Nitrogen Compounds Flashcards
Amines may be classified as…?
primary, secondary, tertiary,
depending on no. of alkyl/aryl grp attached to N atom
(primary is 1 R grp, tertiary is 3 R grp)
NOTE: quaternary ammonium ion is organic equivalent of NH4+ ion where 4 alkyl/aryl grp hv replaced H atoms
- oso can b classified aliphatic, aromatic
Describe nomenclature of amines
aliphatic
- named by specifying alkyl grp attached to N atom, follow by suffix amine
eg
methylamine (1 deg), phenylmethylamine (1 deg), diethylamine (2 deg)
aromatic
- N atom directly attach to benzene ring
- named as derivatives of simplest aromatic amine (phenylamine)
eg
phenylamine (simplest), 4-methylphenylamine, N-methylphenylamine
- in compounds where amine grp is named as substituent, prefix amino- is used
Explain boiling point of amines
- Within homologous series, bp generally increase w increasing e- cloud size (seen by increasing Mr)
bcos
- w increasing Mr, size & polarisability of e- cloud increase
=> increasing strength/extent of intermolecular id-id attract n, require more energy to overcome - for isomeric amines bp tend follow 3 deg < 2 deg < 1 deg
bcos
- more energy needed overcome stronger intermolecular H bonding in primary, secondary amines vs weaker intermolecular pd-pd attract n of tertiary amine
=> tertiary amine hv lowest bp
- primary vs secondary: presence of more alkyl grp bonded to N in secondary amine cause steric effect, decreases availability of lp of e- on N for intermolecular H bonding. Secondary amine hv weaker intermolecular H bonding, needs less energy overcome
=> secondary amine hv lower bp than primary amine - Amines hv higher bp than hydrocarbons of similar e- cloud size
bcos,
- more energy needed to overcome stronger intermolecular H bonding in amines vs weaker intermolecular id-id attract n for hydrocarbon - Amines hv lower bp than alcohols of similar Mr
bcos,
- N-H bond less polar than O-H bond, intermolecular H bond in amines weaker, need less energy overcome than those in alcohols
Explain solubility of amines in water
- Amines w small molecular size soluble in water
bcos,
- amines can form H bond w water molecules
- energy released by form n H bond btw amine, H2O molecules enough overcome intermolecular H bond btw H2O & amine molecules respectively - Amines w larger molecular size less soluble in water
bcos,
- as alkyl/aryl (R) grp(s) bcome bulkier/larger, solubility in water decrease as greater hydrophobic nature of R grp decrease strength of interact n btw amine & water
Name reagent, conditions and type of reaction of nitriles + H
R&C:
1. LiAlH4 in dry ether
or
2. H2 w Ni catalyst, high temp & Pa
or
3. H2 w Pd/Pt catalyst
type: reduct n
NOTE:
-CN –> -CH2NH2 (forms primary smine)
Name reagent, conditions and type of reaction of amides + H
R&C: LiAlH4 in dry ether
type: reduct n
NOTE: H2(g), NaBH4 cnt b used reduce amides,
this rxn forms primary amine
Name reagent, conditions and type of reaction of halogenoalkane and NH3
R&C: excess conc NH3 in ethanol, heat in sealed tube
type: Nu Sub
NOTE:
- forms primary amine
- use of excess NH3 discourage form n of side-pdt, give primary amine as MAIN pdt
- if excess of RX is used, main pdt formed is quaternary ammonium salt
Name reagent, conditions and type of reaction of nitrobenzene and H
R&C:
Sn in excess conc HCl, heat (reduct n),
followed by add n NaOH(aq) (neutral n)
type: reduct n
NOTE:
- other reducing agent cnt b used
- pdt formed in 1st step is C6H5NH3+Cl-
- NaOH(aq) added in 2nd step for acid-base rxn w acidic C6H5NH3+Cl to form required pdt, C6H5NH2
- Oni method to prepare phenylamine in syllabus
In aqueous state, amine behaves as … Thus, any process that … availability of … for bonding with a proton will cause amine to be a …
Bronsted-Lowry base,
increases,
lone pair of electrons,
stronger base
Explain relative basicity of ammonia, ethylamine and phenylamine
order of base strength:
ethylamine > ammonia > phenylamine
ethylamine stronger than ammonia bcos
- e-donating ethyl grp increase e- density of lp of e- on N atom
=> lp e- more available to accept proton (form dative bond w H+)
- ammonia no e-donating grp attached to N atom, so lp e- on N less available accept proton
phenylamine weaker base than ammonia bcos
- in phenylamine, lp e- on N atom delocalised into benzene ring
=> lp e- less available to accept proton
- lp e- on N in ammonia not delocalised so more available to accept proton
Describe effects of substituents on basicity
- e-donate grp cause increase in basicity
Bcos,
- increase e-density of lp e- on N atom
- lp more available accept H+ ion
- stronger base - e-withdraw grp cause decrease in basicity
Bcos,
- decrease e-density of lp e- on N atom
- lp less available to accept H+ ion
- weaker base
Generally,
- more grp —> greater effect
- effect decrease w dist fr N atom
Describe basicity of amines in gaseous phase
- amine behave as Lewis base
- N atom use its lp e- form dative bond w e-deficient species
=> any process increasing availability of lp e- for bonding w proton cause amine bcome stronger base
NOTE:
In gaseous phase, no water molecules to interact w amine molecules so discuss n amines as Bronsted Lowry base not considered
Explain relative basicity of primary, secondary and tertiary aliphatic amines in gaseous phase
Basicity: tertiary>secondary>primary
Order of base strength:
Trimethylamine>dimethylamine>methylamine
Bcos,
- from methylamine to trimethylamine, increase in no of e-donate alkyl grp increases e-density of lp e- on N atom
=> lp more available form dative bond to e-deficient species
Name reagent, conditions and type of reaction of amines and acid
R&C: HCl(aq)/H2SO4(aq) or any aq acid, r.t.p.
Type: neutral n
Note:
Forms salts
Name reagent, conditions and type of reaction of amines and acyl chloride
R&C: anhydrous acyl chloride
Type: condensat n
Note:
- cnt use RCOOH as neutral n occur instead
- ONLY method form amides
Name reagent, conditions, type of reaction, OBSERVATION and APPLICATION of phenylamine and Br2
R&C: Br2(aq)
Type: e- sub
Observ n: decolour n orange Br2(aq) sol n w form n white ppt
Application: chemical test for phenylamine (NOTE: phenol give similar observ n)
As compared to electrophilic substitution of benzene, conditions required for that of phenylamine are considered mild (ie rtp, no catalyst). Why?
- in phenylamine, lp e- on N atom delocalised into benzene ring
=> increased e- density in ring makes it more readily attacked by e-philes (activated towards e- sub)
What are amides?
- derivatives of RCOOH containing structural unit - O= C-N- -
Describe nomenclature of amides
- generally named by replace ‘e’ of alkanes w ‘-amide’
eg ethanamide - C atom of -CONH2 is NUMBERED 1
- substituent grp bonded to N given prefix N-
eg N-methylpentanamide
Explain boiling and melting point of amides
- RCONH2 and RCONHR’ amides can form intermolecular H bond due to presence of N-H bond
=>expected to have higher mp/bp than RCONR’R’’ amides - RCONR’R’’ amides NO H bond due to absence of H atom bonded to N
Explain solubility of amides in water
- amide can form H bond w water molecules
=> expect smaller amides to be soluble in water - larger amides tend hv larger hydrophobic section, cause decreased solubility (id-id)
Explain lack of basicity of amides
Amides are NEUTRAL (not basic, neutral to litmus)
Bcos
- lp e- on N atom delocalised w adjacent C=O thereby reduce e-density on N atom
=> N atom less available accept/ form dative bond to proton (H+)
Name reagent, condition and type of reaction of acyl chloride and NH3 / amine
R&C:
1. To form RCO2NH2 :
RCOCl w NH3, rtp (readily react)
- To form RCO2NHR’ :
ROCl w primary amine - To form RCO2NHR’R’’ :
ROCl w secondary amine
Type: condensat n
Describe overview of hydrolysis of amides
- (adding H2O) involve cleavage of C-N bond in amide
- forms carboxylic acid & ammonia/amine
- in basic medium, acid salt forms (from RCOOH)
- in acidic medium, NH4+/ amine salt forms
Name reagent, condition and type of reaction of amide and acid
R&C: HCl(aq)/H2SO4(aq), heat under reflux
Type: acidic hydrolysis
Name reagent, condition and type of reaction of amide and base
R&C: NaOH(aq)/KOH(aq), heat under reflux
Type: basic hydrolysis
NOTE: in any rxn where ammonia forms, this can be used to test presence of RCONH2
Name reagent, condition and type of reaction of amides + H
R&C: LiAlH4 in dry ether
Type: reduct n
Note:
Cnt use H2 (g) or NaBH4
Describe some reagent and conditions to distinguish RCONH2, ammonium salts and amines
- NaOH(aq), heat
- to distinguish RCONH2 amides fr other organic cpds
- Observ n: pungent NH3 gas evolved turned moist red litmus paper blue
NOTE:
ammonium salts oso liberate NH3(g) on warming w NaOH (see Data Booklet QA for Cation tests) - Br2(aq)
- distinguish Phenylamine fr other cpd (except phenol)
- Observ n: orange Br2(aq) decolourise w form n white ppt - Dilute acid (eg HCl aq)
- distinguish Amines insoluble in water fr other insoluble organic cpd
- observ n: amine insoluble in water but SOLUBLE in dilute acid give homogenous sol n
(Bcos Amine react in neutral n rxn form ionic salt soluble in water due to form n strong ion-dipole interact n btw ion & water)
What is amino acid? How about a α-amino acid? Give general formula
- amino acid contain >= 1 -COOH & 1 amino grp (-NH2)
- α-amino acid is an amino acid where both -NH2 & -COOH grp r direct bonded to same C atom (α-carbon)
- general formula of α-amino acid is
H2N- H-C-R -COOH
Describe nomenclature of amino acid
- named as derivatives of RCOOH
eg
2-aminoethanoic acid (glycine),
2-amino-3methylbutanoic acid (valine)
Define zwitterion
- Amino acid exist predominantly as electrically neutral dipolar ion (zwitterion)
Describe physical properties of amino acids
- Exist as crystalline solids w high mp (> 200 deg C)
Bcos
- large amt energy needed overcome strong e-static force attract n btw zwitterion - Soluble in water
Bcos
- ion-dipole interact n (similar to ionic cpd) formed w water molecules cause release of sufficient energy, cause detachment of zwitterion fr crystal lattice for solvat n
Describe neutralisation reactions of amino acids
- zwitterion hv amphoteric nature as undergo neutral n at respective -COO- or -NH3+ grp
- as acid: -NH3+ grp responsible for acidic properties (reversible rxn)
- as base: -COO- grp responsible for basic properties (reversible rxn)
Describe amino acid zwitterion in aqueous solution
- can act as buffer sol n
- on add n small amt H+,
- added H+ removed
=> [H+] slightly changed so pH fairly constant - On add n small amt OH-
- added OH- removed
=> [OH-] slightly changed so pH fairly constant
Describe behaviour of amino acids in solutions of different pH
- can exist in diff form depending on pH of sol n
- fr low to high pH, can exist as cationic (fully protonated) to (electrically) neutral to anionic
- fully protonated form is acidic in nature & can b titrated w base
How to approach questions dealing with amino acids and multiple pKa values?
- Assign pKa values to each acidic/basic grp
- relevant to groups in protonated form (eg for -NH2, pKa value refer to that of -NH3+)
- if got 2 identical grp, infer which hv lower pKa value (stronger acid)
Eg. W 2 -COOH grp, the one nearer to -NH2 hv lower pKa value as it is stronger acid (bcos -NH2 is e-withdraw grp) - For each grp, compare pH of sol n w its pKa to determine form of gro at that pH
- if pH of sol n < pKa, grp is protonated (eg -COOH/ -NH3+)
- if pH of soo n > pKa, grp is deprotonated (eg -COO-/ -NH2)
How are peptide linkages formed? Describe
By condensat n rxn of amino acids w other amino acid molecules
Resultant molecule aka peptide, each amino acid unit within peptide aka amino acid residue
Warning:
Form n peptide bonds occur in nature under SPECIFIC condit n. Peptide bond CANNOT b formed in lab by simply reacting amine & -COOH grp (bcos in lab, undergo neutral n)
Describe peptides
- contains 2 or more α-amino acids linked by peptide bonds
=> 2 amino acids joined tgt: dipeptide (contain 1 peptide bond)
=> 3 amino acids joined tgt: tripeptide (contain 2 peptide bond)
=> many amino acids joined tgt: poly peptide - polypeptide containing > 400 amino acid residue r termed proteins
- protein can contain up to 400 amino acid residue. Diff protein contain diff sequence of amino acid residue
Name reagent, condition for complete hydrolysis of proteins
R&C:
[acidic] HCl(aq)/H2SO4(aq) or [basic] NaOH(aq), prolonged heat under reflux
NOTE:
- acidic hydrolysis form RCOOH + amine salt
- basic hydrolysis form RCOO- + amine
