C12- Alkanes Flashcards
Naming an Alkane
Isomers
Find longest chain
identify length and number of remaining branches
number chains starting on end closest to the branch
Written in alphabetical order if there are multiple branches
Alkane general formula
Cn H2n+2
Bonding present in Alkanes
Sigma bonds
Single covalent bond formed due to overlap of p orbitals form each bonding atom
Each carbon atom in an alkane has 4 sigma bonds
Shape of alkanes
Each carbon surrounded by 4 pairs of electrons in sigma bonds
3D tetrahedral arrangement around each carbon atom
bond angle of 109.5
Alkanes from crude oil
Crude oil contains many different Alkanes
Crude oil is separated into different fractions by fracrional distillation in a distillation tower
This is becasuie of differences in boiling points of alkanes, increasing as chain length increases
Alkane
Effect of chain length on boiling point
As chain length increases molecules have a larger surface area, so surface area of contact increases between molecules
The London forces get stronger between molecules and require more energy to overcome
Increased chain length increases boiling point
Alkane
Effect of branching on boiling point
Isomers have same Mr
Fewer surface points of contact between molecules of branched alkanes
–> weaker London forces
Shape of molecules prevents the molecules form getting as close to each other as in straight chain molecules, decreasing intermolecular forces further
Why are alkanes unreactive
Non polar
High bond enthalpy
Forced to react by radicals
Radicals
Chemical species with one or more unpaired electrons
These unpaired electrons are very reactive
Represented by a bold dot after an element
Radical substitution
3 key steps
Initiation- creation of radicals where none before
Propagation- Radicals in reactants and products
Termination- two radicals combine to form a covalent bond
Radical substitution
Overall reaction
Combination of propagation reactions
Write out in full then cancel out atoms or radicals of the same type
Side product
Product of an alternative reaction
Not necessarily unwanted
Alkanes combustion
Complete- sufficient oxygen
produce carbon dioxide and water
incomplete combustion- limited supply of oxygen
produces water and carbon monoxide (toxic) or soot
Alkane combustion
Oxygen needed as you progress through the homologous series
As you descend the homologous series, each alkane needs and extra 3/2 O2 molecules
Conditions for alkane to react with halogen
UV light
Provides initial energy for the reaction to take place
