Qualitative Test for Hydrocarbons Flashcards
Hydrocarbons – can be grouped into four structural classes
Alkane
Alkene
Alkyne
Aromatic hydrocarbon
test to distinguish different hydrocarbons:
Bayer’s Test
Br2 Test (light and dark)
Tollen’s Test
Sulfuric acid test
the experiment includes the generation of this
ethyne
used to determine the presence of ethyne
flammability test
oxidizes alkenes to geminal diols
dilute aqueous potassium permanganate
means two -OH groups
diol
OH groups on adjacent carbons
geminal
oxidized to geminal diketones
alkynes
in this reaction, the potassium permanganate is reduced to this of manganese dioxide
brown precipitate
what the potassium permanganate is reduced to
manganese dioxide
which solution is the potassium permanganate soluble in
water
neither water nor potassium permanganate is soluble in this
hydrocarbon
where the water and hydrocarbon reaction takes place and is somewhat slow
water-hydrocarbon interface
except cycloalkanes react with bromine
unsaturated hydrocarbons
these are the only hydrocarbons that is negative in br2 (dark)
alkane
hydrocarbons positive in Br2 (light)
all hydrocarbons
usually used as solvents because they are unreactive toward both bromine and hydrocarbons that have multiple bonds
cyclohexane/carbon tetrachloride/tetrachloromethane
undergo an addition reaction with bromine
alkenes and alkynes
its bond becomes a single bond and one bromine atom becomes bonded to each of the carbons that had shared the double bond
alkene
its bond undergoes addition becoming a single bond but the carbons that had been joined by the number of its bond will now hold two bromine atoms
alkyne
react with concentrated sulfuric acid at room temperature to produce alkyl sulfonic acid
alkenes
highly polar and is soluble in the polar sulfuric acid with sufficient stirring
alkyl sulfonic acid
not soluble in the polar concentrated sulfuric acid concentrated sulfuric acid, but dissolves as it reacts and the mixture is stirred
alkene
not capable of forming
hydrogen bonds, while the sulfuric
acid and the alkyl sulfonic can
alkene
especially strong
type of dipole-dipole attraction
hydrogen bonding
in order for the H-bonding to occur:
➢ one of the molecules must
have a hydrogen (directly)
bonded to a fluorine, oxygen,
or nitrogen
will be strongly attached
to a fluorine, oxygen, or nitrogen in
another molecule
hydrogen
more reactive than saturated counterpart
unsaturated hydrocarbons
strong oxidative
reagent that lead to the formation of
alcohol for alkenes and carboxylic
acid for alkynes
Bayer’s reagent
weak oxidizing
agent composed of ammoniacal
silver nitrate
Tollen’s reagent
Reacts with terminal alkynes
to form silver alkyides
Tollen’s reagent
addition of halide
substituents like bromine
halogenation
Occurs via different
mechanisms and extent with
different hydrocarbons
halogenation
Materials and reagent needed (9)
➢ Hexane
➢ cyclohexene
➢ Calcium carbide
➢ Unknown hydrocarbon 1
➢ Unknown hydrocarbon 2
➢ Bromine in CCl4
➢ Bayer’s reagent
➢ Distilled water
➢ Tollen’s reagent
Equipment needed (6)
➢ Pasteur pipettes
➢ Screw-capped test tubes
➢ Separatory funnel
➢ Distilling flask
➢ Iron ring and clamp
➢ Rubber tubing and pinchcock
are combustible,
avoid skin contact and inhalation of
vapour
hydrocarbons
its generation its highly
exothermic
acetylene
has an explosive
tendency when it forms alkylide
precipitates
Tollen’s reagent
Dilute with copious amount of
water and dispose of it
immediately
Tollen’s reagent
5 drops of the following in separate
test tubes:
➢ Hexane
➢ Cyclohexene
➢ Unknown hydrocarbon 1
➢ Unknown hydrocarbon 2
procedures of bayer’s test (2 steps)
➢ 5 drops of Bayer’s reagent to
each sample
➢ Shake and observe
procedures of bromination (with light)
➢ Add 5 drops of bromine
➢ Shake and observe
procedures of bromination (without light)
➢ Wrap the test tube with paper
➢Add 5 drops of bromine and immediately place the sample in a dark place for 2 minutes
➢Observe
procedures of tollen’s test
Add 5 drops of Tollen’s reagent
Shake and observe
label the setup of acetylene generation
grade yourself accordingly
acetylene generation setup (4) steps
➢Add small increments of water to calcium carbide at a time to initiate formation
➢Collect acetylene into test tubes by inserting the end of the rubber tubbing and opening the pinchcock
➢When enough acetylene is collected, cap the tubes immediately
➢Observe any ensuing reaction
used to identify the presence of unsaturation (double or triple bonds) in hydrocarbon compounds
bayer’s test
Detects the presence of carbon-carbon double or triple bonds
bayer’s test
bayer’s reagent is a mixture of
concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)
nitric acid
HNO3
sulfuric acid
H2SO4
the reaction between the reagent and hydrocarbon if unsaturation is present
reddish-brown color
Baeyer’s reagent result if the solution is pinkish purple
negative (alkene and alkyne are absent)
Bayer’s reagent solution discharges color and a brown sludge is formed
positive (alkene and alkyne are present)
color change in bayer’s test is because of this process in the unsaturated bonds in the hydrocarbon by the nitric acid reagent
oxidation
formed when the unsaturated bond react with the nitric acid that causes a rearrangement of electrons
nitro compound
result of bayer’s test:
Hexane –
Cyclohexene –
Acetylene –
Hexane – negative
Cyclohexene – positive
Acetylene – positive
result of br2 test (light)
all positive
result of br2 test (dark)
Alkane is negative in Br2
used to detect the presence of unsaturation, particularly carbon-carbon double or triple bonds
br2 test
Hydrocarbon is mixed with what in br2 test
bromine water
solution of bromine (Br2) dissolved in water
bromine water
reddish-brown liquid at room temperature
bromine
this result in br2 indicates a positive result for unsaturation in the hydrocarbon
colorless solution or color change
what color in br2 test for saturated carbons
orange colored
what color in br2 test for unsaturated carbons
colorless
Result for bromine water test (dark)
Hexane –
Cyclohexene –
Acetylene –
Hexane – no reaction
Cyclohexene – positive result
Acetylene – positive result
Result for bromine water test (light)
Hexane –
Cyclohexene –
Acetylene –
all positive
used to distinguish between an aldehyde and a ketone
Tollen;s test
positive for tollen’s test
dark grey precipitate or silver mirror
negative for tollen’s test
no result
which hydrocarbon will be positive for tollen’s
alkyne
involves the oxidation of unsaturated bonds in hydrocarbons by the nitric acid reagent
bayer’s oxidation
refers to the reaction between a hydrocarbon and bromine (Br2) in the presence of light.
bromination
state the reaction between alkene and KMnO4 and H2O
grade yourself accordingly
used to detect aldehydes and distinguish them from ketones.
tollen’s test
can be used to distinguish between an alkane and alkene/alkyne through the color test
bromine test
state the reaction between ethylene and bromine
grade yourself accordingly
state the reaction between aldehyde and tollen’s reagent
grade yourself accordingly
