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alkane
a homologous series made up of saturate hydrocarbons which are joined by sigma bonds
what ways can alkanes form
plankton die, fall to the ocean floor→ covered in layers of mud→ fossils over a million of years
general formula of alkanes
CnH2n+2
bonding in alkanes
single covalent bond known as a sigma bond→ a sigma bond is made from the overlapping of 2s orbital from each bonding atom
each carbon atom is joined to 4 other atoms via sigma bond
shapes of alkanes
four bonding pairs of electrons around the carbon atom→ tetrahedral shape, 109.5o
what helps alkanes not be rigid
the sigma bond acts as a point of rotation
forces in alkanes
larger the molecule→ greater surface contact→ more London forces→ more energy to overcome→ higher boiling point
how does bp vary in alkanes?
length of chain branching
how does length of chain branching affect bp in alkanes?
there is less surface are contact between the molecules so fewer London forces= lower energy to overcome so low bp
fractional distillation
process of separating crude oil into groups of hydrocarbons with similar number of carbon atoms
crude oil
mixture of hydrocarbons
process of fractional distillation
crude oil is vaporized at 400o → the vaporized hydrocarbons enter the column and rise upwards(temperature gradient = hot at bottom and cool at top)→ different length hydrocarbons will condense at different levels due to the difference in boiling points
where will larger hydrocarbons condense at in fractional distillation and why?
condense at the bottom due to a higher boiling pint
reactivity of alkanes
alkanes are unreactive
why are alkanes unreactive?
the c-c bond (sigma bond) and the c-h bonds in alkanes are very strong and have a high bond enthalpy
c-c bonds are non-polar so the electronegativity difference between C and H is very small
combustion of alkanes
will react with plentiful supply of oxygen to form carbon dioxide and water
incomplete combustion of alkanes
occurs when alkanes are burnt in a limited supply of oxygen, the products vary either being carbon, carbon monoxide or carbon dioxide
why is carbon monoxide dangerous to humans?
binds to hemoglobin at the same area as oxygen but approx 210 times tighter
bond fission
during reactions bonds break and electrons are redistributed
two types of fission
heterolytic
hemolytic
heterolytic fission
when the bond is broken, both electrons go to one atom, this creates a + and - charged ion
homolytic fission
when the bond is broken, one electron goes to each atoms from the bond, this creates two atoms with an unpaired electron each
what conditions are needed for free radical substitution?
under extreme conditions such as UV light
free radical
an atom with one unpaired electron is called a free radical
photochemical reaction
a reaction bought about by light
what type of reaction is free radical substitution
photochemical
what are the three steps of free radical substitution
initiation
propagation
termination
initiation in terms of radicals
tarts with no radical but ends with a radical
propagation in terms of free radicals
starts with a free radical and ends with one
termination in terms of free radicals
starts with a free radical but does not end with one
do the three steps of free radical substitution for the reaction between methane and chlorine
initiation
Cl2 → 2Cl-
propagation
CH4 + Cl* → HCl + CH3 *
CH3 + Cl2 → CH3Cl + Cl*
termination
2Cl* → Cl2
2CH3* → C2H6
CH3*+ Cl* → CH3Cl
limitations of free radical substitution
there are further substitutions creating a mixture of products
substitutions occur at different positions along the carbon chain