4.1.2 Alkanes Flashcards
What are alkanes and cyclohexanes examples of
Saturated hydrocarbons
How are sigma bonds formed in alkanes
One sp3 orbital from each carbon overlap to form a single C-C bond
Sigma bond
What can occur around a sigma bond
Free rotation
Shape of alkanes
Tetrahedral
Angle: 109.5 degrees
Four bonded pairs repelling equally
What effects boiling points of alkanes
Size
Shape (branching)
How does size of alkanes effect boiling point
Increases as size increases
Increased number of electrons in larger molecules
Increased size of London forces between molecules
Increased energy to break London forces
How does the shape of alkanes effect boiling point
Increased branching decreases the boiling point
Less points of contact as alkanes can’t pack as closely together so decreased London forces
Decreased boiling point to break London forces
Reactivity of alkanes
Low reactivity
High bond enthalpies of C-C and C-H bonds
Low polarity of sigma bonds
Definition of fuel
Releases heat energy when burnt
How are alkanes useful as fuels
Readily burn in presence of oxygen
Highly exothermic
Explains use as fuels
Complete combustion of alkanes
In excess oxygen will completely combust to form carbon dioxide and water
Incomplete combustion of alkanes
In limited oxygen
Produces toxic carbon monoxide
Produces carbon which is soot
What can production of carbon in incomplete combustion contribute to
Global dimming
Reflection of the suns light
Which type of combustion releases more energy per mole
Complete combustion
What can formation of carbon monoxide cause
Death if built up in enclosed environment
Bonds to haemoglobin so prevents oxygen binding
Definition of cracking
Conversion of large hydrocarbons into smaller molecules by breaking C-C bonds
High mr alkanes -> smaller mr alkanes + alkenes + hydrogen
Economic reasons for catalytic cracking
Petroleum fractions with shorter carbon chains are in greater demand
Make use of larger hydrocarbons and supply demand for shorter ones
Products are more valuable than the starting materials
Conditions of catalytic cracking
Slight pressure
High temperature
Zeolite catalyst
Catalytic cracking
Turns straight chain alkanes into branched and cyclic alkanes and aromatic hydrocarbons
Because they burn more cleanly and give fuels a higher octane number
Why does cracking require high temperatures
Involves splitting of strong covalent bonds so needs high temperatures
Synthesis of haloalkanes from alkanes
Uses mechanism of free radical substitution
Presence of UV light
Three steps of free radical substitution
Initiation
Propagation
Termination
Condition of free radical substitution
UV light
Why is UV light required in free radical substitution
Supplies energy To break the halogen bond
Free radical substitution (using methane and chlorine)
Initiation (using UV light): Cl2 -> 2Cl ⏺
Propagation: CH4 + Cl⏺ -> HCl + CH3⏺
⏺CH3 + Cl2 -> CH3Cl + Cl⏺
Termination: ⏺CH3 + Cl⏺ -> CH3Cl
⏺CH3 + ⏺CH3 -> CH3CH3
(Also regenerates Cl from radicals)
What do propagation steps include
A free radical in the reactants and the products
Further substitution
Excess Cl2 could produce products such as:
CH3Cl2
CHCl3
CCl4
