Organic Analysis Flashcards
1
Q
How do you test for carboxylic acids? (2)
A
- Add hydrogen sodium carbonate (NaHCO3).
* Positive result: effervescence- CO2 gas given off.
2
Q
How does infrared (IR) spectroscopy work? (8)
A
- A pair of atoms joined by a chemical bond is always vibrating.
- The system behaves like two balls (the atoms) joined by a spring (the bond).
- Stronger bonds vibrate faster (at a higher frequency).
- Heavier atoms make the bond vibrate more slowly (at a lower frequency).
- Every bond has its own unique natural frequency that is in the IR region of the electromagnetic spectrum.
- When you shine a beam of IR radiation (heat energy) through a sample the bonds in the sample can absorb energy from the radiation and vibrate more.
- Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond.
- The radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample.
3
Q
What happens in the infrared spectrometer? (4)
A
- 1). A beam of infrared radiation containing a spread of frequencies is passed through the sample.
- 2). The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample.
- 3). The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation.
- 4). The frequency is expressed as a wavenumber, measured in cm^-1.
4
Q
What is the infrared spectrum? (3)
A
- A graph in which the dips in the graph represent particular bonds.
- Helps to identify the functional group present in a compound.
- Use the data sheet to identify an organic compound.
5
Q
What is the fingerprint region? (3)
A
- An area of an infrared spectrum below 1500 cm^-1, which usually has many peaks caused by complex vibrations of the whole molecule.
- This shape is unique for any particular substance.
- Used to identify the chemical.
6
Q
How do chemists use a fingerprint region? (2)
A
- By using a computer to match the fingerprint region of a sample with those on a database of compounds.
- An exact match confirms the identification of the sample.
7
Q
How do you test if you have a primary, secondary or tertiary alcohol? (6)
A
- 1). Add 10 drops of the alcohol to 2 cm^3 of acidified potassium dichromate solution in a test tube.
- 2). Warm the mixture gently in a hot water bath.
- 3). Observe the colour change:
- Primary alcohol = orange solution goes green as an aldehyde forms.
- Secondary alcohol = orange solution goes green as a ketone is formed.
- Tertiary = No visible change.
8
Q
How do you collect some of the product formed when a primary or secondary alcohol is oxidised? (3)
A
- 1). Add excess alcohol to 2 cm^3 of acidified potassium dichromate solution in a round bottomed flask.
- 2). Set up the flask as part of distillation apparatus.
- 3). Gently heat the flask- the alcohol will be oxidised and the product would be distilled off immediately.
9
Q
How do you test if you have an aldehyde or a ketone? (5)
A
- 1). Add 2 cm^3 of Fehling’s or Benedict’s solution to a test tube.
- 2). Add 5 drops of the aldehyde or ketone to test.
- 3). Put the test tube in a hot water bath to warm it for a few minutes:
- Aldehyde = blue solution will give a brick red precipitate.
- Ketone = no visible change.
10
Q
Describe the silver mirror test to test for aldehydes and ketones. (6)
A
- 1). Put 2 cm^3 of 0.10 mol dm^-3 silver nitrate solution in a test tube.
- 2). Add a few drops of dilute sodium hydroxide solution- a light brown precipitate should form.
- 3). Add drops of dilute ammonia solution until the brown precipitate dissolves completely = 2Ag(NH3)2OH (aq).
- 4). Place the test tube in a hot water bath and add 10 drops of the aldehyde or ketone and wait for a few minutes:
- Aldehyde = silver mirror forms on the walls of the test tube.
- Ketone = no visible change.
11
Q
How do you test for alkenes? (5)
A
- 1). Add 2 cm^3 of the solution that you want to test to a tube.
- 2). Add 2 cm^3 of bromine water to the test tube.
- 3). Shake the test tube:
- Alkene = the solution will decolourise (go from orange to colourless).
- Not alkene = no visible change.
12
Q
How do you test for carboxylic acids? (5)
A
- 1). Add 2 cm^3 of the solution to the test tube.
- 2). Add 1 small spatula of solid sodium carbonate/solution.
- 3). If the solution begins to fizz, bubble the pas that it produces through some lime water in another test tube:
- Carboxylic acid = solution will fizz as CO2 is released.
- Not carboxylic acid = no visible change.
13
Q
How do you test for halides? (3)
A
- 1). Add dilute nitric acid- it removes ions such as carbonates (it would fizz and also produce a white precipitate) that might interfere with the test.
- 2). Add a few drops of silver nitrate solution (AgNO₃).
- 3). A precipitate is formed (of the silver halide).
14
Q
What is the ionic equation used to show the reaction of silver nitrate solution and halides? (3)
A
- Ag^+(aq) + X^-(aq) —> AgX(s) •X = F^-, Cl^-, Br^- or I^-
- Silver ions are positive.
- Precipitate formed of the silver halide.
15
Q
Does a precipitate form when fluoride is present when AgNO3 is added?
A
•No precipitate is formed.
