Download - Properties of Hydrocarbons
Properties of Hydrocarbons
Physical Properties of Alkanes
Non polar compounds Insoluble in water Less dense than water and so will float on top
of the water Dissolve in organic solvents (eg dry cleaning
fluid) and in each other (eg Petrol is a mixture of alkanes)
Low melting and boiling points
Chemical Properties of Alkanes
Very unreactive Don’t react with concentrated sulfuric acid or
concentrated bases Don’t react with strong oxidants (dichromate or
permanganate ions) Will react with bromine but only if heated or in
strong sunlight
Substitution Reactions
Where an atom or group of atoms is displaced by an atom or group of atoms
CH4 + Br2 → CH3Br + HBr Non specific reaction. Can not control which
hydrogen is substituted or how many are substituted
CH3Br + Br2 → CH2Br2 + HBr
Substitution reactions Very slow with heat or sunlight Product is a mixture of bromoalkanes Products would need to be separated by
fractional distillation giving only a very small yield of each
Combustion of Alkanes
Very useful group as fuels Fuels burn completely when there is sufficient
oxygen, forming CO2 and H2O If oxygen is limited then incomplete
combustion occurs and carbon (soot) and carbon monoxide (CO) are formed as well as CO2 & H2O
Combustion of Alkanes Incomplete combustion is less efficient and so
produces less energy per mole that complete combustion
Balancing Combustion Equations
Write correct formula Alkane + O2 → CO2 + H2O
Balance carbon Balance hydrogen Balance oxygen (Fractional coefficients are
acceptable in combustion equations)
Physical Properties of Alkenes
Non polar molecules so they are insoluble in water
Less dense than H2O, so floats on water Low melting and boiling points
Chemical Properties of Alkenes
Combustion is incomplete in air but will burn completely in pure O2
Burn with a smoky flame in air due to unburnt carbon. Higher carbon to hydrogen ratio means some carbon remains unburnt
More reactive than alkanes because the can undergo addition and oxidation reactions
Addition Reactions The colour of bromine rapidly disappears when
shaken with an alkene C2H4 + Br2 → C2H4Br2
The reaction site is the alkene double bond. One bromine adds to the carbon on one end of the double bond and the other bromine to the other end. The double bond becomes a single bond
Addition Reactions
C
H
H
C
H
H
C
H
H
C
H
H
+
Br Br
Br Br
C
H
H
Br C
H
H
Br
C
H
H
Br C
H
H
Br
1,2 dibromo ethane
Addition Reactions An alkene can be converted to the
corresponding alkane by the addition of hydrogen gas
The reaction conditions for this to occur are heat, high pressure and a nickel catalyst
This reaction is called catalytic hydrogenation
Addition Reactions
C
H
H
H C
H
C H
H
C
H
H
H C
H
C H
H
+
H H
H H
Ni Cat HEAT PRESSURENi Cat
Ni Cat
HEAT PRESSURE
HEAT PRESSURE
C
H
H
H C
H
H
C
H
H
H
C
H
H
H C
H
H
C
H
H
H
This reaction is used to turn a pourable oil into a spreadable fat (margarine)
Other Addition Reactions
Alcohols can also be made from alkenes by the addition of water in the presence of an acid catalyst (conc. Sulfuric acid)
C
H
H
C
H
H
C
H
H
C
H
H
+
H O H
H O H
H+ CatH+ Cat
H+ Cat
C
H
H
H C
H
H
O H
C
H
H
H C
H
H
O H
Other Addition Reactions
Alkenes can add to each other to form polymers (very large molecules)
C
H
H
C
H
H
C
H
H
C
H
H
CatCat
Cat
C
H
H
H C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
.
C
H
H
H C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
.
ethene polyethylene