4.1.2 Alkanes Flashcards
1
Q
What are the bonding properties of alkanes?
A
- Saturated hydrocarbons
- General formula CnH2n+2
- Contains only single C-C and C-H bonds
- These bonds are sigma bonds
- Free rotation of sigma bonds
2
Q
What are sigma bonds?
A
The overlap of orbitals directly between the bonding atoms
3
Q
What are the shape properties of alkanes?
A
- Each C atom is surrounded by 4 electron pairs
- Electron repulsion results in tetrahedral shape around each C
- 109.5° bond angle
4
Q
How does boiling point change with chain length?
A
- As the chain length increases, the boiling point increases
- Greater surface area of contact so stronger London forces
5
Q
How does boiling point change with branching?
A
- More branched isomers have lower boiling points than straight-chain isomers
- Less surface area of contact so weaker London forces
6
Q
Why are alkanes unreactive?
A
- C-C and C-H bonds are strong with high bond enthalpies
- Alkanes are non-polar
7
Q
What is the complete combustion of alkanes?
A
- React in plentiful supply of O2 to produce CO2 and H2O
- Used as fuels
8
Q
What is the incomplete combustion of alkanes?
A
- React in a limited supply of O2 to produce H2O and CO/C
- CO is toxic as it irreversibly combines with haemoglobin
- Soot is a carcinogen, causing respiratory problems
9
Q
How can the problems of incomplete combustion be mitigated?
A
- Use more oxygen
- Use octane as a fuel
- Use a catalytic converter which turns toxic gases into less toxic gases (HC, CO and NOx into N2, H2O and CO2)
10
Q
What are the steps in free radical substitution?
A
- Initiation
- Propagation (1 + 2)
- Termination
11
Q
What is intiation?
A
- Covalent bond in a halogen molecule is broken by homolytic fission
- UV radiation is required to provide energy
12
Q
What is propagation?
A
- Chain reaction with 2 steps
- In prop 1, a halogen radical reacts with a C-H bond in an alkane to form an alkyl radical and a molecule of a hydrogen halide
- In prop 2, the alkyl radical reacts with a halogen molecule to form a haloalkane and a new halogen radical
13
Q
What is termination?
A
- 2 free radicals react to form a molecule with all paired electrons
- There are multiple possible termination steps
14
Q
How do you write the overall equation for free radical substitution?
A
Every replaced H atom requires 1 molecule of halogen and 1 molecule of hydrogen halide
15
Q
What are the limitations of free radical substitution?
A
- Further substitution (until all hydrogen atoms have been substituted)
- Substitution at different positions in the carbon chain (to produce a mixture of monosubstituted isomers)
