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CH10135 > Isomerism > Flashcards

Isomerism Flashcards

1
Q

Configurational isomers: definition:

A

Configurational isomers: are stereoisomers for which interconversion between the different stereoisomers would require the breaking and reforming of covalent bonds

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2
Q

What is E/Z isomerism?

A

a systematic approach for assigning the stereochemistry of variously substituted alkenes.

The E/Z system for naming alkenes is preferable to using cis-/trans- because cis-/trans- is limited to naming simple alkenes (typically 1,2-disubstituted alkenes).

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3
Q

What configuration do alkenes with the higher priority substituents on the opposite sides of the double bond have?

A

If the higher priority substituents are on the opposite sides of the double bond = the alkene has E-configuration.

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4
Q

What configuration do alkenes with the higher priority substituents on the same sides of the double bond have?

A

If the higher priority substituents are on the same side of the double bond = the alkene has Z-configuration.

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5
Q

Steps for determining the relative priorities of substituents in E/Z isomerism:

A
  1. Rank the atoms directly attached to each carbon atom in the double bond in the order of decreasing atomic number so that the atom of highest atomic number has the highest priority.
    For isotopes of the same atom, use the atomic mass number to prioritise the group.
    The isotope with the highest atomic mass number has the higher priority.
  2. If the two atoms directly attached to the same carbon are the same, then rank the second, third, fourth, and so on atoms (working away from the C=C bond) one at a time until a difference is found. This is often called the first point of difference rule.
  3. If substituents contain double and triple bonds, treat the atoms joined by the double or triple bonds as if they were linked to two or three single-bonded atoms.
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6
Q

What are Constitutional isomers (structural isomers)?

A

Constitutional isomers (sometimes called “structural isomers”) differ in their constitution (connectivity)

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7
Q

What are stereoisomers?

A

stereoisomers differ in their stereochemistry* (spatial arrangement of their atoms, i.e., configuration and/or conformation).

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8
Q

2 types of stereoisomers:

A

configurational and conformational stereoisomers

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9
Q

3 types of structural isomers:

A 
chain isomers
position isomers (regioisomers)
functional group isomers
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10
Q

What are chain isomers?

A

molecules in which the carbon chain is connected in different ways.

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11
Q

What are position isomers?

A

molecules in which one (or more) functional group(s) is at a different position on the carbon chain.

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12
Q

What are functional group isomers?

A

molecules with the same molecular formula but different functional groups.

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13
Q

2 types of configurational isomers:

A

E/Z isomers

isomers with chiral centres

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14
Q

What are Conformational stereoisomers?

A

Conformational stereoisomers can be defined as a set of stereoisomers for which interconversion between the different stereoisomers only requires the rotation of covalent bonds

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15
Q

What is a conformation?

A

is any spatial (3D) arrangement of atoms in a molecule.

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16
Q

2 types of molecular projection:

A

‘zig-zag’ projections

sawhorse projections & Newman projections (used interchangeably)

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17
Q

What is a conformer?

A

A conformer is one of several conformations that corresponds to a local/global energy minimum.
(i.e., a conformation that is not at an
energy minimum is by definition not a conformer).

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18
Q

Differences in restriction for a single bonds (C-C) and double bonds (C=C)?

A

For single bonds, such as C–C bonds, there is generally no restriction to rotation because rotation does not change the orbital overlap.

rotation about the C=C bond alkenes is severely restricted because detract this requires breaking the π bond (so high temperatures are needed).

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19
Q

How to draw a sawhorse projection?

A

Sawhorse projection: the C-C bond is drawn at an oblique angle, the C nearest you is on the LHS and is drawn below the C furthest from you.

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20
Q

How to draw a Newman projection?

A

Newman projection: imagine you’re looking directly down the C-C bond, the nearest C to you is shown by a point where the 3 other groups intersect, the C furthest from you in represented by a circle with the attached groups shown bonded to the edge of the circle.

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21
Q

Differences between a staggered and eclipsed conformation?

A

Staggered conformation: H atoms on adjacent carbon atoms are as far apart as possible.

Eclipsed conformations: H atoms on adjacent carbon atoms are as close as possible

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22
Q

What is a Dihedral angle (torsional angle)?

A

the angle between the C-H bonds on the front and back carbons.

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23
Q

Dihedral angle for staggered and eclipsed conformations?

A

Staggered conformation:

smallest dihedral angle = 60°.

Angles are = 60°, 180° and 240°.

Eclipsed conformation:

Smallest dihedral angle = 0° - because the C-H bonds on each C are superimposed.

Angles are = 0°, 120° and 240°.

24
Q

Which is more stable: the staggered or eclipsed conformation and why?

A

Staggered conformations = lower energy = more stable.

25
Q

Which is less stable: the staggered or eclipsed conformation and why?

A

Eclipsed conformations = higher energy = less stable.

Electron pairs in C-H bonds are closer – repulsion between pairs raises the energy.

26
Q

What is Torsional strain (energy)?

A

Torsional strain (energy): difference between the staggered and eclipsed conformations.

27
Q

What is the Energy barrier to rotation?

A

Energy barrier to rotation: due to differences in energy between the staggered and eclipsed conformations, in order to convert one staggered conformation into another by rotation around the C-H bond, more energy is required to overcome the torsional strain.

28
Q

Between what angles is synperiplanar conformations achieved?

A

0 +-30 degrees

29
Q

Between what angles is synclinal conformations achieved?

A

+-30 to +-90

30
Q

Between what angles is anticlinal conformations achieved?

A

+-90 to +-150

31
Q

Between what angles is antiperiplanar conformations achieved?

A

180 +-30

32
Q

What is a rotamer?

A

conformers that differ by rotation about only a single bond. Both staggered and eclipsed rotamers are possible, but only the former are “conformers” (i.e., energy minima)

33
Q

What is the special term “gauche” used for?

A

The special term “gauche” is used for conformers with a 60° torsion angle.

34
Q

Are synperiplanar and anticlinal conformations staggered or eclipsed?

A

Eclipsed

35
Q

Are synclinal and antiperiplanar conformations staggered or eclipsed?

A

Staggered

36
Q

What is Steric strain (steric hinderance)?

A

the repulsive interaction between 2 atoms/groups of atoms that are closer together than their atomic radii allow

37
Q

What is Angle strain?

A

Angle strain: arises when the normal C-C-C bond angle in cycloalkanes is larger than the normal 109.5° tetrahedral bond angle, which is preferred for sp3 hybridised atoms.

38
Q

Why are Conformations of cyclopropane highly strained?

A

Highly strained because:

Torsional strain – due to eclipsed C-H bonds on adjacent carbons.

Angle strain – the C-C-C bond angle is reasonably compressed (109.5° to 60°).

39
Q

What is the name of the conformation that cyclobutane takes?

A

the “butterfly” conformation

40
Q

What does the butterfly conformation lead to?

A

The butterfly conformation leads to C-H bonds being more staggered – reducing the torsional strain.

This conformation leads to higher angle strain – the C-C-C bond angle changes from 90° to 88°.

The relief from the torsional strain compensates for the increase in angle strain.

41
Q

What is the name of the conformation that cyclopentane takes?

A

the envelope conformation.

42
Q

What does the envelope conformation lead to?

A

The envelope conformation, leads to the C-H bonds becoming nearly staggered which reduces the torsional strain.

It slightly increases the angle strain but the relief from the torsional strain compensates for this.

43
Q

What is the name of the conformation that cyclohexane takes?

A

the chair conformation.

44
Q

What does the chair conformation lead to?

A

The chair conformation:

Reduces the angle strain - the C-C-C bond angle is reduced to 111° - closer to the normal tetrahedral value.

Reduces the torsional strain - the adjacent C-H bonds become staggered

45
Q

The two types C-H bonds in the chair conformation:

A
  • axial

- equatorial

46
Q

What is an axial bond?

A

one type of C-H bond points vertically up and down on alternate carbons (when the ring is drawn out in the horizontal plane).

47
Q

What is an equatorial bond?

A

C-H bonds point outward from the ring.

48
Q

What is a ring flip?

A

The conformational interconversion of a cyclohexane chair conformer to its mirror-image form proceeds by rotations of C C bonds within the ring, and is known as a ring-flip.

49
Q

What happens during a ring flip?

A

p, all of the equatorial substituents (just hydrogens on cyclohexane itself) are converted into axial ones (and vice versa

50
Q

What is a configurational enantiomer (mirror-image stereoisomers)?

A

Configurational enantiomers (mirror-image stereoisomers) are said to differ in their absolute configuration

51
Q

What is a configurational diastereomer (non-mirror-image stereoisomers)?

A

configurational diastereomers differ in their relative configuration.

52
Q

What is Chirality?

A

the geometric property of a molecule (or rigid object) of being non-superimposable on it’s mirror image.

53
Q

What does it mean when molecules are said to be non-superimposable?

A

Molecules with chiral centres are normally non-identical with their mirror image—the two structures are said to be non-superimposable.

This means that the two structures cannot be placed in the same space in such a way that all of the atoms in both structures overlap.

54
Q

What is a configuration?

A

the 3D arrangements of atoms and substituents attached to a chiral centre.

55
Q

What is a chiral centre?

A

Chiral centre: organic compounds with 4 different substituents attached to a tetrahedral carbon atom – the tetrahedral carbon atom is known as the chiral centre

56
Q

What is a enantiomer?

A

a chiral molecule and its non-superimposable mirror image are special types of configurational isomers called enantiomers.

57
Q

What is Plane-polarized light?

A

Plane-polarized light: differs from normal light in that the EM waves vibrate in a single plane rather than in a multitude of planes.

CH10135 (7 decks)

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