Organic Chemistry: Alkanes & Alkenes (reaction & polymerization)

Alkane reacts with ...

• Chlorine gas in dark condition

• Iodine solution at 100^oC

• Bromine in inert organic solvent with UV light

• Hot acidified potassium manganate(VII)

Answer explanation

alkane reacts with halogens (chlorine, bromine, and iodine) if got UV ray or about 250^oC.

alkane only reacts with halogens, not other reagents

Cracking of big alkanes ...

• happens at 200^oC

• happens at 400^oC to 500^oC if anhydrous AlCl³ present.

• always forms branched or cyclic hydrocarbon

• not always produce petrol with high Octane number

Answer explanation

if no catalyst, temperature must be around 600^oC to 900^oC

the catalyst is Al₂O₃ or SiO₂, not anhydrous AlCl₃ (that is for benzene)

branched or cyclic hydrocarbons usually formed if got catalyst

if no catalyst, petrol with low Octane number formed

Alkenes do not react with ...

• Cl₂, Br₂, I₂ with or without UV.

• HCl, HBr, or HI

• conc. H₂SO4 and then warmed with water

• haloalkane (RCl, RBr, RI) in the present of anhydrous AlCl₃

• Benzene with H₃PO₄ at 25O^oC

Answer explanation

haloalkane (RCl, RBr, RI) in the present of anhydrous AlCl₃ is the reagent that react with benzene

Alkenes react with ...

• Steam at 300^oC, 60atm in the presence of H₃PO₄ adsorbed on silica

• Hydrogen gas with nickel at 180^oC

• Cold or Hot Acidified Potassium Manganate(VII)

• Oxygen with silver at 200^oC

• Ozone in the presence of concentrated sulphuric acid

Answer explanation

Alkenes react with ozone at 20^oC.

Only then the product will break down if mixed with zinc and hot acid.

Which of the TWO tests are used to identify alkenes? (choose 2 answers)

• add HBr

• add bromine liquid in inert solvent

• add Steam, H₃PO₄ at 300^oC, 60^oC

• add cold acidified potassium manganate (VII)

Answer explanation

HBr: can react but no visible change

Br₂ in inert solvent (cannot use "in CCl₄" anymore): reddish brown liquid decolourised

Steam: can react but no visible change

KMnO₄/H⁺: purple solution decolourised

Which of the following does not match?

• poly(chloroethene) - PVC

• poly(ethylbenzene) - polystyrene

• low density poly(ethene) - LDPE

• high density poly(ethene) - HDPE

Answer explanation

Which of the following is not the condition to produce high density poly(ethene)?

• TiCl₄

• AlCl₃

• lower pressure

• lower temperature

Answer explanation

not AlCl₃, but (CH₃CH₂)₃Al (triethylaluminium)

Which of the following is not related to high density polyethene?

• Ziegler-natta catalyst

• isotactic

• syndiotactic

• atactic

• higher melting point

Answer explanation

atactic = not stereospecific, randomly branched.