Free Radical

Free Radicals



Structure and Geometry of Free Radicals: A free radical is a species which has one or more unpaired electrons. In the species where all electrons are paired the total magnetic moment is zero. In radicals, however, since there are one or more unpaired electrons, there is a net magnetic moment and the radicals as a result are paramagnetic. Free radicals are usually detected by electron spin resonance, which is also termed electron paramagnetic resonance. Simple alkyl radicals have a planar (trigonal) structure, i.e., these have sp2 bonding with the odd electron in a p orbital. The pyramidal structure is another possibility when the bonding may be sp3 and the odd electron is in an sp3 orbital. The planar structure is in keeping with loss of activity when a free radical is generated at a chiral center. Thus, a planar radical will be attacked at either face after its formation with equal probability to give enantiomers. Unlike carbocations, the free radicals can be generated at bridgehead shows that pyramidal geometry for radicals is also possible and that free radicals need to be planar.



Stability of Free Radicals: A As in the case of carbocation, the stability of free radicals is tertiary > secondary > primary and is explained on the basis of hyperconjugation. The stabilizing effects in allylic radicals and benzyl radicals is due to vinyl and phenyl groups in terms of resonance structures. Bond  dissociation energies shown that 19 kcal / mol  less energy is needed  to form the benzyl radical from toluene than the formation of methyl radical from methane. The triphenyl methyl type radicals are no doubt stabilized by resonance, however, the major cause of their stability is the steric  hindrance to dimerization. 






Carbenes



Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non bonding orbitals. When the two electrons are spin paired the carbene is a singlet, if the spins of the electrons are parallel it is a triplet.

Structure of Carbenes : A singlet carbene is thought to possess a bent sp2 hybrid structure in which the paired electrons occupy the vacant sp2 orbital. A triplet carbene can be either bent sp2 hybrid with an electron in each unoccupied orbital, or a linear sp hybrid with an electron in each of the unoccupied p-orbital. It has however, been shown that several carbenes are in a non-linear triplet ground state. However, the dihalogenocarbenes and carbenes with oxygen, nitrogen and sulphur atoms attached to the bivalent carbon, exist probably as singlets. The singlet and triplet state of a carbene display different chemical behaviour. Thus addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than addition of triplet carbenes.

Generation of Carbenes: Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by a-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.

Reactions of Carbenes: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products (a car-benoids –oranometallic or complexed intermediates which, while not free carbenes afford  products expected from carbenes are usually called carbenoids).

When a carbene is generated  in a three membered ring allenes are formed by rearrangement. However, a similar formation at a cyclopropylmethyl carbon gives ring expansion. Carbenes are also involved in Reimer —Tiemann reaction.




Arenium Ions



A considerable amount of experimental evidence indicates that electrophiles attack the p system of benzene to form a delocalized non-aromatic carbocation known as arenium ion or sometimes a s complex CMR spectroscopic evidence is available in favour of  s complex.


Benzynes



It is a reactive intermediate in some nucleophilic aromatic substitutions. It is a benzene with two hydrogen atoms removed. It is usually drawn with a highly strained triple bond in the six membered ring. Benzyne intermediate has been observed spectroscopically and trapped.

By:- NKGupta