Transcript Hydrocarbons (revision)

Some reactions of
hydrocarbons
Substitution of alkanes
Aqueous bromine is
put in a test tube,
then cyclohexane
poured carefully on
top.
The hydrocarbon layer
floats on top of the
aqueous bromine
solution.
When mixed, the nonpolar bromine moves
from the aqueous layer
into the hydrocarbon
layer, turning the alkane
orange. A small amount
of bromine remains in
the aqueous layer,
making it very pale
orange.
No chemical reaction
has occurred at this
stage.
Put the test tube in bright light for
ten minutes.
After 10 minutes sitting
in the bright light of an
overhead projector, the
bromine in the
hydrocarbon layer has
decolourised.
The colour of the
aqueous layer is
unaffected.
The reaction between bromine and alkanes is a
SUBSTITUTION reaction.
• The reaction is slow.
• It requires UV light.
• Two products are formed: a bromoalkane and
hydrogen bromide.
Any hydrogen in the chain could be replaced by a
bromine atom, so in fact there are many different
organic products formed.
CH3CH3 + Br2
→ CH3CH2Br + HBr
Cracking alkanes
Long-chain alkane molecules can be cracked into a
shorter-chain alkane and an alkene:
C12H26
Paraffin—a mixture
of liquid alkanes—can
be cracked in the
laboratory using a
steel wool catalyst.
→
C8H18
+ C4H8
Here the catalyst is Al2O3.
In industry a nickel catalyst is used.
Addition of alkenes
Aqueous bromine is put
in a test tube, then
cyclohexene poured
carefully on top.
The hydrocarbon floats
on the more dense
aqueous solution.
When the test tube is
shaken, the bromine
enters the organic layer
and is immediately
decolourised.
More bromine from the
aqueous layer moves into
the organic layer, until
all the bromine has
reacted.
Bromine water added
dropwise to
cyclohexene is also
instantly decolourised.
The reaction between bromine and an alkene is an
ADDITION reaction.
• The reaction is fast.
• Light is not required.
• Only one product is formed.
cyclohexene + bromine → 1, 2-dibromocyclohexane
CH2=CH2 + Br2
→
CH2Br—CH2Br
Acidified potassium permanganate and hydrocarbons
Compounds containing no double or triple bonds are
called saturated. Alkanes are saturated
hydrocarbons.
Compounds containing one or more C=C or C≡C bonds
are unsaturated because these bonds can be broken
to add in more hydrogen (or anything else).
Purple potassium permanganate solution can be used
to distinguish between saturated and unsaturated
hydrocarbons.
Alkane
Alkene
Saturated
Unsaturated
The hydrocarbons float on top of the aqueous
solution of permanganate.
Alkane
Alkene
When the test tubes are shaken, the alkene
decolourises the acidified permanganate. There is no
reaction with the alkane.
If acid is not added to the permanganate solution, the
purple solution will change to a brown precipitate.
The permanganate oxidises the
double bond to form a diol.
Alkenes undergo addition reactions with several different
reagents:
• with hydrogen to form alkanes (hydrogenation).
This occurs during margarine manufacture or at the
oil refinery where hydrogen is added to stop alkenes
forming in petrol.
• with bromine to form a dibromoalkane (or chlorine)
• with HBr or (HCl) to form haloalkanes
• with H2O to form alcohols.
An acid catalyst is required.
Typically a 50% H2SO4 solution or conc H3PO4 with
water is used.
• with permanganate to form diols.
Markovnikov’s rule
When an unsymmetric reagent, such as HBr or H2O, is
added to an unsymmetric alkene, two different products
will be formed. In these cases one product will be formed
in a greater amount than the other.
The rule is that the hydrogen atom is given to the carbon
atom which already has the greater number of hydrogen
atoms. In summary: the rich get richer.
H H H
H H H
H—C—C
C—H + H
HBr
C=C
Br
H
poor
rich
→
H—C—C—C—H
H
H H H
H—C—C—C—H
H H Br
Major
product
Minor
product
Another look at alkenes with bromine
The experiment shown in the movie which follows this
slide is extremely dangerous and should only be done by
very experienced chemistry teachers if they know how to
handle liquid bromine safely.
Bromine vapour causes deep and painful burns to the skin
which can take many months to heal – even through
gloves…
— which is why we made a movie of the experiment so you
can see it in safety.
Special thanks to Rudi Jansen from Middleton Grange
School who did this reaction for you.
In the test tube there is some bromine water to which we are
now going to add an alkene. You will notice the alkene does not
mix with bromine water, alkene being non-polar and the bromine
being in aqueous solution. There's not much reaction there.
If we were to shake it the colour fairly quickly is removed from
the bromine, as you can see, and the hydrocarbon continues to
sit at the top.
We use this as a test for unsaturation but it's not particularly
impressive, so what we want to do is to show how reactive
bromine really is with the alkene by adding molecular bromine
directly to the alkene.
So this is one of the two liquid elements, bromine itself (you
can see the vapour -- maybe) and we will add some drops of
(bromine to the cyclohexene). You will notice the reaction is
fairly vigorous and bromine is immediately decolourised and,
over time you may see a change in the meniscus of the
alkene...as the reaction proceeds.
This should impress you that the reaction of bromine with
alkenes is certainly rapid, and you can notice too, that the
material has now sunk to the bottom.
Click on the movie to start.
It’s a big file that may take a while
to load.
The audio is a little hard to follow in
places, so it is written for you here.
There is a small amount of bromine still present and if we were
to shake this up now you will see the colour quite disappear.
If you write the formula for this and check out the molar mass
of the cyclohexene and then the cyclohexene with two bromines
added, you will notice a very marked difference in the molar
masses which should account for the change in behaviour from
the layer sitting on top to now at the bottom of the aqueous
layer.
Cyclohexene = C6H10 M(C6H12) = 82 g mol–1
Bromine = Br2
M(Br2) = 160 g mol–1
1, 2-dibromocyclohexane = C6H10Br2
M(C6H10Br2) = 242 g mol–1
So when two bromine atoms are added to cyclohexene
the molar mass triples.
Although cyclohexene is less dense than water, the
haloalkane formed is more dense than water and sinks.