Introduction to Mechanisms: Acid and Base Chemistry Flashcards

1
Q

Define a nucleophile

A

an electron rich species (with a lone pair or pi bond) that reacts by donating an electron pair to an electron-poor species

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2
Q

Define an electrophile

A

an electron poor species (polarised bond or empty orbital) that reacts by accepting an electron pair from a nucleophile

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3
Q

Is a carbonyl carbon a nucleophile or an electrophile?

A

Overall, the molecule is an electrophile.

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4
Q

Electron pairs always move from

A

a nucleophile (high electron density) to an electrophile (low electron density)

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5
Q

when a bond is broken, bonding electrons tend to move toward which atom

A

the more electronegative atom

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6
Q

define polarisability

A

the ability to shift bonding or nonbonding electrons in response to nearly nucleophile or electrophile

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7
Q

as you go down a group

A

size increases and polarisability increases (more reactive bonds)

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8
Q

as you go across a period

A

size decreases and electronegativity increases (stabilizes negative charge better)

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9
Q

why do reactions often involve polar bonds?

A

due to polarisability and differences in electronegativity

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10
Q

intermolecular reactions

A

reactions that occur between two or more molecules

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11
Q

intramolecular reactions

A

reactions that occur between two functional groups on the same molecule

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12
Q

state the equation for the equilibrium constant

A
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13
Q

K(eq) < 1

A

reactants are more favoured

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14
Q

K(eq) > 1

A

products are more favoured

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15
Q

how is the equilibrium constant K related to the Gibbs free energy change?

A

ΔG = -RTlnK(eq)

K(eq)>1 and ΔG<0: products are favoured (reaction is exergonic)
K(eq)<1 and ΔG>0: reactants are favoured (reaction is endergonic)

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16
Q

exergonic

A
  • reactions where there is a net release of free energy
  • spontaneous
  • ΔG<0
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17
Q

exothermic

A
  • reactions where there is a net release of heat
  • ΔH<0
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18
Q

when ΔH is negative:

A
  • exothermic (heat released)
  • bonds formed in product are stronger (more stable) than bonds broken in reactants
19
Q

when ΔH is positive:

A
  • endothermic (heat absorbed)
  • bonds formed in products are weaker (less stable) than bonds broken in reactants
20
Q

bond dissociation energy

A

the amount of energy required to symmetrically break a covalent bond

21
Q

define entropy (ΔS)

A

measure of freedom of movement or disorder

22
Q

what is the effect of ΔS being positive on ΔG?

A

there is more movement/disorder and ΔG becomes more negative

23
Q

ΔS<0

A

entropically unfavourable

24
Q

ΔS>0

A

entropically favourable

25
Q

transition state

A
  • highest energy structure
  • in-between reactants and products
  • from transition state, reaction can go in either direction
  • cannot be isolated or observed
26
Q

ΔG‡

A
  • activation energy
  • energy required to reach transition state
  • determines the rate of reactions (higher activation E = slower process)
27
Q

what is the rate-determining step of the reaction?

A

the slowest elementary step

28
Q

what is the effect of a catalyst?

A

increase reaction rate without changing ΔG of the overall reaction
- catalyst not consumed during the reaction
- activation energy lowered by providing a new reaction mechanism

29
Q

define a Bronsted acid and a Bronsted base

A

Bronsted acid: proton (H+) donor
Bronsted base: proton (H+) acceptor

30
Q

how can the position of an acid-base equilibrium be determined?

A

by comparing the acid strengths. strong acids dissociate more readily than weak acids, so the equilibrium will lie in the direction of the weaker acid and base

31
Q

draw two free energy graphs for strong and weak acids

32
Q

what is the ΔG for strong/weak acids?

A

strong acids have a negative ΔG
weak acids have a positive ΔG

33
Q

how does pKa relate to acid strength?

A

the lower the pKa value, the stronger the acid

34
Q

how does stability of a conjugate base relate to the strength of an acid?

A

conjugate bases that are stabilised have lower free energy than similar bases without the stabilising effect (->strong acid)

35
Q

how does electronegativity impact acid strength?

A

conjugate bases in which the atom carrying the negative charge is more electronegative are more stable (weaker base). this leads to a stronger acid.

high electronegativity -> increase ability to accomodate negative charge

36
Q

how does induction impact acid strength?

A

removal of electron density from an atom by a strongly electronegative atom nearby increases the ability to accommodate negative charge and increases stability. this leads to a stronger acid.

37
Q

how does hybridisation impact acid strength?

A
  • orbitals with higher ‘s’ character are lower in energy because s orbitals experience a greater effective nuclear charge
  • conjugate bases with unpaired electrons in orbitals with greater ‘s character’ are more stable
  • this leads to a stronger acids
38
Q

s character

A

sp = 50%
sp2 = 33%
sp3 = 25%

39
Q

how does resonance impact acid strength?

A
  • charge delocalisation increases the ability to accommodate negative charge and increases stability for the conjugate base
  • this leads to a stronger acid
40
Q

common acids and their pKa values

A

strongest acid
HCl (-7)
H3O+ (0)
carboxylic acid (5)
phenol (10)
water (16-18)
CH4 (>45)
weakest

41
Q

define a Lewis acid and base

A

Lewis acid: electron pair acceptor
Lewis base: electron pair donor

42
Q

examples of Lewis acids

A
  • tricoordinate B and Al
  • cations such as Li+, Mg2+
43
Q

examples of Lewis bases

A
  • lone-pair donors
  • benzene