Chapter 15 Nitrogen compounds Flashcards
(57 cards)
1
Q
Primary and secondary amines can be prepared from different reactions including:
A
- The reaction of halogenoalkanes with ammonia
- The reaction of halogenoalkanes with primary amines
- The reduction of amides
- The reduction of nitriles
2
Q
Primary and secondary amines
A
3
Q
Reaction of halogenoalkanes with ammonia
A
- This is a nucleophilic substitution reaction in which the nitrogen lone pair in ammonia acts as a nucleophile and replaces the halogen in the halogenoalkane
- When a halogenoalkane is reacted with excess, hot ethanolic ammonia under pressure a primary amine is formed
4
Q
Reaction of halogenoalkanes with primary amine
A
- This is also a nucleophilic substitution reaction in which the nitrogen in the primary amine acts as a nucleophile and replaces the halogen in the halogenoalkane
- When a halogenoalkane is reacted with a primary amine in ethanol and heated in a sealed tube, under pressure a secondary amine is formed
5
Q
Reduction of amides
A
- Amines can also be formed from the reduction of amides by LiAlH4 in dry ether
- Whether a primary or secondary amine is formed depends on the nature of the amide
6
Q
Reduction of nitriles
A
- Nitriles contain a -CN functional group which can be reduced to an -NH2 group
- The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH4 in dry ether can be used to form a primary amine
7
Q
Production of Amides
A
- Amides are organic compounds with an -CONR2 functional group
- They can be prepared from the condensation reaction between an acyl chloride and ammonia or amine
- In a condensation reaction, two organic molecules join together and in the process eliminate a small molecule
- In this case, the acyl chlorides and ammonia or amine join together to form an amide and eliminate an HCl molecule
8
Q
Condensation reaction
A
- The chlorine atom in acyl chlorides is electronegative and draws electron density from the carbonyl carbon
- The carbonyl carbon is therefore electron-deficient and can be attacked by nucleophiles
- The nitrogen atom in ammonia and amines has a lone pair of electrons which can act as a nucleophile and attack the carbonyl carbon
- As a result, the C-Cl bond is broken and an amide is formed
- Whether the product is a substituted amide or not, depends on the nature of the nucleophile
9
Q
- Whether the product is a substituted amide or not, depends on the nature of the nucleophile (condensation reaction)
A
- Primary and secondary amines will give a substituted amide
- The reaction of acyl chlorides with ammonia will produce a non-substituted amide
- Note that amides can also be formed from the condensation reaction between carboxylic acids and ammonia or amines
- However, this reaction is slower as carboxylic acids are less reactive than acyl chlorides and the reaction doesn’t go to completion
10
Q
Acyl chlorides undergo condensation reactions with ammonia and amines to form amides
A
11
Q
The nitrogen atom in ammonia and amines can donate its lone pair of electrons to form a bond with a proton and therefore act as a base
A
12
Q
Basicity of Aqueous Solutions of Amines
A
- The nitrogen atom in ammonia and amine molecules can accept a proton (H+ ion)
- They can therefore act as bases in aqueous solutions by donating its lone pair of electrons to a proton and form a dative bond
- For example, ammonia undergoes an acid-base reaction with dilute hydrochloric acid (HCl) to form a salt
NH3 + HCl → NH4+Cl-
13
Q
Strength of ammonia and amines as bases
A
- The strength of amines depends on the availability of the lone pair of electrons on the nitrogen atom to form a dative covalent bond with a proton
- The more readily this lone pair of electrons is available, the stronger the base is
14
Q
Factors that may affect the basicity of amines include:
A
-
Positive inductive effect - Some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available and therefore increasing the amine’s basicity
- Delocalisation - The presence of aromatic rings such as the benzene ring causes the lone pair of electrons on the nitrogen atom to be delocalised into the benzene ring
- The lone pair becomes less available to form a dative covalent bond with ammonia and hence decreases the amine’s basicity
-
Positive inductive effect - Some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available and therefore increasing the amine’s basicity
- For example, ethylamine (which has an electron-donating ethyl group) is more basic than phenylamine (which has an electron-withdrawing benzene ring)
15
Q
Phenylamine is an
A
organic compound consisting of a benzene ring and an amine (NH2) functional group
16
Q
Phenylamine can be produced in a three-step synthesis reaction followed by the separation of phenylamine from the reaction mixture
A
- Step 1- Benzene undergoes nitration with concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at 25 to 60 oC to form nitrobenzene
- Step 2 - Nitrobenzene is reduced with hot tin (Sn) and concentrated hydrochloric acid (HCl) under reflux to form an acidic mixture that contains the organic product C6H5N+H3
- Step 3 - Sodium hydroxide (NaOH) is added to the acidic reaction mixture to form phenylamine
- Step 4 - The phenylamine is separated from the reaction mixture by steam distillation
17
Q
Reactions of Phenylamine
A
- Both the benzene ring as well as the -NH2 group in phenylamine can take part in chemical reactions
- These reactions include
- The bromination of phenylamine
- Formation of a diazonium salt
18
Q
Bromination of phenylamine
A
- Phenylamines react in electrophilic substitution reactions in a similar way as phenols
- The lone pair of electrons on the nitrogen atom in phenylamines donate electron density into the benzene ring
- In phenols, the oxygen atom donates its lone pair of electrons instead
- The delocalisation of the electrons causes an increased electron density in the benzene ring
- The benzene ring, therefore, becomes activated and becomes more readily attacked by electrophiles
- The incoming electrophiles are directed to the 2,4 and 6 positions
19
Q
Phenylamines, therefore, react under what conditions
A
milder conditions with aqueous bromine at room temperature to form 2,4,6-tribromophenylamine
20
Q
Formation of diazonium salt
A
- Diazonium compounds are very reactive compounds containing an -N2+ group
- The amine (-NH2) group of phenylamines will react with nitric(III) acid (HNO3) at a temperature below 10 °C to form diazonium salts
- Since nitric(III) acid is unstable, it has to be made in the test-tube by reacting sodium nitrate (NaNO2) and dilute acid (such as HCl)
- These diazonium salts are so unstable that they will upon further warming with water to form a phenol
21
Q
Ammonia and amines act as bases
A
- as they can donate their lone pair of electrons to form a dative covalent bond with a proton
- The basicity of the amines depends on how readily available their lone pair of electrons is
-
Electron-donating groups (such as alkyl groups) increase the electron density on the nitrogen atom and cause the lone pair of electrons to become more available for dative covalent bonding
- The amine becomes more basic
-
Delocalisation of the lone pair of electrons into an aromatic ring (such as a benzene ring) causes the lone pair of electrons to become less available for dative covalent bonding
- The amine becomes less basic
22
Q
Trends in the basicity of ammonia, ethylamine, and phenylamine
A
23
Q
Azo (or diazonium) compounds are
A
- organic compounds that have an R1-N=N-R2 group
- They are often used as dyes and are formed in a coupling reaction between the diazonium ion and an alkaline solution of phenol
24
Q
Coupling of benzenediazonium chloride with phenol in NaOH
A
- Azo compounds can be formed from the coupling reaction of a benzenediazonium chloride salt with alkaline phenol
- Making an azo dye is a multi-step process
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**Formation of azo compounds table**
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***Reaction mechanism of the formation of azo compounds***
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As a result of the delocalisation of electrons throughout the compound, azo compounds are
**very stable**
* The **delocalised** electrons in the π bonding systems of the two benzene rings are **extended** through the -N=N- which acts as a **bridge** between the two rings
28
**Making other azo dyes**
* Other dyes can be formed via a **similar route** as described above
* For example, the **yellow** **dye** can be formed from the **coupling reaction** between **benzenediazonium chloride** and **C6H5N(CH3)2** instead of phenol (C6H5OH)
29
**Amides** are formed from the
* **condensation reaction** of **carboxylic acids or acyl chlorides** with **ammonia or amines**
* The amide group (CONR2) in these compounds can undergo reactions including
* **Hydrolysis** with **aqueous alkali** or **aqueous acid**
* **Reduction** with **LiAlH4**
30
**Hydrolysis of amides**
* The -CON- group in **substituted amides** links two hydrocarbon sections of their molecules together
* This amide link can be broken down by **hydrolysis** by **refluxing** it with an **acid** or **alkali**
* The products of a **non-substituted amide** are:
* Carboxylic acid
* Ammonia
* The products of a **substituted amide** are:
* Carboxylic acid
* Primary amine
* Ammonia will react in **excess acid** to form an ammonium salt
* Carboxylic acid will get deprotonated in **excess base** to form a carboxylate ion
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***Hydrolysis of substituted and non-substituted amides (diagram)***
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***Amides are hydrolysed to carboxylic acids and ammonia or primary amine when refluxed with acid or alkali***
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**Reduction of amides**
* The C=O group in amides can be **reduced** by the strong reducing agent LiAlH4 to form an amine
* The products of a **non-substituted amide** are:
* A primary amine and water
* The products of a **substituted amide** are:
* A secondary amine and water
34
A **base** is a species that can
**donate** its lone pair of electrons to form a **dative covalent bond** with another species
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**Amines** are acidic or basic
basic as the **nitrogen** atom has a lone pair of electrons which can form a dative covalent bond with an
## Footnote
electron-deficient species (such as an H+ ion)
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The **basicity** of the amine depends on the availability of this lone pair of electrons
* The **more readily** available the lone pair of electrons is for dative covalent bonding, the **stronger the base**
* The **less readily** available the lone pair of electrons is, the **weaker the base**
* **Electron-donating** groups such as alkyl groups **increase** the electron density on the nitrogen atom causing the lone pair to become more available
* **Electron-withdrawing** groups such as aromatic benzene rings, cause **delocalisation** of the lone pair of electrons which become less readily available
37
**Basicity of amides**
* Amides also contain a nitrogen atom with a lone pair of electrons
* Again, the **basicity** of the amide depends on the availability of this lone pair for dative covalent bonding
* Due to the presence of the **electron-withdrawing** oxygen atom in the amide group, electron density is **removed** from the nitrogen atom
* The lone pair on the nitrogen atom, therefore, becomes **less readily** available and is not available to donate to an electron-deficient species
* Since this electron-withdrawing oxygen is characteristic of amides and is **not** present in amines, amides are **much weaker bases** than amines
38
**Amino acids** are
are **organic compounds** that contain two functional groups:
* A basic **amino** (-NH2) group
* An acidic **carboxylic** **acid** (-COOH) group
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Due to the presence of both a **basic** and **acidic** group in amino acids
they are said to be **amphoteric**
* They can act as both acids and bases
40
**Naturally occurring amino acids**
* **2-aminocarboxylic acids** are a type of amino acids in which the amine (-NH2) group is bonded to the carbon atom **next** to the -COOH group
* These type of amino acids form the ‘building blocks’ that make up **proteins**
* There are **20** naturally occurring amino acids with the general structural formula of **RCH(NH2)COOH**
41
***General structural formula of amino acids***
42
The **R** group varies in different amino acids and can be:
* Acidic
* Basic
* Neutral
43
**Acid / base properties of amino acids**
* Amino acids will undergo most reactions of amines and carboxylic acids including acid-base reactions of:
* Amines with acids
* Carboxylic acids with bases
* However, they can also interact **intramolecularly** (within themselves) to form a **zwitterion**
44
A zwitterion is an ion
* with both a **positive** (-NH3+) and a **negative** (-COO-) charge
* Because of these charges in a zwitterion, there are **strong intermolecular forces of attraction** between amino acids
* Amino acids are therefore **soluble crystalline solids**
45
**Isoelectric point**
* A solution of amino acids in water will exist as **zwitterions** with both **acidic** and **basic** properties
* They act as **buffer solutions** as they resist any changes in pH when **small** amounts of acids or alkali are added
46
**Isoelectric point,** If an acid is added (and thus the pH is **lowered**):
* The -COO- part of the zwitterion will **accept** an H+ ion to reform the -COOH group
* This causes the zwitterion to become a **positively charged ion**
47
**Isoelectric point,** If a base is added (and thus the pH is **raised**):
* The -NH3+ part of the zwitterion will **donate** an H+ ion to reform the -NH2 group
* This causes the zwitterion to become a **negatively charged ion**
48
***A solution of amino acids can act as a buffer solution by resisting any small changes in pH***
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the **isoelectric point** of the amino acid
The pH can be slightly adjusted to reach a point at which neither the **negatively charged** or **positively charged** ions dominate and the amino acid exists as a **neutral zwitterion**
50
**Formation of Peptide Bonds**
* Each amino acid contains an amine (-NH2) and carboxylic acid (-COOH) group
* The -NH2 group of **one amino acid** can react with the -COOH group of **another amino acid** in a **condensation reaction** to form a **dipeptide**
* The new **amide bond** between two amino acids is also called a **peptide link** or **peptide bond**
51
**Formation of Peptide Bonds happens with what reaction**
* is a condensation reaction, a small molecule (in this case H2O) is **eliminated**
* The **dipeptide** still contains an -NH2 and -COOH group at each end of the molecule which can again participate in a condensation reaction to form a **tripeptide**
52
A **polypeptide** is formed when
**many** amino acids join together to form a long chain of molecules
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**Electrophoresis** is an
**analytical technique** which separates ions by placing them in an electrical field
* This method is often used in **biochemical analysis** to **identify** and **purify** proteins
54
**Electrophoresis of** A sample of amino acids
* they placed between **two oppositely charged electrodes**
* The positively charged ions will move towards the **negative electrode**
* The negatively charged ions will move towards the **positive electrode**
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The **rate** (how fast) at which the ions move towards the electrodes depends on:
* The **size** of the ions: larger ions move **more** **slowly**
* The **charge** of the ions: highly charged ions move **more quickly**
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An **electropherogram** is the series of
bands which are observed on the paper or gel after **electrophoresis** has occurred
* Each band in the electropherogram corresponds to a particular species
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**Separating mixtures of amino acids by varying the pH**
* The charge on the **amino acid ions** depends on the pH of the solution
* The movement of the ions to the electrodes during electrophoresis will therefore be affected by the pH
