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CHEM2010 - Synthesis and Spectroscopy > Topic 4 - Stereochemistry and Conformations > Flashcards

Topic 4 - Stereochemistry and Conformations Flashcards

1
Q

What is a conformation?

A

The different spatial arrangements that a molecule can adopt due to rotation about single bonds.

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2
Q

What is a conformer?

A

Structures that differ only by virtue of rotations around single bonds.

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3
Q

What is conformational analysis?

A

The study of the energy changes that occur during rotations around single bonds.

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4
Q

What do conformations have consequences on?

A

Reactivity.

Stereoselectivity.

J-coupling values.

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5
Q

What are enantiomers?

A

Molecules that are non-superimposable mirror images of each other.

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6
Q

What are diastereoisomers?

A

Stereoisomers that are not non-superimposable mirror images of each other.

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7
Q

What does staggered mean?

A

Substituents on either side of the bond are in between each other.

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8
Q

What does eclipsed mean?

A

Substituents on either side of the bond are in line with each other.

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9
Q

What does antiperiplanar mean?

A

Atoms are 180° relative to each other.

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10
Q

What does anticlinal mean?

A

Atoms are 120° relative to each other.

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11
Q

What does synclinal mean?

A

Atoms are 60° relative to each other.

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12
Q

What does synperiplanar mean?

A

Atoms are 0° relative to each other.

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13
Q

What four factors are contributions to conformational energy?

A

Hyperconjugation

Pitzer (torsional) strain

Sterics

Hydrogen bonding

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14
Q

What is hyperconjugation?

A

σ-σ* orbital overlap stabilises staggered conformations.

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15
Q

What is Pitzer (torsional) strain?

A

Electrostatic repulsion between electrons in filled orbitals destabilises eclipsed conformations.

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16
Q

What are sterics?

A

Repulsion between substituents destabilises eclipsed conformations.

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17
Q

What is hydrogen bonding?

A

An intermolecular force that stabilises synclinal and synperiplanar conformations.

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18
Q

What is an A-value?

A

The energy difference (in kcal/mol) between the lowest energy conformation and the highest energy conformation of a mono-substituted cyclohexane.

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19
Q

How is an A value linked to an equilibrium constant?

A

A = ΔG = -RTln(K)

K = [Ax] / [Eq] = e-A/RT

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20
Q

How does the size of substituents affect the A value?

A

A larger substituent will have a larger A value.

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21
Q

What is the size of a small saturated ring?

A

3-4 atoms.

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22
Q

What is the size of a normal saturated ring?

A

5-7 atoms.

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23
Q

What is the size of a medium saturated ring?

A

8-14 atoms.

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24
Q

What is the size of a large saturated ring?

A

> 14 atoms.

25
Q

What is ring strain energy?

A

The extra energy a molecule has by virtue of being constrained to a ring.

26
Q

What are extra contributions to conformational energy in ring systems?

A

Baeyer (angle) strain.

Transannular strain.

27
Q

What is Baeyer (angle) strain?

A

Atoms are distorted from their ideal bond angles in order to form a ring.

28
Q

What is transannular strain?

A

Substituents point at each other across the ring leading to additional steric strain.

This only affects medium sized rings (8-14 atoms).

29
Q

How do rings adopt their conformation?

A

They will use the lowest energy conformation. This is usually found by limiting Baeyer (angle) and Pitzer (torsional) strain.

30
Q

What is the structure of cyclopropane?

A

All carbon atoms in one plane and they are eclipsed. This causes high Pitzer strain.

The bond angles are much lower than their ideal values so there is high Baeyer strain. This causes the atoms to rehybridise and the C-C bonds distort (banana bonds).

Carbon-carbon bonds are sp3.7 hybridised and are long and weak bonds. Carbon-hydrogen bonds are sp2.5 hybridised and are strong and short.

31
Q

What is the structure of cyclobutane?

A

Cyclobutane rings pucker (one corner lifts up) to reduce the overall energy.

This is because no bonds are fully eclipsed which causes reduced (but still high) Pitzer strain.

Bond angles are decreased by puckering which increases Baeyer strain.

32
Q

What is the structure of cyclopentane?

A

Cyclopentane adopts an envelope conformation (one corner lifted up). Their is some Pitzer strain and some Baeyer strain.

33
Q

What is the structure of cyclohexane?

A

Cyclohexane adopts the chair conformation. All bonds are staggered and the molecule is essentially strain free.

34
Q

What is the structure of cycloheptane?

A

There is some Pitzer strain and some Baeyer strain.

35
Q

What conformation do substituted cyclohexanes adopt?

A

The chair conformation with the largest group in the equatorial position to minimise 1,3-diaxial interactions.

36
Q

What is meant by ‘conformational locking’?

A

Cyclohexanes that cannot flip from one chair to another.

37
Q

How can 3J values be related to conformation?

A

The Karplus curve can be used.

Dihedral angles of around 60° give a coupling constant of 2-5 Hz whereas dihedral angles of around 180° give a coupling constant of 10-13 Hz.

38
Q

What is the anomeric effect?

A

Electronegative groups at the anomeric position adopt an axial position, despite destabilising 1,3-diaxial interactions, due to strong hyperconjugation.

This effect can be observed for OR, Cl, Br, F etc. at the anomeric position (adjacent to a heteroatom in the ring).

This effect is reduced in other ring sizes due to poor orbital overlap, so sterics dominate.

39
Q

What is irreversible cyclisation?

A

A method that can be used to form small and normal rings (3-7 MRs) from molecules that contain both an electrophile and a nucleophile.

It attacks the electrophilic centre in an SN2 style reaction and stereochemistry undergoes inversion.

The nucleophile and leaving group must be anti-periplanar to each other. This is because the nucleophile must be able to reach the C-LG σ* orbital.

40
Q

How does the rate of ring closure depend on the size of the ring in irreversible cyclisation?

A

3 > 5 > 6&raquo_space; 4.

The entropic cost of ring formation increases with ring size as more order must be imposed for the ends of the chain to be oriented correctly.

The enthalpic cost of ring formation decreases with ring size because normal rings are less strained than small rings.

41
Q

What is reversible cyclisation?

A

A way of forming normal rings (5-7 MRs) whose starting material contains both an electrophile and a nucleophile.

42
Q

What are two methods of synthesising cyclopropanes?

A

Irreversible cyclisation.

Addition of carbenes to alkenes.

43
Q

What are carbenes?

A

Carbenes contain a carbon atom with a lone pair. This makes them extremely reactive and so they must be generated in situ.

They can have a triplet or singlet ground state depending on their substituents.

44
Q

What is a singlet carbene?

A

A carbene where the lone pair of electrons is in the sp2 orbital and the p orbital is empty.

It is favoured if the substituents are halogens.

45
Q

What is a triplet carbene?

A

A carbene where there is one electron in the sp2 orbital and one electron in the p orbital.

It is favoured for alkyl substituents.

46
Q

How can singlet carbenes be synthesised?

A

They can be generated by α-elimination.

47
Q

How do singlet carbenes react with alkenes?

A

They react in a concerted fashion so the alkene geometry is maintained.

As they are electrophilic, they will react preferentially with electron-rich alkenes.

48
Q

How can triplet carbenes be synthesised?

A

Via the decomposition of diazo compounds.

49
Q

How do triplet carbenes react with alkenes?

A

In a stepwise fashion which means alkene geometry may not be maintained in the product.

50
Q

What are carbenoids?

A

Metal bound species that exhibit carbene-like reactivity. They are more stable and often more selective than free carbenes.

51
Q

What is a reaction that uses carbenoids to produce a cyclopropane ring?

A

Simmons-Smith cyclopropanation.

It uses a concerted syn addition, so alkene geometry is conserved in the product.

52
Q

What is an alternative method for synthesising epoxides?

A

The Weitz-Scheffer reaction.

This uses hydrogen peroxide and sodium hydroxide or aqueous sodium hypochlorite.

As a nucleophilic oxidant is used, so an electron deficient alkene is required. Electron neutral or electron-rich alkenes are unreactive.

53
Q

What is the another method of synthesising epoxides?

A

Using an alkene and a peracid.

Peracids, like mCPBA, are electrophilic oxidants, and so only react with electron-neutral or electron-rich alkenes.

As the reaction mechanism is concerted, alkene geometry is maintained.

54
Q

How do epoxides react?

A

The chemistry of epoxides is dominated by nucleophilic ring opening. This is because epoxides have a high ring strain, the C-O bonds are polarized due to electronegativity differences and the C-O bonds are weak due to their high p-orbital character.

55
Q

How do epoxides react under basic conditions?

A

It proceeds via an SN2 mechanism. It will attack the least hindered site and generally requires a relatively reactive nucleophile.

56
Q

How do epoxides react under acidic conditions?

A

It proceeds via an SN2 mechanism with SN1-like character. It proceeds via attack at the site better able to support developing positive charge. This does not require a reactive nucleophile.

57
Q

How can the regioselectivity be controlled for epoxides?

A

The choice of acidic or basic conditions can control regioselectivity.

58
Q

How can the stereospecificity be controlled for epoxides?

A

As epoxide opening follows an SN2-type mechanism, stereochemistry is inverted at the reacting centre.

59
Q
A

CHEM2010 - Synthesis and Spectroscopy (4 decks)

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