3.3.11 Amines

Question 1

What is an amine?

Answer 1

If one or more hydrogens in ammonia (NH₃ is replaced with an organic groups, it is an amine

Question 2

What is a primary amine

Answer 2

When one of the hydrogens in ammonia is replaced with an organic group - e.g. methylamine

Question 3

What is a secondary amine?

Answer 3

When two of the hydrogens in ammonia are replaced with an organic group - e.g. dimethylamine

Question 4

What is a tertiary amine?

Answer 4

When three of the hydrogens in ammonia are replaced with an organic group - e.g. trimethylamine

Question 5

What is a quarternary ammonium ion?

Answer 5

When all of ammonia’s hydrogen’s have been substituted for organic groups, ammonia’s lone pair can also bond with a fourth group - e.g. tetramethylamine

Question 6

What are quarternary ammonium salts?

Answer 6

Quaternary ammonium ions are positively charged, therefore they are attracted to negative ions. The complexes formed are called quarternarty ammonium salts - e.g. tetramethylammonium chloride (CH₃)₄N⁺Cl^-

Question 7

What can quaternary ammonium salts be used for, how?

Answer 7

• Quarternary ammonium salts with at least one long hydrocarbon chain can be used as cationic surfactants.
• The non polar tail will bind to nonpolar substances such as grease, whilst the cationic head will dissolve in water
• Therefore they are useful in fabric cleaners and **hair products **
• Also, the positively charged ammonium ion will binds to negatively charged surfaces such as hair and fibre, this gets rid of static, therefore they can be used in hair and fabric conditionsers

Question 8

Explain how amines act as a base

Answer 8

1. Amines acts as weak bases beacuse they accept protons.
2. There is a lone pair of electrons on the nitrogen atom that can form a dative covalent bond with an H⁺ ion
3. The strength of the base depends on how available the nitrogen’s lone pair of electrons is. The more available the lone pair is the more likely the amine is to accept a proton, and the strongers the bases will be.
4. A lone pair of electons will be more available, if it’s electron density is higher

Question 9

Explain why primary aliphatic amines are stronger bases than ammonia, which is a stronger base than aromatic amines

Answer 9

The more available the lone pair of electrons, the stronger the base

Primary aliphatic amines - the alkyl group push electrons onto the attached groups. So the electron density on the nitrogen atom increases. This is called the positive inductive effect. This makes the lone pair more available

Ammoinia - no/little effect on the lone pair

Primary aromatic amines - The bezene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring. So the electron density on the nitrogen decreases. This is called the negative inductive effect. The lone pair become much less available

Question 10

What makes amine nucleophiles?

Answer 10

They have a lone pair of electrons, therefore they are nucleophiles.

They can react with halogenoalkanes in nucleophillic-substitution reactions, or with acyl chloride/ acid anhydrides in nucleophillic addition elimination reactions

Question 11

Explain how aliphatic amine are made by heating a halogenoalkane with excess ammonia

Use a mechanism

Use ethylamine being produced by reacting ammonia with bromoethane as an example

Answer 11

The mechanism is:

Ammonia + halogenoalkane (CH₃CH₂Br) -> Alkylammonium salt (CH₃CH₂NH₃⁺Br^-)
Ammonia attack the δ+ carbon in the halogenalkane (C-Br bond) creating a bond between the nitrogen and the carbon, the halogen (Br^-) leaves ,this produces an alkylammonium salt

then…

Alkylammonium salt (CH₃CH₂NH₃⁺Br^-) + NH₃ <=> primary amine (CH₃CH₂NH₂ + ammonium salt (NH₄⁺Br^-
A second ammonia molecule donates it lone pair of electrons to a hydrogen, which breaks off from the alkylammonium salt

Question 12

Explain why heating a halogenoalkane with ammonia produces a mixture of aliphatic amine products

Answer 12

Because the primary amine that is first produced has a lone pair of electrons, it’s a nucleophile. This means it can react with the remaining halogenoalkanes in a nucleophillic substiution reaction. As long as there is still halogenoalkane to react with, further substitutions can take place. This will keep happening until a quaternary ammonium salt is produced, which can’t react as it has no lone pair:

Primary amine (RN̈H₂) –Halogenoalkane–> Secondary amine (RN̈HR) –Halogenoalkane–> Tertiary amine (RN̈RR) –Halogenoalkane–> Quaternary amine (RN⁺RRR)

Question 13

Explain how aliphatic amines produced by reducing a nitrile using LiAlH₄

Answer 13

LiAlH₄ in a non-aqueous solvent (e.g. dry ether), followed by dilute acid.

Example R-CH₂-C≡N +4[H] –LiAlH₄/dilute acid–> R-CH₂-CH₂NH₂

[H] is just the reducing agent, here it’s LiAlH₄

Question 14

Why isn’t LiAlH₄ used in industry to reduce a nitrile into an aliphatic amine?

Answer 14

Too expensive

Question 15

How in industry are nitriles reduced by catalytic hydrogentation

Answer 15

The nitriles are reduced using hydrogen gas with a metal catalyst (e.g. platinum or nickel) at a high temperature and a high preasure

Example:

R-CH₂-C≡N + 2H₂ –Nickel catalyst/ High preasure/ High temp–> R-CH₂-CH₂NH₂

Question 16

How are aromatic amines produced

Answer 16

By reducing a nitro compound such as nitrobenzene

1. Heat a mixture of a nitrocompound, tin metal and concentrated hydrochloric acid under reflux - this makes a salt e.g. using nitrobenzene the salt C₆H₅NH₃⁺Cl^- is formed
2. To turn the salt into a aromatic amine, an alkali such as NaOH solution is added

Overall equation:
C₆H₅NO₂ + 6[H] –Tin,conc. HCl, reflux / NaOH–> C₆H₅NH₂ + 2H₂O

Aromatic amine are useful for organic synthesis of things like pharmaceuticals and dyes

Question 17

What is an amide?

Answer 17

Amides contain the functional group -CONH₂.

The carbonyl group has a negative inductive effect on the NH₂ group, so they behave differently

Question 18

What is an N-substiuted amide?

Answer 18

One of the hydrogens is repalced with an alkyl group