Organic Reactions Dr. M. Abd-Elhakeem Faculty of Biotechnology Organic Chemistry Chapter 3
The majority of organic reactions take place at functional groups and are characteristic of that functional group. the reactivity of the functional group is affected by stereo-electronic effects. For example a functional group may be surrounded by bulky groups which hinder the approach of a reagent and slow down the rate of reaction. This is referred to as steric shielding.
Electronic effects can also influence the rate of a reaction. Neighboring groups can influence the reactivity of a functional group if they are electron-withdrawing or electron donating and influence the electronic density within the functional group.
Organic compounds burn exothermically to produce carbon dioxide and water if there is a plentiful supply of oxygen. This is known as complete combustion. e.g. CH 4 + 2O 2 CO 2 + 2H 2 O + Energy 1- Combustion
2- Substitution Reaction Substitution reactions a group or an atom in a particular chemical compound is replaced by another one.groupchemical compound
There are three main types substitution reactions: 1- Electrophilic substitution: A positive charged atom or group attack the electrons richest part of the compound 2- Nucleophilic substitution: A negative charged atom or group attack the part of the lower electron density in the compound.
3- Free radical substitution As a result of homogeneous break down of the attacking reagent a free radical is produced to attack a neutral molecule and replace one part of the molecule. The result is new molecule and new free radical
In the presence of ultraviolet light Alkanes react to form alkyl halide and hydrogen chloride. CH 4 + Cl 2 CH 3 Cl + HCl The mechanism for this reaction is known as free radical substitution. A- Chlorination of Alkanes
CH 4 + Cl CH 3 + HCl Cl 2 Cl + Cl CH 3 + Cl 2 CH 3 Cl + Cl CH 3 Cl CH 3 + Cl initiation step two propagation steps termination step UV Light CH 3 CH 3 CH 3 + CH 3 minor termination step Free radical substitution mechanism Also get reverse of initiation step occurring as a termination step.
2. Electrophilic substitutions of Aromatic compounds
Benzene ring has six electrons circulate inside the ring thus benzene ring has a high electron density. This resonance give a high chemical stability for benzene and cause the difficulty of double bond reaction
So Benzene generally give only substitution reaction except in addition of hydrogen
Friedl Craft reaction is used to introduce any alkyl (R) or acyl (RCO) group into an aromatic ring in presence of AlCl 3 as catalyst
An atom or group already attached to a benzene ring may direct an incoming electrophile to either the ortho-para positions or the meta position. Directing groups
Atoms or groups that make the benzene molecule more reactive by increasing the ring's electron density are called activating groups. Activating groups serve as ortho- para directors when they are attached to a benzene ring. An atom or group that makes the benzene molecule less reactive by removing electron density from the ring acts as a deactivating group. Deactivating groups direct incoming electrophiles to the meta position.
You can further classify activating and deactivating groups or atoms as strong, moderate, or weak in their directing influence. This table lists some typical activating and deactivating groups by the order of their strength.
NH 2 OH OR R NO 2 COOH CN Ortho-Para director Meta director
Halogen atoms show both activating and deactivating characteristics. Because 1- they have three pairs of unshared electrons, halogen atoms can supply electrons toward the ring. 2- due to their high electronegativity, halogen atoms also tend to remove electrons from the benzene ring. These conflicting properties make halogens a weak ortho-para director and also a ring deactivator.