Sec 1 Preparation of RX ■2.Nith HX HX reflux HX is coming from NaX conc.H2SO4
Sec 1 Preparation of RX ■3.From alcohol ROH PCls- →RCl+POCL3+HCI (phosphorus oxychloride) ROH+PCL3→RCI+HPO3 ROH+P+Br2→RBr+H3PO3 We rarely start with PCla,PBra or PI3,since they are easily hydrolysed by moisture in air. Instead,they are made in situ,using red phosphorus in the alcohol and halogen is put into the reaction flask as the reaction proceeds
Sec 1 Preparation of RX 3.From alcohol -cont. ROH+HX→RX+H,O (usually HBr,HI BUT RCI cannot be prepared by this method) Pyridine (as solvent) ROH SOCI2 RCI SO2+HCI (This product is most easily purified)
Reaction of Alcohols with Hydrogen Halides ROH HX RX HOH Hydrogen halide reactivity HI HBr HCI HF most reactive least reactive Alcohol reactivity R3COH R,CHOH RCH,OH CH3OH Tertiary Secondary Primary Methanol most reactive least reactive
Examples SOCl2 CH3CH(CH2)5CH3 K2C03 CH2CH(CH2)sCH3 OH ci (81%) (pyridine often used instead of K2CO) PBr3 (CH3)2CHCH2OH (CH3)2CHCH2Br (55-60%)