Doreen Foy

Organic Chemistry 1 Laboratory

The Recrystallization of Benzoic Acid and Determination of its Melting Point

Hannah Loch

10/01/2012

Abstract

The goal of the experiment was to obtain pure benzoic acid crystals from crude benzoic

acid by using the laboratory techniques of recrystallization. Water was used as the suitable

solvent which was heated using a heating mantle to dissolve the crude benzoic acid crystals.

Charcoal was used to remove any colored impurities and it was filtered out together with any

insoluble impurities to obtain a clear benzoic acid filtrate. The clear filtrate was cooled on an ice

bath for pure benzoic acid crystals to precipitate out of the solution. The percent yield of the

crystals was calculated to be 70.8%. The melting point of the benzoic acid crystals was measured

(120.2 °C) and compared to the literature value (122.4°C) 1

to determine the purity of the

crystals. Since the melting point temperature of the benzoic acid crystals was close to the

literature melting point temperature, it was concluded that the crystals were indeed pure, taking

into account any experimental errors.

Introduction

Chemical reactions often result in a mixture of products, some of which are not of interest

to the chemists; such mixtures are not pure. Chemists therefore need a way to isolate the product

of interest from the original mixture of products, thus, purifying that product of interest from

impurities. For an impure solid mixture, the solid of interest is more concentrated that the

impurities. The impurities may include some combination of insoluble, soluble, and colored

compounds. There are many techniques used by chemist to purify compounds, such as

fractionation, filtration, extraction, and recrystallization. Fractionation refers to a purification

strategy in which some relatively inefficient purification method is repeatedly applied to isolate

the desired substance in progressively greater purity2. Filtration is a mechanical method used to

separate solids from liquids or gases by passing the feed stream through a porous sheet such as a

cloth or filter paper, which retains the solids and allows the liquid to pass through2. Extraction

removes an impurity from a solid, or recovers a desired product, by dissolving it in a solvent in

which other components of that solid are insoluble in that solvent2. Recrystallization separates a

product from an impure liquid, often in extremely pure form, by cooling the liquid or adding

precipitants which lower the solubility of the desired product so that it forms crystals. The pure

solid crystals are then separated from the remaining liquid by filtration2.

Recrystallization is the primary method for the purification of non-volatile solids which

are solids that do not vaporize readily and it is the purification method used in this laboratory.

Recrystallization is possible because of the thermodynamics of the processes that take place

which can be explained using Gibbs free energy (∆G), which is the energy associated with a

chemical reaction that can be used to do work (Equation 1). For a reaction to occur, the value of

∆G has to be negative. ∆G is negative for recrystallization due to enthalpic stabilization in

crystals because the process is going from a less ordered system to a more ordered system. The

enthalpy is negative and larger in magnitude than the positive entropy of the system which

makes ∆G negative, thus the process of recrystallization is favorable.

ΔG = ΔH – T ΔS (Equation 1)

Solubility of solid compounds in different solvents plays a very crucial role in the process

of recrystallization. It largely depends on the intermolecular interactions between the solid

compounds and the solvents. It is often stated that “like dissolves like”, meaning that compounds

that have similar structures will be soluble in one another. That is polar compounds dissolve in

one another while non-polar compounds dissolve in one another. There are three solubility

stages: collision, dissociation and solvation. Collision is when the solvent molecules collide with

the surface of the solid compound. The solid molecules become the solute component of the

solid-solvent solution. Dissociation is when the solute molecules disassociate from each other

and leave their crystal lattice by breaking the intermolecular forces that hold them together.

Temperature affects the dissociation of the molecules because at a higher temperature the solvent

has a higher kinetic energy and is more likely to collide with the solute causing the solute

molecules to separate. Solvation is when the solvent molecules surround and interact with the

solute molecules. At this point, the solute is said to have dissolved in the solvent because the

solute is present in the solution. If a solid compound is less soluble in a given solvent than an

impurity, at higher temperatures, the solid compound will be more saturated in the solution than

the impurity.

The general technique for recrystallization involves dissolving the impure solid in a

minimum amount of an appropriate hot solvent (hot solvation) and cooling the solution slowly to

allow the solid crystals to re-form. The solid compound being purified will recrystallize out

because it is less soluble at a lower temperature and the impurity will still be dissolved in the

solution because it is more soluble and less saturated at a lower temperature, thus the solid

compound can be filtered out. The difference in the solubility of the compounds present in the

impure solid is very crucial for recrystallization. If the desired compound and the impurities

dissolved in a particular solvent have different solubility’s, it becomes possible to isolate the

desired compound from the impurities.

Choosing a solvent determines the outcome of recrystallization and so it is important to

choose the right solvent. When choosing a solvent, it is necessary to choose one in which the

desired compound would be highly soluble at high temperatures but not at low temperatures.

This is important because if the compound is highly soluble in the solvent at low temperatures, it

will be difficult for it to recrystallize when cooled, thus making the process of recrystallization

useless. Also, the boiling point of the solvent should be lower than the melting point of the solid.

The solvent should dissolve all the impurities easily or not at all; if the impurities do not dissolve

at all, they can be removed by hot filtration before the solution is cooled. On the other hand, if

the impurities fully dissolve in the solvent, they will remain in solution when the desired

compound reforms. Also the solvent must be non-reactive with the solid to avoid changing the

property of the solid, relatively volatile in order to evaporate off the formed crystal and cheap.

Sometimes it is difficult to find a solvent that will dissolve the compound when it is hot, but will

precipitate it out when it is cold. If no solvent can be found with the appropriate solubility

parameters, sometimes it is possible to create an appropriate solvent by mixing two inappropriate

miscible solvents. By using two solvents, it is possible to successfully complete the

recrystallization. The solvent pair should be chosen such that one solvent dissolves the solute at

elevated temperature and the other one does not dissolve the solute3.

Charcoal is used in some cases to remove colored impurities which are usually large and

nonpolar and so they stick to the charcoal surface and can be filtered out by hot filtration because

charcoal will be insoluble in the solvent. The filtrate solution is cooled slowly to prevent the

trapping of the impurities in the pure crystals being formed. As the pure crystals begin to form

they require a surface for the molecules to begin to adhere and the process of starting efficient

crystal growth is called nucleation. Nucleation can be induced if the crystals take much time to

recrystallize. Some methods of induced nucleation include; scratching the sides of the flask with

a glass rod, seeding the solution with some pure compound, and forcing the solution to cool on

ice. What these methods have in common is the fact that the presence of few crystals in solution

help anchor other crystals in the solution, thereby speeding the recrystallization process. The

formed crystals are isolated from the solution by vacuum filtration and allowed to air dry.

Since the main purpose is to obtain pure crystals, the purity of the compounds can be

determined by measuring the melting point of the crystals formed using a Mel-Temp apparatus

and comparing it to the literature melting point of that compound. The melting point of a solid is

the temperature at which the solid and liquid phase is at equilibrium and so the rate of the liquid

formed equals the rate of the solid formed. Experimentally, the melting point of a solid is a

range. The lower temperature point is when the crystals just start to melt which is very noticeable

because the crystals are distorted and the higher temperature point represents when all the

crystals have melted into a liquid. If the experimental melting point of a solid is close to the

literature melting point, then the compound can be said to be pure. The smaller the range (up to a

one degree difference,) the more pure the crystals and the larger the range, the less pure the

crystals.

Reagent table 1, 4

Name and

structure

Molecular

weight(g/mol)

Amount

used

Density(g/mL) Melting

point

(Celsius)

Boiling

point

(Celsius)

Benzoic

Acid

122.1g/mol 3.97g 1.32 g/mL 122.4 °C 249 °C

Water

18.01g/mol ≈75mL 1.00g/mL

0 °C 100°C

Experimental

A 3.97g sample of wet crude benzoic acid was placed in a 125mL Erlenmeyer flask and a

minimum amount (≈75mL) of boiling water that was boiled on a heating mantle (scheme1) was

slowly added to it to dissolve the crystals. Charcoal was added to the flask after the crystals had

fully dissolved to decolorize the solution. A hot gravity filtration technique was performed using

fluted filter paper (scheme 2). The Erlenmeyer flask was kept on the heating mantle during the

hot filtration process. The filtrate was removed from the steam bath and allowed to cool at room

temperature in the hood to form pure crystals as the benzoic acid recrystallizes. The filtrate was

cooled with an ice bath to allow the crystals to form a little faster. The pure crystals were

collected by vacuum filtration (scheme 3). The mass of the benzoic crystals collected was 2.81g

(23.0mmol). The melting points of crude and pure benzoic acid were measured simultaneously

using a Mel-Temp apparatus (scheme 5) and they were compared to the literature melting point

of benzoic acid.

Melting point range of crude benzoic acid: 118.3 °C - 119.8 °C.

Melting point range of pure benzoic acid: 119.1°C - 120.2 °C.

Results

The crude benzoic acid crystals were wet and had a light pink color. When hot water was added

to dissolve the benzoic acid crystals, the solution was light pink. Adding charcoal to the solution

changed the color to black and made the solution thick. The hot gravity filtration process was

very slow and the filtrate was very clear. The filtrate looked a little oily. The crystals that started

to form looked like small moth balls. When the ice bath was used to cool the solution, there was

an increase in the rate of formation of crystals. The isolated pure crystals from vacuum filtration

had a white color and as they air dried, they looked like snowflakes. In melting point procedure,

when the capillary tubes where place in the Mel-Temp, the crystals were very visible and so

when the crystals started to melt, it was very noticeable because they looked distorted. When the

crystals had completely melted, the liquid in the capillaries was visible.

Calculations

Discussion

Recrystallization purifies impure solids thus every step is crucial to getting a good

percent yield of pure crystals. Picking the right solvent is crucial to dissolving the solid

compound. Water was chosen as the solvent for the recrystallization of benzoic acid because

benzoic acid is highly soluble in it at high temperatures but insoluble in it at room temperature.

This makes water a good solvent for the recrystallization of benzoic acid, since it meets the

criteria for a good solvent to be used for such a process. Benzoic acid dissolved in water at high

temperatures because the energy applied to the system in the form of heat increased the

temperature of the system which causes an increase in the kinetic energy of the solvent and

solute molecules and thus increase the collision between them. The energy provided is also

needed to break the intermolecular forces between the crystallized benzoic acid molecules and

water molecules, thus dissociating them from their crystal lattice structure. The process for the

recrystallization of benzoic acid is possible because the Gibbs free energy (∆G) of the system is

negative. Another reason why water was chosen was because its boiling point (100 °C) 4

is lower

than the melting point of benzoic acid (122.4 °C) 1

so during the recrystallization process,

benzoic acid did not melt.

It is important that the crystal solution be kept near boiling to maximize solubility which

is why the solution is kept on the steam bath as boiling water is added to it. After the crude

crystals had dissolved, the solution was light pink in color because it contained methyl orange.

Charcoal was used to decolorize the solution because charcoal is finely divided carbon which

absorbs organic molecules that are highly conjugated and thus colored. The insoluble impurities

together with the charcoal and colored impurities were removed by the process of hot filtration.

Hot filtration was performed instead of vacuum filtration because the benzoic solution needed to

be kept hot. Had vacuum filtration been used, the drop in pressure would have caused a decrease

in temperature of the benzoic acid solution which would in turn cause benzoic acid crystals to

start to come out too early. Fluted filter paper was used in the hot gravitational process because it

maximizes the filtering surface area thereby hastening filtration, it minimizes the contact

between the paper and the funnel and it does not have multiple layers of paper that cone filters

have thus minimizing precipitation. The solution had to be cooled slowly to prevent the insoluble

impurities from being trapped in the pure crystals as they come out of the solution because if

they do, the goal of the experiment will be of no use. The benzoic acid crystals formed allow

only other benzoic acid crystals to be incorporated in the crystal lattice structure. Nucleation was

induced by cooling the solution on an ice bath.

The melting point of the recrystallized crystals (120.2°C) was very close to the literature

melting point (122.4°C) 1

which indicates that the crystals formed are pure. Also the magnitude

of the melting point range (1.1°C difference) indicates that the crystals formed are indeed pure. It

makes sense that the range for the melting point of crude benzoic acid is higher than that of pure

benzoic acid, because crude benzoic acid contains impurities which lowers the melting point of

benzoic acid and thus increasing its melting point range. It equally makes sense that the melting

point of the recrystallized benzoic acid is closer to that of the literature value because it contains

little or no impurities.

Since the experimental value of the melting point is lower that the literature value it

suggests that the recrystallized crystals might still contain a few impurities because the solution

was rapidly cooled on an ice bath instead of being allowed to slowly cool at room temperature

due to time constraints, thus trapping some impurities. The experiment yielded a 70.8% of pure

benzoic acid which is due to some experimental errors. Maybe too much water was added to the

crystals to dissolve, thus making fewer crystals to be formed. Other possible error sources might

have been due to incomplete product transfer, for example, when the crude benzoic acid was

transferred from the watch glass to the Erlenmeyer flask and also during the hot filtration

process. Also, some crystals might have started to precipitate early in the filter during the hot

filtration. The 70.8% yield can be considered high if it still contains a significant amount of

impurities due to the rapid cooling on the ice bath.

Works Cited

1. "Benzoic Acid Compound Summary." Pubchem. National Center for Biotechnology

Information, n.d. Web. 28 Sep 2012.

2. "Sidepad Articles." List of Purification Methods in Chemistry. N.p., n.d. Web. 28 Sep

2012. <http://www.sidepad.com/List_of_purification_methods_in_chemistry>.

3. "Everything you want to know about recrystallization and melting points.” Penn State,

n.d. Web. 28 Sep 2012.< http://spot.pcc.edu/~chandy/241/recrystallization.htm>.

4. Padias, A. B. “Making the connections a how-to guide for organic chemistry lab

techniques”. Second. Plymouth: Hayden-McNeil Publishing, 2011.