Chemistry A Level CIE > Chapter 15 Nitrogen compounds > Flashcards

Chapter 15 Nitrogen compounds Flashcards

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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
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2
Q

Primary and secondary amines

A
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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
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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
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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
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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
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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
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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
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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
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10
Q

Acyl chlorides undergo condensation reactions with ammonia and amines to form amides

A
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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
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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-

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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
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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
  • For example, ethylamine (which has an electron-donating ethyl group) is more basic than phenylamine (which has an electron-withdrawing benzene ring)
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15
Q

Phenylamine is an

A

organic compound consisting of a benzene ring and an amine (NH2) functional group

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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
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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
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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
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19
Q

Phenylamines, therefore, react under what conditions

A

milder conditions with aqueous bromine at room temperature to form 2,4,6-tribromophenylamine

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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
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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
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22
Q

Trends in the basicity of ammonia, ethylamine, and phenylamine

A
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23
Q

Azo (or diazonium) compounds are

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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

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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
25
**Formation of azo compounds table**
26
***Reaction mechanism of the formation of azo compounds***
27
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
31
***Hydrolysis of substituted and non-substituted amides (diagram)***
32
***Amides are hydrolysed to carboxylic acids and ammonia or primary amine when refluxed with acid or alkali***
33
**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
35
**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)
36
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
39
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***
49
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
53
**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**
55
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**
56
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
57
**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
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