physical
  properties.
• Ex 1960s drug thalidomide one enantiomer
  reaction was used to relieve symptoms of morning
  sickness, the other caused severe fetus
  malformations

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Sources

• http//www.chemguide.co.uk/basicorg/isomerism/geom
  etric.html
• http//www.chemguide.co.uk/organicprops/alkanes/ba
  ckground.html
• http//www.creative-chemistry.org.uk/molecules/opt
  ical.htm
• Play with this
• http//www.wellesley.edu/Chemistry/Flick/chem341/o
  pticalisomers.html