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. CH4 + 2O2 CO2 + 2H2O + Energy
1- Combustion
2 -Substitution Reaction
Substitution reactions a group or an atom in a particular chemical compound is replaced by another one.
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.
CH4 + Cl2 CH3Cl + HCl
The mechanism for this reaction is known as free radical substitution.
A- Chlorination of Alkanes
CH4 + Cl CH3 + HCl
Cl2 Cl + Cl
CH3 + Cl2 CH3Cl + Cl
CH3ClCH3 + Cl
initiation step
two propagation steps
termination step
UV Light
CH3CH3CH3 + CH3minor termination step
Free radical substitution mechanism
Also get reverse of initiation step occurring as a termination step.
CH3Cl + Cl2 CH2Cl2 + HCl
Overall reaction equations
Conditions
CH2Cl2 + Cl2 CHCl3 + HCl
CHCl3 + Cl2 CCl4 + HCl
ultra-violet lightexcess chlorine
Further free radical substitutions
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
Friedl Craft reaction is used to introduce any alkyl (R) or acyl (RCO) group into an aromatic ring in presence of AlCl3 as catalyst
Cl
AlCl3
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.
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.
NH2
HNO3
H2SO4
Br2
FeBr3
CH3CH2Br
AlCl3
SO3
H2SO4
o,p directing group
NO2
NH2
Br
NH2
NH2
CH3
SO3
NH2
+
+
+
+
NH2
NO2
NH2
BrNH2
CH3
NH2
SO3
NO2
HNO3
H2SO4
NO2
NO2
NO2
Br
Br2
FeBr3
CH3CH2Br
AlCl3
NO2
CH3
SO3
H2SO4
NO2
SO3
R m directing group
CH CHNaNH2 / liq NH3
Ag(NH3)+/
-OH
Cu(NH3)+/
-OH
CH CNa
CH CAg + NH3
CH CCu + NH3
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Terminal alkynes show also substitution reactions. Where the terminal hydrogen is very easy to remove and replaced by a metal.
The last two reactions are used to differentiate between terminal alkynes and mid-alkynes or alkenes
C C CH3 CH3
Ag(NH3)+/
-OH
Cu(NH3)+/
-OH
NO Reaction
The easiness of removing the terminal hydrogen from alkyne molecule give some weak acidic properties of this molecule.
CH2 X
CH3
NaOR / alcohol
NaNH 2 / liq NH3
CH CNa
liq NH3
KCN
CH CH2CH3
CH2 OR
CH3
CH2 CN
CH3
CH2 NH2
CH3
X = Cl, Br, I, F
the halogen atom in the alkyl halide is readily removed by many reagents
The conventional method to replace a halogen is made by reaction with reagent contain alkali metal