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Structure and Reactivity in Organic Chemistry > Ring chemistry > Flashcards

Ring chemistry Flashcards

(77 cards)

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1
Q

What cyclic ring can you never make?

A

cyclic amine

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2
Q

What causes ring strain?

A

non-ideal molecular geometries

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3
Q

What causes torsional strain?

A

repulsion between electrons and substituents on adjacent atoms

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4
Q

What causes bond length strain?

A

bond stretching or compression

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5
Q

What ring is predicted to be most stable based on angles?

A

5-membered ring

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6
Q

What causes more influential strain, bond angle or bond length strain?

A

Bond angle deformation is less costly than changes in bond length

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7
Q

Conformation

A

the different shapes of molecules that result from the deformation of bonds (no bonds are broken or made to make conformational isomers)

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8
Q

Configuration

A

the connectivity of atoms within stereoisomers

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9
Q

Is the chair or boat conformation more strained and why?

A

Boat conformation is more strained as the C-H bonds are eclipsed whereas in the chair conformation they are staggered so there’s almost no strain

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10
Q

What ring size constitutes a small, normal, medium, large ring?

A

3-4: small
5-7: normal
8-11: medium
12+: large

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11
Q

Why are medium rings less stable than cyclohexane?

A

due to bond angle strain, torsional strain and transannular clashing due to the more atoms present

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12
Q

If rings are large enough what characteristic can they adopt?

A

the space in the middle of macrocycles (its cavity) becomes accessible to guests

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13
Q

What does low temperature 1H NMR measure?

A

measure the relative concentrations of the conformers spectroscopically

other spectroscopic techniques such as IR can also be used

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14
Q

A values

A

the free-energy differences between two chair conformations

(the larger the value, the more likely it will lie equitorial)

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15
Q

What group always sits equatorially?

A

tBu will always sit equatorially because it is so bulky - it is a conformational lock

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16
Q

Where does a carbonyl sit in a ring?

A

between axial and equatorial position

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17
Q

General guidelines for predicting lowest energy conformations

A
  • the conformation with fewer axial substituents is lower in energy
  • if there are equal numbers of axial and equatorial in the two conformations, refer to the A values
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18
Q

What position does polar (H-bond accepting) solvents favour?

A

diequatorial conformer

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19
Q

What position does apolar solvents favour?

A

diaxial conformer

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20
Q

What position does high dielectric solvents favour?

A

diequatorial conformer

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21
Q

What position does low dielectric solvents favour?

A

diaxial conformer

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22
Q

Spiro compounds

A

contain two rings linked through a single atom (spiroatom)

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23
Q

Decalins

A

molecules containing two fused cyclohexane rings

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24
Q

What are stereoelectronic effects?

A

they are the chemical consequences (structure, reactivity and properties) of the relative arrangements of orbitals in space

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25
What is a stereoelectronic effect in E2 eliminations?
the proton and the leaving group must be anti-periplanar (preferred) or syn-periplanar for there to be good orbital overlap
26
What is a stereoelectronic effect in nucleophilic addition to a carbonyl?
the trajectory of nucleophilic attack at a carbonyl - 107º
27
Baldwin's rules
- All endo-tet are disfavoured - All exo-tet are favoured - All exo-trig are favoured - All endo-dig are favoured - (3,4,5) endo-trig are disfavoured, (6,7) endo-trig are favoured - (3,4) exo-dig are disfavoured, (5,6,7) exo-dig are favoured
28
When is Baldwin's rules usually disobeyed?
reactions involving second row atoms (e.g. sulfur) often do not follow the rules due to larger variations in their bond lengths and the presence of accessible d orbitals
28
Energetics of different size ring formation
Making a LARGE ring: - high entropic cost due to having to organise many atoms to get reaction - enthalpically okay Making a SMALL ring: - entropically okay (less atoms to organise) - enthalpically costly due to the product formed having very high ring strain
28
What sort of reactions are thermodynamically controlled and which are kinetically controlled?
- irreversible reactions are kinetically-controlled - reversible reactions are thermodynamically controlled
29
General trend for ring closure of saturated rings
Fastest: Slowest: 5 > 6 > 3 > 7 > 4 > 8-11
30
2 most common ring-closing reactions
1) Ionic reactions 2) Pericyclic reactions
31
What sort of reactions do small ring synthesis favour?
irreversible, kinetically-controlled reactions
32
What is a [2+1] cycloaddition?
addition of singlet carbenes to olefins to form cyclopropanes
33
What reaction is the addition of triplet carbenes?
a radical reaction
34
What is a [2+2] cycloaddition?
Cyclisation of two olefins to form cyclobutanes - photochemical excitation is necessary to achieve correct orbital symmetry
35
What are thermal [2+2] cyclisations?
only occur for substrates with two double bonds at the same carbon atom (e.g. ketenes and isocyanates)
36
What are lactams?
cyclic amides
37
What do ketenes and isocyanates react with to form β-lactams?
Ketenes (CCO) react with imines (CN) Isocyanates (OCN) react with alkenes (CC)
38
What molecule does a β-lactam ring form a key structural feature of?
the penicillin class of antibiotics
39
How does a β-lactam in penicillin work as an antibiotic?
During bacterial cell wall synthesis, an enzyme catalyses peptide bond formation between a Gly and a D-Ala residue. The β-lactam 'imitates' the D-Ala and forms a covalent bond to the active site, irreversible inhibiting the enzyme.
40
Two examples of ring contractions?
- Favorskii - Wolff
41
What sort of reaction do normal ring formations occur under?
they are generally favourable so can occur under kinetic or thermodynamic control
42
Nazarov cyclisations
used to form cyclopentanones from divinyl ketones
43
What is the most important way to form a 6-membered ring?
through Diels-Alder cycloaddition [4+2] cycloaddition
44
Reduction of aromatics
- Birch reduction - Catalytic hydrogenation using Pd/ C and H 2 under harsh conditions
45
What are the four strategies to make larger rings?
- high dilution - templation - ring expansion (including fragmentations) - phase separation
46
What impacts the rate of intermolecular reaction?
The rate of intermolecular reaction is concentration dependent, whereas the rate of the intramolecular reaction is not - thus, lower concentration minimises intermolecular reaction
47
What is required for a cyclisation with a low effective molarity?
high dilution conditions in order to favour the cyclic product
48
Pros and cons for high dilution method of forming large rings
Pro: widely applicable - no specific structural features are required Cons: - very large quantities of solvents are required, which can be costly and limits the scalability of reactions - to counteract slow reaction rates, high temperatures and/ or long reaction times are required - not ideal and the reactive functional groups must be robust in order to survive the conditions
49
Olefin Metathesis
allows the exchange of substituents between different olefins - a transalkylidenation
50
Conditions for ring-closing olefin metathesis
the reaction conditions are mild, the reactive functional groups are robust and the catalysts are efficient - this reaction performs well at high dilution
51
Suitable catalysts for ring-closing olefin metathesis
carbene complexes of Ru, W, Mo and other transition metals
52
Do ring-closing olefin metathesis result in a mixture of diastereoisomers?
yes - often a mixture of E and Z diastereoisomers is formed - the E isomer is usually more stable for large rings so its frequently the major product
53
What type of dimerisation is the acyloin reaction?
a radical-radical dimerisation
54
What happens in the acyloin reaction?
an ester is reduced using sodium metal, resulting in a 1,2-diketone after dimerisation and then eventually a ɑ-hydroxyl-ketone after further reduction
55
What reaction is best for making medium and large carbocyclic rings?
the acyloin reaction
56
What is the McMurry reaction?
a radical-radical dimerisation of ketones, promoted by titanium (0), which is formed in situ
57
What do chemical templates do?
they influence the geometry of a substrate, usually through noncovalent bonding interactions such as: - hydrogen bonding - coordination to a metal centre - donor- acceptor interactions
58
What aids the formation of crown ether macrocycles?
it's templated by alkali metal cations (the O lone pairs interact with the positively charged cation, causing the intermediate species to 'wrap around' the metal and bringing the end groups close together in space)
59
Are SN2 reactions faster when nucleophile attacks axially or equatorially?
attack at axial leaving groups are faster - the approach of the nucleophile along the direction of the C-X σ* orbital is less hindered for axial substituents
60
What is the preference for epoxide opening?
attack at a position that results directly in chair formation rather than a twist boat, regardless if it ends up a chair in the end
61
Reduction of cyclohexanones: which reducing agents undergo equatorial attack vs axial attack?
bulky reducing agents undergo equatorial attack whereas small reducing agents attack in an axial manner
62
Why do large nucleophiles not attack axially?
the axial attack of large nucleophiles would suffer from unfavourable steric interactions with other axial groups as the nucleophile approaches the face of the ring
63
What is the Thorpe-Ingold Effect?
adding substituents makes the cyclisation reactions go faster
64
Thorpe-Ingold Effect: enthalpy
Bond angle compression - in the formation of small rings, starting from a smaller bond angle is advantageous because then there is less of a change necessary to form the small ring and therefore less strain is introduced
65
Thorpe-Ingold Effect: entropy
the substituents reduce the number of conformers that are accessible to the acyclic precursor which means that it suffers less of an entropic penalty to form the transition state
66
Geminal relationship
a cation stabilising group attached to the same C atom as a leaving group stabilises the cationic intermediate of an SN1 reaction
67
Vicinal relationship
a functional group at the carbon next to the reaction centre (neighbouring group)
68
What is a fragmentation reaction?
involves the cleavage of a C-C bond, causing tje molecule to break open
69
Common characteristic of spirocyclic systems
often chiral
70
Synthesis of Nornornanes: alkyne as the dienophile
the product contains 2 double bonds
71
How are carbocations stabilised?
by the electrons in neighbouring σ orbitals
72
Why does the bridgehead position not allow for carbocations?
Would need for the σ orbitals to overlap with the vacant p orbital thus the carbocation must adopt a trigonal planar geometry. However the rigid structure of the bridgehead position would result in a massively distortion to form the planar carbocation
73
What chemistry can happen at bridgehead positions?
- no Sn1 or Sn2 - elimination reactions don't tend to happen they are generally very unreactive positions and chemistry only occurs if it involves an anionic or radical species at the bridgehead, rather than a cation or double bond
74
The Cope Rearrangement
a pericyclic [3+3]- sigmatropic rearrangement in which electrons in p orbitals shuffle around a 6-membered ring transition state
75
Bredt's rule
it's 'impossible' for there to be planar bridgehead carbons therefore double bonds can almost never be formed to bridgehead carbons in bicyclic systems

Structure and Reactivity in Organic Chemistry (2 decks)

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