1.First prepare benzyl chloride from benzene by chloromethylation with formaldehyde, HCl in presence of ZnCl2. The benzyl chloride can be converted to benzyl cyanide by reaction with KCN. This on hydrolysis will give phenyl acetic acid.

2.Convert benzene to benzaldehyde by gattermann Koch aldehyde synthesis( reaction of benzene with CO & HCl in presence of AlCl3) or Gattermann formylation ( reaction of benzene with HCN & HCl in presence of AlCl3 followed by hydrolysis). Benzaldehyde is then reacted with HCN to give cyanohydrin which on reaction with HI/Red P gives phenyl acetic acid in good yield.