Hydrocarbons

Question 1

What are the physical properties of hydrocarbons?

Answer 1

• Hydrophobic.
• London Dispersion Forces present only.
• Lower boiling points than compounds the same size but with functional groups.
• Very long alkanes can have very high boiling points due to the sum of all the dispersion forces. A useful reference is that heptane, the 7 membered alkane, has the same boiling point as water.

Question 2

Combustion Reaction

Question 3

What are the free radical initiators?

Answer 3

UV light (hv) or peroxides

Question 4

Stability of free radicals

Answer 4

The more substituted the radical, the more stable it is.

Stability: 3° > 2° > 1° > methyl.

Question 5

Radical Chain Reaction Mechanism

Answer 5

Initiation: Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.

• *Propagation:** (a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
  (b) The methyl radical abstracts a bromine atom from another molecule of Br₂ to form the methyl bromide product and anotherbromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.

Termination: Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle.

Question 6

Ring Strain in Cylic Compounds

Answer 6

Cyclopropane has the highest ring strain and Cyclohexane has the lowest ring strain.

Any ring with greater or equal to 14 carbon atoms has the next lowest ring strain.

Question 7

What is Angle (Baeyer) strain?

Answer 7

Angle (Baeyer) strain is caused by deviation from the ideal sp³ tetrahedral bond angle of 109.5°

Question 8

What is torsional strain?

Answer 8

Torsional strain is caused by the molecule having eclipsed conformations instead of staggered ones.

Question 9

Ring strain of bicylic molecules

Answer 9

Bicyclic molecules have more ring strain than monocyclic molecules. Except for spiro bicyclics, which have similar ring strain as their monocyclic counterparts.

Question 10

Most common molecular geometries

Answer 10

Linear, trigional planar & tetrahedral

Question 11

Rank the following in terms of bond length and bond energy:

SINGLE BOND, DOUBLE BOND, and TRIPLE BOND

Answer 11

SINGLE BOND (longest length, smallest energy)

DOUBLE BOND

TRIPLE BOND (shortest length, highest energy)

Question 12

Name this functional group

Answer 12

Iso-propyl

Question 13

What is this functional group?

Answer 13

Isobutyl

Question 14

Name this functional group

Answer 14

Sec-butyl

Question 15

Name this functional group

Answer 15

Tert-butyl