isomers learning goals: i will understand the difference between structural isomers and...
Post on 16-Dec-2015
226 Views
Preview:
TRANSCRIPT
ISOMERSLearning Goals: I will understand the difference
between structural isomers and stereoisomers
(diastereomers and enantiomers) and be able to
name/draw them accordingly
TYPES OF ISOMERISMTYPES 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 skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point
There are two structural isomers of C4H10. 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.
BUTANEstraight chain
2-METHYLPROPANEbranched
C4H10
STRUCTURAL ISOMERSSTRUCTURAL ISOMERS
STRUCTURAL ISOMERSSTRUCTURAL ISOMERS
DIFFERENCES IN PROPERTIES
Chemical Isomers show similar chemical properties becausethe same functional group is present.
Physical Properties 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 onesthe greater the degree of branching the lower the boiling pointbranching decreases the effectiveness of intermolecular forcesless energy has to be put in to separate the molecules
- 0.5°Cstraight chain
- 11.7°Cbranched
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 C6H14. Can you draw and name them all? Use molecular model kits to help.
STEREOISOMERSSTEREOISOMERS
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 GEOMETRICAL ISOMERS (DIASTEREOMERS) IN ALKENES(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....
CISGroups/atoms are on the
SAME SIDE of the double bond
TRANSGroups/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
DIASTEREOMERSDIASTEREOMERS
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
DIASTEREOMERSDIASTEREOMERS
How to tell if it exists
Two different atoms/groups attached
Two different atoms/groups attached
Two similar atoms/groups attached
Two similar atoms/groups attached
Two similar atoms/groups attached
Two different atoms/groups attached
Two different atoms/groups attached
Two different atoms/groups attached
GEOMETRICAL ISOMERISM
GEOMETRICAL ISOMERISM
Once you get two similar atoms/groups attached to
one end of a C=C, you cannot have geometrical
isomerism
DIASTEREOMERSDIASTEREOMERS
Isomerism in butene
There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.
BUT-1-ENE 2-METHYLPROPENEtrans BUT-2-ENEcis BUT-2-ENE
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING
DIASTEREOMERSDIASTEREOMERS
RESTRICTED ROTATION OF CYCLOALKANES
a) Cis or Trans?b) Name it!
c) Cis or Trans?d) Name it!
DIASTEREOMERSDIASTEREOMERS
OPTICAL ISOMERS (ENANTIOMERS)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
ENANTIOMERSENANTIOMERS
• 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 images ex. hands
What is a non-superimposable mirror image?
Animation doesn’t work in old versions of Powerpoint
ENANTIOMERSENANTIOMERS
17
Learning Check: Classify each of the following pairs as enantiomers or not enantiomers
a)CH3
ClCH3
Br
CH3
ClH3C
Br
b) CH3
ClBr
H
CH3
ClBr
H
F
H Br
F
HBrc)
APPLICATION OF ISOMERS- APPLICATION OF ISOMERS- THALIDOMIDETHALIDOMIDE
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 A Quote from Lewis Carroll’s Lewis Carroll’s ‘Alice Through ‘Alice Through
the Looking the Looking Glass’…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)
22
Distinguishing Isomers Flowchart
How Did We Do?Learning Goals: I will understand the difference
between structural isomers and stereoisomers
(diastereomers and enantiomers) and be able to
name/draw them accordingly
top related