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Fundamnetals of Chemistry for Biochemsists CHE00031C > Organic Structures and Mechanisms 1 > Flashcards

Organic Structures and Mechanisms 1 Flashcards

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1
Q

Define isomer

A

Compounds with the same molecular formula but a different constitution, configuration and confirmation

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2
Q

Define structural/constitutional isomerism

A

Compounds different in nature/sequence of atoms in a molecule (connectivity)

e.g. C4H8O can be in lots of arrangements but as long as all are C4H8O they are constitutional isomers

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3
Q

Define stereoisomerism

A

Two or more compounds whose structure differ only in the 3D arrangement of their constituents (same constitution)

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4
Q

Define conformational isomerism (conformers)

A

Different spatial arrangement of a molecule due to rotation about a single bond (as opposed to a double bond)

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5
Q

Generally what is the more stable configuration of a conformer and why?

A

Staggered, they are 12KJMol^-1 more stable (lower torsional strain) than eclipsed in ethane. Therefore, at any given moment most molecules are staggered

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6
Q

What is torsional strain?

A

The resistance of a bond twisting in a chemical compound, highest (generally) in eclipsed, so staggered is favoured

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7
Q

What difference in eclipsed to staggered torsional strain will cause a slow spin?

A

Anything greater than 80KJMol^-1 will cause a slow spin of the C atoms around the single bond

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8
Q

What causes torsional strain?

A

Electrostatic repulsion between electron clouds

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9
Q

Define antiperiplanar

A

Torsion angle about C-C bond is 180 degrees (staggered)

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10
Q

Define synclinal

A

Torsion angle about C-C bond is 60 degrees (staggered)

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11
Q

Define anticlinal

A

Torsion angle about C-C bond is 120 degrees (eclipsed)

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12
Q

Define synperiplanar

A

Torsion about C-C bond is 0 degrees (eclipsed)

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13
Q

Define steric hinderance

A

The increase in energy as atoms are crowded together

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14
Q

What may cause a molecule to favour synclinal over antiperiplanar?

A

Internal hydrogen bonding will pull the groups towards eachother

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15
Q

What is an sp2 carbon atom?

A

A carbon atom that is bonded to three atoms (2 single bonds and one double bond)

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16
Q

Why can’t C=C double bonds rotate?

A

Rotation will break the overlap of the pi-orbital (overlap of the two p-orbitals, allowed by the planar nature of the C=C)

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17
Q

What is E/Z isomerism?

A

Isomerism found around a C=C bond due to each C atom having 2 different groups attached.

E - Groups are on opposite sides of the C=C double bond to each other
Z - Group are on the same side of the C=C double bond to each other

18
Q

What are the CIP priority sequencing rules?

A
  • Higher atomic number = higher priority
  • Higher oxidation = higher priority
19
Q

What is an sp3 carbon atom?

A

An sp3 carbon atom is a saturated carbon atom (bonded to 4 atoms)

20
Q

What is a chiral carbon?

A

An sp3 carbon with 4 different groups attached

20
Q

Define enantiomer/optical isomer

A

An isomer around a chiral carbon that cannot be superimposed onto its other isomers

21
Q

What is a configurational isomer?

A

The spatial array of atoms that distinguishes a stereo-isomer other than distinctions due to differences in conformation

22
Q

R vs S stereo-isomers

A

R - clockwise
S - anticlockwise

23
Q

How do you determine enantiomeric purity?

A

1) Put sample in polarimeter and irradiate with Na-“D-line”
2) Measure specific rotation @ 25 degrees

Specific rotation = observed angle of rotation (degrees) / path length (dm) x density (gcm-3)

24
Q

What specific rotation value would you get for a racemic mixture?

A

0

25
Q

If a chiral compound is formed from an achiral starting mixture, what ratio of enantiomers would we get?

A

50:50

26
Q

How many stereoisomers would you get for a molecule with 2 chiral centres?

A

4

27
Q

What is a diastereomer?

A

Stereoisomers that are not entantiomers

28
Q

If you attempted to reflect a molecule with two identical chiral carbons to form its other entantiomer what would you actually get?

A

A meso-isomer (no optical rotation)

29
Q

Are meso-isomers also diastereoisomers?

A

Yes

30
Q

Define Tautomers

A

Tautomers are isomers of a molecule that exist in solution or in a cell. They are interchangeable forms because chemical bonds are rearranged many times spontaneously

They are constitutional isomers

31
Q

Define Atropisomer

A

stereochemistry arising from restricted bond rotation that creates a chiral axis

32
Q

Features of cyclopropane

A
  • Smallest cyclic compound
  • Planar
  • Can’t adopt non-planar conformations
  • Bond angle: 60 degrees
33
Q

Features of cyclobutane

A
  • Can adopt a non-planar conformation
  • bond angle: 90 degrees
  • Favours non-planar conformation due to decreased eclipsing
34
Q

Features of cyclopentane

A
  • Can adopt a non-planar conformation
  • Bond angle of 108 degrees
  • No angle strain in planar confirmation but high torsional strain
35
Q

Features of cyclohexane

A
  • Can be planar
  • 2 major non-planar conformations (Boat and chair)
36
Q

Features of Boat confirmation of cyclohexane

A
  • non-planar
  • extensive eclipsing
  • high torsional strain
37
Q

Features of chair conformation of cyclohexane

A
  • non-planar
  • Low torsional strain
  • Low eclipsing
38
Q

What interactions occur in a monosubstituted cyclohexane molecule

A
  • 1,3 diaxil interactions
    • steric hindrance
39
Q

Define racemate

A

a mixture of equal quantities of two enantiomers

40
Q

Define axial

A

a bond which is parallel to the axis of the ring, or a group attached by such a bond

41
Q

Define equatorial

A

a bond which is perpendicular to the axis of the ring (i.e., the bond lies along the equator of the chair), or a group attached by such a bond

Fundamnetals of Chemistry for Biochemsists CHE00031C (5 decks)

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