Acids and Bases

• Bronsted acids are hydrogen ion donors
• Lewis acids are species than can accept an electron pair
• Bronsted bases are hydrogen ion acceptors
• Lewis bases are electron rich species that can donate an electron pair
• Bronsted acids must contain a H atom, the overall charge of the molecule does not matter
• Bronsted bases must be able to form a bond to a proton or cation through containing an available electron pair that can easily be donated to form a new bond
• Acidity is dependent upon the degree of dissociation of the species HX
• Equilibrium lies to the right for a strong acid - strongest acids have large negative values
• Strength of a base is inversely dependent upon the strength of its conjugate acid
• The pKa value is a property of a particular H atom in a molecule - each H in a molecule will have its own pKa value
• Most interested in the most acidic H atoms in a molecule as these are the ones that are most readily removed
• pH of a solution indicates the concentration of positively charged H+ ions in the solution - the pH of a solution can be changed by adding more acid or base
• pH scale is used to describe the acidity of a solution
• pKa is characteristic of a particular compound and indicates the tendency of a compound to give up its proton
• Factors influencing pKa:

  • stability of anion X-
  • electronegativity
• Anything that destabilises X- will lead to decreased acidity and anything that helps to stabilise X- will lead to increased acidity - so influences pKa
• More electronegativity means more stable, so a stronger acid.

  As the halide ion increases in size, the negative charge is spread over a larger volume of space and its stability increases
• Inductive effect is an electronegative halogen pulling electrons towards itself through a single bond, so shows the inductive electron withdrawal.

  Stronger the pull, greater the stability of the anion and stronger the acid and lower the pKa value
• Factors of substituents affecting stability:

  1. larger the number of electronegative substituents, stronger the acid
  2. more electronegative the substituent, stronger the acid
  3. close the electron withdrawing group is to COOH, stronger the acid
• Some substituents are electron withdrawing, some are inductively electron donating
• Different nuclei have different attraction for the shared electron pair, resulting in unsymmetrical distribution of electron density and a dipole (permanent uneven distribution of charge)
• Effect of a permanent dipole may be felt some distance away in a molecule as the dipole in one bond induces a similar dipole in an adjacent bond
• Inductive effect is transmitted through sigma bonds - inductive effect is important in supplying explanations for differences in chemical reactivity
• Inductive effect is typically observed over maximum 3 bonds
• Resonance effect is a stabilising effect - the transmission of electron pairs between atoms in a molecule
• Some compounds structures cannot be shown by a single structure as the molecule will show characteristics of both structures - these resonance structures are separated by a resonance arrow
• Electron withdrawing groups by inductive effects:

  • F
  • NR2
  • NHR
  • NH2
  • CO2R
  • CO2H
  • CN
  • COR
  • CHO
  • OR
  • OH
  • NO2
  • Br
  • Cl
  • SO3H
• Electron donating groups by inductive effects:

  • R (alkyl)
  • metals
• Electron withdrawing groups by resonance effects:

  • COR
  • COOH
  • COOR
  • CRCRR
  • CN
  • NO2
  • COH
  • benzyl
• Electron donating groups by resonance effects:

  • Cl
  • Br
  • OH
  • OR
  • SR
  • NH2
  • NR2
  • CRCRR
  • benzyl
• Inductive effect is the uneven sharing of electrons in a sigma bond, effective through 1-2 bonds
• Resonance effect is the movement of electrons in a molecule, must involve pi-electrons or a pi bond
• Inductive and resonance effects are used to stabilise positive of negative charges
• pKa of the acid and pH of a solution will determine if a compound will lose a proton in an aqueous solution - pKa = pH + log( [non-ionised form]/[ionised form])
• When pH=pKa, the compound concentration HA (in its acidic form) will equal the amount in A- (in its basic form)
• Compound that contain both H and lone pairs may be an acid or base depending on the particular reaction
• Inductive and resonance effects can also be used to stabilise positive charges (carbocations) and negative charges (carbanions):

  • carbocations stabilise electron donating groups
  • carbocations destabilise electron withdrawing groups
  • carboanions destabilise electron donating groups
  • carboanions stabilise electron withdrawing groups
• Only electrons may be delocalised - resonance effects must involve a pi electron or a pi bond, this may be from:

  • negative charge
  • lone pair of electrons
  • double or triple bond

  These effects can stabilise or destabilise carbocations or carbanions
• In a resonance hybrid with carbocation stabilisation by an alkene , the positive charge is spread equally over 2C, C-C bonds are identical and are in-between a double and single bond. A resonance hybrid can from instead of two separate resonance forms. Example, C=C-C or C-C=C are the resonance forms
• Resonance (+M) effect stabilises the carbocation