Question

Exp 24 Report Sheet This is the equivalent of p. 297-298 in your lab manual, but easier to read. Be sure to identify the three unknowns at the bottom of page 2.

Physical Properties of Hydrocarbons

Solubility: Indicate if the substance is soluble or insoluble in water and ligroin. Does this indicate that the substance is polar or non-polar?

Density: Is the substance more dense than water (sinks) or less dense than water (floats). For ligroin, can you tell anything about relative density?

	H2O		Ligroin
Hydrocarbon	Solubility	Density	Solubility	Density
Hexane	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix
Cyclohexene	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix
Toluene	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix
Unknown A	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix
Unknown B	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix
Unknown C	Insoluble	Lighter than water (floats)	Soluble	Cannot determine relative densities when two liquids mix

Chemical Properties of Hydrocarbons

Hydrocarbon	Combustion	Bromine Test	KMnO4 Test	H2SO4 Test
Hexane	Burns clean	Remains red, even after addition of AlCl3. No rxn	Stays purple, no rxn	H2SO4­ does not mix with hexane. No rxn.
Cyclohexene	Some soot	Readily changes from Red to colorless	Brown precipitate forms	H2SO4­ mixes with cyclohexene. Heat detected.
Toluene	Heavy soot	Remains red, changes to colorless only after addition of AlCl3.	Stats purple, no rxn	H2SO4­ does not mix with toluene. No rxn.
Unknown A	Some soot	Readily changes from Red to colorless	Brown precipitate forms	H2SO4­ mixes with Unknown A. Heat detected.
Unknown B	Heavy soot	Remains red, changes to colorless only after addition of AlCl3.	Stats purple, no rxn	H2SO4­ does not mix with Unknown B. No rxn.
Unknown C	Burns clean	Remains red, even after addition of AlCl3. No rxn	Stays purple, no rxn	H2SO4­ does not mix with Unknown C. No rxn.

           

Unknown A is ___________________

Unknown B is ___________________

Unknown C is __________________

Answer 1

On the basis of the chemical properties listed of the unknowns we can determine the identity of the unknown

Unknown A

observations

presence of some soot = this compound may be cyclic alkane/ alkene

br2 test= decolourisation = +ve test = presence of the unsaturation (alkene)

KMnO4 test= decolourisation = +ve presence of the alkene unsaturation

H2SO4 tets= soluble =  compound maybe alkene .Sulfuric acid reacted with the alkene because they are unsaturated forming alkyl hydrogen sulfate. The π-electrons of alkenes are very reactive toward sulfuric acid (and other sources of H+). The π-electrons attack the H+to form a carbocation, which reacts with the bisulfate anion.

therefore the given unkown has most of the physical and chemical properties similar to cyclohexene there fore

Unknown A = Cyclohexene

Unknown B

presence of some soot= compound must be aromatic

Br2 test= no decolourisation = absence of alkene unsaturations

KMnO4 test= no decoulorisation = absence of alkene unsaturations

H2SO4 test= -ve as h2so4 reacts with bases and alkenes alkanes do not react with toluene

from the above observations it is clear that the compound is alkane + aromatic ring it has most of its physical and chemical properties similar to toluene

therefore , Unknown B = Toluene

Unknown C

Absence of soot = Open chain compound

Br2 test= no decolourisation = absence of alkene unsaturation

KMnO4 test=no decolourisation = absence of alkene unsaturation

H2SO4test= -ve as alkanes do not react with acids

from the above observations it is clear that the unknown compound is an open chain alkane its physical and chemical propertiues match mostl;y with hexane\

therefore Unknown C = Hexane