Chapter 13: Hydrocarbons

The negative charged developed on one carbon attacks the proton from NH3 and another sodium atom lose its electron to develop a second negative charge on the atom. This negative charge is finally neutralised by the attack of the second proton to give a trans-but-2-ene.

This but-2-ene produced shows geometrical isomers as cis and trans.

The single bond in ethane is a σ – bond, which is a coaxial overlap of orbitals, so the C ─ C bond can be rotated on its axis. But this rotation is not completely free due to the torsional strain that the bond undergoes due to rotation.

The staggered configuration is more stable as compared to an eclipse, as there is less C – H bond pair repulsion and the atoms are at a maximum distance away from each other.

HI >HBr>HCl is the increasing order of reactivity. The increasing order of reactivity of halogen is same as their increase in bond energies.

This reaction is an example of Friedel Crafts alkylation using a lewis acid.

In the first step, there will be a formation of the carbocation, then a secondary carbocation will be formed which is more stable. There will be a hydride shift. At last nucleophilic attack will happen to the benzene ring which forms the major product cumene and a minor product of primary carbocation.

(i) when bromine is treated with Br2 in presence anhydrous FeBr3 it undergoes electrophilic substitution to give bromobenzene. Again treated with conc. HNO3 and Conc. H2SO4 at 323K we get a p-nitrobromobenzene

(ii) We need to deactivate the benzene by the introduction of a nitro group using conc. HNO3 and H2SO4 and then treat with Br2 in presence of FeBr3 we will get m-nitrobromobenzene.

Halogens have an outer configuration of ns2p5 which shows it can accept one electron and are close to completing its octate; halogens show a strong affinity to attract one electron and thus having a higher electronegativity and show negative inductive effect.

Nitro group has a nitrogen atom bonded to two highly electronegative oxygen atoms. This results in a net decrease of electron density around nitrogen atom and imparts a positive δ+ charge on nitrogen.

(i) By intermolecular condensation method, we can cycle acetylene and then treated at high temperature in a red hot iron tube.

(ii) The aliphatic compound thus converted into benzene is treated with con.HNO3 and Conc.H2SO4 which undergoes nitration to give nitrobenzene.