Laboratory report on organic tests for functional groups PDF

Preview

Summary

Essay type report. ...

Description

Laboratory report on organic tests for functional groups Introduction: Hydrocarbons are molecules which are made up of hydrogen and carbon atoms. Carbons can form four bonds whilst hydrogen can form one bond. They tend to have strong covalent bonds, which would give them a high melting point (McMurry et al 2014). Alkanes are saturated hydrocarbons with the general formula of C nH2n+2. They are non-polar meaning the charge within the molecule is spread out equally since they do not contain electronegative atoms so they cannot undergo electrophilic addition reactions. Alkanes form a tetrahedral structure. On the other hand, a haloalkane is an alkane with an electronegative halogen. An example of an alkane is cyclohexane. Cyclohexane has the molecular formula of C6H14 and can be used in cleaning products. The bigger the alkane, the higher the boiling point due to bigger surface area. Branched alkanes have a lower boiling point than chained alkanes. Primary alkane has 1 methyl group attached to a carbon atom. Secondary alkane has 2 methyl groups attached to a carbon atom. Tertiary alkane has 3 methyl groups attached to a carbon atom (Smith 2013). Alkenes are unsaturated hydrocarbons with the general formula of C nH2n and contain one or more carbon to carbon double bonds (C=C) which does not rotate due to can form E-Z isomers. The double bond is electronegative since the

π

π

orbital so they

orbital contains a

cloud of electrons which means alkenes can react with electrophiles by the electrophilic addition mechanism. There are different types of alkenes. Internal alkenes are when C=C are within the molecule and external are at the end of the molecule. Cycloalkenes are when a ring of hydrocarbon contains a C=C. An example of alkene is cyclohexene which has the molecular formula of C 6H12 and contains 1 double bond. A primary alkene is when 1 methyl group is added onto a carbon atom. Secondary alkene is when 2 methyl groups are added onto a carbon atom and a tertiary alkene is when 3 methyl groups are added onto a carbon atom. Alkenes are trigonal planar. Alkenes can be produced from primary or secondary alcohols using hydrochloric acid via the dehydration mechanism (Tro 2015).

Figure 1 – dehydration of an alcohol which forms an alkene (Wade 2011).

Bromine water is an orange-brown liquid. It is used to test unsaturated solutions such as alkenes. In the presence of unsaturated hydrocarbons, the orange solution will turn colourless since the unsaturated hydrocarbon will undergo electrophilic addition to form a colourless bromine haloalkane. No colour change will occur when bromine solution is added to saturated hydrocarbons since they do not undergo electrophilic addition mechanism with Br-Br molecules (Clugston et al 2000).

Figure 2 – this shows an example of a mechanism of bromine water reacting with an alkene (Clarkson 2009). Carbonyl compound contains a carbon to oxygen double bond (C=O). Examples are carboxylic acid, ketones, aldehydes, acid anhydrides, and acyl halides. Carboxylic acid is a proton donor so has a low pH. This is due to C=O being more electron rich than the O-H bond so the molecule is more likely to give up the H atom. The central carbon atom is double bonded to an oxygen and single bonded to an oxygen. Oxygen and hydrogen are hydrogen bonded to each other. This is because the oxygen atom is more electronegative than the hydrogen atom. Carboxylic acid are held together by dimers which is when hydrogen bonds hold together 2 molecules. Carboxylic acids have strong intermolecular forces so have a higher boiling point than esters and alcohols. An example of carboxylic acid is an acetic acid (CH3COOH) which can be found in vinegar (Hathaway 2011).

Figure 3 – carboxylic acid functional group (Leach 2009). Universal indicator solution can be used to test the pH of a solution. It changes colour depending on the pH of the solution. It starts off as a brown colour and changes to red in acidic conditions and changes to purple in alkaline conditions. It can be used to see if the compound is

a carboxylic acid. Since carboxylic acids are acids, the universal indicator will change to red (Hathaway 2011). Sodium bicarbonate (NaHCO3) can also be used to determine whether a compound is an acid or not. When it is added to an acid such as carboxylic acids, it forms bubbles of carbon dioxide gas (CO2) (Li et al 2007). Ketones and aldehydes are carbonyl compound. Their functional group is C=O, which is unsaturated and polar, so the electron distribution in the molecule is not evenly disturbed giving it a partial charge. This means the double bond is electron rich so is reactive. Aldehydes have one R group and hydrogen attached to a carbon and ketones have two R groups attached to a carbon. Aldehydes are reduced by NaBH 4 and LiAlH4 to form primary alcohols using addition mechanisms. Ketones are reduced by NaBH 4 and LiAlH4 to form secondary alcohols. An example of a ketone is acetone, which can be found in nail polish remover. Benzaldehyde is an example of an aldehyde which can be found in almonds, giving them a sweet smell. Aldehyde and ketone have weaker intermolecular forces which are called van der waal forces meaning they would have a lower boiling point than alcohols and carboxylic acids (Tro 2014). Fehling’s solution is a blue copper (II) sulfate solution which can be used to distinguish between ketones and aldehydes. When added to aldehydes, the solution should turn from blue to brick red. When added to ketones, the solution should remain blue (Tro 2014). Tollens reagent [Ag(NH 3)2]+ and silver mirror test, can be used to distinguish between aldehydes and ketones. Ketones produce no changes and aldehydes produce silver precipitation (Tro 2014). Alcohols functional group is a hydroxyl group (OH). The carbon that contains the OH group and is bonded to one methyl group is known as a primary alcohol, when that carbon is attached to two methyl groups, it is a secondary alcohol, when that carbon is attached to three methyl groups, it is a tertiary alcohol. Ethanol, an example of an alcohol, can be used to produce alcoholic drinks. An example of a primary alcohol is propan-1-ol and an example of a secondary alcohol is propan-2-ol. The oxygen molecule is more electronegative than the hydrogen atom, making the bond polar. The shape of the bond is tetrahedral and has a bond angle of 109.5 o. This means alcohol contains strong intermolecular forces called hydrogen bonds so have a high boiling point. Alcohols have a lower pH range tan water. Alcohols can be formed by the hydration mechanism. First protonation occurs. This is when the alkene C=C is broken by a hydrogen being taken from H 3O+ to form a carbocation and water molecules, which is polar. Since the oxygen atom from the water molecule is more electronegative than the hydrogens, it is attracted to the positively charged carbocation. After this, deprotonation occurs which means the electronegative oxygen on that molecule is attracted to a hydrogen molecule so this will form an alcohol and H3O+ (Hathaway 2011).

Acidified potassium dichromate can be used to distinguish between different types of alcohols. It is an oxidising agent so when added to primary or secondary alcohols, the pale-yellow dichromate (vi) ions (Cr2O72-) will be reduced to form a lime green coloured chromium (iii) ions (Cr3+). When it is added to primary alcohols, they are oxidised into aldehydes or carboxylic acids. This could be tested by reagents that were mentioned earlier. When added to secondary alcohols, it is oxidised into ketones which can also be tested. Tertiary alcohols do not react with acidified potassium dichromate (Hathaway 2011). The aim of the experiment was to find out what functional group the unknown sample belongs to using the results of the experiments carried out of the samples using reagents. In this experiment, orange bromine water should turn unsaturated hydrocarbons colourless. When universal indicator is added to carboxylic acid, it should turn red. When NaHCO 3 is added to the carboxylic acid, bubble of gas should form. This is known as an effervescence. When Fehling’s reagent is added to an aldehyde, the colour should go from blue to brick red and when Tollens reagent is added, a silver mirror precipitate should form. When acidified potassium dichromate is added to primary or secondary alcohols, it should turn light green. Method: Safety procedures were taken by wearing laboratory coat, goggles, and gloves to ensure that substances would not touch the skin. Any glass which broke was cleared up and any substances which were spilt was cleared up. The experiments took place in a fumehood to avoid breathing in chemicals. Water baths used had the temperature of water at 50 oC. 2 ml of 2% of orange bromine was added into three test tubes. In one, cyclohexane was added and in the other, cyclohexene was added and in the third, solution A was added. 1 drop of universal indicator solution was added into solution A, 1 ml of acetic acid and 1 ml of water into a test tube. A spatula of sodium hydrogen carbonate was added into a test tube filled with 2 ml of water, 2 ml of acetic acid and 2 ml of solution A. 2 ml of Fehling’s A was added into three test tube which contained 2 ml of acetone, 2 ml of benzaldehyde and 2 ml of solution A. They were then put into a water bath at 50 oC for 15 minutes. The same was repeated for Fehling’s B solution. Tollens test was carried out by adding 1 ml of Tollens reagent to 3 different test tubes which contained 2 ml of acetone, 2 ml of benzaldehyde. In the third test tube, 2 ml of 0.1M of sodium hydroxide solution was added. 1 ml of Tollens reagent was added into the test tube. All 3 test tubes were then added into a water bath at 50 oC. 1 ml of propan-1-ol and propan-2-ol was added into 2 different test tubes and 5 drops of acidified potassium dichromate was added into both test tubes. They were then added into a water bath at 50 oC. Plastic pipettes were used to measure the solutions. Results: Table 1 – Reagent being added into the sample being tested.

Sample (ml) Cyclohexene Cyclohexane Unknown sample A Acetic acid Unknown sample A Acetic acid Unknown sample A Acetone Benzaldehyde Unknown sample A Acetone Benzaldehyde Unknown sample A Propan-1-ol Propan-2-ol Solution A

Test reagent (ml) 2% bromine solution 2% bromine solution 2% bromine solution Universal indicator Universal indicator NaHCO3 NaHCO3 Fehling’s A Fehling’s A Fehling’s A Fehling’s B Fehling’s B Fehling’s B Acidified K2Cr2O3 Acidified K2Cr2O3 Acidified K2Cr2O3

Observations Orange to colourless No change (remains orange) No change (remains orange) Colourless to red Green Effervescence No change Forms silver mirror No change No change Blue solution Blue solution (supposed to be red) Silver mirror Turns pale green Turns pale green Remains light yellow

Discussion: The advantages of this experiment are that the experiment is quick and not time consuming and cheap since the reagents are not very costly. New pipettes were used for each reagent to avoid contamination. However, they were put down in the fumehood so a used pipette could have been used to put a new reagent into a test tube. To avoid this, the used pipettes can be thrown out. Another advantage to this experiment is that the reagents were prepared before the experiment was carried out, so when added to the reagent was added into the solution being tested, the reagent would work since it would be able to undergo the mechanism. The test tube that contained Fehling’s and Tollens reagent were put into a water bath at 50 oC so rate of reaction increases, making the experiment less time consuming. A disadvantage of this reaction is that a few drops of cyclohexene was added into bromine. This meant that the cyclohexene could have diluted the orange bromine, so the colour could have faded, making the results unreliable. Instead, a drop of the bromine should have been added to the cyclohexene. Another disadvantage to this experiment is that plastic pipettes are not accurate. Gilson pipettes are more accurate at measuring the sample in ml so can be used instead. This experiment is qualitative and not quantitive, so statistical tests cannot be carried out to determine whether the results are accurate or not. The individual that is carrying out the experiment could have judged the colour wrong making the results inaccurate so gas chromatography can be used to determine its functional group. To further improve this experiment, other reagents could be used to try and distinguish the functional group.

When colourless cyclohexene is added to orange bromine water, the solution will turn colourless due to it undergoing addition reaction, as mentioned in the introduction. Bromine will attach to alkene to form a colourless substance called 1,2-dibromocyclohexane. Since alkanes are saturated, so they do not contain a C=O, they cannot undergo addition reaction to form a colourless haloalkane (Clugston et al 2000). Acetic acid is a Bronsted Lowry acid, so donates a proton (H +). A drop of universal indicator was added to water which turned the colour of the solution to green, and was added to acetic acid, which turned the solution red. This was done as a control, so that the unknown sample could be compared to each test tube (Hathaway 2011). NaHCO3 was added to acetic acid to form effervesce, which is when bubbles of carbon dioxide gas is produced in a liquid. This could have been tested by adding lime water which would have formed a cloudy solution. The equation is HC 2H3O2

(aq)

+ NaHCO3

(aq)

→ H 2O(l) + CO2(g) +

NaC2H3O2(aq) (Li et al 2007). Fehling’s reagent, which is a blue solution, oxidises benzaldehyde to form a brick red precipitate of Copper Oxide and benzenecarboxylic acid. When added to ketone, no changes occur since ketones cannot be oxidised (Tro 2014). Tollens reagent was added to acetone and benzaldehyde. The benzaldehyde formed a silver precipitate since it is an aldehyde. This is due to benzaldehyde reacting with the silver nitrate to form a benzenecarboxylic acid and silver. When the Tollens reagent is added to acetone, no reaction occurs (Tro 2014). Unknown sample A is not a saturated hydrocarbon since bromine solution remained orange. It is not a carboxylic acid or an alcohol since when the universal indicator was added into the solution, it turned green and not red. When NaHCO 3 was added, no effervescence formed which also indicates that it is not a carboxylic acid. When Fehling’s reagent was added, no changes occurred which meant that the molecule is not an aldehyde. When Tollens reagent was added to sample A, a silver mirror formed which indicates that the solution was an aldehyde. When acidified potassium dichromate was added into the unknown solution, no change occurred so the solution was not a primary or secondary alcohol. Tertiary alcohols does not undergo reduction reaction so the unknown sample is not a tertiary alcohol. In conclusion, the aim of the experiment was met and the experiment carried out matched the aim of the experiment. The functional group of the unknown solution was worked out using the test reagents and results of other functional groups that were tested out in this experiment. The unknown solution was an aldehyde since the test reagents such as the silver mirror test shows this. References:

Clarkson, R. (2009). Alkenes. [Online] Available from < http://www.chemistryrules.me.uk/candr/alkenes.htm> Clugston, M., Flemming R. (2000) Advanced chemistry. Malaysia; Oxford Hathaway, B. (2011) E-Z Organic Chemistry. (2nd edn) United States of America; Barron’s Educational Series Leach, M. (2009) Organic functional groups. [Online) Available from < http://www.chemistrydrills.com/functional-groups.php?q=simple> Li, J. Limberakis, C., Pflum, D. (2007) Modern organic synthesis in the laboratory . United States of America ;Oxford McMurry, J., Ballantine, D., Hoeger, C., Peterson, V. (2014) Fundamental of General, Organic, and Biological Chemistry. (7th edn) United States of America; Pearson new international edition Smith, J. (2013) General, Organic, & Biological Chemistry. (2nd edn) United States of America; McGRAW-HILL Tro, N. (2014) Introductory Chemistry. (4th end) United States of America; Pearson’s Education Limited Tro, N. (2015) Chemistry: Structure and Properties. China; Pearson Education Limited Wade, L. (2011) Dehydration of alkenes. [Online] Available from

Walker, D. (2007) Acids and Alkalis. China; Evans Brothers...