5.11 alkanes

Question 1

why are cyclopropane and cyclobutane unstable?

Answer 1

they experience ringstrain: sp3 carbon atoms should be at bond angle 109.5, but are forced into 60 and 90

Question 2

why is boiling point higher for straight chains as opposed to branched isomers?

Answer 2

branched isomers have smaller surface area of contact between molecules. hence there is weaker id-id, less energy required

Question 3

why are alkanes soluble in non-polar solvents but not water and polar solvents?

Answer 3

id-id between alkane molecules and non-polar solvent are similar in strength to id-id between alkane molecules and solvent molecules respectively. sufficient energy is released to overcome id-id, allowing molecules to intermingle

Question 4

why are alkanes less dense than water?

Answer 4

id-id between alkane molecules are weaker than hydrogen bonding between water molecules. hence alkane molecules are pulled less closer together compared to water molecules. there is less alkane per unit volume, less dense

Question 5

why are alkanes generally unreactive?

Answer 5

1) all carbon atoms are sp3 hybridised and fully saturated; hence no addition can occur
2) C-H bonds are strong and non-polar
3) lack of electron-rich or poor sites, unreactive to polar reactants

Question 6

what are the reactions that can be undergone by alkanes?

Answer 6

1) combustion
2) free-radical substitution

Question 7

describe the mechanism of free radical substitution

Answer 7

1) initiation: homolytic fission to form halogen radicals
2) propogation: alkyl radicals formed, halogen radicals re-formed with major product
3) termination: combo all radicals

Question 8

predict the ratio of products 1-chlorobutane and 2-chlorobutane

Answer 8

3:2; 1-chlorobutane is synthesised from the radical generated by any of the 6 equivalent H atoms on carbons 1 and 4, while 2-chlorobutane is synthesised from the radical generated by any of the 4 equivalent H atoms on carbons 2 and 3

Question 9

explain the actual ratio due to tertiary VS primary radical

Answer 9

the rate of substitution is much faster at the tertiary carbon than at the primary carbon due to tertiary radical intermediate being more stable than the primary radical intermediate.
[state which compound is formed by which intermediate]
in both alkyl radicals, the carbon atom with unpaired electron is electron deficient .
in tertiary radical, carbon atoms with unpaired electron is bonded to three electron-donating alkyl groups, while that in primary radical is bonded to only one electron-donating alkyl group. hence, tertiary radical is more stable