Chapter 10 Introduction To A-level Organic Chemistry

Question 1

Arenes

Answer 1

• Arenes are aromatic compounds that contain a benzene ring

Question 2

Chemical properties of Arenes

Answer 2

• Due to the delocalised electron ring (π system of electrons), these compounds are electron-rich and therefore can undergo electrophilic attack under the right conditions
• However, because the delocalised electron ring system makes benzene so stable, it is resistant to addition reactions
• This is very different to alkenes, which are very reactive and readily undergo addition reactions

Question 3

Physical properties of Arenes

Answer 3

• Benzene has van der Waals dispersion forces of attraction between the molecules and has a boiling point of 80 C
• The presence of the non-polar hydrocarbon part in the arene functional group means that these compounds are often insoluble in water
• Benzene would have to break many hydrogen bonds between the water molecules to be soluble in water, which does not happen as it is not energetically favourable

Question 4

Halogenoarenes

Answer 4

• These are aromatic compounds that contain a halogen bonded to a benzene ring
• They are also known as aryl halides

Question 5

Chemical properties of Halogenoarens

Answer 5

• These compounds are also prone to electrophilic attack because of the π system of delocalised electrons
• The halogens can also take part in substitution reactions

Question 6

Physical properties of Halogenoarens

Answer 6

• Chlorobenzene, bromobenzene and iodobenzene are all liquid at room temperature with an oily texture
• As you might expect, the boiling points increase as the size of the halogen attached increases, because the number of electrons within the molecule increases
• Like other arenes, halogenoarenes are insoluble in water because of the non-polar hydrocarbon part of the ring
• These molecules are large relative to the size of water molecules, and as with the arenes it is not energetically favourable for the halogenoarene molecules to break the hydrogen bonds between the water molecules, so it does not happen

Question 7

Acyl chlorides

Answer 7

• Acyl chlorides are (carboxylic) acid derivatives containing:
  
  • A chlorine atom attached to a C=O group (replacing what would have been the -OH group of a carboxylic acid)
  • An acyl (hydrocarbon) group attached to a C=O group
• Acyl chlorides are also known as ‘acid’ chlorides

Question 8

Chemical properties of Acyl chlorides

Answer 8

• They are fuming liquids and are colourless, with a strong smell
• Acyl chlorides are extremely reactive and readily take part in substitution reactions in which the chlorine atom is substituted by other species
• This reactivity is why they are fuming liquids and why they have such a strong smell – they react with any water vapour in the air

Question 9

Physical properties of Acyl chlorides

Answer 9

• Acyl chlorides react violently with water, so we cannot say whether or not they would be soluble in water

Question 10

Amines

Answer 10

• Amines are compounds with the -NH₂ (primary amine), -NH (secondary amine) or -N (tertiary amine) group

Question 11

Classification of amines

Answer 11

• In primary amines, the N of the amine group is bonded to one R group (and two hydrogen atoms)
• In secondary amines, the N of the amine group is bonded to two R groups (and one hydrogen atom)
• In tertiary amines, the N of the amine group is bonded to three R groups

Question 12

Chemical properties of Amines

Answer 12

• Due to the lone pair of electrons on the nitrogen, amines are basic compounds

Question 13

Physical properties of Amines

Answer 13

• The lone pair on the N of the amine group means that they can form hydrogen bonds
• They are often soluble in water because they form hydrogen bonds with water molecules
• The smaller amines are very soluble in water, but their solubility decreases as the non-polar hydrocarbon chain gets longer
• They often have a fishy smell, especially as the size of the amines increases

Question 14

Amides

Answer 14

• Amides are compounds containing:
  
  • An amine (-NH₂) group
  • A carbonyl group (C=O)
  • The amide group is -CONH₂

Question 15

Chemical properties of Amides

Answer 15

• Amides are less basic than amines, as the lone pair of electrons on the nitrogen is delocalised

Question 16

Physical properties of Amides

Answer 16

• Amides are often soluble in water as they can form hydrogen bonds with water molecules
• The smaller amides are very soluble in water, but their solubility decreases as the non-polar hydrocarbon chain gets longer

Question 17

Amino Acids

Answer 17

• Amino acids are the building blocks of proteins and consists of:
  
  • An amine (-NH₂) group
  • A carboxyl (-COOH) group

Question 18

Chemical properties of Amino Acids

Answer 18

• Amino acids react with bases to form salts
• They also react with alcohols to form esters
• The reaction of amino acids with amines gives amides

Question 19

Physical properties of Amino Acids

Answer 19

• Most of the amino acids are soluble in water but insoluble in organic solvents
• Amino acids have chiral centres and exhibit optical isomers (except for glycine)

Question 20

Formulae of organic compounds table (Arene, Halogenoarenes, Phenol and Alcyl chloride)

Answer 20

Question 21

Formulae of organic compounds table (amine, amide ad amino acids)

Answer 21

Question 22

The following method can be applied when naming organic compounds

Answer 22

• Identify the longest carbon chain containing the functional group
• Identify the functional group on the chain to determine the suffix or prefix on the compound
• Count along the carbon chain such that the functional group has the lowest number
• Add any side chains or lower priority functional groups as prefixes to the beginning of the name in alphabetic order
• Use the prefixes di-, tri- and tetra- if there are two or more identical functional groups or side chains

Question 23

Nomenclature of simple aliphatic organic molecules with functional groups table

Answer 23

Question 24

Nomenclature of Aromatic Compounds

Answer 24

• The method used to name aromatic compounds is similar to that of aliphatic compounds
• Students are required to use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents

Question 25

Nomenclature of simple aromatic organic molecules with functional groups table

Answer 25

Question 26

Electrophilic substitution

Answer 26

• attack by the electron-deficient species
• Electrophiles are species that are electron deficient and can act as an electron pair acceptor
  
  • Electrophiles are ‘electron loving’ species
• Substitution reactions are reactions that involve the replacement of one atom or group of atoms by another
• Electrophilic substitution reactions are therefore reactions in which an atom or group of atoms are replaced by an electrophile after initial attack by the electron-deficient species

Question 27

An example of an electrophilic substitution reaction is the reaction of benzene with bromine in the presence of anhydrous aluminium bromide catalysts

Answer 27

• The bromine acts as an electrophile and attacks the electron-rich benzene ring
• A hydrogen atom is substituted by a bromine atom to form bromobenzene and hydrogen bromide
• Benzene undergoes substitution reactions rather than addition reactions because of the stability of the benzene ring

Question 28

Addition- elimination Reaction

Answer 28

• Other common organic reactions include addition and elimination reactions
• In an addition reaction, two or more molecules combine to give a single product only
• Elimination reactions are the reverse of addition reactions; a small molecule (such as H₂O or HCl) is removed or ‘eliminated’ from an organic molecule

Question 29

Benzene & Aromatic Molecules are

Answer 29

• Aromatic molecules consist of one or more rings with conjugated π systems
• Conjugated π systems arise from alternating double and single bonds in which the electrons are delocalised
• Aromatic compounds are called ‘aromatic’ as they often have pleasant odours

Question 30

Examples of aromatic compounds including benzene table

Answer 30

Question 31

Shape of benzene & aromatic compounds

Answer 31

• Benzene and other aromatic compounds contain sp² hybridised carbons as two of their p orbitals have mixed with an s orbital
• This means that each carbon atom in benzene and other aromatic compounds has one p orbital
• Each carbon atom in the ring forms three σ bonds using the sp² orbitals
• The remaining p orbital overlaps laterally with p orbitals of neighbouring carbon atoms to form a π bond
• This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring
• Benzene and other aromatic compounds are regular and planar compounds with bond angles of 120^o
• The delocalisation of electrons means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character
• The bonds all being the same length is evidence for the delocalised ring structure of benzene

Question 32

The carbon atoms in aromatic compounds are sp² hybridized as two of their p orbitals mix with an s orbital

Answer 32

Question 33

Like other aromatic compounds, benzene has a planar structure due to the sp²hybridisation of carbon atoms and the conjugated π system in the ring

Answer 33

Question 34

Stereoisomers are molecules that have the

Answer 34

• same structural formula but have the atoms arranged differently in space
• There are two types of stereoisomerism
  
  • Geometrical (cis/tras)
  • Optical

Question 35

Optical isomerism

Answer 35

• A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
• Compounds with a chiral centre (chiral molecules) exist as two optical isomers which are also known as enantiomers
• The enantiomers are non-superimposable mirror images of each other

Question 36

Optical isomerism Physical and chemical properties

Answer 36

• Their physical and chemical properties are identical but they differ in their ability to rotate plane polarised light
  
  • Hence, these isomers are called ‘optical’ isomers
  • One of the optical isomers will rotate the plane of polarised in the clockwise direction
  • Whereas the other isomer will rotate it in the anti-clockwise direction

Question 37

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

Answer 37

Question 38

Biological activity of enantiomers

Answer 38

• Enantiomers also differ from each other in terms of their biological activity
• Enzymes are chiral proteins that speed up chemical reactions by binding substrates
• They are very target-specific as they have a specific binding site (also called active site) and will only bind molecules that have the exact same shape
• Therefore, if one enantiomer binds to a chiral enzyme, the mirror image of this enantiomer will not bind nearly as well if at all
• It’s like putting a right-hand glove on the left hand!

Question 39

Enantiomers differ from each other in their biological activity

Answer 39

Question 40

Optically Active Compounds & Racemic Mixtures

Answer 40

• Enantiomers are optical isomers that are mirror images of each other and are non-superimposable
• They have similar chemical properties but differ from each other in their ability to rotate plane polarised light and in their biological activity

Question 41

Optical activity

Answer 41

• Let’s suppose that in a solution, there is 20% of the enantiomer which rotates the plane polarised light clockwise and 80% of the enantiomer which rotates the plane of polarised light anticlockwise
• There is an uneven mixture of each enantiomer, so the reaction mixture is said to be optically active
  
  • The net effect is that the plane of polarised light will be rotated anticlockwise
• Similarly, if the percentages of the enantiomers are reversed, the reaction mixture is still optically active but now the plane of polarised light will be rotated clockwise
  
  • In this case, there is 20% of the enantiomer, which rotates the plane anticlockwise
  • And 80% of the enantiomer, which rotates the plane clockwise
• A racemic mixture is a mixture in which there are equal amounts of enantiomers present in the solution
• A racemic mixture is optically inactive as the enantiomers will cancel out each others effect and the plane of polarised light will not change

Question 42

When one of the enantiomers is in excess, the mixture is optically active; when there are equal amounts of each enantiomer the mixture is optically inactive

Answer 42

Question 43

Effect of Optical Isomers on Plane Polarised Light

Answer 43

• Molecules with a chiral centre exist as optical isomers
• These isomers are also called enantiomers and are non-superimposable mirror images of each other
• The major difference between the two enantiomers is that one of the enantiomers rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
  
  • The enantiomer that rotates the plane clockwise is called the R enantiomer
  • The enantiomer that rotates the plane anticlockwise is called the S enantiomer
• These enantiomers are therefore said to be optically active
• Therefore, the rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance

Question 44

Each enantiomer rotates the plane of polarised light in a different direction

Answer 44

Question 45

Chirality & Drug Production

Answer 45

• Most of the drugs that are used to treat diseases contain one or more chiral centres
• These drugs can therefore exist as enantiomers which differ from each other in their ability to rotate plane polarised light
• Another crucial difference between the enantiomers is in their potential biological activity and therefore their effectiveness as medicines
• Drug compounds should be prepared in such a way that only one of the optical isomer is produced, in order to increase the drugs’ effectiveness
  
  • Some drug enantiomers can have very harmful side effects

Question 46

Potential biological activity of enantiomers

Answer 46

• If conventional organic reactions are used to make the desired drug, a racemic mixture will be obtained
  
  • In a racemic mixture, there are equal amounts of the two enantiomers
• The physical and chemical properties of the enantiomers are the same, however, they may have opposite biological activities

Question 47

The drug naproxen

Answer 47

• is used to treat pain in patients that suffer from arthritis
  
  • One of the enantiomers of naproxen eases the pain, whereas another enantiomer causes liver damage
• One enantiomer of a drug used to treat tuberculosis is effective whereas another enantiomer of this drug can cause blindness
• Thalidomide is another example of a drug that used to be used to treat morning sickness, where one of the enantiomers caused very harmful side effects for the unborn baby

Question 48

Separating racemic mixtures

Answer 48

• Due to the different biological activities of enantiomers, it is very important to separate a racemic mixture into pure single enantiomers which are put in the drug product
• This results in reduced side-effects in patients
  
  • As a result, it protects pharmaceutical companies from legal actions if the side effects are too serious
• It also decreases the patient’s dosage by half as the pure enantiomer is more potent and therefore reduces production costs
  
  • A more potent drug has a better therapeutic activity

Question 49

Chiral catalysts

Answer 49

• In order to produce single, pure optical isomers, chiral catalysts can be used
• The benefits of using chiral catalysts are that only small amounts of them are needed and they can be reused
  
  • For example, an organometallic ruthenium catalyst is used in the production of naproxen which is used in the treatment of arthritis

Question 50

Enzymes are

Answer 50

• excellent biological chiral catalysts that promote stereoselectivity and produce single-enantiomer products only
  
  • Stereoselectivity refers to the preference of a reaction to form one enantiomer over the other
• Due to the specific binding site of enzymes, only one enantiomer is formed in the reaction
• The enzymes are fixed in place on inert supports so that the reactants can pass over them without having to later separate the product from the enzymes

Question 51

The disadvantage of using enzymes is that it can be

Answer 51

• expensive to isolate them from living organism
  
  • Therefore, more research has recently been carried out into designing synthetic enzymes
• Although using enzymes to produce pure enantiomers in drug synthesis takes longer than conventional synthetic routes, there are many advantages to it in the long run
  
  • For example, using enzymes to synthesise drugs is a greener process as fewer steps are involved compared to conventional synthetic routes