Organic Chemistry l6zs

Title: Properties of Hydrocarbon

Title:

Properties of Hydrocarbon- (n-hexane, cyclohexene and toluene)

Objective:

1. To differentiate the type of hydrocarbon 2. To determine the properties among n-hexane, cyclohexene and toluene

Theory and background: Hydrocarbons are compounds which contain only carbon and hydrogen, can be classified into several types, depending on their structure. Aliphatic hydrocarbons are divided into three classes: alkanes have only single bonds, and are said to be saturated; alkene and alkyle have carbon-carbon double or triple bonds, and are said to be unsaturated. Aromatic hydrocarbons are cyclic compounds whose structure is related to that of the benzene with six-electrons in a delocalised ring. For this experiment, n-hexane was used as an example of saturated hydrocarbon, cyclohexene was used as an unsaturated hydrocarbon and toluene as an aromatic hydrocarbon. These three hydrocarbons are reacted differently toward the same reagent or condition, in which case it may be used to distinguish between them. Three methods that used to determine the properties of hydrocarbons are combustion, reaction with Bromine (Br2) and reaction with Potassium Permanganate (KMnO4).

Discussion: 1. Combustion of n-hexane, cyclohexene and toluene Observation:

Type of flame Time of burning Condition of evaporating dish

n-hexane Vigorous flame Short burning time Remain unchanged

cyclohexene Moderate flame Burn out after hexane Slightly become black

toluene Sooty flame The longest burning time Totally become black in colour

Hexane, C6H14 is a member of alkane family, where cyclohexene, C6H10 belongs to the alkene family and toluene, C7H8 belongs to the aromatic hydrocarbon. Three of them burn incompletely in air with a sooty flame. This is because these hydrocarbons have a high percentage by mass of carbon. Furthermore, air has only 21% of oxygen. Percentage of carbon in hexane:

[6(12)/6(12)+14(1)] * 100%= 83.71%

Percentage of carbon in cyclohexene:

[6(12)/6(12)+10(1)] * 100%= 87.80%

Percentage of carbon in toluene:

[7(12)/7(12)+8(1)] * 100%= 91.30%

CHAN JIUN HAUR

1003825

Title: Properties of Hydrocarbon

Thus, toluene burns with the sooty flame compared to hexane and cyclohexene as its carbon content is much higher. The carbon released while burning has indirectly cause the evaporating dish become black in colour. This has shown that the amount of soot collected while burning of toluene is the highest in this experiment.

2. Reaction with bromine

Exposed to light

Kept in the dark

n-hexane Reddish colour of bromine turns colourless after a few minutes Reddish colour of bromine remain in the same condition

cyclohexene Reddish colour of bromine turns colourless immediately Reddish colour of bromine turns colourless immediately

toluene Reddish colour of bromine remain unchanged Reddish colour of bromine remain unchanged

The colour of bromine solution remain unchanged while reacting with toluene because toluene does not have any double or triple bonds site to undergo addition of halogens. On the other hand, cyclohexene which has a double bond site will react with bromine solution immediately even in the dark condition. Hence, this will decolourise the bromine solution without delay. When hexane is exposed to light and reacted with bromine solution, the reddish colour of bromine will decolourise because of the halogenations of alkane with free-radical mechanism. But when the reaction is kept in the dark, the colour of bromine remain unchanged because energy from UV light is needed to carry out the fission. Mechanism of halogenations Bromination of alkanes is by a free-radical mechanism. Free radicals are formed by hemolytic fission of the bromine molecule. The Br-Br bond cleaves in a symmetrical way, so that each bromine atom retains its own electron from the shared pair. Energy from UV light is needed to carry out the fission. Free radicals are actually bromine atoms. The bromine radical which is also a bromine atom with seven electrons is extremely reactive because of its urgency to complete its octet. When bromine is exposed to light and heat, it will break into two bromine radicals.

CHAN JIUN HAUR

1003825

Title: Properties of Hydrocarbon

The reactive Br. radical abstracts a hydrogen atom from hexane to form hydrogen bromide and CH3CH2CH2CH2CH2CH2. radical will react with another bromine molecule to generate the Br. radical. This reaction will decolourise the bromine solution.

Addition of halogens When cyclohexene reacts with bromine in tetrachloromethane, reddish colour of bromine turns colourless immediately. According to the mechanism of electrophilic addition, although the bromine molecule is non-polar, random movements of electrons within the bromine molecule may cause an unsymmetrical distribution of electrons at one end of the molecule. Even though this occurs only for a short while, a small dipole is formed. This momentary dipole alters the electron distribution of neighboring bromine molecules. The momentary bromine dipole is said to induce an oppositely-oriented dipole to its surrounding bromine molecules. The partially-charged positive ends of the momentary and induced bromine dipoles will behave as electrophiles to attack the double bond of alkene.

The first step in the electrophilic addition mechanism is the attack of an electron-seeking reagent or an electrophilic. The C=C bond is the reactive site that supplies the electron. This has caused the adjacent C atom to lose its electron in the shared pair and to consequently become C+. A positively charged C atom is called a carbonium ion. There is a pair of bonding electrons in the bromine molecule being transferred to the Br- atom. This gives the Br- atom another electron besides its own electron contribution to the bonding pair. The Br- atom breaks away as the Br- ion.Then, Br- ion will combine with the carbonium ion to form 1,2dibromocyclohexane.

CHAN JIUN HAUR

1003825

Title: Properties of Hydrocarbon

3. Reaction with Potassium Permanganate

Changes of KMnO4

n-hexane Purple colour of potassium permanganate remain unchanged.

cyclohexene Purple colour of potassium permanganate disappears and form a yellowish brown colour.

toluene Purple colour of potassium permanganate remain unchanged.

There is only cyclohexene will decolourise the purple colour of potassium permanganate because of the present of double bonding in the hydrocarbon. Addition reaction across the double bond Because of the cyclohexene is unsaturated hydrocarbon, the most important reaction is addition reaction across double bond. The cyclohexene is readily oxidized by potassium permanganate to form glycol. During the oxidation of alkenes, the purple colour of the permanganate solution disappears and the reaction constitutes a test, known as Baeyer’s Test, to detect unsaturation in any compound. Precaution: 1. The combustion experiment should be performed in the hood. 2. The bromine solution should be dispensed under the hood because bromine is an extremely strong oxidizing agent. 3. All of the hydrocarbons in the test tube should be closed by using a stopper so that it will not evaporate into the atmosphere. Conclusion: Hexane, cyclohexene and toluene have different chemical properties. Toluene burns with the sooty flame compared to hexane and cyclohexene as its carbon content is much higher. The colour of bromine solution and potassium permanganate remain unchanged while reacting with toluene because toluene does not have any double or triple bonds site to undergo addition of halogens. Besides, hexane burns with the clean flame as its carbon content is very low. When hexane is exposed to light and reacted with bromine solution, the reddish colour of bromine will decolourise because of the halogenations of alkane with free-radical mechanism. But when the reaction is kept in the dark, the colour of bromine remain unchanged because energy from UV light is needed to carry out the fission. Moreover, the purple colour of potassium permanganate will not be decolourise in hexane because it is a saturated hydrocarbon, so it does not have any bond site to react with KMnO4. Cyclohexene which has a double bond site will react with bromine solution and potassium permanganate immediately by decolourising them.

CHAN JIUN HAUR

1003825