Question

An alkyl halide by the formation of its Grignard reagent and heating with water gives propane. What is the original alkyl halide?
A. methyl iodide
B. ethyl iodide
C. ethyl bromide
D. propyl bromide

Answer 1

Hint: Alkyl halides are also known by the name halo alkanes or halogen alkanes. These are chemical compounds which are derived from alkanes which contains one or more halogen atoms. These are also known as these are a subset of the general class of halocarbons.

Complete step-by-step answer:
Alkyl halides are formed by the replacement of hydrogen atom by halogen atoms i.e. fluoride, chloride, bromide and iodide. Grignard reagent is one of the important reagents used in organic chemistry and it is an organometallic compound. These compounds have the ability to form new carbon-carbon bonds and have strong nucleophilic character.
Grignard reagent is an organomagnesium compound which is represented by the chemical formula $R-MgX$ where R refers to alkyl and aryl groups and X is halogen present in the compound. According to question an alkyl halide by the formation of its Grignard reagent and heating with water gives propane and the reaction by two steps:
1. Alkyl halide react with Grignard reagent which can be shown as:
$C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-Br+RMgX\to C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-MgBr+RX$
2. In second step the product further heats with water and gives propane which can be shown by the following reaction:
\[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-MgBr+{{H}_{2}}O\to C{{H}_{3}}-C{{H}_{2}}-C{{H}_{3}}+Mg(OH)Br\]

From this we can say that original alkyl halide is propyl bromide, option D is the correct answer.

Note: Alkyl halides are colorless in nature when they exist in pure form. But exceptional cases are seen in bromide and iodide which develop color when these are exposed to light and many volatile halogen compounds have a sweet smell like ethers.