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Copyright Steven Bottle, Chemistry, QUT, 1995 1
CHB 242 / 289 OCHB 242 / 289 ORGANICRGANIC
Organic chemistry of biologically important
functional groups containing O, N and S.
Lecture 3
Copyright Steven Bottle, Chemistry, QUT, 1995 2
Alkanes - Alkanes - Alkyl/Aryl HalidesAlkyl/Aryl Halides
General Formula: R-X X = F, Cl, Br, I R = alkyl or aryl group
Little importance in normal biological processes CHCl3 Early Anaesthetic CCl4 Dry Cleaning Fluid CF2Cl2 Freon Gas in
Fridges (Text: Pgs. 37 - 44)
Copyright Steven Bottle, Chemistry, QUT, 1995 3
Alkanes - Alkanes - Alkyl/Aryl HalidesAlkyl/Aryl Halides
Here is an example of a solvent commonly used by University students.
With trichloroethane this a solvent in liquid paper.
QuickTime™ and aGraphics decompressor
are needed to see this picture
Copyright Steven Bottle, Chemistry, QUT, 1995 4
Alkanes, Alkyl/Aryl Alkanes, Alkyl/Aryl HalidesHalides
Many halogenated compounds play a role in biological systems as insecticides.
ChlordaneDDT
CCl3
ClCl
Cl Cl
ClCl
Cl
Cl
Copyright Steven Bottle, Chemistry, QUT, 1995 5
NomenclatureNomenclature
Ordered alphabetically Numbered to give the lowest number
at the first point of difference:
2-chloro-2,3-dibromo-5-fluoro-3-iodo-4-methylpentane
CH2 C C C CH3
F Br
Br
Cl
I
H
H3C
Copyright Steven Bottle, Chemistry, QUT, 1995 6
Nomenclature Nomenclature Alkyl Alkyl HalidesHalides
Ordered alphabetically Numbered to give the lowest number
at the first point of difference:
NOT: 4-chloro-2,3-dibromo-5-fluoro-3-iodo-2-methylpentane!
CH2 C C C CH3
F Br
Br
Cl
I
H
H3C
Copyright Steven Bottle, Chemistry, QUT, 1995 7
Synthesis -Synthesis - Alkyl Halides Alkyl Halides
Addition to alkenes: Of Br2 for instance:
C C + Br2 C C
Br Br
Copyright Steven Bottle, Chemistry, QUT, 1995 8
Synthesis -Synthesis - Alkyl Halides Alkyl Halides
Addition to alkenes: Or of HBr for instance:
C C + HBr C C
H Br
Copyright Steven Bottle, Chemistry, QUT, 1995 9
Synthesis -Synthesis - Aryl Halides Aryl Halides
Don’t forget we can make aryl halides via a substitution reaction:
catalyst
CH3
Br
CH3
Br
65%35%
++ Br Br
CH3
Copyright Steven Bottle, Chemistry, QUT, 1995 10
ReactionsReactions
Mostly involve displacement:
Halides are generally good leaving groups.
R O-
Br -+R O CC Br
Copyright Steven Bottle, Chemistry, QUT, 1995 11
Alkyl halides - ChiralityAlkyl halides - Chirality
Alkyl halides are not very significant biologically, but are useful to introduce the concept of chirality.
“Chiral” - from Greek meaning “hand”.
Look at your left and right hands Same gross physical characteristics Four fingers, one thumb, one palm etc.
(Text: Pgs. 250 - 258)
Copyright Steven Bottle, Chemistry, QUT, 1995 12
ChiralityChirality
The left and right hands have the same gross physical characteristics, but are different!
You cannot wear a left glove on your right hand.
The left and right hands are mirror images of each other.
Two molecules may also be mirror images of each other.
Copyright Steven Bottle, Chemistry, QUT, 1995 13
ChiralityChirality
Chirality arises in organic molecules as each group bound to a carbon occupies a specific place in space.
QuickTime™ and aGraphics decompressor
are needed to see this picture
Copyright Steven Bottle, Chemistry, QUT, 1995 14
ChiralityChirality
These two halomethanes are non-superimposable mirror images of each other and are said to be Chiral.
mirror
BA
Br Cl
FI
Cl Br
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 15
ChiralityChirality
These two halomethanes are non-superimposable mirror images of each other and are said to be Chiral.
Copyright Steven Bottle, Chemistry, QUT, 1995 16
ChiralityChirality The compounds are differentbecause A can not be super-imposed over B.Br Cl
FI
Cl Br
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 17
ChiralityChirality
Two atoms align. But two don’t!
Copyright Steven Bottle, Chemistry, QUT, 1995 18
ChiralityChirality
Superimposing the Iodine and Fluorine atoms means the Chlorine and Bromine do not correspond (and vice versa).
Cl Br
FI
Cl Br
IF
Cl Br
FI
Br Cl
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 19
ChiralityChirality
Not all mirror images are chiral (ie are non-superimposable).
H H
FI
H H
FI
A B
mirror
Copyright Steven Bottle, Chemistry, QUT, 1995 20
ChiralityChirality
Not all mirror images are chiral (ie are non-superimposable).
Copyright Steven Bottle, Chemistry, QUT, 1995 21
ChiralityChirality
These two compounds are super-imposable mirror images.
H H
FI
H H
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 22
ChiralityChirality
These two compounds are super-imposable mirror images.
H H
FI
H H
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 23
ChiralityChirality
The other two are chiral isomers. H H
FI
H H
FI Cl Br
FI
Cl Br
IF
Copyright Steven Bottle, Chemistry, QUT, 1995 24
ChiralityChirality
Rule For Chirality: Four different groups attached to
carbon. H H
FI
H H
FI
Copyright Steven Bottle, Chemistry, QUT, 1995 25
ChiralityChirality
Chiral compounds are special types of isomers.
They have the same molecular formulae and the same functional groups attached to them.
They differ only in the 3D arrangement of those groups.
Copyright Steven Bottle, Chemistry, QUT, 1995 26
ChiralityChirality
Non-superimposable mirror images are special isomers called ENANTIOMERS.
IBUPROFENHOOC
HH3C
Copyright Steven Bottle, Chemistry, QUT, 1995 27
ChiralityChirality
It is important to realise that each enantiomer has exactly the same gross physical properties as its mirror image.
MB, BP, solubility, reactivity etc. are all the same.
However they interact with polarised light differently.
Copyright Steven Bottle, Chemistry, QUT, 1995 28
ChiralityChirality
Chiral substances are called optically active because they rotate the plane of polarised light.
Each enantiomer rotates the plane of polarised light in the opposite direction.
Copyright Steven Bottle, Chemistry, QUT, 1995 29
ChiralityChirality
(+)-2-iodobutane rotates the plane of light clockwise by 15.9°.
Its enantiomer (-)-2-iodomethane rotates the light anti-clockwise by 15.9°.
Copyright Steven Bottle, Chemistry, QUT, 1995 30
ChiralityChirality
(+)-2-iodobutane rotates the plane of light clockwise by 15.9°.
Copyright Steven Bottle, Chemistry, QUT, 1995 31
ChiralityChirality
Its enantiomer (-)-2-iodomethane rotates the light anti-clockwise by 15.9°.
Copyright Steven Bottle, Chemistry, QUT, 1995 32
ChiralityChirality
These compounds are enantiomers: non-superimposable mirror images.
Copyright Steven Bottle, Chemistry, QUT, 1995 33
ChiralityChirality
Rotate the molecule onthe right 180°.
Copyright Steven Bottle, Chemistry, QUT, 1995 34
ChiralityChirality
Rotate the molecule onthe right 180°.
Copyright Steven Bottle, Chemistry, QUT, 1995 35
ChiralityChirality
Rotate the molecule onthe right 180°.
Copyright Steven Bottle, Chemistry, QUT, 1995 36
ChiralityChirality
Rotate the molecule onthe right 180°.
Iodine doesnot align.
Copyright Steven Bottle, Chemistry, QUT, 1995 37
ChiralityChirality
Rotate the molecule onthe right 180°.
Iodine doesnot align.
Copyright Steven Bottle, Chemistry, QUT, 1995 38
ChiralityChirality If a substance rotates the light
clockwise (as you look at the source) it is called Dextrorotatory and the symbol (+) is placed in front of its name.
Enantiomers that rotate the light in the opposite direction are called Levorotatory and have the symbol (-).
Copyright Steven Bottle, Chemistry, QUT, 1995 39
ChiralityChirality
Importance of Chirality. The body (and indeed Nature) is filled
with chiral molecules, especially enzymes, which will only react with molecules of the same chirality.
Think of shaking hands. It must be done left to right or it won’t
work.
Copyright Steven Bottle, Chemistry, QUT, 1995 40
ChiralityChirality - Examples - Examples O
CH3
CH3
CH2CH
(-) Carvone[]20 = -62.5 °D
O
CH3
CH3
CH2CH
(+) Carvone[]20 = +62.5 °D
(Spearmint oil) (Caraway oil)
Copyright Steven Bottle, Chemistry, QUT, 1995 41
ChiralityChirality - Examples - Examples
Minus sugar: The enantiomer of sucrose tastes just
as sweet, but it cannot be broken down by the body!
Unfortunately it must be made by chemists and not plants so it is expensive!
Copyright Steven Bottle, Chemistry, QUT, 1995 42
ChiralityChirality - Examples - Examples
Sucrose and its mirror image.
O
O
O
OH
CH2OH
HOHO
HOCH2
OH
CH2OH
HO
O
O
O
CH2OH
OH
OH
OH
CH2OH
OH
HO
HOCH2
Copyright Steven Bottle, Chemistry, QUT, 1995 43
ChiralityChirality - Examples - Examples
Lindane - a common insecticide. This is the only activecompound!
One of the nine isomers produced
Cl
ClCl
Cl
ClCl
3Cl2light
+
Copyright Steven Bottle, Chemistry, QUT, 1995 44
ChiralityChirality - Examples - Examples
Ibuprofen: Sold as a mixture of the two enantiomers.
Only one is biologically active!
HOOC
HH3C
HOOC
HH3C
Copyright Steven Bottle, Chemistry, QUT, 1995 45
ChiralityChirality - Examples - Examples
Look at this compound. Can it be chiral?
N
H
O
H
O
N
O
O
Copyright Steven Bottle, Chemistry, QUT, 1995 46
ChiralityChirality - Examples - Examples
Remember the rule for chirality is that a carbon must be bound to four different groups.
Copyright Steven Bottle, Chemistry, QUT, 1995 47
ChiralityChirality - Examples - Examples
Remember the rule for chirality is that a carbon must be bound to four different groups.
N
H
O
H
O
N
O
O
Copyright Steven Bottle, Chemistry, QUT, 1995 48
ChiralityChirality - Examples - Examples
Remember the rule for chirality is that a carbon must be bound to four different groups.
N
H
O
H
O
N
O
O
Four!
Copyright Steven Bottle, Chemistry, QUT, 1995 49
ChiralityChirality - Examples - Examples
What would these isomers look like?
Copyright Steven Bottle, Chemistry, QUT, 1995 50
ChiralityChirality - Examples - Examples
What would these isomers look like?
NO O
H
NH
O
O
Copyright Steven Bottle, Chemistry, QUT, 1995 51
ChiralityChirality - Examples - Examples
What would these isomers look like?
NO O
H
NH
O
O
NO O
H
NH
O
O
Copyright Steven Bottle, Chemistry, QUT, 1995 52
ChiralityChirality There is no real need to draw the
reflection to ascertain the structure of the two enantiomers. Chiral compounds exist because of the
differing positions of the four groups in space.
Switching two of the groups will produce the enantiomer for simple systems.
Copyright Steven Bottle, Chemistry, QUT, 1995 53
ChiralityChirality
Draw one enantiomer. NO O
H
NH
O
O
Copyright Steven Bottle, Chemistry, QUT, 1995 54
ChiralityChirality
Then change the orientation of the groups.
NO O
H
NH
O
O
Swap
Copyright Steven Bottle, Chemistry, QUT, 1995 55
ChiralityChirality
These two molecules are now non-superimposable.
NO O
H
NH
O
O
NO O
H
NH
O
O
Swap
Copyright Steven Bottle, Chemistry, QUT, 1995 56
ChiralityChirality
This only makes enantiomers for compounds containing one chiral carbon.
Remember enantiomers are non-superimposable mirror images.
Let us look at an example that demonstrates this.
Copyright Steven Bottle, Chemistry, QUT, 1995 57
ChiralityChirality
Let’s look at Peppermint:
This molecule can be chiral as there is a carbon which has four different groups attached.
OH
Copyright Steven Bottle, Chemistry, QUT, 1995 58
ChiralityChirality
Let’s look at Peppermint:
This molecule can be chiral as there is a carbon which has four different groups attached.
This is indicated by the presence of a star (Asterisk).
OH *
Copyright Steven Bottle, Chemistry, QUT, 1995 59
ChiralityChirality
Here the four groups are in differing positions in space.
The molecules are not super imposable.
And they would bend polarised light. But they are NOT enantiomers!
OH
H*
OH
H*
Copyright Steven Bottle, Chemistry, QUT, 1995 60
ChiralityChirality
Enantiomers are mirror images. These two molecules are not
mirror images!
OH
H*
OH
H*
Copyright Steven Bottle, Chemistry, QUT, 1995 61
ChiralityChirality
Draw the mirror image of the molecule on the left and compare it.
OH
H*
OH
H*
OH
H*
Copyright Steven Bottle, Chemistry, QUT, 1995 62
ChiralityChirality
If you flip the reflected molecule you can attempt to superimpose the two.
OH
H*
OH
H*
OH
H*
H
HO*
Copyright Steven Bottle, Chemistry, QUT, 1995 63
ChiralityChirality
It appears that the two molecules on the right of this slide are the same .
OH
H*
OH
H*
OH
H*
H
HO*
OH
H*
Copyright Steven Bottle, Chemistry, QUT, 1995 64
ChiralityChirality
They are not the same because there is another chiral carbon in the molecule.
OH
H*
OH
H*
OH
H*
H
HO*
OH
H*
Copyright Steven Bottle, Chemistry, QUT, 1995 65
ChiralityChirality
The top methyl group is bound to a carbon that has four different groups.
Scrambling of the bonds to one of the chiral carbons does not produce a mirror image (ie an enantiomer).
CH3H
H
OH*
*
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 66
ChiralityChirality
Take the molecule on the left and make its mirror image.
CH3H
H
OH*
*
CH3H
H
OH*
*
OH
H
CH3H
**
Copyright Steven Bottle, Chemistry, QUT, 1995 67
ChiralityChirality
Take the molecule on the left and make its mirror image.
CH3H
H
OH*
*
CH3H
H
OH*
*
OH
H
CH3H
**
H
HO
CH3H
**
Copyright Steven Bottle, Chemistry, QUT, 1995 68
ChiralityChirality
Flipping the reflection does not give a super-imposable compound!
H
HO
CH3H
**
CH3H
H
OH*
*
CH3H
H
OH*
*
OH
H
CH3H
**
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 69
ChiralityChirality
Flipping the reflection does not give a super-imposable compound!
The methyl groupsdo not line up.
CH3H
H
OH*
*
CH3H
H
OH*
*
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 70
ChiralityChirality
These are examples of yet another type of isomer!
They are optically active (ie chiral). But they are not mirror images. They are called diasteriomers.
CH3H
H
OH*
*
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 71
ChiralityChirality
The situation is actually a little more complex as there is another chiral carbon in the molecule. Can you find it?
For ‘n’ chiral carbons in a molecule there are up to ‘2n’ optical isomers.
CH3H
H
OH*
*
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 72
ChiralityChirality
CH3H
H
OH*
*
CH3H
H
OH*
*
Copyright Steven Bottle, Chemistry, QUT, 1995 73
Chirality - Chirality - RacematesRacemates
What happens when HBr is added to the alkene 1-butene?
An addition reaction occurs. The product is 2-bromobutane.
(The Markovnikov product) Is the product optically active?
Copyright Steven Bottle, Chemistry, QUT, 1995 74
Chirality - Chirality - RacematesRacemates
The result is an addition which gives a chiral carbon.
Copyright Steven Bottle, Chemistry, QUT, 1995 75
Chirality - Chirality - RacematesRacemates
The result is an addition which gives a chiral carbon.
C CH H
H CH2 CH3 HBr C C
BrH
H HH CH2 CH3
*
Copyright Steven Bottle, Chemistry, QUT, 1995 76
Chirality - Chirality - RacematesRacemates
But in fact the addition can occur from either side!
Both compoundsare optically active,but the mixture is not.
C C
H H
H CH2 CH3 HBr C C
BrH
H HH CH2 CH3
* C C HH
CH2H CH3
H Br
*
Copyright Steven Bottle, Chemistry, QUT, 1995 77
Chirality - Chirality - RacematesRacemates
A mixture that contains both enantiomers is called racemic.
Most chemical reactions produce racemic mixtures.
Most biochemical reactions produce only one optical isomer and are not racemic.
Copyright Steven Bottle, Chemistry, QUT, 1995 78
Copyright Steven Bottle, Chemistry, QUT, 1995 79
Chirality - Chirality - ReviewReview
Compounds can be chiral if there is a carbon which has four different groups.
Chiral compounds are said to be optically active as they can rotate the plane of polarised light.
Enantiomers are non super-imposable mirror images.
Copyright Steven Bottle, Chemistry, QUT, 1995 80
Chirality - Chirality - ReviewReview
Diasteriomers are optically active compounds which are not mirror images of each other.
Diasteriomers must have at least two chiral carbons.