Download GRADE 12A: Chemistry 6

GRADE 12A: Chemistry 6
UNIT 12AC.6
11 hours
Aromatic organic chemistry
About this unit
Previous learning
Resources
This unit is the sixth of eight units on chemistry
for Grade 12 advanced.
To meet the expectations of this unit, students should already know how to
distinguish between  and  bonds and recognise the relative unreactivity of
the arene ring.
The main resources needed for this unit are:
The unit is designed to guide your planning and
teaching of chemistry lessons. It provides a link
between the standards for science and your
lesson plans.
The teaching and learning activities should help
you to plan the content and pace of lessons.
Adapt the ideas to meet your students’ needs.
For consolidation activities, look at the scheme of
work for Grade 11A.
You can also supplement the activities with
appropriate tasks and exercises from your
school’s textbooks and other resources.
• 0.01 mol dm–3 potassium manganate(VII), 1 mol dm–3 sulfuric acid, methyl
benzene
• class set of student whiteboards
Expectations
By the end of the unit, students know the fundamental chemistry of
arenes and substituted arenes and describe the production of the more
important derivatives of benzene. They explain the stability of the benzene
ring in terms of electron delocalisation.
Students who progress further understand the mechanism of electrophilic
substitution reactions and are able to predict the effect of substitutions on
the aromatic ring.
Introduce the unit to students by summarising
what they will learn and how this builds on earlier
work. Review the unit at the end, drawing out the
main learning points, links to other work and real
world applications.
489 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
• molecular model kits
• methyl benzoate, concentrated sulfuric acid, concentrated nitric acid, ice
• methylphenol, phenol, phenylamine, sodium nitrate(III), sodium hydroxide,
napthalen-2-ol, ice, fume cupboard
• Internet access
Key vocabulary and technical terms
Students should understand, use and spell correctly:
• arenes, aromatic
• electrophilic substitution
• azo dyes, coupling, diazotisation
© Education Institute 2005
Standards for the unit
11 hours
4 hours
Nomenclature of
arenes
1 hour
Bonding in
arenes
4 hours
Reactions of
arenes
2 hours
Azo dyes
Unit 12AC.6
SUPPORTING STANDARDS
11A.24.1 Know, interpret and use the
CORE STANDARDS
Grade 12 standards
112A.23.1 Interpret and use the nomenclature and structural formulae of the following
nomenclature and molecular and
structural formulae of the following
classes of compound:
• alkanes and alkenes;
• halogenoalkanes;
• alcohols;
• aldhehydes and ketones;
• carboxylic acids, esters and acyl
chlorides;
• amines, nitriles, amides and amino
acids
11A.18.10 Describe covalent bonding in terms of
orbital overlap, giving  (sigma) and 
(pi) bonds; explain bond shape and
angles in ethane, ethene and
benzene in terms of  and  bonds.
11A.24.18 Describe the chemistry of arenes (such
as benzene and methylbenzene) and
show an understanding of the relative
unreactivity of the aromatic ring
compared with an isolated double
bond; know that the chemistry of side
chains is similar to that of aliphatic
compounds.
11A.24.20 Compare the preparation and
properties of bromobenzene with
bromoethane to show the effect of the
benzene ring.
11A.24.19 Know the chemistry of phenol, as
exemplified by its reactions with
bases and sodium, and know of its
common use as a mild disinfectant.
11A.24.20 Show an understanding of the broad
issues relating to social benefits and
environmental costs associated with
the organic chemical industry.
EXTENSION STANDARDS
classes of compound:
• arenes;
• halogenoarenes;
• phenols;
• aromatic aldehydes and ketones;
• aromatic carboxylic acids, esters and acyl chlorides;
• aromatic amines, nitriles, amides and amino acids.
12A.23.2
Describe the shapes of the ethane, ethene and benzene molecules in
terms of  and  carbon–carbon bonds.
12A.23.3 Describe the chemistry of arenes (such as benzene and methylbenzene),
as exemplified by substitution reactions with electrophiles, nitration and
oxidation of the side chain.
12A.23.4 Understand the mechanism of electrophilic substitution in arenes and the
effect of the delocalisation of electrons in arenes in such reactions.
12A.23.5 Know the chemistry of phenol, as exemplified by its reactions with bases
and sodium and by electrophilic substitution in the aromatic ring.
12A.23.6 Describe the formation of aromatic amines by the reduction of nitroarenes.
12A.23.7 Describe the production of azo-dyes from phenylamine and understand
their commercial importance.
490 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
© Education Institute 2005
Activities
Unit 12AC.6
Objectives
Possible teaching activities
4 hours
For each of the classifications of compound listed in the standard (i.e. arenes; halogenoarenes;
phenols; aromatic aldehydes and ketones; aromatic carboxylic acids, esters and acyl chlorides;
aromatic amines, nitriles, amides and amino acids) go through the following method to
consolidate student understanding of nomenclature:
Nomenclature of arenes
Interpret and use the
nomenclature and structural
formulae of the following
classes of compound:
• arenes;
• halogenoarenes;
• phenols;
School resources
Use this column to note
your own school’s
resources, e.g.
textbooks, worksheets.
• Write the name of the relevant class of aliphatic compound previously met in the course
(e.g. halogenoalkanes if you intend teaching halogenoarenes) and a structure of examples of
aliphatic analogues (if appropriate) on the board or OHP.
• Ask students to draw the structure of the named compound and the name of the structure
onto a small whiteboard and hold it up for you to see.
• aromatic carboxylic acids,
esters and acyl chlorides;
• The degree of revision of nomenclature for these compounds will depend on students’
responses. If they appear to be confident, just summarise the rules for nomenclature for this
class of compound. If they are not confident, reinforce the rules and give them more
examples of compound names and structures. Let them work in pairs and mark each other’s
work, explaining any incorrect answers to their partner.
• aromatic amines, nitriles,
amides and amino acids.
• Explain to the whole class the extra information needed to be able to name the aromatic
analogues.
• aromatic aldehydes and
ketones;
Notes
Repeat the above process for the aromatic analogues of the aliphatic compounds you have
covered.
1 hour
Bonding in arenes
Describe the shapes of the
ethane, ethene and benzene
molecules in terms of  and 
carbon–carbon bonds.
Revise, through a question and answer session, the shape and type of bonding in ethane and
ethene molecules.
Give students data relating to the enthalpy change of hydrogenation of cyclohexene and ask
them to draw an enthalpy level diagram for the hydrogenation of ‘cyclohexatriene’. Provide them
with data for the enthalpy change of hydrogenation of benzene and ask them to superimpose
this value onto their enthalpy level diagram. Provide them with data for C–C and CC as well as
the carbon–carbon bond length in benzene. Ask them to explain this.
Ask students to use the library or the Internet to research the work of Friedrich Kekulé and,
working in small groups, produce a poster of his work.
491 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
Two good websites are:
• www.chemguide.co.uk/basicorg/bonding/
benzene1.html
• classes.yale.edu/chem220a/studyaids/
history/chemists/kekule.html
ICT opportunity: Use of the Internet.
Enquiry skills 12A.2.1, 12A.2.4
© Education Institute 2005
Objectives
4 hours
Reactions of arenes
Describe the chemistry of
arenes (such as benzene and
methylbenzene), as
exemplified by substitution
reactions with electrophiles,
nitration and oxidation of the
side chain.
Understand the mechanism
of electrophilic substitution in
arenes and the effect of the
delocalisation of electrons in
arenes in such reactions.
Know the chemistry of
phenol, as exemplified by its
reactions with bases and
sodium and by electrophilic
substitution in the aromatic
ring.
Possible teaching activities
Notes
–3
–3
Tell students to mix equal volumes of 0.01 mol dm potassium manganate(VII) and 1 mol dm
sulfuric acid with a few drops of methyl benzene. Tell them to observe colour changes and so
determine whether the manganate(VII) oxidised the methylbenzene.
School resources
Safety: Methyl benzene is flammable.
Draw the class together and give them the equation for this and other similar reactions.
Explain to the whole class a general mechanism for the electrophilic substitution of benzene.
Ask students to use the library or the Internet to research the conditions needed for bromination,
chlorination, nitration, sulfonation, Friedel-Crafts alkylation and Friedel-Crafts acylation. Ask
them, working individually, to produce a chart identifying the reaction conditions, the mechanism
and the electrophile for each of these reactions.
ICT opportunity: Use of the Internet.
Give students the dipole moments for a number of monosubstituted benzene rings
(e.g. phenylamine, chlorobenzene) and ask them to classify these into two groups: those that
are electron-withdrawing from the aromatic ring and those that are electron-releasing. Ask them
to predict the effect of each group on reactivity in electrophilic substitution reactions.
+
Guide students through drawing canonical forms for the attack of NO at the ortho position of
+
phenol then ask them to draw similar diagrams to show the canonical forms when NO attacks
at the meta position of phenol. Through class discussion and question and answer, explain why
the former is preferential.
+
Repeat the process for the attack of NO2 on benzoic acid.
Ask students, in pairs, to carry out an electrophilic substitution reaction. A suitable one is the
nitration of chilled methyl benzoate in concentrated sulfuric acid, with a chilled nitrating mixture
of concentrated sulfuric acid and concentrated nitric acid, keeping the temperature below 10 °C.
Crystallise the product out over ice.
Safety: Methyl benzoate is harmful,
concentrated sulfuric acid and concentrated
nitric acid are corrosive.
Enquiry skill 12A.4.1
Revise the reactions of phenol with bases and sodium (done as a practical in Grade 11) with a
quiz.
2 hours
Azo dyes
Describe the formation of
aromatic amines by the
reduction of nitroarenes.
Describe the production of
azo-dyes from phenylamine
and understand their
commercial importance.
Ask students to use the library or the Internet to research the work of Otto Witt and describe his
contribution to the azo dye industry. Also ask them to research azo dye production worldwide
and present the data in a suitable format.
Describe to the whole class the stages of synthesising an azo dye (i.e. diazotisation and
coupling). Give students structures of a number of pairs of reactants and ask them (individually)
to draw out the structure of the resultant dye. Also carry out the process in reverse.
Let students produce a range of azo dyes using phenylamine and ethyl-4-aminobenzoate as
amines to prepare as diazonium salts. Tell them to prepare each diazonium salt by adding the
aryl amine to cooled dilute hydrochloric acid, cool below 5 °C. They should then add a cooled
solution of sodium nitrate(III), not allowing the temperature to rise above 5 °C.
To prepare the coupling agents, chill phenol, 3-methylphenol and napthalen-2-ol, each made up
in alkaline solution, to below 5 °C.
Tell students, working in pairs, to add each coupling agent slowly to each diazonium salt in turn
and observe the colours of the resultant dyes.
492 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
ICT opportunity: Use of the Internet.
Video footage of coupling reactions can be seen
on: www.uni-regensburg.de/Fakultaeten/
nat_Fak_IV/Organische_Chemie/Didaktik/
Keusch/D-azo-e.htm.
Safety: Methylphenol, phenol, phenylamine and
sodium nitrate(III) are toxic. Methylphenol,
phenol and sodium hydroxide solution are
corrosive. Sodium nitrate(III) is an oxidising
agent. Napthalen-2-ol is harmful. Gloves and
goggles must be worn at all times. The work
must be done in a fume cupboard.
Enquiry skills 12A.2.2, 12A.2.3, 12A.4.1
© Education Institute 2005
Assessment
Examples of assessment tasks and questions
Assessment
Set up activities that allow
students to demonstrate what
they have learned in this unit.
The activities can be provided
informally or formally during
and at the end of the unit, or
for homework. They can be
selected from the teaching
activities or can be new
experiences. Choose tasks
and questions from the
examples to incorporate in
the activities.
Unit 12AC.6
Notes
School resources
If bromine water is added to a solution of phenylamine, a white precipitate of 2,4,6tribromophenylamine forms.
Explain why the reaction occurs so readily. Suggest why the benzenediazonium ion attacks the
para rather than the ortho position on phenol. Draw the structure of the azo compound
produced in this reaction.
Give a generic mechanism for the electrophilic substitution of benzene with an electrophile E+.
Why is this reaction classified as an electrophilic substitution?
Research and present a paper on the role of azo dyes in the organic chemical industry. Focus
on the reasons for and nature of the development of the industry. Reference clearly any
sources of information.
493 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
© Education Institute 2005
494 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.6 | Chemistry 6
© Education Institute 2005