Alkanes Flashcards
What are the properties of alkanes?
Hydrocarbons
Saturated
All atoms bonded with σ (sigma) bonds - orbitals overlap directly
σ bonds allow free rotation
Shape around each C is tetrahedral (109.5 degrees bond angel) as there are 4 bonding pairs of e- repelling equally
What does a black wedge show in a display diagram of a compound?
That the bonded atom is sticking outside of the plane.
What does a dash line show in a display diagram of a compund?
The bonded atom is going back
What does the solid line in a display diagram show?
The bonded atom is in the plane
How does boiling change for straight chain alkanes?
Increases as the chain gets longer. Induced dipole-dipole interactions get stronger as number of electrons in molecule increases. More energy is needed to overcome intermolecular forces
How does boiling point change for branched alkanes? (with same no. e-)
Boiling point decreases as branching increases. Induced dipole-dipole interactions get weaker. Branching reduces surface contact between molecules (cannot pack closely)
What is the reactivity of alkanes?
Unreactive due:
- High bond enthalpies in molecules (C-C & C-H)
- Very low polarity in σ bonds. C & H have similar electronegativities.
How do alkanes react with oxygen?
Combust completely on plentiful O2 to form C02 and water. So they can be used as fuel.
Combust incompletely when O2 is in limited supply, forming CO/C and H2O
What are the issues when alkanes incompletely combust?
CO is toxic gas - reduces blood ability to carry oxygen
C - soot particulates irritate respiratory system
How do alkanes react with halogens? What is the mechanism?
In the presence of UV light as they are unreactive (strong non-polar bonds).
Radical substitution mechanism.
Why is radical substitution not a good synthetic method to make a single organic compound?
Because substitution can occur anywhere on the carbon chain. In termination, you get multiple products = low atom economy.
Also, if the halogen is in excess further substitution can occur.
What are the 3 steps of radical substitution?
Initiation - making our free radical
Propagation (usually more than 2 steps, forming a new radical)
Termination - 2 radicals combine for form a non radical. Multiple options
What is homolytic fission?
2 e- in a broken covalent bond are shared equally
