Organic 11: Amines Flashcards
What are amines derived from?
Ammonia
At least one hydrogen atom is replaced by an alkyl or aryl group
What is an alkyl group?
An alkane that has lost a hydrogen
E.g. a methyl group (CH3)
What is an aryl group?
An aromatic compound
Why are amines useful?
They are very reactive
So are commonly used as intermediates in organic synthesis
What is a primary amine?
An amine where one hydrogen is replaced by an alkyl or aryl group
What is a secondary amine?
An amine where two hydrogen atoms are replaced by alkyl or aryl groups
What is a tertiary amine?
An amine where all three hydrogens have been replaced by an alkyl or aryl group
What is the general formula for amines?
RNH2
Where R is an alkyl or aryl group
How do you name amines?
The prefix represents the alkyl or aryl group, and the suffix -amine represents the amine.
E.g. methylamine
Describe the shape of ammonia.
Pyramidal
Bond angle of 107
Why is the bond angle of ammonia different to that of a normal tetrahedron?
The lone pair of electrons on the nitrogen repels more than the bonding pair of electrons, reducing the bond angle.
Why are the boiling points of amines lower than the boiling points of their comparable alcohols (e.g. methyl amine and methanol)?
Primary amines can form hydrogen bonds to each other using their NH2 groups
Alcohols can form hydrogen bonds to each other using their OH groups
Nitrogen is less electronegative than oxygen so forms weaker hydrogen bonds
Which need less energy to break, so lower boiling point
What state are amines at room temperature?
Shorter chain amines (such as methyl and ethyl amine) are gases at room temperature
Longer chain amines are volatile liquids (rotting fish and rotting animal flesh smell of di- and triamines which are produced when proteins decompose)
Describe the solubility of amines.
Short chain amines (e.g. up to four carbon chain length) are very soluble in water and in alcohols because they form hydrogen bonds
Why are phenyl amines not soluble?
The benzene ring is not soluble as it cannot form hydrogen bonds as C-H bonds are non-polar
Hydrogen bonds also form more readily between molecules with polar groups like oxygen, fluorine or nitrogen
Why are amines so reactive?
Due to the lone pair on the nitrogen
It may be used to form a bond with a H+ ion when the amine acts as a base
Or form a bond with an electron-deficient carbon atom, when the amine is acting as a nucleophile
Why are amines classed as Bronsted-Lowry bases?
Because they can accept protons
How do amines react with acids?
Amines can react with acids to form salts
Describe and give an equation for the reaction of ethylamine with hydrochloric acid.
Ethylamine reacts with dilute hydrochloric acid to form ethyl ammonium chloride:
C2H5NH2 + H+ + Cl- –> [C2H5NH3]+ + Cl-
Ethyl ammonium chloride is an ionic compound that will crystallise as the water evaporates
Describe how phenylamine reacts with hydrochloric acid.
Dissolves in excess hydrochloric acid
Forms soluble ionic salt: phenyl ammonium chloride
How would you regenerate phenylamine from phenyl ammonium chloride?
Add a very strong base like sodium hydroxide
This removes the proton from the salt and regenerates the insoluble amine
As well as water and a chloride ion
Give the order of base strength for amines.
Tertiary amine > Secondary amine > Primary amine > Ammonia > Aromatic amines
Why are primary alkylamines stronger bases than ammonia?
An inductive effect is created as the alkyl groups release their electrons towards the nitrogen atom, increasing the electron density on the nitrogen atom, which makes it more attractive to protons
So acts as a better proton acceptor (base)
Why are secondary alkylamines stronger bases than primary alkylamines and ammonia?
They have twice as many inductive effects due to the two alkyl groups so have greater electron density on the nitrogen
So it will be a better proton acceptor
Why are tertiary alkylamines stronger bases than secondary alkyl amines?
More inductive effects of alkyl groups to increase electron density on the nitrogen atom to make it a better proton acceptor
Why are aromatic amines weaker bases than ammonia?
The lone pair on the nitrogen is partially delocalised into the benzene ring
Which reduces the electron density on the nitrogen atom
Making it a poor proton acceptor
How are primary aliphatic amines produced from halogenoalkanes? (Describe the reaction and conditions)
Nucleophilic substitution with ammonia
Conditions:
Excess concentrated ammonia solution
Ethanolic
Under pressure
Reaction:
Lone pair on ammonia is attracted to delta positive carbon atom on C-X bond
The electron pair in C-X moves to the X atom, making the halide ion a leaving group
Then, one hydrogen on the NH3 group must leave to form a neutral amine (arrow from N-H bond to N)
A second ammonia molecule reacts with the H+ to form an NH4+ ion
Products
Primary amine
Ammonium ion (NH4+)
Name the steps involved in forming a primary amine from a halogenoalkane via nitriles.
Step 1: Nucleophilic substitution of halogenoalkane to produce nitrile
Step 2: Reduction of nitrile to form primary amine
How are nitriles prepared from halogenoalkanes? (Describe the reaction and conditions)
Nucleophilic substitution with cyanide
Conditions:
Warm
Aqueous, alcoholic solution of KCN (Potassium cyanide)
Reaction:
The lone pair on the cyanide ion is attracted to the delta positive carbon atom on the C-X bond
The electron pair on the C-X bond moves to the X (halogen), so the halide ion is the leaving group
Products
Nitrile compound
Halide ion
How are primary amines prepared from nitriles?
Nitriles can be reduced to primary amines using a nickel/hydrogen catalyst.
R-CN + 2H2 –> R-CH2NH2
Where is benzene naturally derived from?
Crude oil
What is the organic synthesis route to make phenylamine from benzene?
Benzene –> Nitrobenzene –> Phenylamine
Step 1: Nitration (electrophilic substitution)
Step 2: Reduction
How is the electrophile formed for the electrophilic substitution of benzene?
HNO3 + 2H2SO4 –> 2HSO42- + NO2+ + H3O+
(NO2+ is the electrophile)
Describe the mechanism for electrophilic substitution of benzene using a nitronium ion.
Conditions:
Concentrated nitric and sulphuric acid (to produce electrophile)
Reaction:
A bond forms between carbon atom and electrophile using electrons from the delocalised system (very electron dense aromatic ring is attracted to positive electrophile)
The carbon must then lost a H+ with the electron in the C-H bond returning to the delocalised system to regain stability
Products:
Nitro compound (i.e. nitrobenzene)
H+
How is phenylamine produced from nitrobenzene?
Reduction
Conditions:
Tin and hydrochloric acid as the reducing agent (Sn/HCl)
Reaction:
The tin and hydrochloric acid react to form hydrogen, which reduces the nitrobenzene by removing oxygen atoms on the NO2 group and replacing them with hydrogen
Nitrobenzene + 6[H] –> Phenylamine + 2H2O
Why are amines better at reacting with halogenoalkanes than ammonia is?
Because amines have a lone pair on the nitrogen atom which has a higher electron density than on ammonia due to the inductive forces of electrons from the alkyl group
What is a quaternary ammonium salt?
An organic salt containing a positively charged nitrogen atom (ammonium ion) bonded to four organic groups
It does not have a hydrogen bonded to it
R4N+
What are amines used for?
Manufacture of synthetic materials like nylon, dyes, and drugs
What are quaternary ammonium salts used for and why?
Manufacture of hair and fabric conditioners (cationic surfactants)
They have a long hydrocarbon chain and a positively charged organic group, so form cations
Wet hair and fabric pick up negative charges on their surfaces, so positive charges of cations attract them to the surface and form a coating that prevents static electricity
This keeps the surface of the fabric (or hair) smooth
In aqueous solution, their ions cluster with the charged ends in the water and the hydrocarbon tails on the surface
How do primary amines react with acyl chlorides or acid anhydrides?
In addition-elimination reactions
Producing N-substituted amides
The amine adds on to the acyl chloride and eliminates HCl
This is a useful reaction to form polymers like nylon
