Download GRADE 12A: Chemistry 5

GRADE 12A: Chemistry 5
UNIT 12AC.5
11 hours
Organic reaction mechanisms
About this unit
Previous learning
Resources
This unit is the fifth of eight units on chemistry for
Grade 12 advanced.
To meet the expectations of this unit, students should already know the
significance of s, p, d and f orbitals and hybrids in bonding and molecular
shape, and distinguish between  and  bonds. They should have an
understanding of the general chemistry of alkanes, alkenes,
halogenoalkanes, aldhehydes, ketones, acyl chlorides, amines and amides.
The main resources needed for this unit are:
Expectations
Key vocabulary and technical terms
By the end of the unit, students understand the mechanisms of
electrophilic addition and substitution, nucleophilic substitution and
elimination reactions.
Students should understand, use and spell correctly:
Students who progress further show an understanding of the Lewis
theory of acids and bases and relate it to nucleophilic reactions in organic
chemistry.
• reaction mechanism
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 the needs of your class.
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.
Introduce the unit to students by summarising
what they will learn and how this will build on
earlier work. Review the unit at the end, drawing
out the main learning points, links to other work
and real world applications.
483 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
• long thin balloons, Peel models, computer animations of molecular
structures
• L-carvone, D-carvone, tin foil, caraway seeds, spearmint (or spearmint
chewing gum)
• aliphatic, orbitals, electron-pair repulsion
• chiral centre, optical isomerism, enantiomer
• Lewis acid
• hydrolysis, acylation, acylating agent
• phenylamine, butylamine, ammonia solution, dilute hydrochloric acid
© Education Institute 2005
Standards for the unit
11 hours
3 hours
Bonding in
organic
chemistry
SUPPORTING STANDARDS
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.24.5 Illustrate structural and geometric
4 hours
Electrophilic
addition and
nucleophilic
substitution
2 hours
Acylation
isomerism in alkanes and alkenes.
11A.24.9 Describe the chemistry of
halogenoalkanes as exemplified by
substitution reactions and the
elimination of hydrogen halide to
form an alkene.
CORE STANDARDS
Grade 12 standards
EXTENSION STANDARDS
12A.22.1 Describe the shape of aliphatic organic compounds in terms of orbital
overlap and electron-pair repulsion.
12A.22.2 Describe the restricted rotation and the resulting stereochemistry of
multiple bonds in terms of  (sigma) and  (pi) bonds.
12A.22.3 Describe structural isomerism, cis–trans isomerism in alkenes, and how
chiral centres give rise to optical isomerism.
12A.22.4 Describe the mechanisms of electrophilic addition in alkenes and
nucleophilic substitution in compounds such as halogenoalkanes.
11A.24.4 Describe the chemistry of alkenes as
the chemistry of the double bond,
exemplified by addition and
polymerisation.
2 hours
Amines and
amides
Unit 12AC.5
12A.22.5 Show an understanding of the Lewis theory of acids and bases and relate it
to nucleophilic reactions in organic chemistry.
11A.24.13 Describe the chemistry of the
carbonyl group as exemplified by
aldehydes and ketones.
12A.22.6 Describe the chemistry of the carbonyl group in terms of nucleophilic
substitution and show how its reactivity depends on the electronegativity of
the group or groups attached to it.
12A.22.7 Know that acyl chlorides (exemplified by ethanoyl chloride) are readily
hydrolysed and that they are useful agents for acylating alcohols, phenols
and amines.
12A.22.8 Distinguish between amines and amides, recognise that they are both
substituted ammonia compounds and therefore describe their basic
properties.
484 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
© Education Institute 2005
Activities
Unit 12AC.5
Objectives
Possible teaching activities
3 hours
Guide students through the first example given below.
Bonding in organic
chemistry
Arrange students into pairs and get them to inflate long thin balloons and twist them in the
middle. Each ‘lobe’ of the balloon represents a group of electrons in a molecule around a
central atom. A group of electrons might be a lone pair of electrons, a single, double or triple
covalent bond. Ask students to model various atoms in molecules (e.g. the carbon in
methane could be represented by twisting two bilobar balloons together). The lobes
automatically arrange themselves into a pyramidal conformation with a bond angle
approximating to 109°.
Describe the shape of
aliphatic organic compounds
in terms of orbital overlap
and electron-pair repulsion.
Describe the restricted
rotation and the resulting
stereochemistry of multiple
bonds in terms of  (sigma)
and  (pi) bonds.
Describe structural
isomerism, cis–trans
isomerism in alkenes, and
how chiral centres give rise
to optical isomerism
Introduce students to the convention for representing 3D structures in 2D and ask them to
draw out the structure of methane. Reinforce their appreciation of the structure using either
3D Peel models or applets downloaded from the Internet.
Notes
School resources
Use this column to note
your own school’s
resources, e.g.
textbooks, worksheets.
ICT opportunity: Use of the Internet and Java
applets.
Give students the names of a range of organic compounds (e.g. ethanal, trimethylamine) and
ask them to work in pairs, using the technique above, to work out the 3D structure of the
compounds. Differentiate the complexity of the molecules to suit students’ ability. For each
example, ask students to relate the balloon models to the type of bonds (e.g. of  (sigma)
and  (pi) bonds).
Revise the formation of  (sigma) and  (pi) bonds. Ask students, working individually, to make
models of ethane and ethene using a model building kit. Revise the formation of  (sigma) and
 (pi) bonds and ask them to draw diagrams showing the types of bonds involved in the two
different molecules. Give students the bond enthalpy values for the carbon–carbon bonds in
both molecules and ask them to interpret the difference in the bond enthalpies and discuss their
ideas in pairs. Ask each student to build a model of but-2-ene. Ask them to compare their model
with those of the rest of the class and form two groups. Each group will consist of students
producing the same form of but-2-ene. Draw the two different structures on the board or OHP
and discuss with the whole class the difference between the two forms, why they do not readily
interconvert at room temperature and the possible impact on reactivity (steric hindrance).
Then ask students to use their textbook, the library or the Internet to research the differences in
chemical reactivity and physical properties for given cis–trans isomers.
ICT opportunity: Use of the Internet.
Ask students to build a ‘ball and stick’ model using a central carbon with four different coloured
groups attached. (All students must use the same four colours.) Ask them to compare their
model first with the person to the left of them then with the person to the right of them. Ask them
to identify whether the compounds are the same as or different from each other. Ask them to
identify the link between the two non-superimposable models (i.e. they are mirror images of
each other). Ask students to draw a 3D representation of an amino acid. Introduce the terms
enantiomer and chiral centre.
485 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
© Education Institute 2005
Objectives
Possible teaching activities
Notes
School resources
Prepare small tubes wrapped in tin foil and plugged with cotton wool. Place a few drops of
L-carvone in one-third of the tubes and label as ‘compound X’. Place a few drops of D-carvone
in another one-third of the tubes and label as ‘compound Y’. Divide the remaining one-third of
the tubes into two and place freshly ground caraway seeds (contains D-carvone) in half of them
and fresh spearmint (contains L-carvone) in the other half. Label these ‘spearmint’ and
‘caraway’ as appropriate. Let all students smell compound X then compound Y to decide
whether they smell the same or different. Then let them smell the caraway and spearmint to see
whether they smell the same as X or Y. (Caraway seeds contain D-carvone and spearmint
contains L-carvone). Gather data from the whole class. Show students the structures of D- and
L-carvone and ask them to work in pairs to identify the chiral centre. Ask them to try to explain
why not everyone could differentiate between L- and D-carvone.
Ask students to use the Internet to research the use of thalidomide and relate the resultant
problems to optical isomerism. Ask them to prepare a report detailing the case, making
recommendations on how this scenario could be prevented in the future.
4 hours
Electrophilic addition and
nucleophilic substitution
Describe the mechanisms of
electrophilic addition in
alkenes and nucleophilic
substitution in compounds
such as halogenoalkanes.
Show an understanding of
the Lewis theory of acids and
bases and relate it to
nucleophilic reactions in
organic chemistry.
Describe the chemistry of the
carbonyl group in terms of
nucleophilic substitution and
show how its reactivity
depends on the
electronegativity of the group
or groups attached to it.
ICT opportunity: Use of the Internet.
Enquiry skills 12A.2.2, 12A.2.3
In a teacher-led session, teach the mechanism for electrophilic addition in alkenes. Provide a
generic model of the reaction. Ask students to work through a number of reactions they
researched in Grade 11 and draw out the full mechanism for these reactions. Take great care to
reinforce the conventions of single- and double-headed arrows to represent the movement of
one and two electrons, respectively. Let students mark each other’s work.
In a teacher-led session, teach the mechanisms for nuleophilic substitution reactions. Provide a
generic model of each of the mechanisms. Ask students to work through a number of reactions
they researched in Grade 11 and draw out the full mechanism for these reactions. Let students
mark each other’s work.
Revise the definition of a nucleophile in a whole class question and answer session.
Give students the definition of a Lewis acid and Lewis base in terms of electron transfer. Give
them a list of reactions (e.g. the production of butan-1-ol by the reaction of 1-bromobutane with
hydroxide ions) and ask them to identify the Lewis acids and bases in each reaction. Ask each
student to identify another reaction and the Lewis acids and bases involved in the reaction.
Supply data on the reactivity of different carbonyl compounds and electronegativity data for
different elements. Tell students to rank these compounds with respect to order of reactivity.
Ask them to account for the differing reactivities by noting the extent to which the atom (or
group) attached to the carbonyl group tends to oppose or enhance the movement of electrons
away from the carbonyl carbon atom.
486 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
Enquiry skills 12A.3.2
© Education Institute 2005
Objectives
Possible teaching activities
2 hours
Write an equation for the hydrolysis of ethanoyl chloride on the board or OHP. Ask students to
write a similar equation for the hydrolysis of an acyl chloride of their choice.
Acylation
Notes
Know that acyl chlorides
(exemplified by ethanoyl
chloride) are readily
hydrolysed and that they are
useful agents for acylating
alcohols, phenols and
amines.
Ask students to use their textbook, the library or the Internet to research the uses of ethanoyl
chloride in acylating alcohols, phenols and amides. Let them work in small groups to produce
information posters for these reactions.
ICT opportunity: Use of the Internet.
Tell students to carry out the following procedure and record their results. Add ethanoyl chloride
dropwise to 10 drops of butylamine in a dry test-tube and observe any reaction. Add a small
volume (1 cm depth) of water. Add sodium hydroxide solution, warm and test any gases
evolved with damp red litmus paper. Use class discussion to interpret the results
Safety: All work must be carried out in a fume
cupboard. Ethanoyl chloride is corrosive and an
irritant.
Enquiry skills 12A.3.1, 12A.3.4, 12A.4.1
2 hours
Ask students to draw the structure of a primary amine, secondary amine, tertiary amine, primary
amide and secondary amide. Draw up the structure of ammonia on the board or OHP and use
questioning to draw out the relationship between these compounds.
Amines and amides
Distinguish between amines
and amides, recognise that
they are both substituted
ammonia compounds and
therefore describe their basic
properties.
Ask students, working individually, to test the reaction of phenylamine, butylamine and
ammonia solution with water and universal indicator.
Tell them to shake two drops of phenylamine with 2 cm3 of dilute hydrochloric acid, then repeat
with butylamine and record their results. Let them interpret their observations in small group
discussions and ask each group to report back to the whole class for a whole group discussion.
487 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
School resources
Safety: Phenylamine is toxic. Use in small
quantities in the fume cupboard. Butylamine is
flammable and an irritant.
Enquiry skills 12A.3.1, 12A.3.4, 12A.4.1
© Education Institute 2005
Assessment
Examples of assessment tasks and questions
Assessment
a. Write a reaction mechanism for the reaction of ammonia with bromobutane.
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.
b. Describe why the reaction occurs this way.
Unit 12AC.5
Notes
School resources
c. What reaction conditions are needed?
d. Why is this classified as a substitution reaction?
a. Draw a dot and cross diagram for the molecule CH3─N═N─CH3.
b. Draw out the structure of the compound in part a showing all the bond angles.
c. This compound can exist as cis and trans isomers. Draw diagrams to show the two
structures.
a. Draw the structural formula of 3-methylhexane.
b. Identify the chiral carbon in your structure with an asterisk ().
c. Use three-dimensional diagrams to show the structures of the two optical isomers of this
compound.
Write an account of the similarities between the chemistry of ammonia, amines and amides.
488 | Qatar science scheme of work | Grade 12 advanced | Unit 12AC.5 | Chemistry 5
© Education Institute 2005