Alkanes Flashcards
Define alkanes
Staurated hydrocarbons, containing only carbon and hydrogen atoms joined together by covalent bonds.
Describe bonding in alkanes
Single covelent bonds between all c-c and c-h bonds, resulting in sigma (σ) bonds forming between the bonded atoms. The sigma bond is the side on overlap of two orbitals one from each orbital.
why are alkane shapes not rigid?
Sigma (σ) bonds act as axes for free rotation
What is the shape of alkanes and why?
tetrahedral in shape, as each carbon has 4 shared pairs of electrons and 4 sigma σ bonds, the repulsion between the electrons in these bonds leads to a 3D tetrahedral shape.
What two factors change alkanes boiling points?
Chain length and branching.
How does increased chain length change the boiling points of alkanes?
As chain length increases so does the surface area contact between the molecules leading to stronger london forces, requiring more energy to overcome leading to higher boiling points.
How does increased branching change alkane boiling points?
As branching increases, there are fewer surface point of contact between the molecules and the molecules cannot pack as closely together, leading to weaker london forces requiring less energy to overcome and therefore lower boiing points.
Why are alkanes not very reactive?
(3 reasons)
1.The C-H and C-C sigma σ bonds are very strong.
2. The C-C bond is non polar.
3. The C-H bond has an extremely low polarity as Carbon and Hydrogen have very similar electronegativities.
What is needed for complete combustion of alkanes and what is produced
Excess/ plentiful supply of oxygen, producing carbon dioxide and water.
What conditions create incomplete combustion of alkanes and what are the products?
Not enough/limited supplies of oxygen, producing carbon (soot) or carbon monoxide, and always water.
What sort of reaction do alkanes react with chlorine and bromine under uv light?
Radical substitution
Write down the initiation, propigation and termination steps for the raection between methane and bromine
- Initiation Br-Br -> Br* + Br*
- Propigation Br* +CH4 -> *CH3 + HBr
CH3 + Br2 -> CH3Br + Br - Termination Br* + Br* -> Br2
CH3 +Br -> CH3Br
*CH3 + *CH3 -> C2H6
What is the limitation of radical substituion?
A mixture of different haloalkanes will be made.
Why are a mixture of haloalkanes produced in radical substitution?
- Once the product has been made (e.g. CH3Br) it can be further react by substitution (e.g. CH2Br2), and so on producing a mixture of haloalkanes. (e.g. CHBr2, CBr4).
- If the alkanes has a long carbon chain, the halogen will be substituted onto any one of the carbons, producing a mixture with differing positions of the halogen on the carbon chain.
