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Chemistry 101 COPY > Resonance > Flashcards

Resonance Flashcards

1
Q

Which is longer: single bonds, or double bonds?

A

Single bonds.

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2
Q

In acetate (ethanoic acid without the hydrogen), why are the single bonds and double bonds the same length?

A

Due to resonance.

The oxygen atoms/ions share the pi electrons and the negative charge equally (the pie electrons are delocalised)

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3
Q

What is the alternating double bonds in aromatic rings called?

A

Conjugation.

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4
Q

What do partial bonds indicate?

A

The sharing of the pi electrons (the delocalisation of electrons)

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5
Q

Does every bond in benzene have the same length?

A

Yes.

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6
Q

What is the main requirement for resonance?

A

To have adjacent pi-bonds or p-orbitals

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7
Q

What are localised electrons?

A

Electrons that are restricted to a particular region

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8
Q

What type of bond do localised electrons form?

A

Sigma bonds.

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9
Q

What are electrons that are not confined to a single atom or bond called?

A

Delocalised electrons

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10
Q

What must you keep in mind when determining resonance structures?

A

Resonance forms must be valid Lewis Structures. The octet rule applies to resonance.

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11
Q

Are resonance arrows the same as equilibrium arrows?

A

No. Resonance arrows are simply double-headed arrows.

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12
Q

If a carbon only has single bonds around it, what is it’s hybridisation?

A

sp3

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13
Q

If an atom is invovled in one double bond, what is its hybridisation?

A

sp2

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14
Q

If an atom has a negative charge, what is its hybridisation?

A

sp2

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15
Q

Can electrons move about a single bond?

A

No.

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16
Q

What do resonance forms differ in?

A

Resonance forms differ only in the placement of pi electrons (p orbitals associated with sp2 atoms) and non-bonding electrons. The atoms do not move, only the double bonds and lone pairs.

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17
Q

Are electrons in a double bond always delocalised?

A

No. If they are adjacent to an atom which has no p orbitals, they will be localised.

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18
Q

What does resonance lead to?

A

Resonance leads to stability.

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19
Q

When are groups said to be acidic?

A

Groups are acidic if they can stabilise a negative charge.

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20
Q

When are groups said to be basic?

A

Groups are basic if they prefer to keep or accept a proton.

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21
Q

What is basicity and acidity predicted by?

A

Induction and resonance.

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22
Q

Which is more stable: carboxylic acid or carboxylate?

A

Carboxylate. Thus, it is more likely to exist.

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23
Q

Why are carboxylic acids more acidic than alcohols?

A

Carboxylic acids are more acidic compared to alcohols because their conjugate base is stabilised by resonance.

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24
Q

How do resonance structures relate to stability?

A

The more resonance structures, the more stable a compound is.

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25
Q

What type of reaction are acid/base reactions?

A

All acid/base reactions are equilibrium reactions

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26
Q

What is a result of all acid/base reactions being equilibrium reactions?

A

They all have an equilibrium constant for acids; K(a)

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27
Q

What does the equilibrium constant K(a) tell us?

A

Which side of the reaction is favoured

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28
Q

Does it mean if a K(a) value is small?

A

The reactants are favoured

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29
Q

Does it mean if a K(a) value is large?

A

The products are favoured

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30
Q

What is a more appropriate value to use, rather than K(a)?

A

pK(a), where pK(a)=-logK(a)

This is because the K(a) value can be very big.

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31
Q

Does it mean if a pK(a) value is small?

A

The compound is more acidic

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32
Q

Does it mean if a pK(a) value is large?

A

The compound is more basic

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33
Q

Is pK(a) the same as pH?

A

No.

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34
Q

What is pK(a) for and what is pH for?

A

pK(a) is about the molecule

pH is the solution

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35
Q

Is pK(a) variable?

A

No. pK(a) is constant, and each molecule has its own particular pK(a) that never changes.

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36
Q

What is pK(a)?

A

The acid dissociation constant. It represents the equilibrium which generates H+

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37
Q

What is the formula for the equilibrium of an acid?

A

HA <=> A- + H+

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38
Q

What is the formula for the equilibrium of a base?

A

HB+ <=> B + H+

39
Q

If you wanted to measure the equilibrium for the proton being accepted. what value would you be measuring?

A

pK(b), the base dissociation constant. However, to avoid confusion, we only really ever discuss the pK(a)

40
Q

When are carboxylic acids acidic in water?

A

If a proton is lost from carboxylic acid, the negative charge is resonance stabilised, which makes proton loss (ionisation) more likely, hence it acts as an acid in water.

41
Q

What can be delocalised across several sites?

A

A charge (positive or negative), or a double bond can be delocalised over several sites.

42
Q

What is a favourable outcome to have when you have a charged molecule?

A

If you can spread out charge (or double-bonds, which are a source of electron density), that is a favourable outcome.

43
Q

Describe metallic bonding.

A

In metallic bonding, the electrons don’t belong to either metal nuclei, rather they are spread out across all the nuclei.

44
Q

How does resonance relate to metallic bonding?

A

Resonance does not approximate metal-metal bonding, but the concept is the same. The charge is spread out.

45
Q

What effect does resonance have when it operates?

A

A stabilising effect, thus it is favourable.

46
Q

Which is usually the stronger effect, resonance, or inductive effects?

A

Resonance.

47
Q

What is the word ‘conjugation’ used to describe?

A

The word “conjugation” it is used to describe the situation that occurs when π systems (e.g. double bonds) are linked together (i.e. only separated by one bond)

48
Q

What do resonance structures account for?

A

Resonance structures account for the octet on an atom.

49
Q

If an electron is electron-deficient, what delta charge will it have?

A

A delta-positive charge.

50
Q

What factor has a major effect on inductive effects?

A

Distance.

51
Q

How will carbon interact with an electron-deficient atom?

A

A carbon can fill the electron-deficiency of an adjacent atom, but not on one further away.

52
Q

Where are localised electrons found?

A

Localised electrons are in a bond or a lone pair. They aren’t available for sharing.

53
Q

Do bonding electrons participate in a shared system?

A

Bonding electrons do not participate in a shared system. They are located between the two nuclei forming a bond

54
Q

Are there any p orbitals in an sp3 hybridised atom?

A

If an atom is sp3 hybridised, it doesn’t have a p orbital, and thus the electrons can’t delocalise.

55
Q

What is another word for delocalised electrons?

A

Conjugated.

56
Q

What is the exception to the rule that “all resonance forms must be valid Lewis Structures”?

A

Cationic species

57
Q

Describe the mechanism of resonance.

A

Resonance is always a hybrid of different configurations. No matter how much you slow the reaction down, there will always be a hybrid. You can’t isolate one structure.

58
Q

If a carbon has 4 single bonds, what is its hybridisation?

A

sp3.

59
Q

How does a double bond come about (spatially)?

A

A double bond comes about spatially due to the overlap of p orbitals.

60
Q

Do sp2 hybridised atoms have p orbitals?

A

An sp2 atom has a p orbital. This gives a possibility of conjugation.

61
Q

Which atom involved in a double bond will have a p orbital?

A

Both atoms involved in the double bond will have a p orbital.

62
Q

Does each atom have its own corresponding hybridisation?

A

An atom will not always be one hybridisation or another. It depends on what is required to achieve a stable system in the molecule it is present in.

63
Q

What will occur if an atom has a p orbital?

A

If an atom has a p orbital, this will allow it to form side-on overlap to let it take part in a conjugated system. This will be a stable outcome (if it is needed)

64
Q

What hybridisation will have p orbitals?

A

Only sp and sp2 hybridised atoms will have p orbitals

65
Q

How do you represent a single charge shared across two atoms/ions?

A

When showing a charge shared across two atoms, make it a delta charge, so that this represents that there is only one charge and it is shared, rather than two individual charges

66
Q

What are pi electrons?

A

Pi electrons are double bonded electrons or the electrons involved in p orbitals.
This is usually associated with sp2 atoms, but it can just be sp atoms.

67
Q

Other than double and triple bonds, what may give a p orbital?

A

Lone pair electrons can result in a p orbital as well.

68
Q

Can sp3 hybridised carbons participate in the conjugation with an adjacent double bond?

A

sp3 hybridised carbons cannot participate in conjugation with an adjacent double bond

69
Q

What is an atom with localised electrons said to be?

A

An atom with localised electrons is said to be “isolated”

70
Q

What is it important to remember when dealing with double bonds?

A

Remember double-bonds can be conjugated, so if you have double/single/double in a skeletal structure, in truth it will be partial double/partial double/partial double

71
Q

What do sp3 hybridisations do in terms of conjugation?

A

sp3 hybridised atoms “block” conjugation between its adjacent atoms

72
Q

Can carbons in a chain with only sp3 hybridised atoms conjugate?

A

Carbons in a chain with only sp3 hybridised atoms cannot possibly conjugate.

73
Q

If you have a compound composed only of one type of atom, with a double bond present, what will the number of resonance structures be?

A

If you have a compound composed only of one type of atom, with a double bond present, the number of resonance structures will be equal to the number of the connections present

74
Q

What is bond order?

A

The “bond order” refers to what number of bonds the molecule will act like in terms of features of the molecule (e.g. if something will act like one and a half bonds, it’s bond order will be 1.5)

75
Q

How do you determine bond order?

A

If every atom is single-bonded, it will have a bond order of at least one
You then add the share each atom has in the double bond e.g. one double bond amongst three atoms = 1/3

76
Q

What does a carboxylic acid do to act as an acid?

A

Donate its proton

77
Q

What does Le Chatelier’s principle state?

A

Le Chatelier’s principle states that a reaction at equilibrium will want to maintain the equilibrium.

78
Q

What will happen to the conjugate pair of an anion if you stabilise it?

A

If you stabilise an anion, this will make the acid in the conjugate pair stronger (More likely to deliver an atom to the base)

79
Q

If you have alcohols that deprotonate, what do you have?

A

If you have alcohols that deprotonate, you have oxygen-centred anions.

80
Q

What does equilibrium tend to favour?

A

Equilibrium wants to favour stability.

81
Q

Do you need to deprotonate a carboxylic acid for resonance to operate?

A

You don’t need to deprotonate a carboxylic acid for resonance to operate because the lone pair on the single-bonded oxygen can conjugate and form a double bond with an adjacent carbon.

82
Q

When do you lose a proton?

A

You lose a proton when the bond between oxygen and hydrogen becomes very, very polarised.

83
Q

What would a delta-positive oxygen like to do?

A

A delta-positive oxygen would like to liberate its hydrogen, so it can get its electron back, because it is so electronegative. This is why carboxylic acid is acidic.

84
Q

What side of the equilibrium is favoured for carboxylic acids?

A

For carboxylic acid, equilibrium heavily favours the stable anion (deprotonated carboxylate ion)

85
Q

What side of the equilibrium is favoured for alcohols?

A

For alcohol, equilibrium heavily favours the original stable alcohol

86
Q

Is carboxylic acid a strong acid?

A

No. Carboxylic acid is a weak acid but it is still stronger than alcohol

87
Q

What is the ratio of the ionised form against the unionised form?

A

The equilibrium constant.

88
Q

How does the number of resonance structures a compound has relate to its stability?

A

The more resonance structures, the more stable a compound is.

89
Q

What happens if you conjugate an ionic compound?

A

If you have a charged atom which cannot conjugate, it will not be stable and hence equilibrium will favour the other side.

90
Q

What occurs when the lone pair in the single-bonded oxygen of a carboxylic acid conjugates with a double bond on an adjacent carbon?

A

The oxygen then becomes cationic because you are using the lone pair.

91
Q

What is the result of an atom with a lone pair using that pair to bond and hence becoming cationic?

A

This then pushes the electrons away to the formerly double-bonded oxygen, giving that oxygen a delta negative charge, and giving the OH oxygen a delta positive charge.

92
Q

Will oxygen-centred cations participate in conjugation?

A

They won’t participate in conjugation as there is no adjacent molecule to conjugate with. This means it is not as stable, so the equilibrium will favour the opposite side.

93
Q

What happens if you conjugate a molecule (not an ionic compound?

A

If you conjugate a molecule, it will be more stable and hence equilibrium will favour its own side.

94
Q

Why aren’t Alcohols acidic in water if they have an OH group?

A

Alcohols do not have any contributing resonance structures when ionised. It only has induction from the adjoining alkyl groups, giving it more electron density, meaning that if the proton is lost, the resultant negative charge is very, very negative and not stable. Thus, it prefers to receive its own proton back and reverse the reaction. It is an extremely weak acid.

Chemistry 101 COPY (32 decks)

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