4.1.2: Alkanes Flashcards
Intermolecular forces in alkanes:
London forces
London forces
Temporary instantaneous mutually induced dipole-dipole attractions.
Why are alkanes non-polar?
• All bonds are σ bonds
• σ bonds are formed by head on overall of atomic or hybrid orbitals
• Carbon and hydrogen have fairly similar electronegativities
∴ No permanent dipole
General formula for alkanes
C𝒏H₂𝒏+₂
Trends in MBP
- Longer chain = higher MBP
* More branching = lower MBP due to intermolecular forces
Why are alkanes unreactive with many reagents?
- High bond enthalpy
* Very low polarity of the σ bonds
Free radical
A species with a single unpaired electron.
Why do you not get 100% yield in a reaction?
- Reaction may be reversible
- Unfavourable conditions
- Slow reaction
- Byproducts may be formed
- Competing reactions may lead to alternate products
- Products may react with each other to form something else
- Mechanical losses
What is a sigma bond?
Overlap of orbitals directly between the bonding atoms.
Enables free rotation of the σ bond.
Equation to balance complete combustion of any alkane?
CxHʏ +(x+ ʏ/4)O₂ ⟶ xCO₂ + ʏ/2H₂O
Each successive alkane needs how much more oxygen for complete combustion?
1 ½ extra O₂ molecules required per extra CH₂
Why is incomplete combustion bad?
It produces CO which is toxic.
By what mechanism does bromine react with alkanes?
Free radical substitution
Limitations of radical substitution in synthesis
Forms a mixture of organic products
Products can substitute further
Substitution can occur at different points along the carbon chain –> difficult to control
Bromination of alkanes - initiation step
Br₂ ⟶ Br• + •Br
