Understanding Organic Reactions. Writing Equations for Organic Reactions. Equations for organic reactions are usually drawn with a single reaction arrow ( ) between the starting material and product.
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Writing Equations for Organic Reactions
Different ways of writing
When two sequential reactions are carried out without drawing any intermediate compound, the steps are usually numbered above or below the reaction arrow. This convention signifies that the first step occurs before the second step, and the reagents are added in sequence, not at the same time.
Substitution reactions involve bonds: one bond breaks and another forms at the same carbon atom.
Elimination is a reaction in which elements of the starting material are “lost” and a bond is formed.
In an elimination reaction, two groups X and Y are removed from a starting material.
Addition is a reaction in which elements are added to the starting material.
In an addition reaction, new groups X and Y are added to the starting material. A bond is broken and two bonds are formed.
Addition and elimination reactions are exactly opposite. A bond is formed in elimination reactions, whereas a bond is broken in addition reactions.
Classify each of the following as either substitution, elimination or addition reactions.
Regardless of how many steps there are in a reaction, there are only two ways to break (cleave) a bond: the electrons in the bond can be divided equally or unequally between the two atoms of the bond.
To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook.
Three reactive intermediates
resulting from homolysis and
heterolysis of a C – Z bond
Radicals and carbocations are electrophiles because they contain an electron deficient carbon.
Heterolytically cleave each of the carbon-hetratom bonds and label the organic intermediate as a carbocation or carbanion
Use arrows to show the movement of electrons in the following reactions.
Because bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic.
Comparing bond dissociation energies is equivalent to comparing bond strength.
Bond dissociation energies are used to calculate the enthalpy change (H0) in a reaction in which several bonds are broken and formed.
Calculate H for each of the following reactions, knowing H of O2 and O-H = 119 kcal/mol, H of C-H = 104 kcal/ml and H of one C=O = 128 kcal/mol.
C-H = 4 x 104 kcal/mol = 416 kcal/mol
C-O = 2 x -128 kcal/mol = -256 kcal/mol
O-O = 2 x 119 kcal/mol
= 238 kcal/mol
O-H = 4 x -119 kcal/mol = -476 kcal/mol
H = 416 + 238 = +654 kcal/mol
H = -256 + -476 = -732 kcal/mol
H = 654 + -732 kcal/mol = -78 kcal/mol
The size of Keq expresses whether the starting materials or products predominate once equilibrium is reached.
Summary of the relationship
between ∆G° and Keq
G° is related to the equilibrium constant Keq by the following equation:
These equations can be used for any process with two states in equilibrium. As an example, monosubstituted cyclohexanes exist as two different chair conformations that rapidly interconvert at room temperature, with the conformation having the substituent in the roomier equatorial position favored.
This equation indicates that the total energy change is due to two factors: the change in bonding energy and the change in disorder.
In most other reactions that are not carried out at high temperature, the entropy term (TS°) is small compared to the enthalpy term (H0), and therefore it is usually neglected.
The energy of activation is the minimum amount of energy needed to break the bonds in the reactants.
Comparing ∆H° and Ea in two
Complete energy diagram for
the two-step conversion of
The effect of a catalyst
on a reaction