Organics Chemical Tests Flashcards
(24 cards)
Describe and explain which compounds will react with red and/or blue litmus paper.
Base + red litmus = blue litmus
Acid + blue litmus = red litmus
Alkane = Neither
Alkene = Weak base
Alkyne = Acid
Amine = Base
Amides = Neutral
Carboxylic acid = Acid
Alcohol = Neither. However it can act as either according to the aim of the reaction.
Haloalkane = Acid
Ester= Neutral
Aldehydes= Acid
Ketones= Acid
Acyl Chlorides = Acid
How can you can distinguish between acidic, basic, and neutral solutions using both red and blue litmus paper?
Testing for Acids:
* Red Litmus Paper: Remains red.
* Blue Litmus Paper: Turns red.
Testing for Bases:
* Red Litmus Paper: Turns blue.
* Blue Litmus Paper: Remains blue.
Testing for Neutral Solutions:
* Red Litmus Paper: Remains red.
* Blue Litmus Paper: Remains blue.
Describe whether a compound will have a low or high melting/boiling point.
SIZE: Longer/larger/higher molar mass molecules have higher boiling points, (More e-‘s»_space;> stronger IMFA»_space;> more energy/bonds to break»_space;> higher Bp).
SHAPE: Linear molecules have higher Bp’s than rounded molecules, (more surface area»_space;> increased amount of exposed electrons»_space;> stronger IMFA because e-‘s pull molecules together»_space;> more energy to break»_space;> higher Bp).
POLARITY: Polar molecules have higher Bp than non polar molecules of the same size - FONCL. (Stronger IMFA»_space;> more energy to break»_space;> higher Bp).
Describe and explain how bromine water (Br2) can be used to distinguish between alkenes and alkanes.
Orange → colourless if C=C present (addition reaction)
- orange bromine colour of the Br2 will be rapidly decolourised by an alkene if present, due to an addition reaction occurring.
- No UV light is required. However, alkanes will require sunlight (UV light) to decolourise bromine water. Will be a very slow substitution reaction.
- Chlorine reacts even more rapidly in addition reactions than bromine.
When is an organic compound polar or non-polar
“Like dissolves like!”
- Polar: has an overall slight charge to it. Examples: water, alcohols, amines, and carboxylic acids.
- Non-polar: has no overall charge at all. Examples: Alkanes, alkenes, alkynes, and haloalkanes.
Example: Predict whether these substnaces are soluble in each other (yes or no)…..
a) Water and ethanol = Yes
b) Hexane and Tetrachloromethane= Yes
c) Water and hexane = No
describe and explain the
purpose of distillation
What is Distillation?
Imagine you have a mixture of two liquids, like water and alcohol. Now, water boils at 100°C, and alcohol boils at a lower temperature, say 78°C. Distillation uses this difference in boiling points to separate them. i.e “Distillation is a process in which the different boiling points between two liquids is used to separate them.”
The main purpose is to separate a mixture of liquids based on their different boiling points.
For: Purifying products (e.g. ester, alcohol)
How it works:
Heating: You heat the mixture.
Vaporisation: The liquid with the lower boiling point (in our example, alcohol) will turn into a vapor (a gas) first.
Condensation: This vapor then travels through a tube called a condenser, which is cooled down. When the vapor cools, it turns back into a liquid (condenses).
Collection: This condensed liquid (the alcohol) is collected in a separate container.
Remaining Liquid: The liquid with the higher boiling point (water) stays behind in the original container.
Describe and explain the
purpose of reflux
(Re= repeat)
(Flux= go between)
What is Reflux?
The purpose of reflux is to heat a reaction mixture for an extended period of time without the organic substances (reactant, product, or solvent) evaporating away. It allows the reaction to occur at a higher temperature (the boiling point of the solvent) without losing any materials, and any volatile organic substance which evaporates will be condensed and returned to the flask. This ensures the reaction goes to completion.
It involves:
- Heating a reaction mixture in a flask.
- Putting a condenser (a cooling tube) on top of the flask.
- The condenser cools any vapours that rise from the heated mixture, causing them to condense back into a liquid.
- This liquid then drips back down into the flask.
Used in reactions of:
* Esterification
* Amide formation
* Hydrolysis
Describe and explain the
purpose of a separating funnel
A separating funnel is a glass funnel, usually shaped like a cone with a narrow tube at the bottom.
The purpose of a separating funnel is to separate liquids that are “immiscible,” (liquids that don’t mix, like oil and water).
Here’s how it works:
- You pour the two immiscible liquids into the funnel.
- Because the liquids don’t mix, they’ll form two separate layers, with the denser liquid on the bottom.
- You carefully open the stopcock to let the bottom layer drain into a separate container.
- When the bottom layer is completely drained, you close the stopcock, and you’re left with the top layer in the funnel.
Describe the observations
when both polar and non-polar
liquids are added to water
POLAR:
When you add a polar liquid to water, they tend to mix very well. We say they are “miscible.” This is because the positive and negative ends of the water molecules are attracted to the opposite charges on the polar liquid molecules.
So, they kind of “hug” each other and blend together. This will often result in one unified liquid.
NON-POLAR:
When you add a non-polar liquid to water, they don’t mix. Instead, they form separate layers. This is because water molecules are much more attracted to each other than they are to non-polar molecules. Typically, you’ll see the non-polar liquid floating on top of the water, as it’s often less dense.
So you will see two distinct layers of liquid.
In short, “like dissolves like”. Polar substances dissolve polar substances, and non polar substances dissolve non polar substances.
Test #1 for Aldehydes:
Tollens reagent (AgNO3 in excess NH3 solution), silver mirror test:
- Silver mirror forms if aldehyde is present.
This because Aldehydes reduce the Ag+ ions to Ag(s) which forms a silver mirror on the inner walls of the test tube. - Ketones do NOT oxidise with Tollens reagent: NO silver mirror is formed.
Test #2 for Aldehydes:
Benedicts solution (complexed Cu2+ ions and is blue in colour).
- Orange-red precipitate forms if aldehyde present.
This is because Aldehydes reduce the Cu2+ to Cu(1) forming an insoluble orange-red copper (I) oxide, Cu2O, which is seen as a precipitate. - Ketones do NOT oxidise with Benedicts solution: no orange-red precipitate is formed.
Test #3 for Aldehydes:
Fehlings solution (Cu2+ ions in basic solution, and is blue in colour - it behaves same way as Benedicts solution)
- Brick-red precipitate forms if aldehyde present.
Aldehydes reduce the Cu2+ to Cu(1), forming an insoluble reddish-brown copper (I) oxide, Cu2O, which is seen as a precipitate. - Ketones do not oxidise with Fehlings reagent: no reddish-brown precipitate is formed.
Describe and explain how
to identify acyl chlorides by
adding water
Acyl chlorides react violently with water (H2O), producing dense fumes of HCL gas. The resulting solution is highly acidic as some of the HCl gas dissolves in the water forming a hydrochloric acid solution AND a carboxylic acid. Thus, when a stopper on an acyl chloride bottle is removed these dense fumes of HCl gas are seen as they are produced from the acyl chloride reacting with water vapour in the air. This reaction with water is often used as a distinguishing test between acyl chlorides and haloalkanes.
Why are Amides usually odourless?
Amides are usually odourless because they have a carbonyl group and a nitrogen with a lone pair, allowing them to form strong hydrogen bonds with each other. These bonds hold the molecules tightly together, reducing their volatility, meaning the molecules don’t evaporate easily into the air and thus are less likely to reach your nose — which is why they have little to no smell.
Reduction reactions: Ketones and Aldehydes
- Reagent for both = NaBH4
(Sodium borohydride) - Reduce an Aldehyde to a primary alcohol. (Going from THREE oxygen bonds to ONE).
Aldehyde —-> Primary alcohol
- Reduce a Ketone to a secondary alcohol. (Going from TWO oxygen bonds to ONE).
Ketone —-> Secondary Alcohol
- Reduction reactions must be done under strongly alkaline conditions since BH4 decomposes in acidic or neutral conditions.
- BH4 is not a strong enough reductant to reduce a carboxylic acid.
Preparation of Aldehydes
- prepared FROM the oxidation of PRIMARY ALCOHOLS.
- Reagent: H+/Mno4-
Primary alcohol —> Aldehyde
- Aldehydes are rapidly oxidised to carboxylic acids, as the aldehyde is remove from the reaction vessel as it is formed.
- Distillation is required.
- Mixture is heated in a flask.
- The aldehyde has a lower boiling point that the alcohol, so it evaporates first. The aldehyde vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure aldehyde liquid.
Preparation of Ketones
- prepared BY the oxidation of SECONDARY ALCOHOLS
Secondary alcohol —-> Ketone
- reagent: H+/Mno4-
- reaction is slow
- reaction is done under reflux to ensure all the alcohol is oxidised
Chemical test using: Potassium
carbonate (K2CO3)
For: Purification after esterification.
Purpose: Neutralises excess acid, removes acidic impurities (fizzes)
Chemical test using:
Concentrated H2SO4
For: Esterification (Carboxylic acid + Alcohol → Ester)
Purpose: Catalyst + dehydrating agent to push equilibrium toward ester
Chemical test using:
Aqueous acid (e.g.
HCI (aq)
For: Acid hydrolysis of esters or amides
Purpose: Breaks ester → carboxylic acid + alcohol; breaks amide → acid + amine
Chemical test using:
Aqueous alkali (e.g. NaOH)
For: Base hydrolysis / saponification
Purpose: Ester → alcohol + carboxylate salt; amide → salt + amine
Chemical test using:
NaOH (aq) / HCI
(aq)
For: Acid/base reactions, hydrolysis, neutralisation.
Purpose: Convert between functional groups, test solubility or pH
Chemical test using:
Lucas reagent
(ZnCl + HCI)
For: Test for alcohol class
Purpose: Tertiary alcohol reacts fastest → cloudy layer
Test for protein - Biurets test
As proteins are just really polyamides, they have peptide linkages and can be hydrolysed under alkaline conditions. They react with Biurets reagent (contains Cu2+/OH-) which changes from blue to purple - if a protein is present.
