Download Asymmetry and Stereoisomers

Stereochemistry and Chirality
February STAV Workshop
Penny Commons
Brendan Abrahams
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VCAA Study Design …
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Essential knowledge
Structure and nomenclature of organic compounds
• the carbon atom with reference to valence number, bond strength, stability of carbon bonds with
other elements and the formation of isomers to explain carbon compound diversity, including
identification of chiral centres in optical isomers of simple organic compounds and distinction
between cis- and trans- isomers in simple geometric isomers
• structures including molecular, structural and semi-structural formulas of alkanes (including
cyclohexane), alkenes, alkynes, benzene, haloalkanes, primary amines, primary amides, alcohols
(primary, secondary, tertiary), aldehydes, ketones, carboxylic acids and non-branched esters
• IUPAC systematic naming of organic compounds up to C8 with no more than two functional
groups for a molecule, limited to non-cyclic hydrocarbons, haloalkanes, primary amines, alcohols
(primary, secondary, tertiary), carboxylic acids and non-branched esters.
… the formation of isomers to explain carbon compound
diversity, including identification of chiral centres in optical
isomers of simple organic compounds and distinction
between cis- and trans- isomers in simple geometric isomers
VCAA Study Design …
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Metabolism of food in the human body
• metabolism of food as a source of energy and raw materials: general principles of metabolism of
food involving enzyme-catalysed chemical reactions with reference to the breakdown of large
biomolecules in food by hydrolytic reactions to produce smaller molecules, and the subsequent
synthesis of large biologically important molecules by condensation reactions of smaller
molecules enzymes as protein catalysts: active site; modelling of process by which enzymes
control specific biochemical reactions (lock-and-key and induced fit models); consequences of
variation in enzyme-substrate interaction (lock-and-key mechanism) due to the behaviour of a
particular optical isomer; explanation of effects of changes in pH (formation of zwitterions and
denaturation), increased temperature (denaturation) and decreased temperature (reduction in
activity) on enzyme activity with reference to structure and bonding; action of enzymes in narrow
pH ranges; and use of reaction rates to measure enzyme activity
… consequences of variation in enzyme-substrate
interaction (lock-and-key mechanism) due to the behaviour
of a particular optical isomer
Chirality
An object which is non-superimposable on its mirror image
is said to be chiral
https://upload.wikimedia.org/wiki
http://chemwiki.ucdavis.ed
pedia/commons/thumb/4/4c/Ne
u/@api/deki/files/54210/=
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mages?q=tbn:ANd
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http://www.science.oregonstate.edu/~gablek/CH334/Chapter5/chirality.htm
Achiral objects
http://chemwiki.ucdavis.edu/@a
pi/deki/files/54210/=122a3121b
7268f235234e7a9091fc4ea.jpg?
revision=1
http://www.science.oregonstate.edu/~gablek/CH334/Chapter5/chirality.htm
Object
Mirror image
Exercise 1
Indicate whether the following objects are chiral or achiral
Tennis racquet
Wrist watch
Golf club
http://static.giantbo
mb.com/uploads/sc
ale_small/0/140/37
6641tennis_racket.jpg
Stapler
Car
https://upload.wikimedia.or
g/wikipedia/commons/2/2d/
Swingline-stapler.jpg
https://cdn.vecto
rstock.com/i/co
mposite/95,07/g
olf-club-vector879507.jpg
http://stat.homeshop18.co
m/homeshop18/images/pro
ductImages/500/titanmens-wristwatchnd1445yl01large_95d244780622d0c46
1e5cc581730541b.jpg
http://image.cpsimg.com/s
ites/carpartsmc/assets/classroom/ima
ges/dash_gauges.jpg
Chiral Molecules:
Bromochlorofluoromethane
Contains a tetrahedral carbon with four different
substituents (F, Cl, Br and H)
Molecules which are non-superimposable mirror images
are called ‘enantiomers’ – Examples of stereoisomers
Isomers – Molecules with the same molecular
formula but with different structure
Isomers
Constitutional isomers
- different atom to atom
connectivity
Stereoisomers
- same atom to atom
connectivity but the
arrangement in
space is different
Stereoisomers
Same atom connectivity, but have a
different 3-D arrangement of atoms
• Enantiomers are stereoisomers which
are non superimposable mirror images
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSfHnpbZ1Wnx2FGXtMAh53t_LjCEG20O3dm5UcrcxyoLaABuAtHQ
Examples of chiral molecules
3-Methylhexane
* Denotes asymmetric carbon
Exercise 2
Make a chiral model by taking a black tetrahedral
centre and attaching four groups, each of a different
colour to it.
Compare it with your neighbour’s model.
Are they enantiomers or are they the same?
https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTRJdU6GyoEpvLen4hf14BzHlgkSi5BAtnVgnSOIKy9hX370v
Fo
http://image.slidesharecdn.com/isomers-120206102033phpapp01/95/isomers-22-638.jpg?cb=1360324779
https://encryptedtbn0.gstatic.com/images?q=tbn:ANd9GcTi55JoCgJEYb5O_90PFTyU7keY2rs0yxzD
6p3uebAJufLLlviAOw
Exercise 3
Place an asterisk (*) by each of the asymmetric
(stereogenic) centres contained in the molecules
shown below:
CH3
H3C
OH
H2N
O
D
Exercise 3 cont…
Place an asterisk (*) by each of the asymmetric
(stereogenic) centres contained in the molecules
shown below:
How do the chemical and physical
properties of enantiomers differ?
Enantiomers have identical physical properties except
for the direction of rotation of plane-polarised light
(PPL).
They will react in the same way with symmetric
molecules but differently with asymmetric (often
biological) molecules.
Polarimeter – An instrument for measuring
the rotation of the plane of polarised light
https://www.google.com.au/imgres?imgurl=http://wps.prenhall.com/wps/media/ob
jects/3313/3393071/imag2403/AAAYPEV0.JPG&imgrefurl=http://wps.prenhall.co
m/wps/media/objects/3313/3393071/blb2403.html&h=197&w=495&tbnid=xe1hfryGOTdM:&docid=02qRCyvnP6RXIM&ei=BIPWVpzWLcut0ATj_bFg&tbm=isch&ved=0
ahUKEwic6LyFq6HLAhXLFpQKHeN-DAwQMwhCKBswGw
Demonstration of optical rotation using the enantiomers of
Limonene
Some Terminology:
Molecules which rotate PPL are said to be optically active
Specific rotation (-) 102°
anticlockwise
levorotatory
(l) Limonene
or (-) Limonene
(+) 102°
clockwise
dextrorotatory
(d) Limonene
(+) Limonene
A 1:1 mixture of enantiomers is known as a racemic mixture
or racemate.
A racemate is optically inactive (rotations of each enantiomer
cancel out).
Exercise 4
Consider how each of these objects interacts with a right
hand compared to a left hand.
Tennis racquet
Wrist watch
http://static.giantbo
mb.com/uploads/sc
ale_small/0/140/37
6641tennis_racket.jpg
http://stat.homeshop18.co
m/homeshop18/images/pro
ductImages/500/titanmens-wristwatchnd1445yl01large_95d244780622d0c46
1e5cc581730541b.jpg
Golf club
Stapler
https://upload.wikimedia.or
g/wikipedia/commons/2/2d/
Swingline-stapler.jpg
https://cdn.vectorstock.com/i/compo
site/95,07/golf-club-vector879507.jpg
Many objects are made for right handed people…
https://33.media.tumblr.com/ce82ef29988e9971356a03486c9d26d1/tumblr_i
nline_mklhe3Kqqz1qz4rgp.jpg
(Simpson’s Leftorium)
Artificial Vs Natural Chemical Synthesis
When synthetic chemists make chiral
molecules it is common to obtain a mixture of
enantiomers – a racemic mixture.
When nature makes a chiral molecule the
products are almost always a pure enantiomer
e.g. amino acids and sugars
Chiral molecules docking with a biological system
Enzymes are chiral molecules that will interact differently
with pairs of enantiomers
Analogous to a left hand not fitting a right glove.
Enantiomers differ in their interactions with
other chiral molecules – particularly
important in biological systems
http://www.herbmuseu
m.ca/files/images/artic
le-1138955002F8EF600000258941_233x423.jpg
Assignment of configuration
1. Assign priorities in order of decreasing atomic number
of the immediately attached atom to the asymmetric
carbon ie. highest atomic number is ranked first, lowest
(often H) is fourth.
2. If a decision can’t be reached by ranking the first
atoms consider second, third and fourth atoms outwards
until a difference is found.
3. Orient the lowest ranked substituent away from you.
4. If the order of 1, 2 and 3 is clockwise then it is R,
if the order is anticlockwise then it is S.
Assigning configuration to
bromochlorofluoromethane
(R)-bromochlorofluoromethane
(S)-bromochlorofluoromethane
Assigning configuration to 3-methylhexane
R-3-methylhexane
S-3-methylhexane
Lactic acid
2
1
4
3
4
1
S configuration
3
2
R configuration
The S configuration is the one of biological
importance.
Axial Chirality
Allenes
Where A has higher
priority than B
Name this structure
R
S
Axial Chirality
Axial Chirality
M Helicity
P Helicity
Youtube video:
http://www.youtube.com/watch?v=h_DROS_NNx4
Ethene –
Overlap of p orbitals to give p-bond
https://encryptedtbn1.gstatic.com/images?q=tbn:ANd9GcSNXaTkXIhJ0r
PZ3PfAlFkOxDvEYalAPtO7MUG7z6tkMBJBxpXv4g
Stereoisomerism in Alkenes
Geometry of Ethene
No rotation around
double bond
Consider structure of 2-butene CH3CH=CHCH3
These cannot
interconvert
Cis (Z)
An example of
geometric
Trans (E) isomerism
Stereoisomers which arise due to hindered rotation
of bonds are known as geometrical isomers.
Alkenes which have two different groups at either end
of a C=C double bond display geometrical isomerism.
Nomenclature for alkene geometrical
isomers
For simple, unsubstituted internal alkenes, e.g 2-butene, the
cis – trans nomenclature was developed early on.
If the carbon substituents are on the same
side of the C=C double bond, the geometry
is designated as cis.
This compound is called cis-2-butene
If the carbon substituents are on the
opposite side of the C=C double bond, the
geometry is designated as trans.
This compound is called trans-2-butene
cis-hex-2-ene
trans-oct-3-ene
The early cis-trans nomenclature does not work well for all
alkenes, and has been replaced by the more general E / Z
nomenclature:
(1)
(1)
(2)
(2)
(1)
(2)
Z-3-methyl-hex-2-ene
(2)
(1)
E-3-methyl-hex-3-ene
Thank you for your attention.
[email protected]