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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Fundamentals > Electrophiles & Nucleophiles

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Electrophiles & Nucleophiles

Brønsted and Lowry Definitions

An acid is a proton donor and a base is a proton acceptor. For example a hypothetical acid, HA dissociates into H+ and A-   

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. 

Slide1.jpg

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 25o

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 < > 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:

  1. Electronegativity, and thus acidity, increases as we go left to right across the periodic table. Great electronegativity results in a more acidic proton, and the more polar bonds. 
  2. Molecules get larger as we go down a column in the periodic table. Dissociation of a large A is favored because the 1s hydrogen and larger outer-shell overlap is poor, creating a weak H-A bond. This can be further explained through electron electron repulsion, which is reduced when outer shell orbitals allow electrons to occupy a greater volume of space. 
  3. Delocalization of charge onto several atoms is accomplished via resonance of A-. The presence of additional electronegative atoms in A- increases the effect.  

Resonance Effect on Acidity.jpg

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.

Peridoic Table .gif

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) 

 Slide2 (1).jpg

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:

 

Slide3 (1).jpg

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. 

Problems

  1. Which is a stronger acid H2O or H2S? Which is a stronger base, HO- or HS-? 
  2. Rank the following in order of increasing acidity. CH4, HF, H2O, NH3. Provide each acids conjugate base?  
  3. Using electron pushing arrows draw the Lewis acid-Lewis base reaction between boron trifluoride and dimethyl sulfide [(CH3)2S]. Show all formal charges. 
  4. Salicylic Acid has a Ka of 1.06 x 10-3, what it its pKa? 
  5. From the following equation identify the electrophile and nucleophile. (please reason conclusion) 

CH3I + NH3 --> CH3NH2+ + I-

Resources

  1. Tully, T. J. "Waste Acid Neutralization." Sewage and Industrial Wastes 30 (1958): 1385-390.
  2. Wynne-Jones, W. F. Kenrick. "Acid Strenght and Its Dependence Upon the Nature of the Solvent." Proceedings of the Royal Society of London 140 (1933): 440-51.
  3. Zander, Richard H. "Acid-Base Color Reactions: The Status of Triquetrella ferrugnea, Barbula inaequalifolia and B. calcarea." The Bryologist 83 (1980): 228-33.

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Contributors

  • Kristen Kelley and Brittani Farquharson 

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Last Modified
15:31, 2 Oct 2013

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