S1-L13: Isomerism In Organic Molecules

Question 1

What are isomers?

Answer 1

• molecules which have same molecular formula BUT different arrangement of atoms
• ->like C4H8O
• figure 1- examples

Question 2

Define and describe structural isomers

Answer 2

• differ in bonding sequence/ molecular skeleton
• usually different compounds with different physical & chemical properties
• figure 2- examples of structural isomers of butane

Question 3

Outline the 4 different types of structural isomers

Answer 3

• 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

Question 4

What are stereoisomers?

Answer 4

-same structural formula BUT different 3-D arrangement of atoms in space

Question 5

State the two types of stereoisomers

Answer 5

• geometric (cis-trans, E/Z) isomers

- optical isomerism

Question 6

Define and describe geometric isomerism

Answer 6

• resulted from restricted rotation around a bond
• requires different groups at each end of bond
• different physical & chemical properties

Question 7

What do figures 7 & 8 show respectively?

Answer 7

• figure 7- single (sigma) bond has free rotation
• figure 8- double (pi) bond- rotation breaks bond
• ->unable to happen without energy input

Question 8

Explain how geometrical isomers are distinguished from each other (figure 9)

Answer 8

• 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

Question 9

What is Optical Isomerism? (figure 10)

Answer 9

• optical isomers have ability to rotate plane-polarised light in opposite directions
• ->to the right (d or +)
• ->to left (l or -)

Question 10

What is Chirality? (figure 11)

Answer 10

• 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

Question 11

What does the perspective view show when drawing chiral centres AND the different lines?

Answer 11

• perspective view shows tetrahedral carbon
• normal lines are in plane of page
• wedges come out of page
• dashed lines go in to page

Question 12

Outline what “R” and “S” mean in relation to chirality?

Answer 12

• R & S- systematic nomenclature for chiral centres

- ->assigned by looking at arrangement of groups around chiral centre

Question 13

In appropriate detail explain how chirality of amino acids & sugars is often described

Answer 13

• described by D & L
• ->assigned by relating their configuration to glyceraldehyde
• ->can be assigned by studying structure
• ->complicated to generally apply

Question 14

What is neither system related to?

Answer 14

-neither system related to (+) AND (-)

Question 15

Define Fischer projections

Answer 15

• flat drawings which represent 3D molecule

- commonly used for sugars

Question 16

State the key features of Fischer Projections (figure 12)

Answer 16

• 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

Question 17

Outline the Fischer rules

Answer 17

• 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

Question 18

Why are Fischer mirror images useful? (figure 13)

Answer 18

• easy to draw
• easy to identify enantiomers
• easy to find internal mirror planes

Question 19

Explain the relationship between molecules with chiral centres AND optical activity

Answer 19

• 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

Question 20

Would compounds with more than one chiral centre show optical activity?

Answer 20

-may OR may not show optical activity depending on whether OR not non-superimposable on their mirror image (chrial) OR superimposable (achiral)

Question 21

Case of two chiral carbons (refer to figure 14):

Example: 1,2-Dimethylcyclopentane

What does Trans 1,2-dimethylcyclopentane show?

Answer 21

• no plane of symmetry

- molecules shown pair of optical isomers (enantiomers)

Question 22

Similarly, what does Cis 1,2-dimethylcyclopentane show?

Answer 22

• each molecule has plane of symmetry
• mirror images superimposable
• not chiral
• is a meso compound
• ->non-optically active member set of stereoisomers

Question 23

Outline what enantiomers are

Answer 23

• 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

Question 24

What are the key features of enantiomers and the way they behave?

Answer 24

• 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

Question 25

Define a racemic mixture (refer to figure 15)

Answer 25

- 1:1 mixture of two enantiomers of compound - ->equal quantities of d- and l- enantiomers - ->notation (d, l) OR (+/-)

Question 26

State the key features of racemic mixtures

Answer 26

- no optical activity | - ->mixture may have different b. points AND m. point from enantiomers

Question 27

What are diastereomers?

Answer 27

- structure with n chiral centres has 2^n possible isomers | - stereoisomers which not mirror image of each other are diastereomers

Question 28

Outline the types of molecule which are examples of diastereomers

Answer 28

- 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

Question 29

Define an epimer

Answer 29

-diastereomers which differ at one C atom only

Question 30

Describe the physical and chemical properties of diastereomers

Answer 30

- physical: two diastereomers will have different physical properties like m. point/ b. point - chemical: have different chemical reactivity with both achiral AND chiral reagents

Question 31

Diastereomers (figure 16)

Answer 31

- molecules with two or more chiral centres where at least one nut not all differ - ->example: Tartaric acid has two chiral centres

Question 32

What is the importance of enantiomers in proteins/ receptors and enzymes?

Answer 32

- made up of L-amino acid | - ->chiral environment- differentiates between isomers

Question 33

How are the components of bacterial cell walls significant? (figure 17)

Answer 33

- components of bacterial cell wall not found in mammalian proteins - protects bacteria from proteases

Question 34

What does figure 18 show?

Answer 34

- (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

Question 35

How do the following properties differ between enantiomers and diastereomers?: 1-rotation of plane polarised light 2-physical & chemical properties 3-separation

Answer 35

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

Question 36

Differing between enantiomer AND diastereomer properties continued: 4-interaction with chiral/achiral molecules 5-taste/smell 6-ratio

Answer 36

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