Four structures are given in options (a) to (d). Examine them and select the aromatic structures.
The molecules having dipole moment are ............. .
Why do alkenes prefer to undergo electrophilic addition reaction while arenes prefer electrophilic substitution reactions? Explain.
Alkenes are rich source of loosely held pi $(\pi)$ electrons, due to which they show electrophilic addition reaction. Electrophilic addition reaction of alkenes are accompanied by large energy changes so these are energetically favourable than that of electrophilic substitution reactions. In special conditions alkenes also undergo free radical substitution reactions.
In arenes during electrophilic addition reactions, aromatic character of benzene ring is destroyed while during electrophilic substitution reaction it remains intact. Electrophilic substitution reactions of arenes are energetically more favourable than that of electrophilic addition reaction.
That's why alkenes prefer to undergo electrophilic addition reaction while arenes prefer electrophilic substitution reaction.
Alkynes on reduction with sodium in liquid ammonia form trans alkenes. Will the butene thus formed on reduction of 2 - butyne show the geometrical isomerism ?
Trans-2-butene formed by the reduction of 2-butyne is capable of showing geometrical isomerism.
Rotation around carbon-carbon single bond of ethane is not completely free. Justify the statement.
Alkanes can have infinite number of conformations by rotation around $\mathrm{C}-\mathrm{C}$ single bonds. This rotation around a C-C single bond is hindered by a small energy barrier of 1-20 kJ $\mathrm{mol}^{-}$due to weak repulsive interaction between the adjacent bonds. such a type of repulsive interaction is called torsional strain. In staggered form of ethane, the electron cloud of carbon hydrogen bonds are far apart. Hence, minimum repulsive force. In eclipsed electron cloud of carbon-hydrogen become close resulting in increase in electron cloud repulsion. This repulsion affects stablity of a conformer.
In all the conformations of ethane the staggered form has least torsional strain and the eclipsed form has the maximum torsional strain. Hence, rotation around $\mathrm{C}-\mathrm{C}$ bond in ethtane is not completely free.
