PH1125 - Physical properties of drug molecules Flashcards
what does an acid do in a reaction?
- donates protons in a reaction
what does a base do in a reaction?
- accepts protons in a reaction
what does the term amphiprotic mean?
- substance that can donate or accept H+ ions
what is a weak acid associated with?
- a strong base
what is a weak base associated with?
- a strong acid
what is Kw?
- ion product constant for water
what is pKa?
- a measure of the weakness of an acid
what does a low pKa mean in terms of the strength of an acid?
- low pKa means a stronger acid
what is the equation for pKa?
- -logKa
what is the henderson-hasselbalch equation?
- pH = pKa + log [base]/[acid]
what does the Ka tell us about the strength of an acid?
- higher the Ka the stronger the acid and vice versa
what are the factors affecting acidity? (5)
- electronegativity
- bond energies
- inductive effects
- hybridization effects
- resonance / delocalisation effects
what is the most important factor to consider when assessing acidity?
- 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
why is the conjugate base more stable when the compound is more acidic? (3)
- 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
how does the electronegativity change as you go across a period?
- electronegativity increases
how does the electronegativity change as you go down a group?
- electronegativity decreases
how does the bond strength change as you go down a group? (2)
- weakening of bond strength on descending group
- increasing atom size correlates with ability to disperse negative charge of the conjugate base
what are the inductive effects of an electron-withdrawing group? (2)
- stabilisation (or destabilisation) of the conjugate base through electron-withdrawing (or electron-donating) groups
- transmitted through sigma bonds
what are the inductive effects of an electron-donating group? (2)
- they destabilise the conjugate base leading to weaker acids
- inductive effect from alkyl groups is rather small
how does hybridisation affect acidity? (3)
- 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
what does the stabilisation of conjugate bases do to the acidity and how? (2)
- stabilisation by resonance / delocalisation
- increases acidity
what are enolates anions ad what is their importance? (2)
- conjugate bases of carbonyl compounds
- they are the most important reactive species in organic chemistry
