Organic Chemistry: 4 Stepping Stones

The four stepping stones to organic chemistry come from general chemistry you already know.

 

Atoms

 

Each element passes important properties through all levels of organization:

 

• • electronegativity

     

    

    The positive charge from the nucleus compared to the distance of the outer electron gives a different ability to attract electrons in a chemical bond, Figure 1.

 

• • orbitals

     

    The period of an element dictates the energy of  outermost valence electrons. 

     

     

    

     

    • • s orbitals

     

    • • p orbitals

     

  • number valence electrons

     

     

  • 

     

    Neutral atoms possess the same number of electrons as protons. The valence electrons are the outer electrons able to participate in bonds.

     

    • • single electrons

         

        Unpaired electrons must share an electron with another element .

     

    • • paired electrons

         

        Electron pairs cannot bond to other elements. 

  

  • Octet Rule

     

    

     

     

    Elements share electrons until both elements have a total of eight electrons within their valence shell.

 

• • polar bonds

     

    Elements do not share electrons equally. A dipole exists, where the more electronegative element attracts more electron density than a less electronegative element.

 

• • polar molecules

     

    Molecules which possess polar bonds with dipoles which do not directly oppose one another adopt an overall dipole.

 

• ions

   

  Molecules with more electrons than bonds have a negative charge. If a molecule has fewer electrons than the number of possible bonds it has a positive charge.

 

 

 

Electrons

 

 

Electrons exert an influence on nearby neighbors. This occurs through \sigma-bonds or \pi-bonds, which can work at cross purposes to each other.

 

 

• inductive effects

   

  single bonds donate or withdraw electron density when an element is more electronegative or electropositive than its immediate neighbor.

   

• delocalization effects
  Electrons spread along double bonds and empty p orbitals. This allows electron density and charge to be shared among  atoms.Resonance shares p electrons  among atoms and bonds through multiple atoms in a molecule. 

Equilibrium

 

Equilibrium occurs when a reaction mixture no longer shows any observable change. It means the rate of the forward reaction and reverse reaction happen at an equal rate. The concentrations of the reactants are governed by the ratio of reactants to products. The ratio holds a fixed value, where the numerical ratio is the equilibrium constant K_eq.

 

• Enthalpy
  Enthalpy, \Delta H, compares the energy required to break bonds with the energy released to form chemical bonds.

 

 

 

 

• Exothermic reactions release energy.

 

• Endothermic reactions require the input of energy.

 

• • Gibb’s free energy

     

    Reactions do not depend only on the balance between bond making and bond breaking. Entropy,\Delta S, must also be considered.

     

    Reactions favored by entropy means the reaction goes from a more ordered state to less ordered state. When the reaction, such as a synthesis, goes from a less ordered to more ordered state, entropy works against the forward reaction.

     

    At lower temperatures, the entropy term makes a small contribution to the free energy. At higher temperatures, entropy has more influence.

     

    When bond energy and entropy are both factored together, \Delta G, Gibb’s free energy, determines whether the reaction is favorable (\Delta G < 0) or unfavorable (\Delta G > 0).

 

\Delta G =\Delta H - T \Delta S 

• • A reaction is exergonic when it releases energy and is favored

 

• • A reaction is endergonic when it requires energy be absorbed.

• Equilibrium ConstantsThe ratio of products to reactants when a reaction does not have any observable change is K_eq. In general, the expression for a reaction:

 

 

aA + bB \rightarrow cC + dD 

 

The equilibrium constant expression relates the concentration of all the species in solution :

 

K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b} 

 

As example of how this plays out, the reaction of zinc with hydrochloric acid to give zinc chloride and hydrogen gas.

 

Zn + 2 HCl \rightarrow ZnCl_2 + H_2  

 

The equilibrium constant expression takes the following form.

 

K_{eq} = \frac{[ZnCl_2][H_2]}{[Zn][HCl]^2} 

• • • Acid-Base Reactions

      Acid-base equilibria play a major roll in organic chemistry, due to the importance of acid-base equilibrium constants, Ka. The expression excludes the concentration of water in the expression since it remains constant throughout any reaction.HA is the protonated form the acid and acid and H⁺¹ and A^-1 are the dissociated form of the proton and the anionic base.

 

 

 

CH_3COOH \leftrightharpoons H^{+1} + CH_3COO^{-1} 

 

The equilibrium expression when the acid dissociation constant Ka = 1.84 x 10^-5:

 

 

 

1.85 x 10^{-5} = \frac{[H^{+1}][CH_3COO^{-1}]}{[CH_3COOH]} 

Reaction Rates

 

Reaction rates hold a place of central importance in organic chemistry. A study of reaction rates tells you whether a reaction is bimolecular or unimolecular, and reveal the presence of intermediates.

 

• Activation EnergyThe activation energy of a reaction controls the rate of reaction. A high activation energy causes a slower reaction rate. Lowering the activation energy increases the rate of reaction. 

 

 

• Transition StatesThe transition state represents the moment at which a chemical reaction is at its highest energy. The lower the stability of the transition state, the higher the activation energy. Increasing the stability of the transition state increases the reaction rate.

 

• MechanismsA mechanism gives the step-by-step description of each step, which leads from the initial reactants to final product. It could occur in a single step, or be composed of multiple steps.Elementary steps can only have two or one reactant, and sum total of all the reactants and products must equal the overall chemical reaction.