Aromatics Directing Effects 1+2

Question 1

What type of compound is benzene?

Answer 1

An planar, aromatic compound

Question 2

What makes benzene stable?

Answer 2

The delocalised electrons within the benzene ring.

Question 3

The resonance forms of benzene are of equal energy true or false?

Answer 3

True

Question 4

Define Electrophile?

Answer 4

A species which attracts electrons towards itself

Question 5

Define Nucleophile?

Answer 5

A lone electron pair donor.

Question 6

Define Oxidation?

Answer 6

The removal of electrons from a molecule (OIL)

Question 7

Define Reduction?

Answer 7

The addition of electrons to a molecule (RIG)

Question 8

Define Bases

Answer 8

Electron Rich species (Can be negatively charged)

Question 9

Define Acids

Answer 9

Electron Deficient species (Can be positively charged)

Question 10

What is a negative charged species?

Answer 10

contains electrons (nucleophiles/bases)

Question 11

What is a positive charged species?

Answer 11

devoid of electrons (electrophiles/acids)

Question 12

What are arrows used for?

Answer 12

A full bladed arrow represents the movement of 2 electrons

Question 13

What are dots used for?

Answer 13

one dot represents 1 valence electron (e.g. radicals)

2 dots represents 2 valence electrons (e.g. nucleophiles)

Question 14

Why do the resonance forms of benzene have equal energies?

Answer 14

because of the uninterrupted cyclic cloud of p electrons above and below the plane of benzene (a planar molecule) and there is an odd number of pairs of p electrons.

Question 15

What is Huckel’s Rule?

Answer 15

For a planar cyclic compound to be aromatic, it must contain and uninterrupted p cloud with (4n+2) p electrons. Where n is any whole number.

Question 16

What is the aromaticity of benzene?

Answer 16

pi system

Question 17

What is aromaticity?

Answer 17

a term used to describe a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms.

Question 18

What structure is preferred for drawing mechanisms using the benzene ring?

Answer 18

Kekule’s structure.

Question 19

Why does benzene not react with electrophiles like alkenes do?

Answer 19

Because it is less reactive than alkenes due to it’s aromatic stabilisation.

Question 20

What mechanism is used to brominate benzene?

Answer 20

Electrophilic Substitution

Question 21

What is required to brominate benzene?

Answer 21

An acid catalyst e.g. AlCl3

Question 22

How does the presence of an acid catalyst allow for benzene to do an electrophilic substitution reaction?

Answer 22

The acid catalyst makes Br2 more electrophilic (look at slide 12 for mechanism)

Question 23

What is a wheland intermediate?

Answer 23

The carbocation formed after the addition of an electrophile to the benzene ring.

Question 24

How is the wheland intermediate stabilised?

Answer 24

By the resonance forms

Question 25

Wha drives re-aromatisation?

Answer 25

Aromatic stabilisation

Question 26

What is used to analyse the wheland intermediate?

Answer 26

NMR

Question 27

What two substances are required for the nutrition of benzene? And draw the mechanism

Answer 27

Nitric acid and sulfuric acid (the catalyst)

Question 28

What three substances are needed for the sulfonylation of benzene to form crystalline sodium benzene sulfonate? And draw the mechanism!

Answer 28

SO3 Excess H2SO4 NaCl

Question 29

What is required for alkylation and acylation reactions?

Answer 29

An acid catalyst (mainly Lewis acids)

Question 30

What is the difference in reactivity of the starting material and product after undergoing the Friedel-Crafts alkylation reaction?

Answer 30

The product is more reactive than the starting material

Question 31

What are the three types of precursors that can be employed in the Friedel-Crafts alkylation reaction?

Answer 31

- alkyl halide - aliphatic alcohols - alkenes

Question 32

What is used as the catalyst for reacting benzene with methyl or ethyl halide?

Answer 32

FeCl3

Question 33

What is used as the catalyst to react alkyl halides, from propyl onwards, with benzene

Answer 33

AlCl3 (a Lewis acid)

Question 34

Why is there a mixture of products when reacting benzene with alkyl halides with 3 or more carbons?

Answer 34

Because the alkyl halide and AlCl3 react to form varying carbocations (primary and secondary carbocations).

Question 35

Why is there more of the products formed by the secondary carbocation than the primary carbocation when reacting alkyl halides with benzene?

Answer 35

Because the secondary carbocation is more stable than the primary carbocation.

Question 36

What happens to the stability and energy as you go from methyl to primary to secondary and then to tertiary alkyl carbocations?

Answer 36

Stability increases but energy decreases

Question 37

How do we get a monoalkyl product?

Answer 37

Use excess benzene

Question 38

What is required in the alkylation process with alcohols?

Answer 38

alcohol, strong acid (e.g. H2SO4) and this is used to produce the carbocation to react with benzene.

Question 39

How does alkylation with olefins work?

Answer 39

Alkenes react with benzene in the presence of a suitable catalyst (e.g. HF, BF3) to produce alkylbenzenes.

Question 40

How does friedel-crafts acylation work?

Answer 40

Acid chlorides (acyl halides) and acid anhydrides can both react with benzene in the presence of molar amounts of Lewis acids to produce ketones.

Question 41

What product is formed from Friedel-Crafts acylation reaction?

Answer 41

Aryl ketone which is more basic than the starting product (acyl halide)

Question 42

Why is water and acid added at the end of the Friedel-Crafts acylation reaction?

Answer 42

To hydrolyse any Lewis acid remaining hence liberating the ketone.

Question 43

What is the reactive intermediate, formed from the reaction of acyl chloride with AlCl3, of the Friedel-Craft acylation mechanism known as?

Answer 43

Acylium ion

Question 44

Which mechanism happens faster acylation or alkylation?

Answer 44

Acylation

Question 45

What is electronegativity?

Answer 45

A measure of an elements ability to pull a pair of bonding electrons towards itself.

Question 46

What scale is used to measure electronegativity?

Answer 46

Pauling's scale

Question 47

What causes a bond to be polar?

Answer 47

Where there's a difference in the electronegativitiies of the elements bonded together.

Question 48

What is a dipole moment?

Answer 48

The permanent electric moment of a molecule

Question 49

What is the negative inductive effect?

Answer 49

When a species pulls electrons towards itself

Question 50

What is the positive inductive effect?

Answer 50

When a species pushes electrons away from itself.

Question 51

What happens to the inductive effect as the distance increases?

Answer 51

The effect falls off (weakens)

Question 52

Why is the meta derivative of benzoic acid stronger than the para isomer (derivative)

Answer 52

Because of resonance. It is a result of conjugation of a lone pair of electrons with the pi-system of the ring.

Question 53

What do the resonance structures show us?

Answer 53

The regions of increased/decreased electron density.

Question 54

What can stop resonance occurring?

Answer 54

Bulky substituents

Question 55

What is hyperconjugation?

Answer 55

A mechanism of both electron release and withdrawal. (happens via C-H and C-C bonds).

Question 56

Alkyl substituents donate electron density's via two mechanisms, what are they?

Answer 56

positive inductive effect and hyperconjugation

Question 57

What is the effect of the positive inductive effect and hyperconjugation on the alkyl substituted benzene?

Answer 57

They make it more susceptible to electrophilic attack than benzene, making it more reactive than the starting material.

Question 58

Three electronic effects that operate when looking at substituted benzene derivatives?

Answer 58

Inductive effect Resonance Effect Hyperconjugative Effect

Question 59

Three compounds needed for the bromination of a phenol?

Answer 59

Br2, Ethanol, H2o

Question 60

What directing effect does -OH have on the benzene ring?

Answer 60

Ortho-Para directing (look at slide 46 of lecture for more info)

Question 61

When looking at HNMR to analyse directing effect, what does the proton shift indicate?

Answer 61

If the HNMR value decreases, then it shows an increase in electron density at those positions.

Question 62

When a benzene ring has an O substituent, where does the reacting elements attach to?

Answer 62

At the ortho and para sites. (look at slide 48 for more info)

Question 63

Effect of nitrogen substituent on the reactivity of benzene in comparison to the oxygen substituent?

Answer 63

Makes benzene even more reactive than the oxygen substituent did. (shown by the lower HNMR values at the para and ortho positions with nitrogen than with oxygen-slide 51)

Question 64

How can we reduce the effect of the nitrogen substituent on the benzene ring?

Answer 64

By reducing the electron donating power of the nitrogen substituent e.g. pull electrons away from the ring, making it a poorer nucleophile, hence less reactive.

Question 65

Effect of a methyl substituent on the reactivity of benzene?

Answer 65

It increases the reactivity of benzene at the ortho and para positions but not as much as the oxygen and nitrogen substituents do.

Question 66

How does methyl increase the reactivity of benzene?

Answer 66

It is a weak conjugative electron donor hence increases electron density in the benzene ring. (The C-H sigma bond of the methyl overlaps with the pi-system of benzene donating its electron density to benzene's pi-system. The electrons flow between the orbitals.)

Question 67

What intermediate is formed by the substitution of an electrophile on a benzene ring?

Answer 67

A wheland intermediate

Question 68

What is the most favourable resonance form of the benzene with a methyl substituent?

Answer 68

The one with the positive charge at the carbon bonded to the methyl group that is pushing electrons towards it.