CHEMISTRY
ORGANIC
UNIT3 PPAs1-5
Thisisa dehydration
reaction
andnota
condensation
reaction
because
thewatermolecule
hasbeenremoved
from
justonereactant
molecule
andnottwo.
Toprepare
yield.
cyclohexene
fromcyclohexanol
anddetermine
thepercentage
Introduction
Cyclohexene
canbeprepared
bydehydrating
phosphoric
usingconcentrated
ryclohexanol
acid.Thecyclohexene
canbeseparated
from
thereaction
mixture
bydistillation
and,
afterpurification,
it canbeweighed
andthe
percentage
yieldcalculated.
HH
H\ Y ,H
H-t-L-c/-oH
|
|
H-!-.-c,-H
H, A'H
HH
HH
H \/
,-\6-c-6-H
___> |
ll
H-!-..C-r.,
H, A
HH
+H,O
Cyclohexanol
is dehydrated
toproduceEclohexene.
Procedure
About209of cyclohexanol
to a pre-weighed
wasadded
round-bottomed
flask,andtheflaskandcontents
were
reweighed
accurately.
Approximately
8 cm3of concentrated
phosphoric
acidwasaddeddropwise
to thecyclohexanol.
granuleS
Afteradding
a fewanti-bumping
thereaction
mixture
wasslowlydistilled
usingtheapparatus
shown.
reaction
mixture
anti-bumping
granu
res
I
I
I
I
I
t
heat
Whena reaction
mixture
is heated,
thereis a
tendency
for it to boil
violently
aslargebubbles
of superheated
vapour
suddenly
eruptfromthe
mixture.
Thisis orevented
bytheaddition
of theantigranules.
bumping
Theliquidthatcameoverbetween
70oCand90oC
was
collected.
Thedistillate
waspouredintoa separating
funnel
andaboutanequalvolume
of saturated
sodium
chloride
solution
wasadded.
Theseparating
funnelwas
shaken
thoroughly
andleftto stand,
allowing
thetwo
layers
to separate.
Theloweraqueous
layerwasrunoff and
- was
disposed
of,Theupperlayer- thecruderyclohexene
runoffintoa smallconical
flaskcontaining
a fewpieces
of anhydrous
calcium
chloridqa dryingagent.
Thedried
cyclohexene
wasthendecanted
intoa round-bottomed
flasktogether
witha fewanti-bumping
granules.This
crudecyclohexene
wasslowlydistilled
andtheliquidwhichcameoverbetween
80oCand85oCwascollected
in a pre-weighed
receiver
flask.Theflaskwhichnowcontained
purecyclohexene
wasreweighed
accurately,
A fewdropsof thepurecyclohexene
wereaddedto bromine
solution
to testfor unsaturation.
Results
= 30.46g
Massof round-bottomed
flask
Massof round-bottomed
flask+ cyclohexanol= 50.25g
= 19'799
Massof ryclohexanol
= 33'96g
Massof receiver
flask
= 42.489
Massof receiver
flask+ cyclohexene
= 8.52g
Massof cyclohexene
From
thebalanced
eouation:
In orderto calculate
the
1 molcyclohexanol -+
1 molcyclohexene
percentage
yield,youneed
to knowtheaccurate
mass
100.0g
82.0g
(the
of thelimiting
reactant
1e.7eg <--------------- 1e'tex*#$ = $.23s
onewhichis notin excess),
andtheaccurate
mass
=ffix 100
= 52%
%yield
of thepureproduct
that
Thecyclohexene
thatwasprepared
in thisexperiment
wasa colourless
liquidwitha boiling
isformed.
pointof 80-85"C,
lt turned
theoranqe-red
bromine
solution
colourless.
86
I