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CLS3003-B: Chemistry (Semester 1) > L13 - Organic Chemistry (3) > Flashcards

L13 - Organic Chemistry (3) Flashcards

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1
Q

Define Isomer.

A

Molecules which have the same molecular formula, but different arrangement of atoms.

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2
Q

Describe the different types of isomers.

A
  • Structural (Constitutional) Isomers: Differ in their bonding sequence / molecular skeleton
  • Stereoisomers: Differ only in the arrangement of atoms in space
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3
Q

Define Structural Isomer.

A

Usually different compounds with different physical and chemical properties that differ in their bonding sequence / molecular skeleton.

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4
Q

Describe the different types of structural isomers.

A
  • Chain / Skeletal Isomerism: Carbon skeleton
  • Position Isomerism: Position of the functional group
  • Functional Isomerism: Identity of the functional group
  • Tautomerism: Movement of bonds and a proton
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5
Q

Define Stereoisomer.

A

Same structural formula but different 3D arrangement of atoms in space.

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6
Q

Describe the different types of stereoisomers.

A
  • Geometric (cis-trans, E/Z) Isomerism

- Optical Isomerism

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7
Q

What causes Geometric Isomerism?

A

Restricted rotation around a double bond

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8
Q

Describe the possibility of rotation around a double (pi) bond.

A
  • Rotation breaks the double (pi) bond

- Not allowed without an energy input

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9
Q

Describe the possibility of rotation around a sigma bond.

A

Free rotation

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10
Q

What does Geometric Isomerism require?

A

Different groups at each end of the bond

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11
Q

What does Geometric Isomerism result in?

A

Different physical and chemical properties

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12
Q

What is a Cis Isomer?

A

When two alkyl groups are on the same side of the double bond.

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13
Q

What is a Trans Isomer?

A

When two alkyl groups are on opposite sides of the double bond.

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14
Q

Describe Cis and Trans Isomers with reference to steric repulsion.

A
  • Cis Isomer: Greater steric repulsion

- Trans Isomer: Less steric repulsion

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15
Q

Define Optical Isomers.

A

Non-superimposable mirror images.

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16
Q

What do Optical Isomers have the ability to do?

A

To rotate plane-polarised light in opposite directions

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17
Q

Describe the different types of isomers and the direction in which they rotate plane-polarised light.

A
  • Dextrorotatory: d-isomer / + isomer rotates plane polarised light to the right, clockwise
    (bold wedge)
  • Levorotatory: l-isomer / - isomer rotates plane polarised light to the left, anti-clockwise
    (dashed wedge)
18
Q

Define Chiral Molecule.

A

A molecule which has no plane of symmetry.

19
Q

Define Chiral Centre / Asymmetric Carbon Atom.

A

A carbon atom bonded to four different substituent groups which lacks a plane of symmetry.

20
Q

Compounds with one chiral centre will show _______ ________.

A

Compounds with one chiral centre will show optical activity.

(Optical isomers of these compounds are chemically identical).

21
Q

Compounds with more than one chiral centre may or may not show _______ ________ depending on…

A

Compounds with more than one chiral centre may or may not show optical activity depending on whether or not they are non-superimposable on their mirror image (chiral) or superimposable (achiral).

22
Q

What is another word for Optical Isomers?

A

Enantiomers

23
Q

What is a meso compound?

A

A meso compound is a molecule with multiple stereocenters that is superimposable on its mirror image.

Has an internal mirror plane / plane of symmetry.

A molecule which contains chiral carbon atoms but is not chiral (no plane of symmetry) overall.

24
Q

Describe the different lines/wedges used when drawing optical isomers.

A
  • Normal Lines: In the plane of the page
  • Bold Wedges: Come out of the plane of the page
  • Dashed Lines: Go into the plane of the page
25
Q

Describe the systematic nomenclature for chiral centres.

A
  • R: Right-handed / Clockwise

- S: Left-handed / Anti-Clockwise

26
Q

How is the chirality of amino acids and sugars often described?

A

Using d and l

27
Q

What are Fischer Projections?

A
  • Flat drawing that represents a 3D molecule
  • Commonly used for sugars
  • Carbon chain is vertical
  • Horizontal lines are forward, out of the plane
  • Vertical lines are behind, into the plane
28
Q

What are Enantiomers?

A

Non-superimposable mirror image forms of a chiral molecule.

  • Represent 2 optical isomers: (+) and (-)
  • Their opposite rotatory powers are due to the opposite arrangements of groups around each asymmetric carbon atom
29
Q

Describe the features of Enantiomers.

A
  • Enantiomers have identical physical properties in all respects except in their interaction with plane of polarised light
  • Enantiomers interact differently with other chiral molecules
  • Generally, enantiomers are not interconverted under ordinary conditions
30
Q

Define a Racemic Mixture.

A

A 1:1 mixture of the two enantiomers of a compound.

31
Q

Describe the features of Racemic Mixtures.

A
  • Equal quantities of d- and l- enantiomers
  • Notation: (d, l) or (+, -)
  • No optical activity
  • The mixture may have different b.p. and m.p. from the enantiomers
32
Q

Define Diastereomer.

A

Molecules with 2 or more chiral centres where at least one but not all differ.

(A structure with n chiral centres, which has 2n possible isomers).

  • Stereoisomers that are not mirror images of each other
33
Q

Describe the relationship between two Diastereomers.

A
  • Two diastereomers will have different physical properties e.g. melting point, boiling point, stability
  • They have different chemical reactivity with both chiral and achiral reagents
34
Q

Diastereomers which differ at only one carbon atom are called _______.

A

Diastereomers which differ at only one carbon atom are called epimers.

35
Q

Describe the importance of Enantiomers.

A

Proteins / receptors / enzymes made up of L-amino acids = chiral environment, differentiates between isomers

36
Q

What is D-alanine?

A
  • A component of bacterial cell wall not found in mammalian proteins
  • Protects bacteria from proteases
37
Q

What is (S)-Tirofiban?

A
  • Anti-platelet drug
38
Q

What is (R)-Tirofiban?

A
  • Anti-platelet drug that is a thousand-fold less active than (S)-Tirofiban
39
Q

What is S-thalidomide?

A
  • Anti-angiogenic

- Teratogen

40
Q

What is R-thalidomide?

A
  • Sedative

CLS3003-B: Chemistry (Semester 1) (13 decks)

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