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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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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.
  • The reagent, the chemical substance with which an organic compound reacts, is sometimes drawn on the left side of the equation with the other reactants. At other times, the reagent is drawn above the arrow itself.
  • Although the solvent is often omitted from the equation, most organic reactions take place in liquid solvent.
  • The solvent and temperature of the reaction may be added above or below the arrow.
  • The symbols “h” and “” are used for reactions that require light and heat respectively.

Figure 6.1

Different ways of writing

organic reactions


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.


Kinds of Organic Reactions

  • A substitution is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms.
  • In a general substitution, Y replaces Z on a carbon atom.

Substitution reactions involve  bonds: one  bond breaks and another forms at the same carbon atom.

  • The most common examples of substitution occur when Z is a hydrogen or a heteroatom that is more electronegative than carbon.

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.

  • Two  bonds are broken, and a  bond is formed between adjacent atoms.
  • The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.

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.








Bond Making and Bond Breaking

  • A reaction mechanism is a detailed description of how bonds are broken and formed as starting material is converted into product.
  • A reaction can occur either in one step or a series of steps.

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.


Homolysis and heterolysis require energy.

  • Homolysis generates uncharged reactive intermediates with unpaired electrons.
  • Heterolysis generates charged intermediates.

To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook.

  • A full headed curved arrow shows the movement of an electron pair.

Homolysis generates two uncharged species with unpaired electrons.

  • A reactive intermediate with a single unpaired electron is called a radical.
  • Radicals are highly unstable because they contain an atom that does not have an octet of electrons.
  • Heterolysis generates a carbocation or a carbanion.
  • Both carbocations and carbanions are unstable intermediates. A carbocation contains a carbon surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom.

Figure 6.2

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.

  • Carbanions are nucleophiles because they contain a carbon with a lone pair.

Heterolytically cleave each of the carbon-hetratom bonds and label the organic intermediate as a carbocation or carbanion






Bond formation occurs in two different ways.

  • Two radicals can each donate one electron to form a two-electron bond.
  • Alternatively, two ions with unlike charges can come together, with the negatively charged ion donating both electrons to form the resulting two-electron bond.
  • Bond formation always releases energy.
5 3 radical reactions
5.3 Radical Reactions
  • Not as common as polar reactions
  • Radicals react to complete electron octet of valence shell
    • A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule
    • A radical can add to an alkene to give a new radical, causing an addition reaction
steps in radical substitution
Steps in Radical Substitution
  • Three types of steps
    • Initiation – homolytic formation of two reactive species with unpaired electrons
      • Example – formation of Cl atoms form Cl2 and light
    • Propagation – reaction with molecule to generate radical
      • Example - reaction of chlorine atom with methane to give HCl and CH3.
    • Termination – combination of two radicals to form a stable product: CH3. + CH3. CH3CH3
5 4 polar reactions
5.4 Polar Reactions
  • Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities
  • This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom
  • The more electronegative atom has the greater electron density
  • Elements such as O, F, N, Cl more electronegative than carbon
  • Polarization is a change in electron distribution as a response to change in electronic nature of the surroundings
  • Polarizability is the tendency to undergo polarization
  • Polar reactions occur between regions of high electron density and regions of low electron density
generalized polar reactions
Generalized Polar Reactions
  • An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species
  • An electrophile is a Lewis acid
  • A nucleophile is a Lewis base
  • The combination is indicate with a curved arrow from nucleophile to electrophile
5 5 an example of a polar reaction addition of hbr to ethylene
5.5 An Example of a Polar Reaction: Addition of HBr to Ethylene
  • HBr adds to the  part of C-C double bond
  • The  bond is electron-rich, allowing it to function as a nucleophile
  • H-Br is electron deficient at the H since Br is much more electronegative, making HBr an electrophile
mechanism of addition of hbr to ethylene
Mechanism of Addition of HBr to Ethylene
  • HBr electrophile is attacked by  electrons of ethylene (nucleophile) to form a carbocation intermediate and bromide ion
  • Bromide adds to the positive center of the carbocation, which is an electrophile, forming a C-Br  bond
  • The result is that ethylene and HBr combine to form bromoethane
  • All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

Bond Dissociation Energy

  • The energy absorbed or released in any reaction, symbolized by H0, is called the enthalpy change or heat of reaction.
  • Bond dissociation energy is the H0 for a specific kind of reaction—the homolysis of a covalent bond to form two radicals.