612
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Read each question carefully before beginning your
answer. If the question is divided into parts, be
clear how many parts you must answer. Understand
clearly what is being asked. Jot down headings to
help you structure your answer. To be sure to get
maximum credit, organize your answer, setting the
solution neatly and legibly. Cross out any rough work
or mistakes. Take special care with multiple-choice
questions, and do not make guesses if you are going
to be penalized for wrong answers.
Answer the question asked. Easy marks are lost by
not answering the question asked. Do not spend time
giving information that is not required whilst missing
out other aspects that are clearly asked for. Do not try
to copy out your lecture notes. The examiner expects
you to be selective and to formulate a critical answer
to the question.
Be quite clear you fully understand the meanings
of the terms ‘discuss’, ‘evaluate’, ‘illustrate’, and
‘outline’, which are frequently used in examination
questions. You must appreciate the subtle differences,
and provide the appropriate level of detail to answer
the question effectively.
A title that includes ‘discuss’ or ‘evaluate’ requires
you to sum up both positive and negative evidence
or aspects. A title that says ‘illustrate’ or ‘outline’ is
only asking for evidence or aspects that are consistent
with the proposition.
Most organic chemistry questions do not require
descriptive answers, but can be adequately answered
by generous use of structures and mechanisms. These
can be annotated with pertinent points as appropriate.
Budget your time. Keep an eye on the clock, and do
not spend more than the appropriate amount of time
on any question. It may not be worthwhile trying to
get a few extra marks on one question at the expense
of gaining easy marks by starting another. You must
attempt the full number of questions specified on the
paper. Note any breakdown of marks allocated for
each part of a question, and use this to apportion the
time and effort spent on the component parts.
Check your answers. Try to finish a little early
so that you can read through your answers. Make
sure your answers are complete. It is easy to make
simple errors. Check for any missing answers to parts
of questions, especially multiple-choice questions.
Make sure your answers make scientific sense, and
in numerical answers check that you have included
units and considered significant figures.
Mechanisms or calculations that appear unnecessarily complicated are probably wrong and should be
checked for obvious errors. Incorrect transcription of
structures or figures is a common cause of problems.
16.2 How to approach the problem:
‘Propose a mechanism for . . .’
You will usually be provided with the reactants,
reagents, conditions, and the products formed, though
in some questions you may be required to predict the
nature of the products.
• Carry out a quick analysis of the carbon skeletons
of reactants and products to ascertain what the
reaction represents.
Is it merely modification of substituents, e.g. as
in substitution or elimination?
Is it formation of a bigger skeleton, e.g. as in an
addition reaction?
Is it formation of a smaller skeleton, e.g. as in a
cleavage reaction?
Is it a modification that has involved a rearrangement of the basic skeleton?
Register this change, but do not try to accommodate the reaction at this stage. Instead, move
on to the reagents and conditions, as follows.
• Consider the essential principles of mechanism.
Look for nucleophiles, electrophiles, and leaving
groups.
• Consider the conditions. If the reaction requires
a catalyst, e.g. acid or base, it is almost certain
that this needs to be used in the first step. For
example, acid may protonate an electronegative
atom, making a more reactive species. Thus, a
protonated carbonyl becomes a better electrophile,
and a protonated alcohol now has a better leaving
group. Base may remove an acidic proton to
generate a better nucleophile, although in some
reactions it may itself act as the nucleophile.
• At this stage, the mechanism should develop
logically by interaction of a nucleophile with an
electrophile, or by displacement of a leaving group.
If your mechanism is to make sense though, certain
aspects, which are really rather obvious, need to
be considered. You cannot sensibly use X
− as a
nucleophilic reagent under acid conditions, and
there is not going to be any H
+ under basic
conditions. In the former case, you must use HX
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
Read each question carefully before beginning your
answer. If the question is divided into parts, be
clear how many parts you must answer. Understand
clearly what is being asked. Jot down headings to
help you structure your answer. To be sure to get
maximum credit, organize your answer, setting the
solution neatly and legibly. Cross out any rough work
or mistakes. Take special care with multiple-choice
questions, and do not make guesses if you are going
to be penalized for wrong answers.
Answer the question asked. Easy marks are lost by
not answering the question asked. Do not spend time
giving information that is not required whilst missing
out other aspects that are clearly asked for. Do not try
to copy out your lecture notes. The examiner expects
you to be selective and to formulate a critical answer
to the question.
Be quite clear you fully understand the meanings
of the terms ‘discuss’, ‘evaluate’, ‘illustrate’, and
‘outline’, which are frequently used in examination
questions. You must appreciate the subtle differences,
and provide the appropriate level of detail to answer
the question effectively.
A title that includes ‘discuss’ or ‘evaluate’ requires
you to sum up both positive and negative evidence
or aspects. A title that says ‘illustrate’ or ‘outline’ is
only asking for evidence or aspects that are consistent
with the proposition.
Most organic chemistry questions do not require
descriptive answers, but can be adequately answered
by generous use of structures and mechanisms. These
can be annotated with pertinent points as appropriate.
Budget your time. Keep an eye on the clock, and do
not spend more than the appropriate amount of time
on any question. It may not be worthwhile trying to
get a few extra marks on one question at the expense
of gaining easy marks by starting another. You must
attempt the full number of questions specified on the
paper. Note any breakdown of marks allocated for
each part of a question, and use this to apportion the
time and effort spent on the component parts.
Check your answers. Try to finish a little early
so that you can read through your answers. Make
sure your answers are complete. It is easy to make
simple errors. Check for any missing answers to parts
of questions, especially multiple-choice questions.
Make sure your answers make scientific sense, and
in numerical answers check that you have included
units and considered significant figures.
Mechanisms or calculations that appear unnecessarily complicated are probably wrong and should be
checked for obvious errors. Incorrect transcription of
structures or figures is a common cause of problems.
16.2 How to approach the problem:
‘Propose a mechanism for . . .’
You will usually be provided with the reactants,
reagents, conditions, and the products formed, though
in some questions you may be required to predict the
nature of the products.
• Carry out a quick analysis of the carbon skeletons
of reactants and products to ascertain what the
reaction represents.
Is it merely modification of substituents, e.g. as
in substitution or elimination?
Is it formation of a bigger skeleton, e.g. as in an
addition reaction?
Is it formation of a smaller skeleton, e.g. as in a
cleavage reaction?
Is it a modification that has involved a rearrangement of the basic skeleton?
Register this change, but do not try to accommodate the reaction at this stage. Instead, move
on to the reagents and conditions, as follows.
• Consider the essential principles of mechanism.
Look for nucleophiles, electrophiles, and leaving
groups.
• Consider the conditions. If the reaction requires
a catalyst, e.g. acid or base, it is almost certain
that this needs to be used in the first step. For
example, acid may protonate an electronegative
atom, making a more reactive species. Thus, a
protonated carbonyl becomes a better electrophile,
and a protonated alcohol now has a better leaving
group. Base may remove an acidic proton to
generate a better nucleophile, although in some
reactions it may itself act as the nucleophile.
• At this stage, the mechanism should develop
logically by interaction of a nucleophile with an
electrophile, or by displacement of a leaving group.
If your mechanism is to make sense though, certain
aspects, which are really rather obvious, need to
be considered. You cannot sensibly use X
− as a
nucleophilic reagent under acid conditions, and
there is not going to be any H
+ under basic
conditions. In the former case, you must use HX