Download Chem 2641 Chapter 5 Understanding Organic Reactions I. Writing

Chem 2641
Chapter 5
Understanding Organic Reactions
I. Writing equations
A. The reactant or starting material(s) are on the left of the arrow,
and the product(s) are on the right.
B. The reagent, which causes the transformation to occur, can be
written on the left of the arrow with the starting materials, or it can be
written on the arrow as shown below.
reagent
product
Starting material
C. Reagents often contain metals, which never appear in the product.
D. Reagents may sometimes include a catalyst, which is drawn below
the arrow. The term h means light, and the term  means heat.
II. Types of organic reactions
A. Substitution reactions – one atom or group of atoms is replaced by
another. Only sigma bonds are involved.
R–X
R–Y
B. Elimination reactions – two atoms or groups of atoms are removed,
and a double bond forms between the carbons that held those groups.
loss of XY
C
C
X
Y
C
C
C. Addition reactions – two atoms or groups of atoms are added
across a double bond.
C
addn of XY
C
C
C
X
Y
III. Making and breaking bonds
A. The description of how bonds are broken and formed in a chemical
reaction is called the mechanism of the reaction.
B. Reactions can be one step or more than one step.
1. Reactions that are one step are called concerted reactions.
The reactant is converted directly to product.
2. Reactions that require more than one step contain species
called intermediates. These species often have formal charges.
C. Bonds can be broken homolytically or heterolytically.
1. Homolytic bond cleavage means that the two electrons of the
bond divide, so that each atom gets one electron:
A
B
A.
B.
+
An atom that has one electron is called a radical.
2. Heterolytic bond cleavage means that the two electrons of
the bond go with one of the atoms (the more electronegative one), which
becomes negatively charged, and the other becomes positively charged.
A
B
A- +
B+
a. A carbon atom that is positively charged is called a
carbocation.
carbanion.
b. A carbon atom that is negatively charged is called a
IV. Radical reactions – involve single electron transfer; they occur in
biochemistry but are primarily used in industrial processes.
A. A radical is a neutral but highly reactive species because it
requires another electron to fulfill its octet.
B. Examples
C. If a radical reacts with a neutral species, another radical is
formed.
D. Radical reactions have three stages – initiation, propagation and
termination.
E. For the reaction CH4 + Cl2 + uv light
CH3Cl + HCl
The initiation stage – Cl2 reacts with the uv light to form Cl.
Propagation stage – The Cl. reacts with CH4 to form CH3. and HCl
The CH3. reacts with Cl2 to form CH3Cl and Cl.
Termination stage – any two radicals can combine to form a stable
product.
V. Polar reactions – involve Lewis acids (called electrophiles) and Lewis bases
(called nucleophiles)
A. Examples of electrophiles
H 3O +
CH3 – Br
B. Examples of nucleophiles
OH-
NH3
C. Example of a polar reaction
H 2O
Cl-
VI. Arrow notation - The movement of electrons is denoted using curved
arrows.
A. A curved, two-headed arrow is used to show 2 electrons in motion,
either to form a bond or to break one:
A nucleophile is negatively charged, a lone pair, or a pi bond.
An electrophile is positively charged, or partially positively charge.
B. A curved, one-headed arrow is used to show the movement of 1
electron, either to break a bond or to form one:
VII. Thermodynamics
Many organic reactions are equilibrium reactions. The equilibrium will
favor products that are more stable than the starting materials. In other
words, the equilibrium favors products when the G of the reaction is
negative (spontaneous reaction).
Also, the more stable product is favored, if more than one product is
possible.
VIII. Energy diagram - An energy diagram shows the energy of the reaction
as it progresses.
A. An energy diagram for a one-step reaction is shown below. It has
one peak, called the transition state.
transition state
Energy
Eact
product
Hrxn
starting material
Reaction Coordinate
B. The Eact determines the rate of the reaction. The higher the hill,
the slower the reaction.
C. The Hrxn determines whether the reaction is exothermic or
endothermic. If it is negative, the reaction is exothermic. If it is positive,
the reaction is endothermic. The reaction shown above is endothermic.
D. The energy diagram for a multi-step reaction shows a peak for
each transition state in the reaction.
E. A catalyst speeds a reaction by lowering its Eact. It does this by
changing the nature of the transition state.