Q. Describe how to form an amine from a halogenoalkane.
Heat the halogenoalkane in a sealed tube with concentrated ammonia and an ethanol solvent.
Q. Why is excess ammonia used when forming a primary amine from a halogenoalkane?
To prevent further substitution, if ammonia is not in excess, secondary, tertiary, or quaternary ammonium salts may form.
Q. Describe how to form a primary amine from a amide.
Reduction using LiAlH4 as a reducing agent.
Q. Describe how to form a primary amine from a nitrile.
- Reduction using LiAlH4 as a reducing agent.
- Reduction using nickel catalyst and hydrogen gas.
Q. Describe how to form phenylamine.
Reduction of nitrobenzene with tin and concentrated hydrochloric acid. Reflux for 30 mins.
Q. Why are amines basic?
The lone pair on the nitrogen in the amine group can accept a proton.
Q. What affects the strength of a base?
- How easily the lone pair can accept a hydrogen ion.
- The stability of the ions formed.
Q. Compare the relative ease of basicity: ammonia, ethylamine and phenylamine.
Ethylamine > Ammonia > Phenylamine
Ethylamine is the most basic, while phenylamine is the least basic.
Q. Why is ethylamine a stronger base than ammonia?
Alkyl groups tend to puh electrons away from themselves. This is the positive inductive effect. This increases the negative charge of the nitrogen in ethylamine, making the lone pair more attracted to hydrogen ions. The electron-pushing effect also spreads the charge more, meaning the ethyl ammonium ion is more stable than the ammonium ion.
Q. Why is phenylamine a weaker base than ammonia?
In phenylamine the amine group is directly attached to the benzene ring so the lone pair on nitrogen is delocalised into the pi system. This makes the lone pair is less available. The very electronegative nitrogen atom draws electrons towards itself. This charge is much weaker than in ammonia. If the lone pair combined with a proton, the pi system would be disrupted making the ion less stable.
Q. Why is phenylamine more reactive than benzene?
The -NH2 group in phenylamine activates the benzene ring when the lone pair on the nitrogen is delocalised into the pi system. This makes phenylamine more reactive because the electron density of the pi system is increased so electrophiles are more attracted.
Q. Describe the conditions for the reaction between the phenylamine and aqueous bromine.
Room temperature, no catalyst.
Q. Describe what would be observed when phenylamine reacts with aqueous bromine.
Bromine water is decolorised. White precipitate forms.
Q. When reacting phenylamine with nitrous acid, why is the nitrous acid typically made in situ? How is this done?
Nitrous acid is made in situ because it decomposes rapidly. Nitrous acid is made by reacting hydrochloric acid with sodium nitrate so phenylamine is added to hydrochloric acid and sodium nitrate at the same time.
Q. Describe the formation of dyes from benzene diazonium chloride and phenol.
Phenol is reacted with sodium hydroxide to form a solution of sodium phenoxide. Sodium phenoxide is cooled in ice. A cool solution of benzene diazonium chloride is added. A yellow-orange precipitate forms. This is the azo compound.
Q. What is an azo compound?
A compound containing 2 benzene rings joined by a nitrogen bridge.
Q. During the formation of azo compounds, at which position does coupling typically take place?
At the 4th position. If the 4th position is occupied, coupling will take place the 2nd position.
Q. Describe how an amide can be formed from ammonia.
React ammonia with an acyl chloride. Hydrochloric gas is also produced.
Q. Describe the acid hydrolysis of amides.
When heated with acid, an amide breaks down into a carboxylic acid and ammonium ions (RNH3+).
Q. Describe the alkaline hydrolysis of amides.
Upon heating with sodium hydroxide, an amide will be broken down into a carboxylate salt and either ammonia or an amine.
Q. Describe the reduction of amides.
LiAlH4 reducing agent followed by treatment with dilute acid. Forms an amine and water.
Q. Why can amino acids act as both acids and bases?
The carboxylic acid group can donate a proton and amine group can accept a proton.
Q. What is a zwitterion? How do amino acids form zwitterions?
A zwitterion is an ion containing a positive and negative charge. It has no overall charge. Amino acids form zwitterions when the carboxylic acid group donates a proton to the amine group.
Q. What happens when an alkali is added to an amino acid zwitterion?
The NH3+ group donates a hydrogen ion to the OH- ions of the alkali to form water. The organic compound is no longer a zwitterion because it only contains a negative charge.
Q. What happens when an acid is added to an amino acid zwitterion?
The COO- group accepts a hydrogen ion from the acid. The organic compound is no longer a zwitterion as it only contains a positive charge.
Q. How does a peptide bond form?
A peptide bond is formed during a condensation reaction between 2 amino acids. A water molecule is lost.
Q. What is the difference between a dipeptide and tripeptide?
A dipeptide is formed from 2 amino acids while a tripeptide is formed from 3 amino acids.
Q. Describe how electrophoresis can be used to separate amino acids.
1. Place a piece of moist filter paper on a microscope slide.
2. Attach crocodile clips to each end of the paper and connect to a battery.
3. Add a drop of amino acid solution to the middle of the paper.
4. Leave the apparatus for separation to occur.
5. Spray the paper with ninhydrin to make the colourless amino acid solution visible. Dry and gently warm the paper to make the amino acids visible as coloured spots.
Q. What must the pH be for a specific amino acid to form its zwitterion?
The pH must be the isoelectric point of that amino acid.
Q. When the pH is the isoelectric point of an amino acid, why doesn't this amino acid move during electrophoresis?
The amino acid forms its zwitterion meaning it has no overall change. As a result, it is not attracted to the positively charged anode or the negatively charged cathode.
Q. When do amino acids move towards the cathode during electrophoresis?
When they are positively charged. This may occur when an amino acid has an extra hydrogen added to the amine group.
Q. When do amino acids move towards the anode during electrophoresis?
When they are negatively charged. This may occur when an amino acid has lost a hydrogen ion from the carboxylic acid group.
Q. How is the size of ions related to the speed at which they travel during electrophoresis?
Smaller ions travel faster because there is less resistance to their movement through the fibres of the paper.
Q. In terms of peptides, what can electrophoresis be used for?
To separate peptides in order of relative molecular mass. To estimate the relative molecular mass of a peptide.
Q. What must be done before peptides can undergo electrophoresis? Describe how the molecules are changed.
- Treat the peptides with SDS and denature them by heating.
- The secondary and tertiary structures are lost. The molecules become amino acid chains surrounded by negative charges.
Q. During electrophoresis of peptides, what do the molecules move towards? What sized molecules move fastest?
- Move towards the anode because the molecules are negatively charged.
- Smaller molecules move fastest.
No comments:
Post a Comment