Chapter 12 - Alkanes Flashcards
What are sigma (σ) bonds?
The strong covalent bonds that join a carbon atom to other atoms in organic compounds.
Sigma bonds can freely rotate.
e.g. C-C and C-H
A σ bond is the result of…
…the overlap of orbitals directly between the bonding atoms.
Describe and explain the shape of alkanes.
Each carbon atom is surrounded by four bonded electron pairs. The four bonded electron pairs (sigma bonds) repel each other equally, resulting in a tetrahedral shape around each carbon atom, and bond angles of 109.5°.
As the chain length of a hydrocarbon increases, the boiling point ________.
Explain why.
As the chain length of a hydrocarbon increases, the boiling point increases.
As the chain length increases, the molecules have more electrons and more points of contact between molecules. The strength of the London forces between the molecules increases and so more energy is required to overcome the intermolecular forces.
How does UV radiation cause alkanes to react with halogens?
What is the name of this reaction?
UV radiation provides the initial energy for a reaction to take place.
Radical substitution.
Describe the initiation stage of the bromination of methane (example of radical substitution).
The reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission.
The energy for this bond fission is provided by UV radiation.
Each bromine atom takes one electron from the electron pair, forming two highly reactive bromine radicals.
Br-Br → Br• + •Br
Describe the propagation stage of the bromination of methane (example of radical substitution).
- A bromine radical reacts with a C-H bond in the methane, forming a methyl radical •CH3, and a molecule of hydrogen bromide HBr.
CH4 + Br• → •CH3 + HBr - Each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, CH3Br, (the intended product) together with a new bromine radical Br•.
•CH3 + Br2 → CH3Br + Br• - The new bromine radical then reacts with another CH4 molecule as in the first propagation step, and the two steps can continue to cycle through in a chain reaction.
In practise propagation is terminated whenever two radicals collide. It has been estimated that up to a million propagation cycles take place before a termination step stops the reaction.
Give the equation for the initiation, propagation and termination stages of the bromination of methane.
Initiation stage:
Br-Br → Br• + •Br
or
Br2 → 2Br•
Propagation stage:
CH4 + Br• → •CH3 + HBr
•CH3 + BR2 → CH3Br + Br•
Termination stage:
Br• + Br• → Br2
•CH3 + •CH3 → C2H6
•CH3 + •Br → CH3Br
Describe the termination stage of the bromination of methane (example of radical substitution).
Two radicals collide, forming a molecule with all electrons paired. There are a number of possible termination steps with different radicals in the reaction mixture.
Br• + Br• → Br2
•CH3 + •CH3 → C2H6
•CH3 + •Br → CH3Br
When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction.
What is the general formula of an alkane?
CnH2n+2
State whether alkanes are soluble and explain your answer.
Alkanes are non-polar molecules and do not dissolve in polar solvents like water. However, they will dissolve in non-polar solvents.
What are the limitations of using radical substitution for organic synthesis?
A large mixture of products can be formed.
The product CH3Cl of one of the possible termination reactions contain hydrogen atoms which can be replaced by further substitution with bromine to produce a mixture of other products.
For alkanes with a chain of three or more carbon atoms, substitution can take place at different positions along the carbon chain giving a mixture of structural isomers.
Why do alkanes have a low reactivity?
High bond enthalpy and very low polarity of the non-polar C-C and C-H bond.
As branching of a hydrocarbon increases, the boiling point ________.
As branching of a hydrocarbon increases, the boiling point decreases.
