Topic 4: alkanes Flashcards
What are the other names for alkanes?
1) Saturated hydrocarbons
2) Aliphatic compounds
3) Paraffins
What are the 2 principal reactions of alkanes?
1) With O2 = combustion
2) With Cl2 = chlorination = 1 of the Cl from Cl-Cl swaps with the H from CH4 = CH3Cl + H-Cl
Describe chlorination of methane
- Can keep chlorinating until CCl4
- If want CH3Cl + want it to stop = excess CH4 + little Cl2 + remove HCl by reacting with base = will cause shift to right + produce more CH3Cl as product
- If want to go through CCl4 = excess Cl2 + remove HCl to cause shift to right = produce more CCl4
Describe the boiling points of alkanes
- Smoothly increase as number of C increase = increase in surface contact = increase in LDP
- 1st 4 = gas at room temp
- All liquids until 18C
Describe the difference in boiling point for isomers
- n-pentane = longer molecule = more surface contact = more LDF = higher bp
- As it gets more branched = less surface contact = lower LDF = lowering bp
How does shape affect LDF?
- Longer = more surface contact
- More branched and bundled = less surface contact
What are the isomers of pentane?
- n-pentane
- 2-methylbutane
- 2,2-dimethylpropane
What are the isomers of hexane?
- n-hexane = highest bp
- 2-methyl pentane
- 3-methyl pentane
- 2,3-dimethylbutane
- 2,2-dimethyl butane = lowest bp
Explain the difference in bp between cycloalkane stereoisomers
- Cis > Trans
- Cis = higher surface contact on the opposite side to bigger groups = more LDF
- Polar = dipole-dipole
- Trans = bigger groups are above + below = low close contact
- Non-polar
What is the Adolf Von Baeyer theory?
- 3-4 carbon cyclo too strained to exist
- C in cycloalkanes sp3 = tetrahedral of 109
- 3 C = 60
- 4 C = 90
- 5 C = 108
- 6 C = 120
- 3-4 have deviation from tetrahedral angle = greater the deviation = greater strain
How can we measure total strain?
- By combusting the cycloalkane in pure O2
- If highly strained = more energy released
- Cyclopropane + butane = very strained
- Cyclopentane = less strain
- Cyclohexane = strain free
Describe the C-C bonds in cyclopropane
- C-C bonds are bent = banana bonds
- Sp3 don’t overlap due to bond angles
- Reduced overlap = weaker sigma bonds = bond easier to break = more reactive
Describe the torsional strain in cyclopropane
- All adjacent C-H bonds = eclipsed
- 6 H-H eclipses = 6x4=24 kJ
Describe the structure of the cyclobutane
- 3 form a plane + 1 is out of plane
- It folds to reduce torsional strain = reduces the eclipse between H-H
Describe the structure of cyclopentane
- Single fold = 4C on 1 plane + 1 other plane = reduces eclipsing = reduces torsional strain
- Double folding = reduces strain even more
- 26 kJ
Describe the structure of cyclohexane
- Boat shape = reduces torsional + angle strain BUT H-H steric repulsion = not most stable shape
- Chair shape = no eclipsing = all C-H staggered = 0 torsional strain + no angle strain = stable
