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
CO2 + H2O
Alkane + O2 →
 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
H
H
C
H
+
C
H
Br
Br
Br
H
H
C
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
H
H
C
H
C
H
+
C
H
H
H
H
H
H
C
C
C
H
H
H
Ni Cat
HEAT PRESSURE
H
H
This reaction is used to turn a pourable oil
into a spreadable fat (margarine)
H
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)
H
H
C
H
+
C
H
H
O
H
H
H
C
C
H
H
H+ Cat
H
O
H
Other Addition Reactions
 Alkenes can add to each other to form polymers
(very large molecules)
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
Cat
C
H
C
H
H
ethene
polyethylene
.