Introduction to Mechanisms: Acid and Base Chemistry

Question 1

Define a nucleophile

Answer 1

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

Question 2

Define an electrophile

Answer 2

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

Question 3

Is a carbonyl carbon a nucleophile or an electrophile?

Answer 3

Overall, the molecule is an electrophile.

Question 4

Electron pairs always move from

Answer 4

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

Question 5

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

Answer 5

the more electronegative atom

Question 6

define polarisability

Answer 6

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

Question 7

as you go down a group

Answer 7

size increases and polarisability increases (more reactive bonds)

Question 8

as you go across a period

Answer 8

size decreases and electronegativity increases (stabilizes negative charge better)

Question 9

why do reactions often involve polar bonds?

Answer 9

due to polarisability and differences in electronegativity

Question 10

intermolecular reactions

Answer 10

reactions that occur between two or more molecules

Question 11

intramolecular reactions

Answer 11

reactions that occur between two functional groups on the same molecule

Question 12

state the equation for the equilibrium constant

Question 13

K(eq) < 1

Answer 13

reactants are more favoured

Question 14

K(eq) > 1

Answer 14

products are more favoured

Question 15

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

Answer 15

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)

Question 16

exergonic

Answer 16

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

Question 17

exothermic

Answer 17

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

Question 18

when ΔH is negative:

Answer 18

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

Question 19

when ΔH is positive:

Answer 19

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

Question 20

bond dissociation energy

Answer 20

the amount of energy required to symmetrically break a covalent bond

Question 21

define entropy (ΔS)

Answer 21

measure of freedom of movement or disorder

Question 22

what is the effect of ΔS being positive on ΔG?

Answer 22

there is more movement/disorder and ΔG becomes more negative

Question 23

ΔS<0

Answer 23

entropically unfavourable

Question 24

ΔS>0

Answer 24

entropically favourable

Question 25

transition state

Answer 25

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

Question 26

ΔG‡

Answer 26

- activation energy - energy required to reach transition state - determines the rate of reactions (higher activation E = slower process)

Question 27

what is the rate-determining step of the reaction?

Answer 27

the slowest elementary step

Question 28

what is the effect of a catalyst?

Answer 28

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

Question 29

define a Bronsted acid and a Bronsted base

Answer 29

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

Question 30

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

Answer 30

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

Question 31

draw two free energy graphs for strong and weak acids

Question 32

what is the ΔG for strong/weak acids?

Answer 32

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

Question 33

how does pKa relate to acid strength?

Answer 33

the lower the pKa value, the stronger the acid

Question 34

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

Answer 34

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

Question 35

how does electronegativity impact acid strength?

Answer 35

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

Question 36

how does induction impact acid strength?

Answer 36

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.

Question 37

how does hybridisation impact acid strength?

Answer 37

- 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

Question 38

s character

Answer 38

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

Question 39

how does resonance impact acid strength?

Answer 39

- charge delocalisation increases the ability to accommodate negative charge and increases stability for the conjugate base - this leads to a stronger acid

Question 40

common acids and their pKa values

Answer 40

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

Question 41

define a Lewis acid and base

Answer 41

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

Question 42

examples of Lewis acids

Answer 42

- tricoordinate B and Al - cations such as Li+, Mg2+

Question 43

examples of Lewis bases

Answer 43

- lone-pair donors - benzene