Chemistry Form 6 Sem 3 08
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CHEMISTRY FORM 6 ORGANIC CHEMISTRY CHAPTER 8 : AMINE
9.1 Introduction and nomenclature Amines are organic compounds with functioning group –NH2 (aminO) The naming of amine depends on the classification of amine. Table below shows different class of amine and their naming Classificati on
Example CH3CH2CH2NH2
propylamine
2-methylpropylamine
Primary amine
cyclopentylamine
4,4-dimethyl cyclohexylamine
3-ethyl-1methylpentylamine
Phenylamine or Aniline
Secondary amine
N-propylaniline N-ethylpropylamine
N-cyclopentyl cyclohexylamine
Tertiary amine
N-ethyl-N-methyl propylamine
N,N-diphenyl methylamine
N,N-dipropyl ethylamine
Quaternary salt
Ammonium chloride
Tetramethylammonium Tetraethylammonium hydroxide bromide
9.1 Physical properties of amine Boiling point of increasing in homologous series Amine Boiling point oC Boiling point trend Explanation :
CH3NH2
C2H5NH2
C3H7NH2
C4H9NH2
C5H11NH2
4
17
52
83
106
Boiling point increase
When going down to homologous series, the boiling point increase. This is due to the increase in relative molecular mass, which increase the weak Van DerWaals forces causing boiling point increase. Classification of amine Example (same RMM)
Primary amine
Secondary amine
Tertiary amine
CH3CH2CH2NH2
CH3CH2NHCH3
(CH3)3N
Boiling point (oC)
52
34
2
Explanation : Amine can form hydrogen bond between molecules. Straight chain molecule has
a larger total surface area compare to a branched chain molecule. Hence, greater the total surface area exposed, greater the Van DerWaals forces, higher the boiling point. However, tertiary amine cannot form hydrogen bond, so their boiling point decrease significantly.
•Boiling point of different organic compounds Organic
CH3CH2CH2CH3 CH3CH2CH2NH2 CH3CH2CH2OH
CH3COOH
RMM
58
57
60
60
Boiling point oC
4
52
78
118
Ethanoic acid has 2 hydrogen bond within their molecules, so their hydrogen bond is the strongest. Both propylamine and propan1-ol has hydrogen bond, but propan1-ol has a stronger hydrogen bond since O is more electronegative than N. Butane is a non-polar molecule, which is held by weak Van Der Waals forces B)
Solubility of amine Amine
Solubility trend
CH3NH2
C2H5NH2
C3H7NH2
C4H9NH2
C5H11NH2
Solubility DEcrease
methylamine, ethylamine and propylamine are completely miscible in water as they can form hydrogen bond with water. However, the LONGER the ALKYL GROUPS ATTACHED, molecule become MORE HYDROPHOBIC. As a result, HYDROGEN BOND BECOME LESS SIGNIFICANT and cause the solubility decrease.
Explanation
C) Basicity of amine Amine is a weak base. When dissolve in water, it undergoes partial dissociation where
Kb =
+ − [ RNH 3 ][OH ]
[ RNH 2 ]
pK b = − lg K b
The pKb of some amine is given in the table below Amine
CH3NH2
C2H5NH2
C3H7NH2
CH3CH2NHCH3
pKb
3.42
3.27
3.03
2.88
(CH3)3N 2.64
9.41
9.58
All alkyls are electron donating group, which donate electron density to N in NH2, making N to be more readily to accept proton. Hence, equilibrium shift more to right, increasing the basicity. Longer the alkyl chain, greater the electron donating effect, greater the basicity.
Basicity increase from 10 amine < 20 amine < 30 amine. This is due to, the more the alkyl group surrounding N, greater the electron donating effect, where N has large electron density and are more readily to accept proton, causing equilibrium to shift more to right, increasing the basicity of amine.
phenylamine is a weaker base compare to alkyl amine, due to the benzene ring is an electron withdrawing group. As a result, N in amine has lesser electron density and is less readily to accept proton (conversely more readily to donate proton) which caused equilibrium shift more to left. This results the basicity of phenylamine is lesser than alkyl amine. When a ring activator is bonded to phenylamine, ita activate further the ring and caused the ring to be more readily to accept proton, hence increase the basicity. Conversely, if a ring deactivator is bonded to phenylamine, it deacticate the ring and caused benzene to be less readily to accept proton, decreasing the basicity
9.2
Preparation of amine
Name of reaction
Reagent used and condition
Equation
Alkylation Concentrated of 1-bromopropane NH3 haloalkane
Reduction of nitrile
conc. Ammonia
Lithium aluminium tetrahydride 2-methylbutylnitrile LiAlH4
Lithium Reduction tetrahydridoof amide aluminate, LiAlH4
propanamide
propylamine
2-methylbutylamine
propylamine
9.2.1 Chemical reaction of amine Name of reaction
Reagent used and condition
Reaction with mineral acid
Mineral acid such as HCl ; H2SO4
Nitrosation of amine
HNO2 under certain condition
Reaction with acyl chloride, R–COCl
R–COCl, acyl chloride
Equation
CH3CH2CH2 NH2 + O
N
OH
CH3CH2CH2OH + H2O + N2
(A) Reaction with mineral acids : Formation of salts Amine dissolves in aqueous solution of mineral acid to form salt of ammonium. Example Class of amine
Primary amine Secondary amine Tertiary amine
Example
Mineral acid
Salt
CH3CH2NH2
HCl
CH3CH2NH3+Cl–
(CH3CH2)2NH
HCl
(CH3CH2)2NH2+Cl–
(CH3CH2)3N
HCl
(CH3CH2)3NH+Cl–
The amine salt form is white crystalline solid dissolve readily in water When dissolve in alkaline solution, amine is form back CH3CH2NH3+Cl– (aq) + OH– (aq)
+ Cl– (aq)
CH3CH2NH2 (aq) + H2O (l)
(B) Reaction of amine with nitrous acid Nitrous acid (or nitric (III) acid) is unstable. It can be prepared by reaction of sodium nitrite and hydrochloric acid Equation :
NaNO2 (s) + HCl (aq) NaCl (aq) + O=N–OH (aq) The organic products formed from the reaction between nitrous acid and amine depend on 2 factors Class of amine used Condition of the reaction When aliphatic primary amine reacts with nitrous acid, nitrogen is evolved rapidly and alcohol is produce as major product along with some side product such as alkene and in some case, ether. The reaction of nitrous acid follows the mechanism below
Primary amine : Amine act as nucleophile and attack nitrosyl cation, the following mechanism of reaction take place.
diazonium ion The reaction after the formation of diazonium ion is complex. Depend on the condition of the reaction, it form various organic compound such as alcohol or alkene. Example : Secondary amine Secondary amine reacts with nitrous acid (nitrosyl cation) under room condition to form nitrosoamine. Unlike primary amine, there’s no nitrogen gas evolve. Example
N–nitrosoamine
Tertiary amine Tertiary amine dissolve readily in acidic solution of nitrous acid to form salt according to the equation
(C) Reaction with acyl chloride (refer Chap 8.4) When acyl chloride reacts with primary amine, it will form a secondary amide
When react acyl chloride with a secondary amine, it will form a tertiary amide
Acyl chloride will not be acylated as it does not has a hydrogen atom to
be substituted.
9.3 Aromatic amine The simplest form of aromatic amine is phenylamine, which is better known as aniline Some of the common naming of aromatic amines are shown below
aniline
p-nitroaniline
o-methylaniline
N-methylaniline
N-ethyl-Nmethylaniline
N,N-diethylaniline
phenylmethylamine
H N
diphenylamine
9.3.1 Preparation of aniline Aniline can be prepare by using nitrobenzene. Reagent : conc. HCl catalysed by tin, Sn. 9.3.2 Chemical reaction of aniline 1. Reaction with acid : the basic properties of aniline Aniline is considerably weak acid as the pKb = 9.3
This is due to the electron withdrawing effect of the benzene, causes the
electron density of nitrogen decrease. When this occur, nitrogen are more readily to donate proton rather than accept and equilibrium shift more to the left (Kb decrease) Though, aniline is able to react with acid by accepting the H+ from acid and form salt
Aniline
Acid
Salt
HCl H2SO4 H2SO4 in excess HNO3 •
Salts of aniline are colourless, crystalline solid and mostly are soluble in water. When react with alkali, it give back the phenyl amine with salt and water
2. Reaction of aniline with haloalkane If aniline is reacted with iodomethane (CH3I) under ethanolic, a secondary amine is first formed and eventually turns out to become salt
If excess haloalkane is used, the reaction will further to form a tertiary amine and
quaternary salt.
3. Reaction of aniline with acyl chloride (Chapter 8.4) This is one of the method use to prepare amide as discussed earlier. When acyl chloride react with amine, a secondary amide is formed
When a secondary amine reacts with acyl chloride, it will form a tertiary amide.
Tertiary amine will not react with acyl chloride as there’s no H substitute out
4. Reaction of aniline with nitrous acid Similar to the reaction of nitrous acid with alkyl amine, aniline react with the electrophile nitrosyl cation where the formation of ion follow the mechanism below
When aniline attacks the electrophilic nitrosyl cation, it form benzenediazonium
ion.
Benzenediazonium chloride formed is unstable and tends to react with other
substances. Diagram below shows the type of reaction that the benzenediazonium can undergoes
5. Formation of dyes by coupling reaction The reaction of forming azo dye is via coupling reaction. Diazonium ion react with benzene attached with strong ring activate group, under alkaline condition, to give bright colouration of azo dye. Strong ring activate group direct at ortho-para position.
Benzenediazonium chloride phenol 4-hydroxyphenylazobenzene When react with 1-naphthol, it form a bright scarlet azo dye
When react with another aniline, it form an orange-red precipitate dye.
9.4 Chemical tests for aniline (1) Reaction with bromine Similar to phenol, aniline is a strong ring activate group. So, when aniline is reacted with aqueous bromine solution, it forms a white precipitate.
(2) Reaction with sodium chlorate (I), NaClO When aniline is added to NaClO, a purple colouration is obtained (3) Reaction with phenol under alkaline solution As discuss earlier, when aniline is react phenol under NaOH, a yellow precipitate is observed.
10.1 Introduction Amino acid has the general formula of NH2 – CHR – COOH. From the general above, we can tell that there are 2 functioning groups of –NH2 (amine) and –COOH (carboxylic acid). There are 22 types of amino acids and almost all naturally occurring amino acids are α-amino acid The terms α- mean both NH2 and COOH are attached at the same C. Considering –COOH act as the functioning group, α-amino acid has the general naming as 2-aminocarboxylic acid The simplest form of amino acid is called glycine (2aminoethanoic acid) All amino acids are optical active (except glycine) as they have 4 different functioning groups (chiral carbon atom).
Acidic amino acid
Neutral amino acid
Basic amino acid
O H2N
CH C
OH
CH2 CH2 C
O
OH
The number of COOH is more than NH2
The number of COOH is equal to the number of NH2
The number of COOH is less than NH2
Since amino acid has both acidic (–COOH) and basic (–NH2) fuctional
group, hence amino acid undergoes 3 types of chemical reaction : reactions characteristic of the amino group reactions characteristic of the carboxylic group reaction due to the presence of both group Because of both (–COOH) and (–NH2) group, it makes amino acid very soluble in water. Neutral amino acid such as alanine and glycine are slightly miscible on ethanol and epoxyethan. This is due to amino acid exist as polar ion.amino acid exist as a zwitterions, which carries both positive and negative charge in its molecule. A zwitterions is formed when –COOH donated its proton to – NH2, according to
Condition
Equation
Charge
Acidic
Positive charge
Neutral
Zwitterions
Basic
Negative charge
Isoelectric point – the pH at which amino acid has a net charge of zero. Amino acids with one –COOH and one –NH2 have isoelectric points in the
pH range 5.5 – 6.5 Acidic amino acid (eg : aspartic acid and glutamic acid) has lower isoelectric point (lower pH value) while basic amino acids (eg : lysine and arginine) have higher isoelectric point (higher pH value)
10.2 Chemical Properties of Amino acid 10.2.1 Preparation of amino acid Name of reaction
Reagent used and condition
Reaction of ammonia with α-halo carboxylic acid
Concentrated ammonia
Equation
10.2.2 Formation of peptide lingkage The reaction which link 2 amino acids together is a condensation reaction where water is given off and formed a dipeptide. Peptide link is a secondary amide group where the C- from one amino acid is linked to the –N of another amino acid.
If 3 amino acids joined together, the product is ………………….. When a lot of
amino acids join together, the product is a …………….. When the many chain of peptide formed …………..…....
10.2.3 Hydrolysis of Protein Protein could be broken into its constituent amino acids by heating under reflux with aqueous NaOH or dilute H2SO4 via hydrolysis reaction.
Hydrolysis of protein can be completely or partially hydrolyse. Complete hydrolysis of protein gives the number and relative amount of
amino acids, but does not provide the information of the order of how they joined together (the sequence of R)
Partially hydrolysis give fragments which still retain sequence information.
When these fragments are separated and identified, the overall protein structure can be deduced. Example
10.3 Chemical Test for amino acid Aminoethanoic acid (NH2CH2COOH) gives the following reactions : It liberates CO2 when treated with Na2CO3 and NaHCO3. Form dark red solution when treated with neutral iron (III) chloride solution. It gives a deep blue solution when treated with copper (II) solution due to the copper (II) complex. 10.4 Function of Proteins All proteins are made up of long chains of amino acids. Each protein is determined by its amino acid sequence (which ranges from 44 amino acids in the hormone insulin to giant molecules with more than 50,000 amino acids). Proteins are essential in living systems - animals, plants and micro-organisms. Proteins are responsible for growth, repair and maintenance of the body.
Types of proteins
structural proteins
muscle fibres
Function • provide the framework which defines the size and shape of cells • e.g. elastin (in ligament) a -keratins (in skin, hair, nails) collagen (in connective tissue, tendon, cartilage) • produces mechanical force and movement, • e.g. actin and myosin
transport proteins
• move metabolites around the cell or around the whole organism • e.g. haemoglobin which transports oxygen in blood
Some hormones
• control the level and type of cell activity, • e.g. insulin which regulates glucose metabolism
Enzymes
• catalyse metabolic processes which produce energy, build up new cell structures and destroy old ones
10.5 Protein Structures The structure of proteins are categorised into four levels of complexity: > primary (1°) structure > secondary (2°) structure > tertiary (3°) structure > quaternary (4°) structure 10.5.1 Primary (1°) structure The primary (1°) structure of a protein shows the order (or sequence) of amino acids in a protein chain
By convention, amino acid sequence is written with free –NH2 on left &
free –COOH on right. The primary structure includes any covalent cross-linkages, e.g. –S–S– bonds. The primary structure determines -what the protein is, how it folds, and its function. The sequence of R groups determines whether the protein chain will 1. coil with hydrogen bonding between N-H and C=O groups within each chain (α–helix), or 2. fold with hydrogen bonds formed between two protein chains (β– pleated sheet).
Conc. HNO3 catalysed by H2SO4 under reflux Conc. HCl under tin
C6H5NH2 + HCl C6H5NH3Cl Phenylamine is a weaker base than ammonia Phenyl is an electron withdrawing group, which decrease the electron density of N Making –N more readily to donate proton
Sodium nitrite, NaNO2 + hydrochloric acid, HCl 0 – 5 0C Sodium hydroxide
Iodine in Sodium hydroxide Yellow precipitate is formed No changes occur CH3CH(OH)COOH + I2 + OH- -OOC–COO- + CHI3 + 3 HI
J (chloropropanoic acid) is stronger acid than propanoic acid Due to the inductive effect caused by –Cl in the carboxylic acid
CH3CH(NH2)COOH + NaOH CH3CH(NH2)COONa + H2O
peptide linkage / peptide bond / amide
C6H5COCl HCl or H2SO4 or NaOH (aq) + heat/reflux
Electrophilic aromatic substitution reaction Conc. HNO3 catalysed by conc. H2SO4 under reflux
NO2+
+
H NO2
H+
reduction
Conc. HCl under tin PCl5 under room temperature amide
amide O
O
NH2 CH C
NH CH C
CH2 CH3 CH CH3
CH2 OH
OH
2 Ag+ + HO–Ph–NH2 + 2 OH- O==O + H2O + NH3 + 2 Ag Rodinol is a weaker base than ammonia, since rodinol has an electron withdrawing benzene ring to reduce the basicity.
Br
Br NH2
HO Br
Conc. HNO3 in conc. H2SO4 under reflux reduction Conc. HCl in tin
Br
Phenol and amide CH3COCl
Bromine water
amide
NaNO2 in HCl under 0-50C
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