Model Answers in Organic Chemistry. For 'A' Level and Ordinary National Certificate Students

116 Pages·1965·1.85 MB·English

by  A. J. Showler, T. A. Brown and C. W. Schofield (Auth.)

T H E C O M M O N W E A L T H A N D I N T E R N A T I O N A L L I B R A R Y
Joint Chairmen of the SIR ROBERT ROBINSON, O.M., F.R.S.,
Honorary Editorial Advisory Board London
DEAN ATHELSTAN SPILHAUS,
Minnesota
Publisher ROBERT MAXWELL, M . C , M.P.
C O M M O N W E A L T H L I B R A R Y O F M O D E L A N S W E R S
Editor C . W . SCHOFIELD
Model Answers in
Organic Chemistry
This book is sold subject to the
condition that it shall not, by way
of trade, be lent, re-sold, hired out
or otherwise disposed of without
the publisher's consent, in any
form of binding or cover other
than that in which it is published.
MODEL ANSWERS
IN
ORGANIC CHEMISTRY
FOR Ά * LEVEL A N D
O R D I N A R Y N A T I O N A L C E R T I F I C A T E S T U D E N T S
BY
A. J. SHOWLER, B.Sc., Ph.D., A.R.I.C.
Lecturer in Organic Chemistry, Medway College of Technology
AND
T. A. BROWN, B.Sc, B.Com.
Senior Science Master, St. Bees School
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Copyright © 1965
Pergamon Press Ltd.
First edition 1965
Library of Congress Catalog Card No. 65-18372
Printed in Northern Ireland at the Universities Press, Belfast
Introduction
IN WRITING this book, the aim has been to present to the student
(particularly the student who is working on his own or who is
inexperienced in sitting for examinations) a series of answers which
show what is required and expected in the General Certificate of
Education Advanced Level and Ordinary National Certificate
examinations. Opinions differ in many cases as to what is required
in answer to a given question, and the authors would not suggest
that their answers are perfect. Nor should the student use them as
such, or assume that all the organic chemistry he needs is to be
found in this book. The answers are intended only to supplement
the information in the normal textbook used in preparing for the
examination, and to serve as a guide to the reader, in that they
show, for example, how long an answer should be, how information
should be presented, the type of sketch required, the essential
equations and details of a preparation.
Nine examining boards at present set Advanced Level examina-
tions in chemistry, and all have substantially the same syllabus.
Ordinary National Certificate is of a similar standard, but with
generally rather more emphasis on aromatic chemistry, and a
wider variation in syllabus, since most technical colleges have their
own and set their own examinations. Nevertheless, the Ordinary
National Certificate candidate could well find any question repro-
duced on the following pages in his examination, though the Ad-
vanced Level student would not expect to be called on to answer
some of the Ordinary National Certificate questions given here.
For this reason, and since many more students sit for Advanced
Level, emphasis has been laid on these questions, which have been
reproduced from the past papers of all the Boards prepared to allow
this. Ordinary National Certificate questions come from four
sources only, three Technical Colleges and the Welsh Joint Education
vii
v i i i INTRODUCTION
Committee. Unless otherwise stated, questions are from the Summer
Advanced Level examination in the year stated.
Naturally, the layout of examination papers varies from one
examining board to another, but all are set out so that the student
answers questions on all three branches of chemistry. It cannot be
too strongly stressed that in studying for the examination, no one
branch, organic, inorganic or physical, should be sacrificed to learn
more of the other two. Such a course may well end in disaster, and
even if the student passes, his basic knowledge may then be so poor
that further progress at a higher level is found to be impossible.
In this respect, Ordinary National Certificate is the more efficient,
since such a course is prevented by the setting of three papers, one
in each branch of chemistry.
It is assumed that a student will either have a copy of the syllabus
of the examination he is to sit for, or will be taught by someone
familiar with it, but, nevertheless, a selection of past papers may also
be useful, so that the candidate can familiarize himself with the
layout of the paper and the type of question set by his board.
(Details of appropriate addresses and costs are set out on pages
107-108.) It should be noted that the University of London Advanced
Level and the three College Ordinary National Certificate papers
(but not the Welsh Joint Education Committee) allow 3 hours for
5 questions, and in all other cases the candidate answers 6 questions
in 3 hours, or 5 questions in 2\ hours. Consequently, answers to
the former questions are a little longer than the others.
In the course of preparation, many alternative answers to parts
of questions presented themselves, and it was felt that these too
should be reproduced, but not in the answer itself, which would be
unduly lengthened by text which would not normally appear in it.
Such alternatives have, therefore, been placed in the notes after the
appropriate question, and the student would put one or other of
the alternatives (but not both) in his answer.
Other interesting points which arise in connection with an answer
are also to be found in the notes, generally because it was felt that
in the time allowed they were not sufficiently relevant to the question
to be included, even though of importance in the general considera-
tion of the topic.
Further questions are reproduced without answers, to enable the
student to obtain practice in answering examination questions, to
INTRODUCTION IX
emphasize points already made, to stress the important parts of the
syllabus, and to fill gaps which inevitably occur when questions are
selected as has been done here.
In answering questions, the student is reminded to take notice of
all the advice given to him by his teachers. In particular, when the
examination comes, remember that the first marks are the easiest to
obtain, so the full number of questions should always be attempted.
Do not overrun your time on any question; keep an eye on the
clock, and at the end of the appropriate time commence the next
question, whether or not the previous one is finished (except perhaps
when nearing the end of a calculation). If you have time at the end,
come back to the unfinished questions. Always allow time to plan
your answer, jotting down a few notes first; this avoids missing
out essential items and putting in irrelevant matter. Finally, leave
time to read through the finished paper; many a small error can
be found and corrected by doing this.
In conclusion, remember that neither this book nor any other
will enable you to pass any examination unless you are prepared to
do the necessary work to learn the chemistry required.
Acknowledgements
THE authors gratefully acknowledge the help of the Examining
Boards and Authorities listed below, in granting permission to
reproduce questions set by them in past examinations.
It is expressly to be noted that copyright still, in all cases, rests
with the Examining body.
Associated Examining Board for the General Certificate of
Education
Southern Universities' Joint Board for School Examinations
Welsh Joint Education Committee
University of Durham School Examination Board
University of London
Bournemouth Municipal College of Technology and Commerce
Medway College of Technology
Whitehaven College of Further Education
These Examining bodies are in no way responsible for the answers
contained in this book, which are solely the work of the authors.
xi
Structure and Isomerism
Explain clearly and concisely how the following are carried out
in the laboratory: (a) Lassaigne sodium fusion, (b) the Carius
method for the quantitative determination of chlorine.
(Welsh Joint Educat ion Commit tee, O . N . C . Syl labus A , 1963, qu . 8)
The most reliable method of detecting nitrogen, sulphur and
halogens in organic compounds is by means of the Lassaigne
sodium fusion. A small amount of the compound under investiga-
tion is gently warmed in a soda-glass tube with a small piece of
sodium. When reaction appears to have ceased and charring is
complete, the tube and its contents are strongly heated and then
plunged into cold water. (During this time the eyes should be
carefully shielded, or goggles worn.) In this way any halogen
present is converted to sodium halide, any sulphur to sodium sul-
phide, and any nitrogen (by combining also with carbon) to cyanide.
The tube is crushed in the water, which is boiled to extract all
the soluble material. The solution is then filtered or decanted off,
and divided into three parts. Sulphide and cyanide are tested for
first, since if present they must subsequently be removed before
testing for halide.
(a) A very dilute solution of sodium nitroprusside is added to one
portion. A purple colour confirms the presence of sulphur.
(b) To a second portion is added ferrous sulphate solution (extra
if sulphide is present). A green precipitate of ferrous hydroxide is
produced, since excess sodium will have rendered the solution
alkaline, due to the formation of the oxide and peroxide.
F e S 0 4 + 2 NaOH = Fe(OH)2 + N a 2 S 0 4
The mixture is then boiled, so that any sodium cyanide present
will produce ferrous cyanide, and after subsequent acidification
1
2 MODEL ANSWERS IN ORGANIC CHEMISTRY
(with concentrated hydrochloric acid) sodium ferrocyanide.
2 NaCN + F e S 0 4 - Fe(CN) 2 + N a 2 S 0 4
Fe(CN)2 + 4 NaCN - Na 4Fe(CN) 6
A few drops of ferric chloride are now added, when a blue or
green colour (or precipitate) due to the formation of ferric ferro-
cyanide results if nitrogen was originally present in the organic
compound (see Note 1). Alternatively the presence of blue specks
on a filter paper after filtration is also positive.
m n
FeCl3 + Na 4Fe(CN) e - N a F e [ F e ( C N ) 6 ] + 3 NaCl
(c) If either or both sulphur and nitrogen are detected, the sulphide
and/or cyanide are now removed from the third portion by acidifica-
tion with sulphuric acid and boiling in an evaporating basin until
no hydrogen sulphide or hydrogen cyanide can be detected in the
vapours.
Na 2S + H 2 S 0 4 - N a 2 S 0 4 + H 2SÎ
2 NaCN + H 2 S 0 4 - N a 2 S 0 4 + 2 H C N |
Silver nitrate solution is added to the remaining solution, and any
precipitate identified as chloride, bromide or iodide by colour (white,
cream or yellow) and ease or otherwise of solubility in ammonium
hydroxide.
NaX + A g N 0 3 - N a N O a + A g X |
If chlorine (or other halogen) has been identified qualitatively,
and it is then desired to carry out a quantitative estimation, the
Carius method is frequently used. This involves heating a weighed
amount of the organic material with fuming nitric acid and silver
nitrate in a sealed, hard glass tube.
In practice the tube is a long one, and 2-4 ml of acid and about
0-2 g of solid silver nitrate are first placed in the tube. The sample
(about 0*15 g) is weighed out in a smaller glass tube which is lowered
into the other, and this is then sealed by carefully drawing it out
and sealing in a blowpipe flame.
The sealed tube is encased in a length of iron pipe, inverted once
to ensure mixing of the sample with the acid and then placed in a
furnace and maintained at 200°C for 4 or 5 hours. Stable sub-
stances may require higher temperatures and longer times.
The tube is opened by playing a very small flame on the tip,
until the glass blows out, after which the end may be safely broken