Electrophiles & Nucleophiles
Brønsted and Lowry Definitions
HA ↔ H+ + A-
An acid, a proton donor, donates protons in water forming hydronium ions (protonated water, H3O+). A base, proton acceptor, removes protons from water forming hydroxide ions (deprotonated water, OH-) Using curved arrows to demonstrate the mechanism by which a proton is transferred from hydrochloric acid to the base water. The arrows demonstrate the movement of electrons but the key feature of the Bronsted Lowry reaction is the transfer of protons.
A lone pair from the base creates a new bond with an acidic proton and the electron pair originally linking the proton to the remainder of the acid shifts to becomes a lone pair on the departing conjugate base.
Water is a neutral compound ( # hydronium ions = # hydroxide ions via self-dissociation). Equilibrium constant Kw (self-ionization constant) describes this process at 25oC
H2O + H2O< Kw > H3O+ + OH-
Kw = [H3O+][OH-] = 10-14 mol2L-2
pH = the negative logarithm of the value of [H3O+]. The concentration of H3O+ in pure water = 10-7 mol L-1
pH = - log [H3O+]
pH in pure water = +7
pH>7 = basic
pH<7 = acidic
acidity of a general acid (HA) is conveyed by a general equation:
HA + H2O < K > H3O + A- K= [H3O+][A-] / [HA][H2O]
Acidity constant (Ka) = K[H2O]= [H3O+][A-] / [HA] mol L-1
like H3O+, Ka can be put to a logarithmic scale
pKa = -log Ka
pKa describes the pH of an acid at 50% dissociation. If pKa<1 = strong, pKa>4 = weak acid. Please refer to the chart below for pKa of common acids.
A- derived from acid HA, is referred to as the conjugate base
A- derived from base HA, is referred to as the conjugate acid
acid + base <---> conjugate base + conjugate acid
Conjugate acid and bases are inversely related:
strong acid = weak conjugate base
strong base = weak conjugate acid
Ex. HCl (strong acid) ↔ H+ + Cl- (weak conjugate base)
CH3OH (weak acid) ↔ H+ + CH3O- (strong conjugate base)
Estimating Relative Acid Base Strengths
Relative strength of an acid (HA) and weakness of conjugate base can be estimated using three structural properties:
Summary: Basicity of A- decreases to the right and down the periodic table, acidity of HA increases to the right and down the periodic table.
Several molecules have the ability to act as acids or bases under differing conditions, thus they are Amphoteric.
ex. water, nitric acid, acetic acid
H3O+ <water accepts a proton (base) H2O water donates a proton (acid)> OH-
Lewis acids and bases interact by sharing an electron pair
Lewis acid = electron pair acceptor
Lewis Base = electron pair donator
Lewis base share its lone pair electrons with a lewis acid to form a new covalent bond, thus can be expressed by an arrow moving in the direction of electron movement (base to acid)
Electrophiles and Nucleophiles interact through movement of an electron pair
Processes that exhibit very similar characteristics as acid-base reactions and are described using the same electron pushing arrows.
Electrophile "electron loving": An electron deficient atom, ion or molecule that has an affinity for an electron pair and will bond to a base or nucleophile. (all lewis acids are electrophiles)
Nucleophile "nucleus loving": An atom ion or molecule that has an electron pair that may be donated in bonding to an electrophile or lewis acid. (all nucleophiles are lewis bases)
The diagram below demonstrates the flow of electrons using electron pushing arrows:
Haloalkanes (compounds with carbon-halogen bonds) are general nucleophilic substitution reactions. Despite differing halogens and arrangement of substituents , all arrangements/combinations behave similarly allowing us to conclude that it is the actual presence of the carbon-halogen bond that controls the behavior of the haloalkane. The C-X bond is the functional group/controlling factor of reactivity.
CH3I + NH3 --> CH3NH2+ + I-
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