S1-L13: Isomerism In Organic Molecules Flashcards
What are isomers?
- molecules which have same molecular formula BUT different arrangement of atoms
- ->like C4H8O
- figure 1- examples
Define and describe structural isomers
- differ in bonding sequence/ molecular skeleton
- usually different compounds with different physical & chemical properties
- figure 2- examples of structural isomers of butane
Outline the 4 different types of structural isomers
- chain/ skeleton isomerism–> carbon skeleton (figure 6)
- position isomerism–>position of functional group
- ->figure 3
- functional group isomerism–>identity of functional group
- ->figure 4
- tautomerism–>movement of bonds AND a proton
- ->figure 5
What are stereoisomers?
-same structural formula BUT different 3-D arrangement of atoms in space
State the two types of stereoisomers
- geometric (cis-trans, E/Z) isomers
- optical isomerism
Define and describe geometric isomerism
- resulted from restricted rotation around a bond
- requires different groups at each end of bond
- different physical & chemical properties
What do figures 7 & 8 show respectively?
- figure 7- single (sigma) bond has free rotation
- figure 8- double (pi) bond- rotation breaks bond
- ->unable to happen without energy input
Explain how geometrical isomers are distinguished from each other (figure 9)
- terms cis AND trans (E/Z) distinguish geometrical isomers
- ->cis isomer: 2 alkyl groups same side of double bond
- ->trans isomer: 2 alkyl groups on opposite side of double bond
What is Optical Isomerism? (figure 10)
- optical isomers have ability to rotate plane-polarised light in opposite directions
- ->to the right (d or +)
- ->to left (l or -)
What is Chirality? (figure 11)
- molecule which has no plane of symmetry
- ->C atom bonded to 4 different substituents lacks plane of symmetry
- ->this called chiral centre OR asymmetric C atom
What does the perspective view show when drawing chiral centres AND the different lines?
- perspective view shows tetrahedral carbon
- normal lines are in plane of page
- wedges come out of page
- dashed lines go in to page
Outline what “R” and “S” mean in relation to chirality?
- R & S- systematic nomenclature for chiral centres
- ->assigned by looking at arrangement of groups around chiral centre
In appropriate detail explain how chirality of amino acids & sugars is often described
- described by D & L
- ->assigned by relating their configuration to glyceraldehyde
- ->can be assigned by studying structure
- ->complicated to generally apply
What is neither system related to?
-neither system related to (+) AND (-)
Define Fischer projections
- flat drawings which represent 3D molecule
- commonly used for sugars
State the key features of Fischer Projections (figure 12)
- C chain vertical
- a carbon is at vertical intersection of horizontal AND vertical lines
- horizontal lines are forwards out-of-plane
- vertical lines are behind plane
Outline the Fischer rules
- C chain is on vertical line
- rotation of 180 degrees in plane not change molecule
- DO NOT rotate 90 degrees
- DO NOT turn over out of plane
Why are Fischer mirror images useful? (figure 13)
- easy to draw
- easy to identify enantiomers
- easy to find internal mirror planes
Explain the relationship between molecules with chiral centres AND optical activity
- compounds without chiral centres not normally show optical activity
- compounds with 1 chiral centre will show optical activity
- ->optical isomers of these compounds chemically inert
Would compounds with more than one chiral centre show optical activity?
-may OR may not show optical activity depending on whether OR not non-superimposable on their mirror image (chrial) OR superimposable (achiral)
Case of two chiral carbons (refer to figure 14):
Example: 1,2-Dimethylcyclopentane
What does Trans 1,2-dimethylcyclopentane show?
- no plane of symmetry
- molecules shown pair of optical isomers (enantiomers)
Similarly, what does Cis 1,2-dimethylcyclopentane show?
- each molecule has plane of symmetry
- mirror images superimposable
- not chiral
- is a meso compound
- ->non-optically active member set of stereoisomers
Outline what enantiomers are
- non-superimposable mirror image forms of chiral molecules
- ->2 optical isomers (+) AND (-)
- ->their opposite rotatory powers due to opposite arrangements of group around each asymmetric C atom
What are the key features of enantiomers and the way they behave?
- identical physical properties in all respect except their interaction with plane polarised light
- enetiomers interact differently with other chiral molecules
- generally enantiomers not interconverted (two things to be converted in to each other) under ordinary conditions
Define a racemic mixture (refer to figure 15)
- 1:1 mixture of two enantiomers of compound
- ->equal quantities of d- and l- enantiomers
- ->notation (d, l) OR (+/-)
State the key features of racemic mixtures
- no optical activity
- ->mixture may have different b. points AND m. point from enantiomers
What are diastereomers?
- structure with n chiral centres has 2^n possible isomers
- stereoisomers which not mirror image of each other are diastereomers
Outline the types of molecule which are examples of diastereomers
- include cis-trans isomers AND other examples involving rotation
- most common example is molecules with 2 or more chiral C’s where minimum 1 but not all differ
Define an epimer
-diastereomers which differ at one C atom only
Describe the physical and chemical properties of diastereomers
- physical: two diastereomers will have different physical properties like m. point/ b. point
- chemical: have different chemical reactivity with both achiral AND chiral reagents
Diastereomers (figure 16)
- molecules with two or more chiral centres where at least one nut not all differ
- ->example: Tartaric acid has two chiral centres
What is the importance of enantiomers in proteins/ receptors and enzymes?
- made up of L-amino acid
- ->chiral environment- differentiates between isomers
How are the components of bacterial cell walls significant? (figure 17)
- components of bacterial cell wall not found in mammalian proteins
- protects bacteria from proteases
What does figure 18 show?
- (S)-Tirofiban–>Anti-platelet drug
- (R)-Tirofiban–>thousand-fold less active
- (S)-thalidomide–>Anti-angiogenic; teratogen
- (R)-thalidomide–>sedative
- so shows how R and S form of same molecule affect function of molecule
How do the following properties differ between enantiomers and diastereomers?:
1-rotation of plane polarised light
2-physical & chemical properties
3-separation
1-E: rotate the light in opposite direction equally
-D: may rotate plane of the light BUT by different amounts
2-E: identical physical & chemical properties
-identical NMR spectroscopy
-D: different physical & chemical properties
-NMR spectra visible different
3-E: impossible to separate via normal (achiral) physical methods
-D: possible to separate by normal physical methods
Differing between enantiomer AND diastereomer properties continued:
4-interaction with chiral/achiral molecules
5-taste/smell
6-ratio
4-E:interact differently with chiral molecules
–>allows separates via chiral technique
-D: interact differently with both chiral AND achiral molecules
5-E/D: differ in taste AND smell
6-E: 1:1 mixture–> a racemate
