Chapter 12 Alkane Flashcards
How are alkanes bonded ?
By single covalent , or SIGMA bonds, thus saturated
What is a sigma bond
A shared pair of electrons as a result of an OVERLAP OF ORBITALS , where the electron density is highest in a line directly between the TWO ATOMS
What is the shape or alkanes?
109.5°, Always tetrahedral as 4 electron groups of saturated
The fact that alkanes are held by sigma binds means what about rotation?
Sigma bonds act as axes that lets each bond ROTATE, so not RIGID
How does boiling point increase in alkanes?
If chain length increased, or less branch
Why does a higher chain length = higher boiling point ?
2, but 2nd point in particular
- As the chain increases, the bigger the London forces in size and strength due to more electrons
- More surface area allows More contact points meaning more and stronger London forces can be made
Why do less branches = more boiling point?
1) More branched alkanes leads to Less surface area, meaning less contact points and thus WEAKER and fewer London forces (due to distance between), as THEY CANT PACK AS CLOSE
How does fractional distillation work ?
1) in a column with different chambers, a temperature gradient established such that it is hotter at bottom and colder at the top
2) as different fractions of crude oil will have different boiling points, when heated, these will evaporate at different temperatures
3) when one evaporated it will then rise the column as a gas until it reaches an area cold enough to condense again. This will then be collected separately in tanks
4) repeated
Why are alkanes not reactive?
Wha had this allowed them to do
- c-c and c-H SIGMA BONDS ARE STRONG
- C-C and C-H bonds can be considered non polar
2) stay in earth for millions of years
Why are alkanes used as fuels?
- readily available
- easy to transport
- burn in plentiful supply of oxygen
How does initiation of radical substitution work?
sunlight or UV RADIATION IS NEEDED , and this allows alkanes tk react with HALOGENS
For cl2 + CH4
Cl2-> cl. + cl.
Here the covalent bond is broken in HOMOLYTIC FISSION ! GIVING TWO RESCTIVE SPECIES WITH ONE UNPAIRED ELECTRON ESCH = RADICAL
Propagation?
How lead to chain reaction?
1) CH4 + Br.-> Ch3. + Hbr
2) Ch3. + Br2 -> CH3Br + Br.
This can repeat in chain reactions , as new bromine radical can react with another CH4 now
When does propagation end tho?
So if it ends when radicals collide, what’s the point”
Either when
- all the reactants are used up
- or termination step = when radicals collide
2) by then like half a million reactions would have happened first
What are the limitations of radical substitution tho
1) further substitution
1) further substitution
- this is when your product CH3Br can react with another bromine radical to give you CH2Br2, and this is not the product you desire
- this continues until Hydrogens used up and gives you iStudiez if different compounds now , not desire so BAD
Limitations of radical susbtiron part 2
Substation can happen at different positions in chain
Here substitution can produce different STRUCTURAL ISOMERS OF A COMPOUND, giving you a product you don’t desire
- this is even worse if you did further substitution first, as these can go different places
From both limitations you are left with PRODUCTS YOU DONT DESIRE
