4.6 amines Flashcards
what is a primary amine?
an amine that has 1 carbon bonded to the N
H I R - N I H
what is a secondary amine?
an amine that has 2 carbons bonded to the N
when drawing compounds, need to add H to the skeletal formula if bonded to anything other than carbon
how do you name amines?
- amino-
e.g 1-aminopropane or 2-amino-2-methylpropane
what is the functional group of amines?
-NH2
what is the type of reaction for the formation of aliphatic amines from halogenoalkanes?
nucleophilic substitution
what are the reagents/conditions needed for the formation of aliphatic amines from halogenoalkanes?
(excess, alcoholic) ammonia
- reflux in excess, alcoholic solution under pressure
(the nucleophile is ammonia (NH3)
what is an example of an equation for the formation of aliphatic amines from halogenoalkanes?
e.g
C2H5Br + NH3 (alc) —> C2H5NH2 + HBr
why is excess ammonia used in the formation of aliphatic amines from halogenoalkanes?
- ammonia attacks halogenoalkanes because it has a lone pair and is a nucleophile
- the amine produced also has a lone pair C2H5NH2 so can also attack a halogenoalkane; this leads to the formation of substituted amines
- using excess ammonia ensures that all the halogenoalkane molecules react with the ammonia before having the chance to react with any amines produced
.
- if want to create a monosubstituted amine, use excess
- bc lots of CH3CH2Br left over if NH3 not in excess
what is the product of the formation of aliphatic amines from halogenoalkanes?
amine (or its salt due to a reaction with the acid produced)
how can ammonium salts be converted to the amine?
by adding alkali (e.g NaOH)
if e.g BrCH2CH2Br undergoes nucleophilic substitution to form an amine, what amine is formed?
BrCH2CH2Br + 2NH3 —> NH2CH2CH2NH2 + 2HBr
what is the preferred reducing agent when reducing nitriles to amines?
LiAlH4
(but hydrogen with a nickel catalyst can also be used)
what are some properties of amines?
- the LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them:
• basic - can be proton acceptors
• nucleophiles - provide a lone pair to attack an electron deficient centre - their boiling point increases with molecular mass
- amines have higher boiling points than corresponding alkanes because of their intermolecular hydrogen bonding
- therefore, lower mass compounds are soluble in water due to the H bonding
- solubility decreases as the molecules get heavier
- soluble in organic solvents
- basic
- high b.p
- high solubility
explaining the base strength of amines:
- amines are weak bases
- lone pair on N can accept a proton
- base strength depends on how well N lone pair can accept H+
- the higher the electron density of the N lone pair, the stronger the base
- CH3CH2CH2NH2 vs CCl3CH2CH2NH2
- A is a stronger base as B has Cl- which is electronegative so withdrawing e- density from N
- alkyl groups push electrons towards the N better than N (remember carbocation stability), hence say that the lone pair is more ‘available’ in tertiary amines
- 3° > 2° > 1° > NH3
amines are (strong/weak) (bases/acids)
weak bases
what does the base strengh of amines depend on?
- how well the N lone pair can accept H+
- (the greater the electron density of the N lone pair, the stronger the base - this is affected by the groups attached to the nitrogen)
why are the lone pair in tertiary amines said to be more ‘available’ than secondary or primary amines?
- alkyl groups push electrons towards the N better than H (remember carbonation stability)
- hence say that the lone pair is more available in tertiary amines than 2° or 1°
what is an aromatic amine?
the amino group must be directly bonded to the benzene ring
aromatic amines are (more/less) basic than tertiary/secondary/primary amines?
less basic
why are aromatic amines less basic than tertiary/secondary/primary amines?
as the lone pair on the N atom delocalises into the ring, making it less available to accept a proton
(therefore not very basic)
what is the general rule of base strength of amines?
3° > 2° > 1° > NH3 > aromatic
what are the 2 reactions of primary amines?
- ethanoylation (acylation) of primary amines
- making alcohols
what is the reaction of the ethanoylation (acylation) of primary amines?
- acid chloride + amine —> N-substituted amide + HCl
- primary amines react quickly with ethanoyl chloride to from substituted amides (e.g N-ethylethanamide)
what is the reaction for the formation of alcohols from aliphatic amines?
- add cold nitrous acid, HNO2 at 5°C
- nitrous acid is made in situ by adding sodium nitrite, NaNO2 and HCl at or below 5°C
- NaNO2 + HCl —> HNO2 (HONO) + NaCl
- an aliphatic amine reacts with nitrous acid (nitric (III) acid) to produce the corresponding alcohol
- bubbles of nitrogen gas are seen as an observation
how is nitrous acid made in the reaction for the formation of alcohols from aliphatic amines?
- made in situ
- by adding sodium nitrite, NaNO2 and HCl at or below 5°C
NaNO2 + HCl —> HNO2 + NaCl
what is seen as an observation from the reaction for the formation of alcohols from aliphatic amines?
bubbles of nitrogen gas
what are the three reactions of amides?
(-C =O
-NH2)
- amide to amine
- amide to nitrile
- amide to carboxylic acid
what type of reaction is the one from amides to amines?
reduction
what is the reducing agent in the reaction from amides to amines?
LiAlH4 in dry ether solvent
what type of reaction is the one forming a nitrile from an amide?
dehydration
what is the reagent in the reaction forming a nitrile from an amide?
P4O10
(phosphorus (V) oxide)
what type of reaction is the one forming carboxylic acids from amides?
- acid/alkaline hydrolysis
what are the reagents/conditions for the reaction forming carboxylic acids from amides?
- reflux
- dilute NaOH / dilute H/SO4
- if using alkaline hydrolsis, need to add acid to protonate the carboxylate salt
what can be used as a test to confirm something is an amide?
- add HNO2 (nitrous acid)
- bubbles of nitrogen gas given off
how can you test between an aliphatic and an aromatic amine?
- add HOMO below 5°C
- if if fizzes it’s aliphatic, if not it’s aromatic
what is step 1 in azo dye formation?
- benzene to nitrobenzene
- cH2SO4
- cHNO3
- reflux
- 55°C
what is step 2 in azo dye formation?
- nitrobenzene to phenylamine
- reduction
- tin + cHCl
- followed by NaOH to deprotonate the salt to form the phenylamine
- reflux
- filtration is used to remove the tin (Sn)
what type of reaction is the one from nitrobenzene to phenylamine in step 2 of the formation of azo dyes?
reduction
what are the conditions for the reaction from nitrobenzene to phenylamine in step 2 of the formation of azo dyes?
- tin + cHCl
- followed by NaOH
- reflux
why is NaOH added after tin + cHCl in the reaction from nitrobenzene to phenylamine in step 2 of the formation of azo dyes?
to deprotonate the salt
how is the tin removed in the reaction from nitrobenzene to phenylamine in step 2 of the formation of azo dyes?
by filtration
what is step 3 in azo dye formation?
- react the phenylamine with nitrous acid (nitric (III) acid) (HONO) to form a diazonium salt
- 5°C
- (HONO (nitrous acid) is prepared in situ by reacting sodium nitrite with hydrochloric acid)
what is formed when you react phenylamine with HONO?
a diazonium salt
what does step 4 of the azo dye formation depend on? (what happens to the diazonium salt because of ____)
the temperature
if the temperature is raised to above 10°C, what happens to the diazonium salt in step 4 of azo dye formation?
- the diazonium salt loses a nitrogen gas and an OH group joins to make a phenol and N2
- bubbles of nitrogen gas seen
if the temperature remains below 5-10°C, what happens to the diazonium salt in step 4 of azo dye formation?
- the diazonium salt remains stable
- (an aliphatic diazonium salt is very unstable, so only aromatics are used)
- the lone pair present in the salt can participate in the benzene ring, making it more stable
- this is due to the overlap of p-orbitals in the diazo group with the p-system in the ring
the diazonium salt is unstable above __°C?
10°C
- so the reaction is normally carried out in ice
what is step 5 in the azo dye formation?
(coupling reactions- with phenols)
- below 5/10°C
- product = 4-hydroxyazobenzene
- = yellow in colour
- the colour arises from the overlap of the pi-electrons from the -N=N- and the pi-electrons of the two benzene rings
- this extended electron system is called a conjugated system
- such systems absorb electromagnetic radiation and when this takes place in the visible region, the electron system is known as a chromophore
- such systems absorb strongly in the UV-vis region of the electromagnetic spectrum and can be measured by UV-vis spectroscopy
at what temperature should coupling reactions take place at?
below ~5°C
what colour is 4-hydroxyazobenzene?
yellow
how does the yellow colour of 4-hydroxyazobenzene arise?
- from the overlap of the pi-electrons from the -N=N- and the pi electrons of the two benzene rings
what is step 5 in the azo dye formation?
(coupling reactions- with napthalen-2-ol (2-napthol))
- done in alkaline conditions
- produces an azodye
- that’s red in colour
what is a chromophore?
- a part of a molecules responsible for the absorption of radiation in the UV-visible region of the EM spectrum
- a group or atom responsible for the colour of a compound
what is the chromophore for azo dyes?
the -N=N group
(the diazo group)
- this group helps conjugation within the molecule which increases the wavelength of light that the molecule absorb
(- but many molecules containing double bonds can also absorb light and therefore appear coloured)
- an organic molecule that had a chromophore will absorb light of a specific frequency (and colour as talking about visible light)
- this will cause the organic molecule to REFLECT/TRANSMIT (not emit) the complementary colour. this is the colour we see
- to deduce what the complementary colour is, we use a colour wheel
e.g chlorophyll reflects green light (as green is the complementary colour)
- it therefore absorbs red/violet/blue light
- if you shine green light on chlorophyll, it will appear _____
- if you shine red/violet or blue light on it, it will appear ____
- green (as it all gets reflected)
- black (as it all gets absorbed)
what can be used to see exactly what wavelength the organic molecule absorbs?
absorption spectras
carotene reflects ____ light so therefore absorbs ________ light
- reflects orange light
- so absorbs purple/blue light
many azo dues are used in colourings
- however, many have been banned under EU law due to suspected carcinogenic effects
give two common methods of producing aliphatic amines?
- nucleophilic substitution of halogenoalkanes with ammonia
- reduction of nitriles
what conditions are required for amines to be formed from halogenoalkanes?
- warmed/reflux
- with excess ethanolic ammonia
when a halogenoalkane reacts with ammonia, why do you get a mixture of products?
- the reaction will produce a mixture of primary, secondary and tertiary amines and quaternary ammonium salts
- this is because when a primary amine is produced it acts as a nucleophile in further reactions
- when it reacts in further nucleophilic substitution reactions, secondary amines are produced
- these substitution reactions continue taking place until the quaternary ammonium salt is produced
why can primary, secondary and tertiary amines act as nucleophiles when quaternary ammonium ions can’t?
- in 1°, 2° and 3° amines, the nitrogen has a lone pair of electrons which allows it to act as a nucleophile
- the nitrogen atom in quaternary ammonium ions does not have a lone pair of electrons so cannot act as a nucleophile
how can the amine be released from an amine salt?
treat the amine salt with an alkali e.g NaOH
what is the chemical equation for the reduction of ethanenitrile to ethylamine?
CH3CN + 4[H] —> CH3CH2NH2
what compounds can aromatic amines be reduced from?
- aromatic amines are produced from the reduction of nitro compounds - like nitrobenzene
what is the reducing agent used for the reduction of nitro compounds to aromatic amines?
tin and concentrated hydrochloric acid
what’s the equation for the reduction of nitrobenzene to phenylamine?
- nitrobenzene + 6[H] —> phenylamine + 2H2O
why do amines act as bases?
- the nitrogen atom in amines has a lone pair of electrons
- this means amines can form a dative covalent bond with a hydrogen ion
- therefore amines can act as bases as they accept protons
how does the strength of the base depend on the availability of the lone pair of electrons?
- the more available a lone pair of electrons is, the more likely they are to accept a proton and so the stronger a base it will be
- the higher the electron density of a lone pair, the more available the lone pair is
why are primary aromatic amines weaker bases than ammonia?
- primary aromatic amines have a benzene ring
- this has a delocalised ring of electrons which draws electrons towards itself
- this means the lone pair on nitrogen gets partially delocalised into the ring which decreases the electron density of nitrogen
- the lone pair is therefore much less available
why are primary aliphatic amines stronger bases than ammonia?
- primary aliphatic amines have an alkyl group which ‘pushes’ electrons towards the nitrogen atom
- this increases the electron density of nitrogen, making the lone pair more available
name the mechanism for the reaction between amines and ethanoyl chloride?
nucleophilic addition-eliminiation
what can be used to test for primary amines?
cold nitric (III) acid
what do primary aliphatic and aromatic amines produce when they react with nitric (III) acid at room temperature?
- aliphatic amines = produce alcohols
- aromatic amines = produce phenols
what are the observations when a primary aliphatic amines reacts with cold nitric (III) acid?
- there is a burst of nitrogen - which is a colourless and odourless gas
what is produced when primary aromatic compounds react with nitric (III) acid at temperatures below 10°C?
- benzenediazonium compounds
what are diazonium ions?
ions which contain an N2+ group
what are coupling reactions?
- organic reactions which involve the joining together of two chemical species
- an alkaline solution is required for coupling to occur
describe the coupling reaction of benzenediazonium chloride with phenol
- first, a solution of sodium phenoxide is produced by dissolving phenol in sodium hydroxide
- next, the solution is cooled and added to benzenediazonium chloride
- a yellow-orange solution/precipitated is formed
- the product is known as an azo dye
what are azo dyes?
compounds in which two benzene rings are joined by a nitrogen bridge
describe the coupling reaction of benzenediazonium chloride with naphthalen-2-ol
- first, naphthalen-2-ol is dissolved in sodium hydroxide
- the solution is then cooled and added to benzenediazonium chloride
- a bright red precipitate forms
- this is an azo dye
describe the coupling reaction of benzenediazonium chloride with phenylamine
- phenylamine is added to a cold solution of benzenediazonium chloride
- a yellow solid of azo dye is produced
how do coloured compound arise?
- coloured compounds arise due to the absorbance and reflection of light by the compound
- when white light shines on a substance, some of the wavelengths are absorbed but the remaining wavelengths are reflected and transmitted
- these reflected wavelengths correspond to a specific colour which is then observed
how can methyl orange be used to distinguish between acids and bases?
- methyl orange is an azo dye which is red in acids and yellow in bases:
• in acidic conditions, the nitrogen group gains a hydrogen causing it to interact differently with light, making it appear red
• in basic conditions, a hydrogen ion is lost so the nitrogen group reflects yellow wavelengths, making it appear yellow
