Alkanes Flashcards
What is the general formula
CnH2n+2
At rtp which of these are gases, liquids or solids
(In the case of straight chains)
C1-C4 C5-C16 C17+
C1-C4 gas
C5-C16 liquid
C17+ solid
As the size of the chain increases so does the mp and bp
But why?
The forces of attraction between molecules is stronger
In alkanes what are the intermolecular forces that occur (also occur in other homologous series)
London forces
What is the effect of branching
Branched isomers have: Lower mp and bp Increased volatility Decreased density Lower viscosity
Why do branched isomers have a lower mp and bp
Branching causes the molecules to pack less closely together therefore there is a smaller surface area of contact between molecules
Therefore reduced induced dipole
Therefore weaker London forces
Therefore lower mp and bp
What colour are alkanes when pure
Colourless
Are they soluble or insoluble in water and why
Insoluble they are non polar, water is very polar.
They are less dense than water and form an immiscible upper layer
They dissolve very well in other non polar solvents like each other
All bonds in alkanes are…….
A) sigma bonds
B) pi bonds
C) delta bonds
A) sigma bonds
What is the characteristic bond angle in alkanes between hydrogen and hydrogen
109.5 degrees
What is the shape of an alkane
Tetrahedral
Bonding electrons repel each other and the tetrahedral shape allows for the greatest separation of them
Are alkanes:
a) very reactive
b) very unreactive
Towards ionic and polar substances
B) very unreactive
Why are alkanes so unreactive
C-C and C-H bonds very strong and non polar
Relatively high bond enthralpies
Why are alkanes used as fuel for combustion
All alkanes react with a plentiful supply of oxygen and don’t produce toxic products
Readily available
Easy to transport
Are combustion reactions of alkanes endothermic or exothermic
Very exothermic
Are combustion reactions of alkanes spontaneous or not and why
Not (activation energy is required) because combustion involves a free radical mechanism
What is the general equation of complete combustion of alkanes
CxHy + (X+Y/4)O2 –> XCO2 +y/2 H2O
What products for when alkanes incompletely combust
Carbon or carbon monoxide
Water
Alkanes react with halogens in what conditions
Uv light (sunlight)
What is the reaction called between alkanes and halogens
Substitution reaction
What is the mechanism called for bromination of alkanes
Free radical substitution
What are the three steps called for the bromination of alkanes
Initiation
Propagation
Termination
What happens in the initiation step
Primary free radicals form
What is the activation energy for the initiation step and why is it needed
Uv light to break bonds by homolytic fission
Finish this initiation step
Uv light Br2 ----------> ?
2Br^. ( free radical)
What happens in the propagation step
Secondary free radicals form, this is described as a chain reaction
Finish these equations
Br ^. + CH4 -> ? + HBr
? + Br2 -> CH3Br + ?
?1 = ^.CH3 ?2= ^.CH3 ?3= Br^.
What happens in the termination stage
Two radicals collide and react, both radicals are removed from the reaction mixture stopping the reaction
Complete these termination reactions
Br^. + Br^. -> ?
^.CH3 + ^.CH3 ->?
^.CH3 +^.Br -> ?
Br2
C2H6
CH3Br
If the molar ration of Cl2:CH4 (halogen:molar ratio) what is the major product
CH3Cl
How would you purely obtain the major organic product from the free radical mechanism
Distillation
What are the limitations of obtaining haloalkanes via free radical substitution
Many different products are made not just the desired haloalkane
Further substitutions may occur
In larger alkanes substitution may occur in different positions
Free radicals occur in a ? Phase only, ? And solid alkanes Must be ? first
Gas
Liquid
Vapourised
