Eduqas Chemistry for A Level Year 2: Student Bk

124 Eduqas A Level Chemistry Year 2: Component 2 The comparative strength of C–Cl bonds in chlorobenzene and chloroalkanes Chloroalkanes such as 1-chlorohexane, react with aqueous sodium hydroxide on heating under reflux, giving hexan-1-ol as the main organic product. CH 3 (CH 2 ) 4 CH 2 Cl + NaOH CH 3 (CH 2 ) 4 CH 2 OH + NaCl The mechanism for this nucleophilic substitution reaction is CH 3 (CH 2 ) 4 C Cl CH 3 (CH 2 ) 4 C O H + Cl H H H H O – H δ + δ – – Benzene tends to react by electrophilic substitution. It has little tendency to react with nucleophiles, as these would be repelled by the stable π-system of electrons. The bond energies and bond lengths of aliphatic and aromatic carbon-to-chlorine bonds are shown in the table. Bond Bond length / nm Bond energy / kJmol –1 aliphatic C Cl 0.177 346 aromatic C Cl 0.169 399 The stronger (and shorter) bond between carbon and chlorine in chlorobenzene results from a non-bonding p electron pair on chlorine overlapping with the ring π-system of electrons. The resulting bond needs much more energy to be broken and is stronger that an aliphatic C–Cl bond. Forcing conditions are therefore needed to produce phenol from chlorobenzene. Cl OH + NaCl NaOH(aq) 300ºC/increased pressure This reaction is not an environmentally viable method of producing phenol commercially. In industry phenol is generally made from (1-methylethyl)benzene (cumene). This method has the advantage that propanone is a useful co-product of the reaction. YOU SHOULD KNOW › › › ››› the relative bond strengths of aliphatic and aromatic C–Cl bonds and the reason for this difference in bond strengths 9 Knowledge check A sample of 1-bromo-4-(bromomethyl)benzene was heated under reflux with aqueous sodium hydroxide. Give the displayed formula of the most likely organic product. & S C Stretch & Challenge Compound M contains both aliphatic and aromatic C–Cl bonds. On heating with aqueous sodium hydroxide, it gives a new compound of formula C 7 H 3 Cl 5 O. Deduce the structural formula of compound M, giving reasons for your answer.