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CHEMISTRY-FY > S1-L13: Isomerism In Organic Molecules > Flashcards

S1-L13: Isomerism In Organic Molecules Flashcards

1
Q

What are isomers?

A
  • molecules which have same molecular formula BUT different arrangement of atoms
  • ->like C4H8O
  • figure 1- examples
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2
Q

Define and describe structural isomers

A
  • differ in bonding sequence/ molecular skeleton
  • usually different compounds with different physical & chemical properties
  • figure 2- examples of structural isomers of butane
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3
Q

Outline the 4 different types of structural isomers

A
  • 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
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4
Q

What are stereoisomers?

A

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

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5
Q

State the two types of stereoisomers

A
  • geometric (cis-trans, E/Z) isomers

- optical isomerism

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6
Q

Define and describe geometric isomerism

A
  • resulted from restricted rotation around a bond
  • requires different groups at each end of bond
  • different physical & chemical properties
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7
Q

What do figures 7 & 8 show respectively?

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

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

A
  • 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
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9
Q

What is Optical Isomerism? (figure 10)

A
  • optical isomers have ability to rotate plane-polarised light in opposite directions
  • ->to the right (d or +)
  • ->to left (l or -)
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10
Q

What is Chirality? (figure 11)

A
  • 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
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11
Q

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

A
  • perspective view shows tetrahedral carbon
  • normal lines are in plane of page
  • wedges come out of page
  • dashed lines go in to page
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12
Q

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

A
  • R & S- systematic nomenclature for chiral centres

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

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13
Q

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

A
  • described by D & L
  • ->assigned by relating their configuration to glyceraldehyde
  • ->can be assigned by studying structure
  • ->complicated to generally apply
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14
Q

What is neither system related to?

A

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

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15
Q

Define Fischer projections

A
  • flat drawings which represent 3D molecule

- commonly used for sugars

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16
Q

State the key features of Fischer Projections (figure 12)

A
  • 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
17
Q

Outline the Fischer rules

A
  • 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
18
Q

Why are Fischer mirror images useful? (figure 13)

A
  • easy to draw
  • easy to identify enantiomers
  • easy to find internal mirror planes
19
Q

Explain the relationship between molecules with chiral centres AND optical activity

A
  • 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
20
Q

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

A

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

21
Q

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

Example: 1,2-Dimethylcyclopentane

What does Trans 1,2-dimethylcyclopentane show?

A
  • no plane of symmetry

- molecules shown pair of optical isomers (enantiomers)

22
Q

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

A
  • each molecule has plane of symmetry
  • mirror images superimposable
  • not chiral
  • is a meso compound
  • ->non-optically active member set of stereoisomers
23
Q

Outline what enantiomers are

A
  • 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
24
Q

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

A
  • 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
25
Q

Define a racemic mixture (refer to figure 15)

A
  • 1:1 mixture of two enantiomers of compound
  • ->equal quantities of d- and l- enantiomers
  • ->notation (d, l) OR (+/-)
26
Q

State the key features of racemic mixtures

A
  • no optical activity

- ->mixture may have different b. points AND m. point from enantiomers

27
Q

What are diastereomers?

A
  • structure with n chiral centres has 2^n possible isomers

- stereoisomers which not mirror image of each other are diastereomers

28
Q

Outline the types of molecule which are examples of diastereomers

A
  • 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
29
Q

Define an epimer

A

-diastereomers which differ at one C atom only

30
Q

Describe the physical and chemical properties of diastereomers

A
  • physical: two diastereomers will have different physical properties like m. point/ b. point
  • chemical: have different chemical reactivity with both achiral AND chiral reagents
31
Q

Diastereomers (figure 16)

A
  • molecules with two or more chiral centres where at least one nut not all differ
  • ->example: Tartaric acid has two chiral centres
32
Q

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

A
  • made up of L-amino acid

- ->chiral environment- differentiates between isomers

33
Q

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

A
  • components of bacterial cell wall not found in mammalian proteins
  • protects bacteria from proteases
34
Q

What does figure 18 show?

A
  • (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
35
Q

How do the following properties differ between enantiomers and diastereomers?:

1-rotation of plane polarised light
2-physical & chemical properties
3-separation

A

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

36
Q

Differing between enantiomer AND diastereomer properties continued:

4-interaction with chiral/achiral molecules
5-taste/smell
6-ratio

A

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

CHEMISTRY-FY (11 decks)

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