Analysis of Hydrocarbons

Analysis of Hydrocarbons

Jessa Grace B. Castillo* and Stephanie Chua

Department of Chemistry, University of Santo Tomas, Manila, Philippines

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Hydrocarbons are organic compounds containing only hydrogen and carbons and further divided in major types. Hydrocarbons react differently in reagents. The basis of determining an unknown hydrocarbon is due to the differences in reactions of hydrocarbons. An unknown compound is determined using tests. The unknown gives a positive result in nitration test and gives no brown precipitate in oxidation test. The unknown is an aromatic and non-alkylated compound.

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Introduction:

Hydrocarbons are organic compounds that composed of only hydrogen and carbon. There are two major types of hydrocarbons; aliphatic (saturated and unsaturated) and aromatics.

Aliphatic compounds are characterized by a continuous or branched chain arrangement aliphatic compounds are subdivided into two; saturated and unsaturated. Saturated hydrocarbons are characterized by single bonds between adjacent carbon atoms. Saturated hydrocarbons are alkanes and cycloalkanes, this compounds are relatively inert and do not react with common laboratory reagents. Alkanes do, however react with oxygen, halogens and other substances under control conditions. All members of alkane series have the general chemical formula CnHn-1. Alkanes are insoluble in water; other physical properties vary with molecular weight. Members C1 to C4 are gasses at room temperature and pressure; C5 to C17 are liquids. Higher members are solids and usually called as paraffin. Cycloalkanes or alicyclic compounds are rings of carbons. Cycloalkanes have a general formula of CnH2n. They are less flexible than the open chains, because open chain molecules can rotate to reduce stress while alicyclic compounds have less freedom of rotation. Larger cycloalkanes have increasingly more rotational freedom. In naming saturated compounds, names ends with -ane.

Unsaturated hydrocarbons on the other hand readily undergo addition reactions and oxidation reactions. Unsaturated Hydrocarbons are characterized by double and triple bonds between adjacent carbon atoms; alkenes and alkynes. Alkenes, often called olefins, have the general formula CnH2n. Alkenes are more chemically active than the saturated hydrocarbons because the double bond is susceptible to attack by other reagents. Systematic names for alkanes are derived by appending the suffix -ene to the root name. Rotation around the double bond cannot occur and substituted alkenes can therefore exist as cis-trans stereoisomer. The geometry of the double bond can be specified by application of the Cahn-Ignold-Prelog sequence rule; assigning higher priority with the atom with higher atomic number.

Aromatic compounds do not readily undergo addition reactions but are characterized by substitution reactions in which another atom or group of atoms replaces a ring hydrogen. The compounds were named aromatic because many of them have a strong odor. Aromaticity means a distinctive type of chemical stability or low reactivity because of resonance. According to Huckel 4n + 2 rule, a molecule is aromatic only if it has a total of 4n + 2 π electrons, where n is an integer. Aromatic compounds react with electrophiles to give substitution products, in which cyclic conjugation is retained, rather than addition products, in which conjugation is destroyed.

Hydrocarbons react differently in different reagents, there are three known tests that can be used to determine what type of hydrocarbon a molecule is: nitration, basic oxidation and the bromine test.

In this experiment, the various types of hydrocarbon were characterized using parallel chemical tests and a scheme was devised to distinguish hydrocarbons from each.

Results and Discussion:

Hydrocarbons, although all contains only carbon and hydrogen, react differently in reagents due to the difference in properties. The difference in the reactions of hydrocarbons is the basis of determining an unknown hydrocarbon. Parallel chemical test such as nitration test, oxidation and bromine test are used to determine what type of hydrocarbon is a compound.

Table 1.1 Results of chemical tests

Nitration Test

Basic Oxidation Test

Bromine Test

Hexane

2 Layers: upper layer- clear

lower layer-clear

Violet

Cloudy soln

Cylohexane

2 layers: upper layer- clear

lower