Amines

Question 1

What is the structure of ammonia?

Answer 1

• has a nitrogen atom bonded to three hydrogen atom
• the nitrogen atom also has a lone pair of electrons
• repulsion between the pairs of electrons means that an ammonia molecule is tetrahedral

Question 2

What two main properties of amines and ammonia is the lone pair on nitrogen responsible for?

Answer 2

• their tendency to act as nucleophiles

* their action as Bronsted-Lowry bases

Question 3

What are the three classes of amines?

Answer 3

Primary, secondary and tertiary

Question 4

How are amines derived from ammonia?

Answer 4

When one, two or all three hydrogen atoms are replaced by alkyl or aryl groups to make primary, secondary or tertiary amines

Question 5

How are quaternary ammonium ions produced?

Answer 5

They are produced from tertiary amines when the nitrogen atom’s lone pair of electrons forms a dative bond to a carbocation to form a fourth hydrocarbon group. They are like an ammonium ion (NH4+)

Question 6

How are primary amines named?

Answer 6

By identifying the longest hydrocarbon chain and adding the suffix ‘amine’ or the prefix ‘amino’

Question 7

How are secondary and tertiary amines named?

Answer 7

First identity the longest hydrocarbon chain attached to the nitrogen. Then identify the other smaller hydrocarbon groups attached to the nitrogen and list these at the beginning of the name. The prefix N- is used to show that the smaller groups are attached to the main chain via the nitrogen atom

Question 8

What do ammonia an amine molecules do in aqueous solution?

Answer 8

They accept a proton from a water molecule producing an ammonium ion (NH4) and a substituted amminiym ion (R - NH3+) and a hydroxide (OH-) ion. The solution is alkaline because of the OH- ions

Question 9

Why are ammonia and amines weak bases?

Answer 9

Because in an aqueous solution the reactants and products are in equilibrium and don’t ionise completely

Question 10

Why does the addition of alkyl groups make it easier for an amine to accept a proton?

Answer 10

Because alkyl groups push electron density away from the alkyl group towards the amine group. Because of this, the electron density on the nitrogen atom increases increasing it’s ability to accept a proton

Question 11

Why do aryl groups decrease the ability of nitrogen to accept a proton?

Answer 11

Because in aromantic amines overlap occurs between the lone pair on the nitrogen and the delocalised electron system of the benzene ring, and the nitrogen lone pair is delocalised towards the benzene ring. The electron density on the nitrogen atim is lowered and this reduces the ability of nitrogen to accept a proton

Question 12

Why does adding dilute acid helo to dissolve weak aromantic amines in water?

Answer 12

Because acid removes OH- and shifts the position of equilibriym to the right. The amine dissolves because it is converted to it’s ionic salt

Question 13

How can primary aliphatic amines be prepared?

Answer 13

By the nucleophillic substitution of the halogen atom in a halogenoalkane with ammonia

Question 14

What are the reaction conditions for the nucleophillic substitution to create primary amines?

Answer 14

• Excess ammonia dissolved in ethanol and heated with the halogenoalkane in a sealed vessel (this increases the pressure which leads to a faster rate of reaction)
• the halogenoalkane must have the same number of carbon atoms as the required amine product
• the halogen atom(s) must be positioned on the carbon atom(s) to which the amino group(s) need to be bonded

Question 15

What three stages does the nucleophillic substitution reaction involve?

Answer 15

1) the lone pair on the nitrogen atom of the ammonia molecule is attracted to the delta + charge on the carbon atom of the polar carbon-halogen bond
2) the lone pair forms a covalent bond between the nitrogen atom and the carbon atom and the carbon atom and the carbon-halogen bond is broken (with the halogen leaving the electron pair. This is the nucleophillic substitution
3) the amine is released from the intermediate salt by removal of a proton by a base. This base can be added to the reaction mixture or the excess ammonia can act as a base

Question 16

Why are chloro and bromoalkanes preffered as sources of the hydrocarbon groups in mucleophillic substitution?

Answer 16

Because they are more readily available than iodoalkanes and more reactive than fluoroalkanes

Question 17

Why are mixtures of primary, secondary and tertiary amines and quaternary salts always produced by nucleophilic substitutions?

Answer 17

Because further nucleophillic substitutions by halogenoalkanes always take place. Because alkyl groups push electron density towards the nitrogen, the product of each succesive alkylation is a better nucleophile than the starting material

Question 18

How can you adjust the initial reaction mixture to favour the primary amine?

Answer 18

Have excess ammonia. This means it is less likely that a second halogenoalkane molecule will react with the primary amine when there are very many unreacted ammonia molecules still available

Question 19

How can you adjust the initial reaction mixture to favour quarternary saltsM

Answer 19

Use an excess of halogenoalkane. This ensures that each ammonia molecule will react with four halogenoalkane molecule

Question 20

What is a different approach to produce primary amines in a higher yield and how does this take place?

Answer 20

Introduce the -NH2 group indirectly into a molecule

First reflux an appropriate halogenoalkane with a solution of potassium cyanide dissolved in a mixture of water and ethanol. The lone pair of the cyanide ion (:CN-) allows nucleophilic substitution of the halogen atom and puts a cyanide group (-CN) into the organic molecule. Organic compounds with a cyanide group are called nitriles. The nitrile group contains a carbon-nitrogen triple bond. Reduction of this triple bond by the addition of two molecules of hydrogen gives the primary amine in good yield. The reduction can be achieved by using lithium tetrahydridoaluminate, LiAlH4, dissolved in dry ether, followed by hydrolysis. Alternatively, hydrogen gas with a nickel catalyst can bring about the reduction.

Question 21

How can Aliphatic primary amines be made?

Answer 21

By reducing nitro compounds and amides

Question 22

What are primary aromatic amines?

Answer 22

Compounds with an -NH2 functional group directly bonded to the benzene ring. The simplest example is phenylamine

Question 23

What are the two reasons that halogenobenzenes do not react strongly with ammonia?

Answer 23

1) the nitrogen atom in ammonia is nucleophillic but benzene usually reacts with electrophiles - the high electron density of the delocalised electron cloud of the benzene ring repels nucleophiles such as ammonia
2) the aromantic ring is an electron-attracting group. The lone pairs of the halogen aton are delocalised towards the benzene ring. This has the effect of shortening and strengthening the C-Cl bond, making it less reactive than in an aliphatic halogenoalkane

Question 24

How are primary amines made from benzene?

Answer 24

Phenylamine is made by nitrating benzene with concentrated nitrix acid and sulfuric acid at 50°c. The nitro group is then reduced to a primary amine group using an acid with a metal. On an industrial scale scrap iron and HCl acid are used but tin and HCl are used in the laboratory. Since excess acid is present the initial form is the protonated form of phenylamine, C6H5NH3+Cl-. The amine is released by addition of a base such as sodium hydroxide. This removes the proton from the C6H5NH3+ ion. This reduction can also be achieved using hydrogen gas and a nickel catalyst

Question 25

What industry is the conversion of aromatic niteo compounds to aromantic amines of huge importance?

Answer 25

In the dye industry. It is used to introduce primary amine groups into a wide variety of aromantic molecules used for making dyes or into dye molecules themselves, causing changes in colour and ability to bond to fabrics

Question 26

What do ammonia and primary amines react with to give amides?

Answer 26

Acyl chlorides and acid anhydrides

Question 27

Why are two moles of ammonia or amine required for each mole of acid anydride or acyl chloride?

Answer 27

Because the by-products of the reaction are acidic. So they need two moles. The first acts as the nucleophile while the second acts as the base and reacts with the generated acid

Question 28

What is the general mechanism for the nucleophilic addition-elimination reactions of ammonia and primary amines?

Answer 28

Check snap camera roll

Question 29

What are cationic surfactants?

Answer 29

A detergent like soap where the polar head is positively charged

Question 30

Why can quarternary ammonium salts be used as cationic surfactants?

Answer 30

Because the positive charge on the nitrogen atom of the quaternary ammonium ion is attracted to the negatively charged surfaces.

Question 31

What are cationic surfacants used in?

Answer 31

Fabric conditioners, leather softeners, sewage flocculants, corrosion inhibitors, hair conditioners, anti-static agents and emulsifiers

Question 32

What are sone quartenernary ammonium salts wjth shorter carbon chains used in and what do they do?

Answer 32

Disinfectants where they combine surfactant action with germicidal properties