Exercise 7: HYDROCARBONS Flashcards by Blueberry Cheesecake

organic compounds composed of carbon and hydrogen atoms
only

HYDROCARBONS

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 most hydrocarbons are used as — because of its flammable property

fuel (petroleum products)

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 have a special property to catenate forming seemingly endless
chain of carbons (potential combinations estimated up to )

HYDROCARBONS
10*6

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HYDROCARBONS are — in nature, thereby most of them are — in
water

non-polar

insoluble

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HYDROCARBONS
 can be classified based on the presence of multiple bonds:

– carbon atoms are bonded with single bond
– carbon atoms are bonded with double or triple bonds

• saturated

• unsaturated

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single bond possesses — which is relatively a strong type
of bond compared to double and triple bonds which possesses a —
; are relatively weak type of bond

one sigma bond

sigma and pi bonds (one pi bond for those with double bonds and two
pi bonds for those with triple bonds)

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this makes unsaturated hydrocarbons - than saturated
ones

more reactive

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based on their structures, hydrocarbons can be classified as:

• open-chain –
• closed-chain–

acyclic; linear structure; could be straight or branched
chains

cyclic; ring structure; could aliphatic or aromatic

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HYDROCARBONS are generally classified into
two:

aliphatic and aromatic hydrocarbons.

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 are cyclic hydrocarbons with delocalized pi electrons between carbon
atoms of ring

 their natural characteristics are described as

AROMATIC HCs

aromaticity

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 criteria for aromaticity:

• must be cyclic in structure
• must be flat or planar in configuration
• must have conjugated double bonds
• must follow Hückel’s rule of aromaticity

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AROMATIC HCs
 contain - bonds, that is alternating double bonds
 this special arrangement of pi bonds exhibits resonance which causes
the conjugative stability of the aromatic compounds

conjugated pi bonds

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 according to, a compound is
particularly stable if all of its bonding molecular orbitals are filled with
paired electrons

• the number of pi electrons =

Hückel’s Molecular Orbital Theory

4n + 2, where n = 0 or any positive
whole integer

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being the most common example of an aromatic compound, generally undergo electrophilic aromatic substitution reactions

 it undergoes substitution reaction wherein the ring
system is not destroyed and therefore resist addition of
substituent groups within the pi bonds

 this explains the reason why aromatic compounds
resembles reactions of - rather than -

benzene

saturated, unsaturated hydrocarbons

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IGNITION TEST results

FLAME* PRESENCE OF SOOT
benzene
cyclohexane
gasoline
kerosene
n-hexane

luminous present

non-luminous absent

luminous present

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*Luminous flame burns - flame. Non-luminous flame burns - flame.

brightly yellow

almost invisible blue

– hydrocarbon reacts with oxygen to produce carbon dioxide, water, and heat

𝐶𝑥𝐻𝑦 + 𝑛 𝑂2 ↔ 𝑥 𝐶𝑂2 + 𝑦2 (𝐻2𝑂)

combustion reaction

flame color
yellow

combustion
incomplete

soot
present

heat
produced do not produced
much energy; do not
produce much heat

LUMINOUS FLAME

flame color
Blue

combustion
Complete

soot
Absent

heat
burns efficiently;
produces hotter flame

NON-LUMINOUS FLAME

incomplete combustion reaction – hydrocarbons
react with insufficient amount of oxygen, instead of
producing CO2, produces as
products

CO, H2O and C

when a fuel (e.g. gasoline, kerosene) burns in plenty
of air, it receives enough oxygen for complete
combustion, thereby producing

 when a fuel burns in a limited space (e.g. car
engine), there is no enough oxygen to completely
oxidize the fuel, thereby producing

non-luminous flame

black smoke/soot

BAEYER’S TEST FOR UNSATURATION results

benzene
cyclohexane
gasoline
kerosene
n-hexane

purple-colored solution

dark-brown precipitate

reddish-brown precipitate

purple-colored solution

*is a positive result. Most of the time, solution turns to (still indicate a positive result).

Brown precipitate, reddish-brown

named after Adolf von Baeyer

 used as a qualitative test for unsaturation (presence of double or triple
bonds)

BAEYER’S TEST FOR UNSATURATION

 Baeyer's reagent is an alkaline solution of (strong oxidizer)  reaction with double or triple bonds (-C=C- or -C≡C-) in an organic material causes the color to fade from

potassium permanganate (KMnO4) purple to brown precipitate

oxidation reaction – an alkene/alkyne is oxidized by KMnO4 producing products of

a diol (for alkene), an alkane with four hydroxyl groups (for alkyne), a manganese dioxide (MnO2) and permanganate ion (MnO42-)

BROMINE TEST FOR UNSATURATION results benzene cyclohexane gasoline kerosene n-hexane

yellow-colored solution yellow-colored solution clear pinkish solution** clear colorless solution yellow-colored solution

* is a positive result. ** may yield clear pinkish color because of its innate color.

Clear colorless solution Gasoline

used as a qualitative test for unsaturation (presence of double or triple bonds)

BROMINE TEST FOR UNSATURATION

 bromine is dissolved either in and the alkene/alkyne sample is added to it  reaction with double or triple bonds (-C=C- or -C≡C-) in an organic material causes the color to fade from

dichloromethane, chloroform, or carbon tetrachloride brown to clear colorless solution

addition reaction – an alkene/alkyne reacts with Br2 producing products of a

dibromo for alkene a tetrabromo for alkyne

 the - in alkenes and alkynes are susceptible to addition reactions

weak pi bond

TEST FOR AROMATICITY (NITRATION) results benzene cyclohexane gasoline kerosene n-hexane

pale yellow-colored layer in solution clear colorless solution deep yellow-colored layer in solution yellow-colored layer in solution clear colorless solution

* colored layer in solution is a positive result.

Yellow

benzene is nitrated using a - to produce nitrobenzene  this nitration test predicts the presence of aromatic ring through the formation of - in solution after warming the test sample  only - will proceed through electrophilic aromatic substitution

nitrating acid (HNO3 + H2SO4) yellow colored layer aromatic compounds

➢ nitration of benzene firstly involves the formation of a very powerful electrophile, the - , which is linear ➢ this occurs following the interaction of sulfuric and nitric acid ➢ - is stronger and it protonates the nitric acid on the OH group so that a molecule of water can leave ➢ - attacks the positively charged nitrogen atom of the electrophile, where one of the N=O bonds is broken at the same time ➢ this is followed by rapid loss of a proton to regenerate the aromaticity

electrophilic aromatic substitution reaction: nitronium ion sulfuric acid benzene