Transcript Halogenation, geometric and optical isomers

Review: Addition
• So called because it makes a bigger molecule (adds
more stuff onto the alkene/alkyne)
• Things that will undergo addition with an alkene/alkyne:
– H2 – hydrogenation (usually requires a catalyst & high
temperature)
– X2 – I.e. Br2, Cl2, F2 – halogenation I.e. bromination (occurs
rapidly at room temperature)
– HX I.e. HCl, HBr, HI (hydrohalogenation – occurs at moderate
rates at room T)
– H2O – hydration (requires an acid catalyst, i.e. H2SO4)
• Addition is always exothermic
– from energy released by breaking C=C bond
Addition Examples
• Benzene +X2  no reaction!
• Aromatics don’t undergo halogenation (or any
type of addition)!
– Because the product would be less stable than the
resonance double bonds of the benzene!
Uses: The Iodine Index
• The degree of unsaturation in oils can be
measured by measuring the amount of iodine
that can react with the unsaturated fat or oil by
addition. Each mole of C=C requires one mole
of I2 to react.
• The haloalkane chain is nearly colorless
Therefore unsaturated hydrocarbon chains will
destroy the purple brown color of iodine solutions
as long as there are C=C bonds present.
Uses: Bromine Test for
Unsaturation
• Like Iodine Index, we test for unsaturation
with Br2
– Usually done for non-fats, however (i.e.
cyclohexene, below, is a non-polar solvent)
H
H
+ Br2
H
Br
H
Br
Review: Geometric Isomers
• Geometric Isomers are another name for cis and trans isomers
• These can only occur around bonds that don’t move easily
– Double or triple bonds
– Single bonds in ring structures
• On linear single bonds, the bond rotates too easily to have cis/trans
isomers.
New Material
(Not Review)
Geometric Isomers
• Geometric Isomers of the same molecule
(cis & trans) often have different physical
and chemical properties
– Due to differences in overall molecule shape
(affects intermolecular bonding
– Also because of differences in dipole
moment (affects polarity)
Geometric Isomers
• Differences in physical properties:
• 1,2-dichloroethane
• cis boiling point: 60.3°C
vs.
• trans b.p.: 47.5°C
• The cis isomer is overall more polar (all polar groups on
one side)
– Therefore more temporary polar interactions between molecules
(adds to London dispersion Forces, or induced dipole-dipole
forces) – stronger intermolecular bonds 
Geometric Isomers
• Differences in chemical properties:
• But-2-ene-1,4-dioic acid
• cis: decomposes (melts) when
heated to 130-131ºC
– Because the H of the OH and the =O of
the carboxylic acid are close together;
they have more H-bonding, and are
more likely to react with each other than
other molecules (makes molecule into a
ring)
VS
• trans: only melts at 286ºC
– OH and =O of carboxylic acid far apart;
is more tightly bonded to other
molecules than to itself; higher m.p.
http://fds.oup.com/www.oup.com/pdf/13/9780199151424.pdf
Optical Isomers/Enantiomers
• A subset of
stereoisomers
– Stereoisomers: 2
molecules with the same
sequence of bonded
atoms, but different 3D
orientations in space (i.e.
cis/trans)
• Enantiomers/optical
isomers are mirror
images of each
other
Optical Isomers/Enantiomers
• “Wait…doesn’t that make
enantiomers the same
molecule?”
– Nope! 
• Their actual 3D appearance
is different
• Like your left and right hand;
mirror images, but put one on
top of the other so they face
the same direction; they’re
different.
– Or: your left shoe won’t fit
on your right foot
Optical Isomers/Enantiomers
• Enantiomers happen around a
carbon bonded to 4 different
atoms/groups
– This is called the chiral
carbon/asymmetric carbon
– Because the bonds to a C are
tetrahedral, they are not
symmetrical if you look at the
3D molecule
– (I.e. bolded – means “coming
toward you in 3D”, --- means
“going away from you in 3D”)
• So when you interchange the
position of 2 “opposite” groups,
you get the enantiomer
Optical Isomers/Enantiomers
• A molecule that has
enantiomers must have at
least 1 chiral carbon (can have
more)
• A chiral carbon does not
always have to have 4
different atoms/groups
bonded to it
I.e. --------------------------
– Rule: Make the mirror image
(switch 2 opposite groups
bonded to a carbon.) If the
mirror image can’t be
superimposed on the original
molecule (doesn’t look exactly
the same when you lay them
on top of each other,) the two
are enantiomers
Terminology
• Chirality – The property of a molecule
having a mirror image that can’t be
superimposed on the original molecule
(because of different 3D structures
between the two)
• Racemic Mixture (/racemate) – A mixture
of 50% of each enantiomer
Why we care: Amino Acids
• All amino acids are chiral (except glycine, has H as its R
group)
• I.e. D-alanine and L-alanine:
• All organisms can only include one enantiomer of amino
acids in their proteins (termed the L-enantiomer)
– But the D-enantiomers are made in nature.
– I.e. part of bacterial cell walls (not as a protein), and D-serine is a
neurotransmitter in our brain
• Why?
– Probably just evolution
Why we care: Medicine
•
•
•
In the 1950s, Thalidamide was
put on the market as a sedative
drug (also given to pregnant
women to fight morning sickness)
In 1961, it was taken off the
market because it caused birth
defects in children born from
women who took it while pregnant
The Thalidomide given medically
was a racemic mixture
– The R enantiomer is harmless and
useful against morning sickness
– The S enantiomer causes birth
defects by obstructing
development of blood vessels
– Even if pure R-Thalidamide is
ingested, the body will convert it to
a racemic mixture
•
(This is why med school really
cares if you know your organic
chem…)
How do we detect Chirality?
• Enantiomers are not
different in their physical
properties (are only
different in their chemical
properties if they interact
with enantiomers of
another molecule)
BUT
• If you shine polarized
light onto them (light
that vibrates in only 1
dimension), they will
deflect the light differently
How do we detect Chirality?
• All molecules deflect light
– A pure sample of one
enantiomer will deflect
polarized light θº to the
right
– A pure sample of the other
enantiomer will deflect it θº
to the left 
• A racemic mixture has
50% of each enantiomer,
therefore the 2
deflections cancel each
other out – the racemic
mixture will appear
“optically inactive”