Consider the following statements about the preparation of haloalkanes.
(A) Free radical chlorination of an alkane gives a complex mixture of isomeric mono- and polyhaloalkanes, so the yield of any single compound is low.
(B) Addition of bromine in CCl4 to an alkene gives gem-dibromides.
(C) Good yields of alkyl iodides are obtained by heating alcohols with potassium iodide in 95% sulphuric acid.
(D) Phosphorus tribromide is usually generated in situ by the reaction of red phosphorus with bromine.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (C) only
Haloalkanes are made from alcohols (the –OH group is replaced by halogen) or from hydrocarbons (substitution in alkanes, addition to alkenes). Each method has its own reagent and its own limits.
(A) Free radical chlorination of an alkane gives a complex mixture of isomeric mono- and polyhaloalkanes, so the yield of any single compound is low. — Correct statement. The products are difficult to separate as pure compounds, so the method is poor for making one haloalkane.
(B) Addition of bromine in CCl4 to an alkene gives gem-dibromides. — Incorrect statement. The two bromine atoms add to the two carbons of the double bond, which are adjacent, so vic-dibromides are formed. The reddish brown colour of bromine is discharged:
\( \displaystyle \ce{CH2=CH2 + Br2} \xrightarrow{\ce{CCl4}} \ce{BrCH2-CH2Br} \)
(C) Good yields of alkyl iodides are obtained by heating alcohols with potassium iodide in 95% sulphuric acid. — Incorrect statement. The acid used is 95% orthophosphoric acid. Sulphuric acid cannot be used: it converts KI to HI and then oxidises HI to I2.
(D) Phosphorus tribromide is usually generated in situ by the reaction of red phosphorus with bromine. — Correct statement. ‘In situ’ means it is produced in the reaction mixture itself; PI3 is made the same way from red phosphorus and iodine.
NCERT (2026-27 edition):
6.4.2 From Hydrocarbons – “Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds. Consequently, the yield of any single compound is low.”
“(ii) Addition of halogens: In the laboratory, addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless…”
6.4.1 From Alcohols – “…Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid. … Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively.”
Answers to Some Intext Questions – “6.2 (i) H2SO4 cannot be used along with KI in the conversion of an alcohol to an alkyl iodide as it converts KI to corresponding acid, HI which is then oxidised by it to I2.”