A:B
A. + .B free radicals
A:B
A:B
These reactions involve the direct displacement of an atom or group by another atom or group.
These reactions involve the removal of atoms or groups of atoms from two adjacent atoms to form a multiple bond. The most commonly encountered elimination reactions are those which involve the removal of atoms or groups from adjacent carbon atoms to yield alkenes.
These reactions are so called since the attacking reagent simply adds itself across an unsaturated bond of the substrate to yield a saturated product, or at least one in which the degree of unsaturation is reduced. Addition is essentially the reversal of elimination.
These involve the migration of an atom or group from one site to another within the same molecule.
- The rearrangement may refer to the migration of a functional group from one position to another:
- or it may simply relate to the rearrangement of the carbon skeleton of the molecule:
Rearrangement may followed by elimination or addition to form a more stable product.
d-, and the other a relatively positive charge, d+.A covalent bond is usually polarized to some extent, since the atoms almost invariably have different electronegativity values, i.e. different powers of attracting the electrons in the bond. Consequently, displacement of the electrons towards the more electronegative atom creates a certain degree of polarity within the bond, causing one atom to acquire a relatively negative charge,
Permanent polarizing effects in single bonds, as exemplified by the carbon-chlorine bond, are known as INDUCTIVE(I) EFFECTS.
If the carbon atom attached to the polarizing atom or group is itself attached to further carbon atoms, the inductive effect is transmitted along the chain, although it tends to be insignificant beyond the second carbon.
The inductive effect of C1 upon C2 is significantly less than the effect of the Cl atom upon C1. Most atoms or groups tend to be electron-withdrawing with respect to the carbon atom to which they are attached, and exert an inductive -I effect.
However, certain groups, notably alkyl groups, exert an inductive effect in the opposite direction, i.e. they are electron-donating and exert a +I effect. Tertiary alkyl groups exert a greater +I effect than secondary which in turn exert a greater effect than primary.
p- electrons in multiple bonds towards the more electronegative atom is referred to as the MESOMERIC (M) EFFECT, and is analogous to the inductive effects in single bonds. The carbonyl group, >C=O, in which the effective electron density is greater in the region of the more electronegative oxygen atom, provides a simple exampleThe shift of
although it is probably generally more convenient simply to indicate the relative charge on each atom, thus :
The effect is transmitted along a chain in a way similar to the inductive effect.
The mesomeric effect is of paramount importance in conjugated chains of carbon atoms (i.e. systems in which multiple bonds occur between alternate carbon atoms, as in buta-1,3-diene, CH2=CH-CH=CH2), and it is for this reason that it is sometimes referred to as the CONJUGATIVE effect.
HOMEThe spatial distribution of atoms or groups in a molecule may alter, or in some cases completely prevent, a reaction occurring despite favourable electronic effects. The most commonly encountered steric effect is that of hindrance or blocking, and it is particularly effective when the reaction site is crowded with large bulky groups. For example, the rate of esterification of ethanoic (acetic) acid with propan-2-ol is half that of the rate with ethanol.
In the reaction with propan-2-ol, the reaction rate is retarded by the presence of the two methyl groups on the a-carbon atom of the alcohol.
Steric factors do not always inhibit the rate of reaction since they sometimes promote more favourable electronic effects by donating or withdrawing electrons to or from the reaction site.
