Question

4. An unknown compound X is one of the five compounds listed in the table below. A mixture of X with 2-methyl-3-pentanol melts at 118-125 °C, a mixture of X with 2-nitro-4- methoxyaniline melts at l 10-120°C, a mixture of X with 2-chloroethanol melts at 127-120°C, a mixture of X with 4-methyl-2-pentanol melts at 115-122 °C, and a mixture of X with 1- naphtylacetic acid melts at 108-121 °C. Give the identity of X and explain your reasoning. Compound 2-methyl-3-pentanol 2-nitro-4-methoxyaniline 126-129 2-chloroethanol 4-methyl-2-pentanol 1-naphtylacetic acid m.р., ос 126-128 128-130 131-132 130-132

Answer 1

4) Melting point of a compound can be defined as the temperature required to convert one solid substance to it's liquid state.

Now melting point of pure crystalline substance is sharp and has narrow temperature range i.e 2° or less than 2°. And melting point of mixture gets depressed and the temperature range becomes larger.

Now if the temperature range of melting point of a mixture is nearly 2° and have a melting point nearly equal to that of the one pure known compound , then the mixture is actually the mixture of two same compound.

Here , 2-chloroethanol has a melting point of 128°-130°C.On mixing compound X with this pure compound the mixture has a melting point of 127°-129°C.The two temperature range are quite equal i.e the mixture act as a pure compound.

So the compound X is 2-chloroethanol.