alkanes

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

C_ⁿH_2n+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, 109.5^o

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 400^o → 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

1. initiation

2. propagation

3. 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