Preparation of Pure Diphenyl

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Experiment 2Date: 24 January 2006

Title:Crystallization of a Crude Sample
Objective:Preparation of pure diphenyl via crystallization of crude diphenyl dissolved in ethanol.

Structural Formula of Organic Substances Involved:

DiphenylEthanol

Apparatus:Round-bottom flask (100 mL), measuring cylinder (50 mL), conical flask (50 mL), filter flask, beaker (100 mL), stemless glass funnel, Hirsch funnel, capillary tube, long glass tube, watch glass, glass vial, water condenser, retort stand, retort clamps, spatula, cork stand, fluted filter paper, filter paper for Hirsch funnel, electronic weigh, electrothermal heater, oven, aspirator, electrothermal device to determine melting point.

Chemicals:Crude diphenyl (3.0 g), ethanol (20 mL), decolourising charcoal (1.0 g), boiling chips, ice

Procedure:

Crude diphenyl (3.0 g) is measured and obtained using a beaker and a laboratory electronic weigh. The crude diphenyl is then put into a quickfit flask, together with 2 to 3 pieces of boiling chips. Ethanol (5 mL) is measured using a measuring cylinder and poured into the flask. A vertical water condenser is fitted to the mouth of the flask, which is then heated with an electrothermal heater. Water is let to run throughout the condenser. Meanwhile, a piece of fluted filter paper is inserted into a stemless glass funnel in a beaker, which is later put in an oven to be heated. Ethanol (15 mL) is again measured using a measuring cylinder while decolourising charcoal (1.0 g) is weighed using the electronic weigh. When the mixture in the quickfit flask is dissolved completely, the heater is removed and the ethanol is added to the mixture followed by the decolourising charcoal. The mixture is then boiled under reflux for 10 – 15 minutes. The preheated glass funnel, beaker and fluted filter paper are taken out of the oven upon reaching 15 minutes while an ice bath is being prepared. The mixture in the quickfit flask is filtered by gravity through the fluted filter paper immediately after the reflux boiling time has elapsed. The filtrate is allowed to cool in the ice bath. The pure diphenyl is filtered through a Hirsch funnel and sucked dry using the aspirator for 10 minutes. The dry crystals obtained are transferred to a watch glass and weighed using the electronic weigh. The weight of the crystals is recorded. A small sample of the crystals is being used to determine the melting point using a capillary tube and an electrothermal device. The remaining crystalline products are retained in a labelled vial for presentation to the lecturer.

Observation:

Mass of crude diphenyl=2.99 g

Mass of empty vial + cap=12.50 g
Mass of empty vial + cap + pure
diphenyl crystals=14.41 g

Net mass of pure diphenyl crystals= (14.41 – 12.50) g
=1.91 g

Colour of pure diphenyl crystals: White, shiny flakes

Melting point range: 65.1°C – 69.0°C

Crystallization of crude diphenyl (2.99 g) gave pure diphenyl (1.91 g, 63.88 %) in the form of white, shiny flakes with melting point 65.1°C – 69.0°C (reference: 69.0°C).

Discussion:

Organic compounds in solid state at room temperature are often purified using the crystallization method. The process of crystallization can be rapid and non-selective. Therefore, this method incorporates the dissolving of the compound in a hot solvent that it is soluble in, usually an organic one like ethanol, filtration of the mixture for the removal of insoluble impurities and later gradual cooling of the solution mixture to form the desired pure compound in crystalline form. The solvent is usually heated under reflux to minimize its evaporation into the air.

In this experiment, ethanol is used as the solvent since it is chemically inert and does not react chemically with the compound used, namely diphenyl. It is also a volatile solvent and therefore, an electrothermal heater is used instead of a Bunsen burner to avoid any fire accidents. Owing to...
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