Organic Nitrogen Compounds Flashcards
What are the different classes of amines
There are three classes of amines:
Primary amines have an -NH2 group bonded to an alkyl or aryl group.
Secondary amines have two alkyl or aryl groups attached to >NH group
Tertiary amines have three alkyl or aryl groups attached to the same nitrogen atom.
In A2 we only look at reactions of primary and secondary amines.
Describe and explain the basicity of amines
Ammonia and the amines act as bases because of the lone pair of electrons on the nitrogen atom. Remember that a base is a proton (H+ acceptor). The nitrogen atom atom donates its lone pair to an H+ ion, forming a co-ordinate (dative) bond.
Dilute Hydrochloride acid reacts with ammonia and with amines to produce salts.
Ammonia and amines have different strengths as bases.
Compare the basicity of ammonia and amines
The strength of ammonia and amines as bases depends on the availability of the lone pair of electrons on their N atom to bond with an H+ ion.
Ethylamine>ammonia>phenylamine
Ethylamine is a stronger base than ammonia because the ethyl group is electron donating: it has a positive inductive effect. By releasing electrons to the N atom, the ethyl group makes the the lone pair more available to bond with an H+ ion than it is with ammonia.
However ammonia is a stronger base than phenylamine because one of the p orbitals on the nitrogen atom in phenyl amine overlaps with the π bonding system in the benzene ring. This causes the lone pair of the N atom in phenylamine to be delocalised into the benzene ring. This then makes it less available to form a co-ordinate (dative bond) with an H+ ion than it is in ammonia.
What are the three different ways we can form an Amine?
- Bromoethane undergoes nucleophilic substitution with ammonia to form a mixture of amines. In order to prepare ethylamine while avoiding the formation of secondary and tertiary amines we use excess hot ethanolic ammonia under pressure:
CH3CH2Br + NH3 ➡️ CH3CH2NH2 + HBr
HBr which could react with ethylamine is removed by excess ammonia. It forms ammonium bromide NH4Br. The excess ammonia also reduces the chances of bromoethane being attacked by ethylamine as ethylamine is also a nucleophile. However if your aim s to prepare secondary amine we can start with halogenoalkane and a primary amine and react them, again in ethanol, heated in a sealed tube.
- A solution of potassium cyanide, KCN in ethanol is heated under reflux with the halogenoalkane for the formation of a nitrile. Note: the cyanide group adds a carbon atom to the alkyl group.
We can then reduce (add Hydrogen) the nitrile to make an amine. The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH4 in dry ether can be used for reduction. For example
CH3Br + CN- ➡️ CH3CN + Br-
CH3CN + 4[H] ➡️ CH3CH2NH2
- We can also use LiAlH4 in dry ether to reduced amides to amines.
So the carbonyl group >C=O in the ethanamide will be reduced to ethylamine:
CH3CONH2 + 4[H] ➡️ CH3CH2NH2 + H2O
How can we prepare phenyl amines
Phenyl amine is made by reducing nitrobenzene. This reaction is carried out by heating nitrobenzene under reflux with tin (Sn) and concentrated HCl. The organic product in the acidic reaction mixture is the ion C6H5N+H3, which is then converted to phenylamine, C6H5NH2, by adding sodium hydroxide solution.
The reduction is summarised by:
Nitrobenzene + 6[H] ➡️ Phenylamine+ H2O
The phenyl amine is separated from the reaction mixture by steam distillation
Describe the reaction of phenyl amine with aqueous bromine
The reaction is similar to the reaction of aqueous bromine with phenol: a white ppt is formed.
The nitrogen in the -NH2 group in phenylamine has a lone pair of electrons that can be delocalised into the benzene ring so that the π bonding system extends to include the C-N bond. The extra electron density in the benzene ring makes it more readily attacked by electrophiles. Remember that the 2, 4 and 6 positions around the benzene ring are activated when electron-donating groups such as -NH2 or -OH are attached to the ring.
Describe how we can use phenylamine to make an azo dye
- Reaction between phenylamine and nitric (lll) acid (nitrous acid), HNO2 to give a diazonium salt (benzenediazonium chloride). We can make nitric acid using sodium nitrate(ll) (sodium nitrite), NaNO2 and dilute hydrochloric acid. Nitric acid is unstable so it has to be made in the test tube. Then the phenylamine is added.
NaNO2 + HCl ➡️ HNO2 + NaCl
The reaction between nitrous acid and phenylamine is called diazotisation.
Phenylamine + HNO2 + HCl ➡️ benzene diazonium chloride + 2H2O
Ionic equation:
Phenyl amine + HNO2 + H+ ➡️ diazonium ion + 2H2O
The reaction mixture must be kept below 10 degrees Celsius because the diazonium ion is unstable and will decompose easily, giving off nitrogen gas at higher temperatures.
- The positively charged diazonium ion acts as an electrophile and reacts with an alkaline solution of phenol in a coupling reaction. The diazonium ion acts as an electrophile. It substitutes into the benzene ring of phenol at the 4 position directly opposite the -OH group.
The orange dye formed is called an azo dye.
What is an azo dye
Azo dyes are coloured compounds formed on the addition of phenol to a solution containing a diazonium ion.
Its delocalised π bonding system extends between the two benzene ring through the -N=N- azo group, which acts like a bridge between the two rings. This makes the azo dye very stable which is an important characteristic of a good dye. The azo dye forms immediately on addition of the alkaline solution containing the diazonium ion.
By using an alternative aryl compounds to phenol we can make a range of brightly coloured dyes.
Describe amino acids
Amino acids all contain:
* the basic amino group (-NH3)
* the acidic carboxylic acid group (-COOH)
This makes the amino acids amphoteric, as they can behave as both acid and base,
The general structural formula of an amino acid is RCH(NH2)COOH.
The R group can vary in different amino acids. The simplest amino acid is glycine in which the R group is an H atom.
The R group can be:
*Acidic (e.g. it contains another carboxylic acid group -COOH)
*Basic (e.g. it contains another amine group, -NH2)
*Neutral (e.g. when R is an alkyl group)
What is a zwitterion?
The H from -COOH is lost and gained by the -NH2.
The -COOH becomes -COO- and the -NH2 becomes -NH3+
The ionic nature of the zwitterions gives amino acids relatively strong intermolecular forces of attraction. They are crystalline solids that are soluble in nature.
A solution of amino acids in water contains zwitterions that have both acidic and basic properties (they are amphoteric). They will resist changes in pH when small amounts of acid or base are added to them. The solution is called a buffer solution.
If the pH is lowered by adding acid, the -COO- part will accept an H+ ion, reforming the undissociated -COOH group. This leaves a positively charged ion.
If the pH is raised by adding a base, the -NH3+ part of the zwitterion will donate an H+ ion to the hydroxide ion (H+ + OH- ➡️ H2O), reforming the amine -NH2 group. This leaves a negatively charged ion.
By adjusting the pH by small amounts we can reach a point where neither positive nor negative ions shown above dominate, so the amino acid has no charge overall. This pH is called the isoelectronic point of an amino acid.
What is an isoelectronic point?
the pH value at which there is no overall charge on a particular amino acid in its aqueous solution.
The isoelectronic point can be found by measuring the pH value of an amino acids solution when it is not attracted to either a positive or negative electrode.
What is a dipeptide?
A compound formed from the condensation reaction between two amino acids. The -COOH group of one amino acid reacts with the -NH2 group of another amino acid.
An amide bond between two amino acid molecules is also called a peptide bond or peptide link. The reaction is a condensation reaction as a small molecule (water) is eliminated when the reactant molecules join together.
The dipeptide still has an amine and carboxylic group at each end so the reaction can continue to form a tripeptide and longer chains of amino acids. The longer molecules are called polypeptides and then proteins.
Describe the structure of amides
The structural formula of the amide group is: -CONH2
Unlike the basic amines, amides are neutral compounds. The presence of the electron withdrawing oxygen-atom in the amide group means that the lone pair on the nitrogen atom is not available to donate to electron deficient species such as H+ ions.
How can we prepare an amide
Acyl chloride + concentrated ammonia solution can form amides.
For example: ethanoyl chloride with concentrated ammonia solution to form ethanamide
CH3COCl + NH3 ➡️ CH3CONH2 + HCl
An amine can also react with an acyl chloride to form a substituted amide.
For example: ethylamine + acyl chloride ➡️ substituted amide (N-ethylbutanimide)
CH3H7COCl + C2H5NH2 ➡️ C3H7CONHC2H5 + HCl
The italic letter N is used in naming substituted amides to denote which alkyl (or aryl) group/groups are bonded to the nitrogen atom. For example in the example above the ethyl group (C2H5-) had replaced an H atom in the amide group. If both the H atom are removed the two N’s are used in the name for example N,N-diethylbutanimide
Describe acid hydrolysis of substituted amides
The characteristic -CONH- group in substituted amides links the two hydrocarbon sections of their molecules together. This amide link is broken when the amide is refluxed with an acid such as hydrochloric acid.
The products of hydrolysis of a substituted amide with acid are carboxylic acid (R1COOH) and a primary amine (R2NH2). The amine formed will react with excess acid in the reaction vessel to make its ammonia salt, e.g. R2NH3+Cl-
If we reflux an unsubstitued amide (RCONH2) with acid, the products are the corresponding carboxylic acid and ammonia. The ammonia reacts with excess acid to make an ammonium salt.
