Chapter 12 - Alkanes Flashcards
Where can alkanes be found?
Components of natural gas and crude oil
What is a use of alkanes?
Often used as fuels
What is the general formula of alkanes?
CnH2n+2
What is the bonding in alkanes?
They are saturated hydrocarbons
Only single covalent bonds
Only sigma bonds
What is a sigma bond?
Two overlapping orbitals
Two electrons shared between the bonding atoms
What is the shape of an alkane?
Each bond angle is approximately 109.5
How does boiling point in alkanes?
Allows for fractional distillation
Increases for bigger alkanes
Why does boiling point in alkanes change?
Increases as chain length increases
Greater surface area of contact for more IMF
Stronger London forces
How does branching affect boiling point on alkanes?
More branches = lower boiling point
Less surface area of contact
Weaker London forces
Describe the reactivity of alkanes.
Fairly unreactive
C-C and C-H sigma bonds are strong
C-C bonds are non-polar
C-H electronegativity difference is very low so considered non-polar
What is needed and produced in the complete combustion of an alkane?
Needs - Oxygen
Products - Carbon dioxide and water
What can be formed in the incomplete combustion of alkanes?
CO or C
Reaction always prioritises oxidising the Hydrogen to form water
How do alkanes react with halogens?
Radial substitution
Presence of UV
What are the 3 steps of radical substitution?
Initiation
Propagation
Termination
What is initiation in radical substitution?
When the covalent bond in the halogen molecule breaks to form two radicals by homolytic fission
e.g. Br2 -> 2Br^.
What is propagation in radical substitution?
The reaction propagating through two propagation steps in a chain reaction
First - halogen radical reacts with a C-H bond in the alkane to form a CH3 radical and a hydrogen halide molecule
Second - CH3 radical reacts with halogen molecule to form a haloalkane and a new halogen radical
e.g. 1. CH4 + Br^. -> .^CH3 + HBr
2. .^CH3 + Br2 -> CH3Br + Br^.
What is termination in radical substitution?
Two radicals colliding to form a molecule with fully paired electrons
e.g. 2Br^. -> Br2
2CH3^. -> C2H6
CH3^. + Br^. -> CH3Br
What are the limitations of radical substitution?
There are many different products than can form meaning it doesn’t just produce the desired product
Very hard to control - you are unable to choose exactly where on the molecule the halogen substitutes
