PH1125 - Physical properties of drug molecules

Question 1

what does an acid do in a reaction?

Answer 1

• donates protons in a reaction

Question 2

what does a base do in a reaction?

Answer 2

• accepts protons in a reaction

Question 3

what does the term amphiprotic mean?

Answer 3

• substance that can donate or accept H+ ions

Question 4

what is a weak acid associated with?

Answer 4

• a strong base

Question 5

what is a weak base associated with?

Answer 5

• a strong acid

Question 6

what is Kw?

Answer 6

• ion product constant for water

Question 7

what is pKa?

Answer 7

• a measure of the weakness of an acid

Question 8

what does a low pKa mean in terms of the strength of an acid?

Answer 8

• low pKa means a stronger acid

Question 9

what is the equation for pKa?

Answer 9

• -logKa

Question 10

what is the henderson-hasselbalch equation?

Answer 10

• pH = pKa + log [base]/[acid]

Question 11

what does the Ka tell us about the strength of an acid?

Answer 11

• higher the Ka the stronger the acid and vice versa

Question 12

what are the factors affecting acidity? (5)

Answer 12

• electronegativity
• bond energies
• inductive effects
• hybridization effects
• resonance / delocalisation effects

Question 13

what is the most important factor to consider when assessing acidity?

Answer 13

• the stabilization of the conjugate base (the conjugate base is able to gain or absorb a proton in a chemical reaction)
• the more stable a conjugate base the more acidic the compound

Question 14

why is the conjugate base more stable when the compound is more acidic? (3)

Answer 14

• acidic molecules have an anion in the conjugate base to delocalise the charge over a larger space
• delocalisation of the negative charge such that one atom doesn’t have to bear the full negative charge makes the molecule more stable
• equilibrium shifts to the right

Question 15

how does the electronegativity change as you go across a period?

Answer 15

• electronegativity increases

Question 16

how does the electronegativity change as you go down a group?

Answer 16

• electronegativity decreases

Question 17

how does the bond strength change as you go down a group? (2)

Answer 17

• weakening of bond strength on descending group

- increasing atom size correlates with ability to disperse negative charge of the conjugate base

Question 18

what are the inductive effects of an electron-withdrawing group? (2)

Answer 18

• stabilisation (or destabilisation) of the conjugate base through electron-withdrawing (or electron-donating) groups
• transmitted through sigma bonds

Question 19

what are the inductive effects of an electron-donating group? (2)

Answer 19

• they destabilise the conjugate base leading to weaker acids
• inductive effect from alkyl groups is rather small

Question 20

how does hybridisation affect acidity? (3)

Answer 20

• s orbitals held closer to +ve charged nucleus
• more s character relative to p so more acidic C-H bond
• sp is more acidic than sp2

Question 21

what does the stabilisation of conjugate bases do to the acidity and how? (2)

Answer 21

• stabilisation by resonance / delocalisation

- increases acidity

Question 22

what are enolates anions ad what is their importance? (2)

Answer 22

• conjugate bases of carbonyl compounds

- they are the most important reactive species in organic chemistry