Alkanes Flashcards
First functional group of the lot....and know it WELL
1
Q
Where are alkanes mostly found?
A
- Natural gas and crude oil deposits (fossil fuels)
- Due to unreactivity, remains put form millions of years!
2
Q
What is a sigma bond?
A
- This is a covalent bond that occurs when 2 S-Orbitals per atom form an OVERLAP!
- e- density present in LINE BETWEEN BONDING ATOMS, and always present during SINGLE BONDS!
3
Q
Bond angle of alkanes?
A
- 109.5’ !!
- Due to tetrahedral shape, 4 bonding region, 0 lone pairs
- Equal repulsion of e- pairs in bonded region
4
Q
What makes alkanes very unreactive?
A
- Due to having VERY STRONG sigma bonds, a lot of energy needed to break!
- Being non-polar molecules overall, less able to react with other molecules!
5
Q
Increasing boiling point and SA?
A
- Due to greater London Forces able to form = more enrgy required to break = increase in boil point
- Hence, straighter chain = more SA = more bioling point!
6
Q
Possible products for alkene incomplete and complete combustion?
A
- Incomplete = CO or C + H2O
- Complete = CO2 + H2O
7
Q
Formula for complete combustion of alkanes?
A
CxHy + (x + y/4)O2»_space; (x)CO2 + (y/2)H2O
8
Q
How can we describe reacting alkenes with halogens, with steps?
A
- Called FREE RADICAL SUBSITUTION!
- Includes initiation, propagation and termination
9
Q
What takes place in Initiation step?
A
- Covalent bond in diatomic molecule brocken via homolyctic fission, leaving 2 free radicals, one e- transfers to each
- Uses energy of UV RADIATION!
10
Q
What takes place in Propagation step?
A
- Radical will react with a C-H bond in alkane, forming alkane radical!
- Each alkane radical reacts with halogen molecule, to form desired organic product
11
Q
Why can Propagation be described as chain reaction?
A
- In final step, halogen radical as product is formed, which THEN can be used to react with alkane as reactant, repeating same cycle!
12
Q
What takes place in Termination step?
A
- Any 2 radicals will react collide to form a stable molecule!
- This removes radicals from reaction mixture, terminating the reaction overall (per stable molecule)
13
Q
What are the limits of Free Radical Subsitution in Organic Synthesis?
A
- Will form, many, MANY side products!!! = Free radicals reacting multiple times with alkane to form di, tri, tetra ect….. = further radical substition takes place
- Longer chanin can form man, MANY isomers!!! = Substutions with specific H atom cant be controlled = 1 desired alkane very hard to make!
e.g. monosubsutions…..
14
Q
Why does Propagation happen SO MUCH MORE than Termination?
A
- Propagtion MUCH MORE LIKLEY to occur!
- As chances of 2 radical reacting compared to radical and molecule is much less!
